SlideShare une entreprise Scribd logo
1  sur  8
Télécharger pour lire hors ligne
SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                             Engg.Chemistry




UNIT –II
                                                                                     INTRODUCTION

                                                                                             Metals have a natural tendency to revert back to combined states. During this process mostly, oxides are
                                                                                     formed though in some cases sulphides, carbonates, subparts etc. may result due to presence of impurities. Any
                                                                                     process of deterioration and loss of solid metallic material by chemical or electrochemical attack by its



  SCIENCE OF                                                                         environment is called corrosion. Corrosion is the reverse process of metallurgy.



                                                                                               Metal
                                                                                                           Corrosion(Oxidation)


                                                                                                                                   Metallic Compound + Energy
                                                                                                           Metallurgy(Reduction)




  CORROSION                                                                          Eg: Rusting of iron when exposed to atmospheric conditions. Rust is hydrated oxide (Fe 2O3.xH2O).

                                                                                     CORROSION

                                                                                           It may be defined as “the process of gradual deterioration or disintegration or eating away of metal by a
                                                                                     chemical or electrochemical reaction in its environment ”.

                  P. V. NARAYANA REDDYM.Sc.,. Ph.D                                   Rusting of Iron based on Electrochemical Theory of corrosion

                                                                                     Eg: Yellow rust, actually corresponds to Fe2O 3. 3H2O on the surface of iron.

                                                                                         Greenish layer formation on the surface of Copper

                                                                                      Theory of corrosion
                                                                                      There are three theories of corrosion: (i) Acid theory, (ii) dry or chemical corrosion and
                                                                                      (iii)Electrochemical or wet corrosion.

                                                                                      (i)Acid theory of corrosion

                                                                                             This theory suggests that corrosion of a metal (iron) is due to the presence of acids surrounding it.
                                                                                     According to this theory, iron is corroded by atmospheric carbon di- oxide, moisture and oxygen. The corrosion
UNIT-II
                                                                                     products are the mixture of Fe(HCO3)2, Fe(OH)CO3 and Fe(OH)3. The chemical reactions suggested are given
Science of Corrosion: Definition, Types of corrosion: Dry Corrosion, (Direct         below
Chemical attack), Wet Corrosion, Theories of Corrosion and Mechanism, Electro
Chemical Theory of Corrosion. Galvanic Series, Galvanic Corrosion, Concentration
Cell Corrosion, Oxygen absorption type. Factors Influencing Corrosion. Control of
Corrosion – Cathodic Protection – Sacrificial anode and Impressed Current. Uses of
Inhibitors. Electro Plating, and Electro less plating (copper and nickel)                    This theory is supported by the analysis of rust that gives the test for CO32- ion. Further, the process of
                                                                                     rusting is reduced by the presence of lime and caustic soda (these two can absorb CO2, thus reducing corrosion).



1                                                                                    2 Narayana Reddy
SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                                  Engg.Chemistry        SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                                Engg.Chemistry


Types Of Corrosion: Two types of Corrosion occur base on the environment                                                                       e.g., Ag, Au and Pt do not undergo oxidation Corrosion.

1.Dry (or) Chemical Corrosion                                                                                                       c)      Volatile that is oxide layer volatilizes after formation and as such leaves the underlying metal
                                                                                                                                    surface exposed further attack. This causes continuous which is excessive. e.g. Molybdenum oxide (MoO3)
        This type of Corrosion occurs mainly through the direct chemical action of atmospheric gasses such as O2,
                                                                                                                                    d)      Porous that is the oxide layer formed having pores or cracks. In this case the atmospheric oxygen
halogens, H2S, SO2, N2 and anhydrous inorganic liquid with metal surfaces in its immediate proximity. Three types
                                                                                                                                    passes through the pores or cracks of the underlying metal surface. This causes continuous corrosion till
of chemical Corrosion are as follows..
                                                                                                                                    complete conversion of metal into its oxide.
                                                                                                                            Pilling Bedworth Rule: If volume of metal oxide on the surface of a metal is more than or equal to the volume of
(i)Oxidation Corrosion:        This is carried out by the direct action of oxygen low or high temperatures on metals
                                                                                                                            metal, the oxide layer will be protective. For example, for Al2O3, Fe, Ni, ZnW, Cr. It will be non-protective if volume
in absence of moisture at ordinary temperature metals or very slightly attacked. The Exceptions are Alkali metals
                                                                                                                            of oxide is less than volume of metal. (The specific volume ratio of W is 3.6, Cr = 2.0, Ni = 1.6. Hence, the rate of
and Alkaline earth metals. At high temperature all metals are oxidized. The exception is Ag, Au and Pt.
                                                                                                                            corrosion is very less in tungsten.) It is called Pilling Bedworth rule.
                   2M                    2Mn++2ne-      (De-electronation)
                                                                                                                            (ii) Corrosion by other gases:Gases such as Cl2, SO2, H2S, NOx : In dry atmosphere, these gases react with metal
                                      (Metal ion)
                                                                                                                            and form corrosion products which may be protective or non-protective. Dry Cl2 reacts with Ag and forms AgCl
              nO2+2ne-                   2nO2-        (Electronation)                                                       which is a protective layer, while SnCl4 is volatile. In petroleum industries at high temperatures, H2S attacks steel
                                     (Oxide ion)                                                                            forming FeS scale which is porous and in- terferes with normal

            2M+ nO 2                2Mn+ + 2nO2- (Metal oxide)                                                              operations.
Mechanism: At the surface of metal oxidation occurs and the resulting metal oxide scale forms a barrier which
                                                                                                                            (iii) Liquid metal corrosion: In several industries, molten metal passes through metal- lic pipes and causes
restricts further oxidation. For oxidation to continue either the metal must diffused outwards through the scale to
                                                                                                                            corrosion due to dissolution or due to internal penetration. For example, liquid metal mercury dissolves most
the surface or the oxygen must defuse inwards through the scale to the underlying metal. Both the cases are
                                                                                                                            metals by forming amalgams, thereby corroding them.
possible. But the outward diffusion of metal is generally more rapid than inward diffusion of oxygen since metal
ion is appreciably smaller than the oxygen ion and hence more mobile.                                                       2.Wet or Electrochemical Corrosion:

                   Metal + Oxygen                Metal oxide                                                                It is a common type of corrosion of metal in aqueous corrosive environment. This type of corrosion occurs when
                                                                                                                            the metal comes in contact with a conducting liquid or when two dissimilar metals are immersed or dipped partly
        When oxidation starts, a thin layer of oxide is formed on the metal surface and the          nature of this film
                                                                                                                            in a solution. According to this theory, there is the formation of a galvanic cell on the surface of metals. Some parts
decides further action.
                                                                                                                            of the metal surface act as anode and rest act as cathode.
        If the film is,
                                                                                                                              The chemical in the environment and humidity acts as an electrolyte. Oxidation of anodic part takes place and it
a)         stable, it behaves has a protective coating in nature e.g., the oxide films on Al, Pb, Cu, Pt etc., are stable   results in corrosion at anode, while reduction takes place at cathode. The corrosion product is formed on the
           and therefore further oxidation correction of prohibited.                                                        surface of the metal between anode and cathode.
 b)        Unstable that is the oxide layer formed decomposes back into metal and oxygen. So, oxidation Corrosion
                                                                                                                              To understand the wet theory, let us take the example of corrosion of iron. Oxidation of metal takes place at
is not possible.
                                                                                                                            anode while the reduction process takes place at cathode. By taking rusting of iron as an example, the reaction can
                                Metal oxide               Metal + Oxygen
                                                                                                                            be explained as that it may occur in two ways:

3 Narayana Reddy                                                                                                            4 Narayana Reddy
SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                          Engg.Chemistry       SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                               Engg.Chemistry

  (i) evolution of hydrogen type (ii) absorption of oxygen type                                                           This type of corrosion causes displacement of hydrogen ions from the solution by metal ions. All metals
                                                                                                                   above hydrogen in electrochemical series have a tendency to get dissolved in acidic solution with simultaneous
At anode: oxidation occurs. At cathodic part, reduction reaction (electro nation) occurs. It does not affect the
                                                                                                                   evolution of H2 gas. The anodes are large areas, whereas cathodes are small areas.
cathode, since most metals cannot be further reduced. At cathodic part, the dissolved constituents in the
conducting medium accepts the electrons forming ions (OH-, O2-). The metallic ions formed at anodic part and the   Absorption of oxygen: - For example, rusting of iron in neutral aqueous solution of electrolytes in presence of
ions formed at cathodic part diffuse towards each other through conducting medium and form a corrosion product     atmospheric oxygen. Usually the surface of iron is coated with a thin film of iron oxide. If the film develops cracks,
somewhere between anode and cathode.                                                                               anodic areas are created on the surface. While the metal parts act as cathodes. It shows that anodes are small areas,
                                                                                                                   while the rest metallic part forms large cathodes.
Mechanism:

Electrochemical corrosion involves flow of electrons between anode and cathode. The anodic reaction involves
dissolution of metal liberating free electrons.

                      M                 Mn+ + ne-

The cathodic reaction consumes electrons with either evolution of hydrogen or absorption of oxygen which
depends on the nature of corrosive environment.

(i) Evolution of hydrogen: This type of corrosion occurs in acidic medium e.g., considering the metal Fe, anodic
reaction is dissolution of iron as ferrous ions with liberation of electrons.


                                                                                                                   At anode:      Fe              Fe2+ + 2e- (Oxidation)
              Fe                 Fe2+ + 2e- ( Oxidation)
                                                                                                                   At cathode:

                                                                                                                   The released electrons flow from anode to cathode through iron metal.

                                                                                                                                        ½ O2 + H2O + 2e-                2OH-(Reduction)

                                                                                                                                               Fe2+ + 2OH-              Fe(OH)2

                                                                                                                      (a) If oxygen is in excess, ferrous hydroxide is easily oxidized to ferric hydroxide.
                                                                                                                                    4Fe(OH)2 + O 2 + 2H2O                  4Fe (OH)3

                                                                                                                      The product called yellow rust corresponds to Fe2O3. xH2O.
The electrons released flow through the metal from anode to cathode, whereas H+ions of acidic solution are
eliminated as hydrogen gas.
                                                                                                                   Concentration Cell Corrosion :
                          2H++2e-          H2
                                                                                                                                  This type of corrosion is due to electrochemical attack on the metal surface exposed to an
       The overall reaction is
                                                                                                                   electrolyte of varying concentrations or of varying aeration. The most common type of concentration cell corrosion
                          Fe+2 H+               Fe2+ + H2                                                          is the differential aeration corrosion which occurs when one part of metal is exposed to different air concentration
                                                                                                                   from other part. This causes a difference in potential between the differently aerated areas. Experimentally it has
5 Narayana Reddy                                                                                                   6 Narayana Reddy
SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                               Engg.Chemistry       SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                                Engg.Chemistry

been observed that poor oxygenated parts are anodic. Differential aeration of metal causes a flow of current called     particular atmosphere, i.e. sea water. In galvanic series, oxidation potential of metals is arranged in the decreasing
the differential current.                                                                                               order of activity of a series of metals. The series is towards the increasing noble nature.

Differential Aeration Corrosion: It occurs when a metallic surface is partially immersed in an electrolyte and                  More anodic: Mg, Mg alloys, Zn, Al, Cd, Fe, Pb, Sn, Ni–Mo–Fe alloys) Brasses, Cu, Ni, Cr–steel alloy, Ag,
partially exposed to air as shown in the figure below. Poorly oxygenated metallic part becomes anodic and               Ti, Au, Pt towards noble nature.
undergoes oxidation. Well oxygenated part becomes cathodic. At the cathode, O 2 takes up electrons to form OH-
ions.                                                                                                                                   Electrochemical series                                    Galvanic series
                  If a metal e.g., Zn is partially immersed in a dilute solution of a neutral salt e.g., NaCl and the
solution is not agitated properly, then the parts above and adjacent to the waterline are strongly aerated and hence             a. This series consists of metals and non-      a.     This series consist of metals and alloys.
                                                                                                                                    metals                                       b.     Position of pure metal and when present in
become cathodic. Whereas parts immersed show a smaller oxygen concentration and become anodic. so there is a                     b. The position of a metal in this series is           the form of alloy is different.
difference of potential which causes flow of current between two differentially aerated areas of same metal. Zinc                   permanently fixed.                           c.     It predicts the relative corrosion
                                                                                                                                 c. It predicts the relative displacement               tendencies.
will dissolve at anodic areas and oxygen will take up electrons at the cathodic areas forming hydroxyl ions.                        tendencies.                                  d.     Corrosion of metals and alloys is studied in
                                                                                                                                 d. Electrode potentials are measured by                unpolluted sea water.
                    Zn               Zn2+ + 2e-          (Oxidation)                                                                dipping pure metals in their salt
                                                                                                                                    solution of 1M concentration.
                  ½ O2 + H2O + 2e-                2OH-   (Reduction)

Following are the facts about differential aeration corrosion:

    (a) Less oxygenated part is the anode. Therefore cracks serve as foci for corrosion.
    (b) Corrosion is accelerated under accumulation of dirt, scale or other contaminations. This restricts the access
        of oxygen resulting an anode to promise greater accumulation. The result is localized corrosion.
    (c) Metals exposed to aqueous media corrode under blocks of wood or glass which restricts the access of
        oxygen.
GALVANIC SERIES

        Electrochemical reactions are predicted by electrochemical series. A metal having higher position can
replace (reduce) other metals that have lower position in the series. For example,

                    Zn + CuSO 4          ZnSO 4   + Cu

 that is,           Zn + Cu++             Zn ++ + Cu

    Or in other words, zinc will corrode faster than copper.

Some exceptions have been observed in this generalization. For example, Ti is less reactive than Ag.

Galvanic series is the series of metals that is made keeping in view the process of corrosion of a metal in a

7 Narayana Reddy                                                                                                        8 Narayana Reddy
SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                                 Engg.Chemistry      SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                                 Engg.Chemistry

    Active (Anodic)                                                 1.     Mg                                                a) Metal surface are not homogeneous.
                                                                    2.     Mg alloys
                                                                    3.     Zn                                                b) External environment is not homogeneous.
                                                                    4.     Al                                                c) Films are not perfectly uniform.
                                                                    5.     Cd
                                                                    6.     Al alloys                                         d) Crystallography directions are not equal in the reactivity.
                                                                    7.     Mild steel
                                                                    8.     Cast Iron                                         e) Environment is not uniform with respect to concentration.
                                                                    9.     high Ni cast Iron                             Pitting is usually the result of the breakdown or cracking of the protective film on a metal at specific points. This
                                                                    10.    Pb-Sn solder
                                                                    11.    Pb                                            gives rise to the formation of small anodic and large cathodic areas. In process of correct environment this
                                                                    12.    Sn
                                                                    13.    Iconel                                        produces corrosion current.
                                                                    14.    Ni-Mo-Fe alloys
                                                                    15.    Brass                                         e.g., Stainless steel and aluminum show characteristic pitting on chloride solution. Pitting is caused by the presence
                                                                    16.    Monel
                                                                    17.    Silver solder                                 of sand, dust scale and other extraneous impurities present on the metal surfaces. Because of differential amount of
                                                                    18.    Cu                                            oxygen in contact with the metal, the small part (underneath the impurity) becomes the anodic areas and the
                                                                    19.    Ni
                                                                    20.    Cr stainless steel                            surrounding large parts become the cathodic areas. Intense corrosion takes place in the anodic areas underneath
                                                                    21.    18-8 stainless steel
                                                                    22.    18-8 Mo stainless steel                       the impurity. Once a small pit is generated, the rate of corrosion will be increased.
                                                                    23.    Ag
                                                                    24.    Ti
                                                                    25.    Graphite
                                                                    26.    Au
    Noble(Cathodic)                                                 27.    Pt



                                                  Table: Galvanic series

GALVANIC CORROSION :

       When two dissimilar metals are electrically connected and exposed to an electrolyte, the metal higher in
electrochemical series undergoes corrosion. This type of corrosion is called Galvanic corrosion .e.g., Zinc (higher in
electrochemical series) forms the anode and is attacked and gets dissolved; whereas copper (lower in                     Factors Influencing Corrosion:
electrochemical series) acts as cathode.
                                                                                                                             The rate and extent of corrosion depends on
Mechanism: If the solution is acidic then corrosion occurs by hydrogen evolution process and if the solution is
                                                                                                                             (i) Nature of the metal
neutral or slightly alkaline in nature then corrosion occurs by oxygen absorption process. The electrons flow from
                                                                                                                             (ii) Nature of corroding environment.
the anodic metal to the cathodic metal.
                                                                                                                         Nature of metal:
                                             Zn                Zn2+ + 2e-        (Oxidation)
                                                                                                                         (a)Position in galvanic series: when 2 metals or alloys are in electrical contact in presence of an electrolyte the more
               Thus the corrosion is a localized accelerated attack resulting in the formation of pits, holes or         active metal having higher position in the galvanic series undergoes corrosion. The greater is the difference in
       cavities. Pitting corrosion therefore results in the formation of pinholes, pits and cavities in the metal. The   position, the faster is the corrosion.
       pitting corrosion may be due to following reasons:
9 Narayana Reddy                                                                                                         10 Narayana Reddy
SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                                  Engg.Chemistry       SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                                  Engg.Chemistry

(b)Over voltage: reduction in overvoltage of the corroding metal accelerates the corrosion rate. E.g. Zn in 1N H2SO4       (c) Impurity of atmosphere: Pollutants like H2S, SO2, CO2 and acid vapours cause more pollution where they
undergoes corrosion slowly because of high overvoltage of zinc metal (0.7 V) which reduces the effective potential         dissolve. In sea water (salty in nature which acts as an electrolyte) corrosion rate increases. Some suspended
to a small value. In presence of CuSO4 the corrosion rate of zinc is accelerated.
                                                                                                                           particles are dissolved in humidity and form electrolyte which helps in corrosion.
(c) Relative areas of anodic and cathodic parts: When 2 dissimilar metals are in contact, the corrosion of the anodic      (d) pH Value: pH value means concentration of H+ (acidic nature). In acidic medium (pH less than 7),
part is directly proportional to the ratio of areas of cathodic part and the anodic part. Corrosion is rapid and           corrosion is faster. Also, in basic medium pH > 7, some metals such as Pb, Zn, Al, etc. form complexes and hence
localized if anodic area is small, because the current density at a smaller anodic area is greater.                        they corrode. Pourbiax relation between pH of medium and potential of metal deals with the corrosion process and
(d) Purity of Metal: Impurities in a metal generally cause heterogeneous state forming minute electrochemical cells        it gives idea how to reduce corrosion.
resulting corrosion of anodic part. E.g. Zinc metal with impurities Pb or Fe. Corrosion resistance of a metal may be       Example: Zn corrodes minimum at pH 11, but at higher pH (more than 11) it corrodes faster. At pH 5.5, Al
improved by increasing its purity.                                                                                         corrodes minimum.
(e)Physical state of Metal: The rate of corrosion is influenced by physical state of the metal such as grain size,         (e) Nature of ions present: Cu++ ions present in the vicinity of Fe, accelerate corrosion, while silicates present in the
stress; orientation of crystals etc., The smaller the grain size of even in pure metal becomes the anode undergoing        vicinity resist corrosion.
corrosion.                                                                                                                 (f) Conductance effect: Due to presence of salts and water in earth, it is of con- ducting nature. More conductance
(f) Nature of surface film: In aerated atmosphere, all metals get covered with a thin surface film of metal oxides. The    leads to more stray current and hence fast corrosion. Dry sandy soil is less conducting and hence less corrosion,
ratio of the volumes of metal oxides to the metal is known as specific volume ratio. Greater is this value lesser is the   while mineralised clay soil is more conducting hence more corrosion occurs.
oxidation corrosion rate.                                                                                                  (g) Oxygen concentration cell: Oxygen is one of the important element responsible for corrosion. It forms oxides
(g) Passivity of Metal: Passive metals are resistant to corrosion due to the formation of highly protective but very       and hydroxides (in presence of H2O) on the surface of metal as corrosion product. Oxygen concentration cell
thin film on the metal or alloy surface E.g. Corrosion resistance of stainless steel is due to passivity character of      is formed on the surface of metal due to difference in oxygen concentration (iron rod half dipped in water corrodes
Chromium present in it.                                                                                                    due to this effect). Dipped portion will be anode and outer portion will be cathode.
(h) Solubility of corrosion products: In electrochemical corrosion if the corrosion product is soluble in the corroding
medium then corrosion is rapid. If the corrosion product is insoluble, then acts as barrier thereby suppressing
further corrosion.
(i) Volatility of corrosion products: If the corrosion product is volatile, then the underlying surface is exposed for     Cathodic Protection :
further attack. This causes rapid and continuous corrosion. E.g. MoO3 is volatile.
                                                                                                                                   The cathodic protection of metals is used to control corrosion metals where it is impractible to alter the
(ii) Nature of corroding environment:
                                                                                                                           nature of the corrosion medium. The principle involved in this method is to protect metals and alloys from
(a)Temperature: As the temperature of environment is increased the reaction rate is increased thereby accelerating
                                                                                                                           corrosion by making them completely cathodic. Since there will not be any anodic area on the metal, therefore
corrosion.
                                                                                                                           corrosion does not occurs.
(b)Humidity of air: critical humidity is defined as the relative humidity above which the atmosphere corrosion rate
                                                                                                                           The following are 2 types of cathodic protections.
of metal increases sharply. The value of critical humidity depends on nature of metal and corrosion products.
                                                                                                                               1. Sacrificial anodic protection
Corrosion of a metal is furnish in humid atmosphere because gases (CO2, O 2) and vapours present in atmosphere
                                                                                                                               2. Impressed current cathodic protection
furnish water to the electrolyte essential to establish an electrochemical corrosion cell. The oxide film on the metal
                                                                                                                           Sacrificial anodic protection:
surface has the property to absorb moisture. In presence of this absorbed moisture, corrosion rate is enhanced.
                                                                                                                               In this method, the metal structure can be protected from corrosion by connecting it with wire to a more
Rain water may also wash away the oxide film from the metal surface. This leads to enhanced atmospheric attack.
                                                                                                                           anodic metal. As this more active metal is sacrificed in the process of saving metal from corrosion, it is known as
The exceptions are Cr, Al.

11 Narayana Reddy                                                                                                          12 Narayana Reddy
SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                              Engg.Chemistry       SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                                  Engg.Chemistry

sacrificial anode. The metals which are commonly used as sacrificial anodes are Mg, Zn, Al and their alloys. The       Inhibitors are mainly classified into types
important applications of this method are                                                                              Anodic inhibitors:
   1. Protection of underground cables and pipelines from soil corrosion.                                              Anodic inhibitors are those which prevent the corrosive reaction occurring at anode by reacting with the ions of
   2. Protection of ships and boat hulls from marine corrosion.                                                        the anode and forming insoluble precipitates. The precipitate thus absorbed on the surface of the metal and forms
   3. Prevention of rusty water by inserting Mg sheets or rods into domestic water boilers or tanks.                   a protective coating resulting in reducing the corrosion rate. If insufficient inhibitors are used, it results in certain
                                                                                                                       area unprotected leading to severe local attack. The anodic inhibitors used are phosphates, chromates, molybdates,
                                                                                                                       alkalis, tungstates etc.
                                                                                                                       Cathodic Inhibitors:
                                                                                                                       The cathodic reaction occurs in acidic solution is the evolution of hydrogen given as
                                                                                                                                                                2 H+ + 2e-  H2
                                                                                                                               The diffusion of hydrogen ions in the acidic solution can be slowed by using organic inhibitors like a mines,
Impressed current cathodic protection:                                                                                 mercaptanes, and heterocyclic nitrogen compounds etc. This organic inhibitor is absorbed at the metal surface and
   As the name implies, an impressed current is applied to convert the corroding metal from anode to cathode.          reduces the corrosion rate.
The applied current is in opposite direction since to nullify the corrosion current. This can be accomplished by               The cathodic reaction occurs at the neutral solution is
applying sufficient amount of direct current source like battery or rectifier to an anode like graphite, high silica                                    H2O + ½ O 2 + 2e-  2OH-
iron, stainless steel or platinum buried in the soil or immersed in the corrosion medium. And connected to the                 In this, the corrosion reaction rate can be controlled by reducing the diffusion of oxygen to the cathodic
corroding metal structure which is to be protected as shown in the diagram below..                                     area or by removing oxygen from the corroding medium. For this, Na2SO3 is used to eliminate O2 and Mg, Zn or Ni
                                                                                                                       salts are used to reduce the diffusion of O2 to the corroding area.
                                                                                                                               These inhibitors react with OH- ions at the cathode forming a layer of insoluble hydroxides which are
                                                                                                                       impermeable and hence reduce the diffusion of oxygen to the cathode area.
                                                                                                                       ELECTROPLATING:

                                                                                                                       Principle of Electroplating:

                                                                                                                               This process involves coating of a thin layer of one metal over another metal by passing direct current
                                                                                                                       through an electrolytic solution. The base metal to be plated is made of cathode whereas the anode is made of
                                                                                                                       either coating metal itself or an inert material in the electrolytic cell.

       In impressed current cathodic protection, electrons are supplied from an external cell, so that the object      Procedure:
itself becomes cathodic and not oxidized. This type of cathodic protect ion has been applied to buried structures
                                                                                                                       The article to be electroplated is first treated with organic solvent to remove oils, greases etc. then, it is made free
such as tanks and pipelines, transmission line-towers, marine piers, laid-up ships etc. since, their operating and
                                                                                                                       from surface scales, oxides, etc., by treating with dil. HCl or H2SO 4. The cleaned article is then made cathode of an
maintenance costs are less, they are well suited for large structures and long term operations.
                                                                                                                       electrolytic cell. The anode is either the coating metal itself or an inert material of good electrical conductivity.
INHIBITORS:                                                                                                            The electrolyte is a solution of a soluble salt of the coating metal. The electrolytic solution is kept in an
       A corrosion inhibitor is “a substance which when added in small quantities to the aqueous corrosive             electroplating tank. The anode and cathode are dipped in the electrolytic solution. When direct current is passed,
environment effectively decreases the corrosion of a metal”.                                                           coating-metal ions migrate to the cathode and get deposited there. Thus, a thin layer of coating-metal is obtained

13 Narayana Reddy                                                                                                      14 Narayana Reddy
SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                            Engg.Chemistry       SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR                                  Engg.Chemistry

on the article, made as the cathode. For brighter and smooth deposits, favourable conditions such as low
temperature, medium current-density and low metal-ion concentration are used.




                                                                                                                       “Coming together is a beginning; keeping together is progress; working together is
                                                                                                                                                           success.”


ELECTROLESS PLATING:
Definition: The process of producing a thin, uniform and hard deposit of metal on an activated substrate (Metal or
non-metal) by using suitable soluble reducing agents without any electrical energy, and the driving force for the
deposition is auto catalytic redox reactions.

        The reducing agent reduces the metallic ions to metal, which gets plated over the catalytically activated
surface giving a uniform thin coating.

                 Metal ions + Reducing agent             Metal + Oxidised products

Process:

The process involves

    1. Pretreatment or activation of work piece to be plated.
    2. Preparation of bath composition.
1. Pretreatment or activation of work piece to be plate

   (i) Metals like Cu, Ag etc. are known as non-catalytic metals. Surface of such metals need activation. They are
   activated by using steel or iron pieces for initiating the reactions.
                                                                                                                      “Time is the coin of your life. It is the only coin you have, and only you can determine how it will
   (ii) Non-metals like glasses, ceramic, plastics are activated by dipping in SnCl2, PdCl2 in HCl. This process                       be spent. Be careful lest you let other people spend it for you.”
   produces a thin film of palladium coating on non-metal surfaces which in turn causes the work piece to get
   activate for electroless plating.

                                                     ΩΩΩΩΩΩ


                       www.pvnreddy.blogspot.com
15 Narayana Reddy                                                                                                    16 Narayana Reddy

Contenu connexe

Tendances

Unit 3 corrosion & batteries
Unit 3                                       corrosion & batteriesUnit 3                                       corrosion & batteries
Unit 3 corrosion & batteriesprudhvi8143
 
Corrosion, standard grade chemistry
Corrosion, standard grade chemistryCorrosion, standard grade chemistry
Corrosion, standard grade chemistryacalero
 
Chapter 5: Corrosion & Non-ferrous Metal
Chapter 5: Corrosion & Non-ferrous MetalChapter 5: Corrosion & Non-ferrous Metal
Chapter 5: Corrosion & Non-ferrous Metalsyar 2604
 
Metal corrosion and its prevention
Metal corrosion and its preventionMetal corrosion and its prevention
Metal corrosion and its preventionDr. Sandip Thorat
 
Loss of corrosion and prevention of corrosion
Loss of corrosion and prevention of corrosionLoss of corrosion and prevention of corrosion
Loss of corrosion and prevention of corrosionAl Fahad
 
Corrosion and its Control
Corrosion and its ControlCorrosion and its Control
Corrosion and its ControlDr. Arun Sharma
 
Corrosion Process and Control
Corrosion Process and ControlCorrosion Process and Control
Corrosion Process and ControlAnsar Lawi
 
Corrosion of material - Engineering Metallurgy
Corrosion of material - Engineering MetallurgyCorrosion of material - Engineering Metallurgy
Corrosion of material - Engineering MetallurgyMechXplain
 
Types of corrosion & selection of moc
Types of corrosion  & selection of mocTypes of corrosion  & selection of moc
Types of corrosion & selection of mocSudeep Paralkar
 
Corrosion and its control
Corrosion and its controlCorrosion and its control
Corrosion and its controlvraju6
 

Tendances (20)

Corrosive Damage In Metals & Its Prevention
Corrosive Damage In Metals & Its PreventionCorrosive Damage In Metals & Its Prevention
Corrosive Damage In Metals & Its Prevention
 
Unit 3 corrosion & batteries
Unit 3                                       corrosion & batteriesUnit 3                                       corrosion & batteries
Unit 3 corrosion & batteries
 
Chapter 5
Chapter 5Chapter 5
Chapter 5
 
Corrosion & its control measures
Corrosion & its control measuresCorrosion & its control measures
Corrosion & its control measures
 
Corrosion, standard grade chemistry
Corrosion, standard grade chemistryCorrosion, standard grade chemistry
Corrosion, standard grade chemistry
 
Forms of corrosion
Forms of corrosionForms of corrosion
Forms of corrosion
 
Chapter 5: Corrosion & Non-ferrous Metal
Chapter 5: Corrosion & Non-ferrous MetalChapter 5: Corrosion & Non-ferrous Metal
Chapter 5: Corrosion & Non-ferrous Metal
 
Theories of corrosion
Theories of corrosionTheories of corrosion
Theories of corrosion
 
Metal corrosion and its prevention
Metal corrosion and its preventionMetal corrosion and its prevention
Metal corrosion and its prevention
 
Corrosion control
Corrosion control Corrosion control
Corrosion control
 
Corrosion
CorrosionCorrosion
Corrosion
 
Loss of corrosion and prevention of corrosion
Loss of corrosion and prevention of corrosionLoss of corrosion and prevention of corrosion
Loss of corrosion and prevention of corrosion
 
Corrosion
CorrosionCorrosion
Corrosion
 
Corrosion and its Control
Corrosion and its ControlCorrosion and its Control
Corrosion and its Control
 
Corrosion Process and Control
Corrosion Process and ControlCorrosion Process and Control
Corrosion Process and Control
 
Corrosion Prevention
Corrosion PreventionCorrosion Prevention
Corrosion Prevention
 
Corrosion of material - Engineering Metallurgy
Corrosion of material - Engineering MetallurgyCorrosion of material - Engineering Metallurgy
Corrosion of material - Engineering Metallurgy
 
Types of corrosion & selection of moc
Types of corrosion  & selection of mocTypes of corrosion  & selection of moc
Types of corrosion & selection of moc
 
Ate 102 corrosion
Ate 102 corrosionAte 102 corrosion
Ate 102 corrosion
 
Corrosion and its control
Corrosion and its controlCorrosion and its control
Corrosion and its control
 

En vedette

C16 alcohols and carboxylic acids
C16 alcohols and carboxylic acidsC16 alcohols and carboxylic acids
C16 alcohols and carboxylic acidsChemrcwss
 
Chromium and insulin sensitivity
Chromium and insulin sensitivityChromium and insulin sensitivity
Chromium and insulin sensitivityfpgg
 
40 cfr 261.4(b)(6) The RCRA Exclusion From Hazardous Waste for Trivalent Chro...
40 cfr 261.4(b)(6) The RCRA Exclusion From Hazardous Waste for Trivalent Chro...40 cfr 261.4(b)(6) The RCRA Exclusion From Hazardous Waste for Trivalent Chro...
40 cfr 261.4(b)(6) The RCRA Exclusion From Hazardous Waste for Trivalent Chro...Daniels Training Services
 
Removal of chromium
Removal of chromiumRemoval of chromium
Removal of chromiumAmr Elshikh
 
Nouveau microsoft word document
Nouveau microsoft word documentNouveau microsoft word document
Nouveau microsoft word documentkarimfpk
 
10 major industrial applications of sulfuric acid
10 major industrial applications of sulfuric acid10 major industrial applications of sulfuric acid
10 major industrial applications of sulfuric acidrita martin
 
A SHORT REVIEW ON ALUMINIUM ANODIZING: AN ECO-FRIENDLY METAL FINISHING PROCESS
A SHORT REVIEW ON ALUMINIUM ANODIZING: AN ECO-FRIENDLY METAL FINISHING PROCESSA SHORT REVIEW ON ALUMINIUM ANODIZING: AN ECO-FRIENDLY METAL FINISHING PROCESS
A SHORT REVIEW ON ALUMINIUM ANODIZING: AN ECO-FRIENDLY METAL FINISHING PROCESSJournal For Research
 
Chromium problems
Chromium problemsChromium problems
Chromium problemscrazyaxe
 
Brochure Meca-19102016-bd
Brochure Meca-19102016-bdBrochure Meca-19102016-bd
Brochure Meca-19102016-bdCamille Volant
 
Protection des métaux contre la corrosion
Protection des métaux contre la corrosionProtection des métaux contre la corrosion
Protection des métaux contre la corrosionCHTAOU Karim
 
TRANSITION METALS
TRANSITION METALSTRANSITION METALS
TRANSITION METALSDrix78
 
TALAT Lecture 5203: Anodizing of Aluminium
TALAT Lecture 5203: Anodizing of AluminiumTALAT Lecture 5203: Anodizing of Aluminium
TALAT Lecture 5203: Anodizing of AluminiumCORE-Materials
 

En vedette (19)

Corrosion.ppt
Corrosion.pptCorrosion.ppt
Corrosion.ppt
 
Prevention of corrosion
Prevention of corrosionPrevention of corrosion
Prevention of corrosion
 
Corrosion
CorrosionCorrosion
Corrosion
 
final report.edit
final report.editfinal report.edit
final report.edit
 
CORROSIVE DAMAGE IN METALS AND ITS PREVENTION
CORROSIVE DAMAGE IN METALS AND ITS PREVENTIONCORROSIVE DAMAGE IN METALS AND ITS PREVENTION
CORROSIVE DAMAGE IN METALS AND ITS PREVENTION
 
C16 alcohols and carboxylic acids
C16 alcohols and carboxylic acidsC16 alcohols and carboxylic acids
C16 alcohols and carboxylic acids
 
Chromium ppt
Chromium pptChromium ppt
Chromium ppt
 
Chromium and insulin sensitivity
Chromium and insulin sensitivityChromium and insulin sensitivity
Chromium and insulin sensitivity
 
40 cfr 261.4(b)(6) The RCRA Exclusion From Hazardous Waste for Trivalent Chro...
40 cfr 261.4(b)(6) The RCRA Exclusion From Hazardous Waste for Trivalent Chro...40 cfr 261.4(b)(6) The RCRA Exclusion From Hazardous Waste for Trivalent Chro...
40 cfr 261.4(b)(6) The RCRA Exclusion From Hazardous Waste for Trivalent Chro...
 
Removal of chromium
Removal of chromiumRemoval of chromium
Removal of chromium
 
Nouveau microsoft word document
Nouveau microsoft word documentNouveau microsoft word document
Nouveau microsoft word document
 
10 major industrial applications of sulfuric acid
10 major industrial applications of sulfuric acid10 major industrial applications of sulfuric acid
10 major industrial applications of sulfuric acid
 
A SHORT REVIEW ON ALUMINIUM ANODIZING: AN ECO-FRIENDLY METAL FINISHING PROCESS
A SHORT REVIEW ON ALUMINIUM ANODIZING: AN ECO-FRIENDLY METAL FINISHING PROCESSA SHORT REVIEW ON ALUMINIUM ANODIZING: AN ECO-FRIENDLY METAL FINISHING PROCESS
A SHORT REVIEW ON ALUMINIUM ANODIZING: AN ECO-FRIENDLY METAL FINISHING PROCESS
 
Santosh_Kr_Yadav_RAIM08
Santosh_Kr_Yadav_RAIM08Santosh_Kr_Yadav_RAIM08
Santosh_Kr_Yadav_RAIM08
 
Chromium problems
Chromium problemsChromium problems
Chromium problems
 
Brochure Meca-19102016-bd
Brochure Meca-19102016-bdBrochure Meca-19102016-bd
Brochure Meca-19102016-bd
 
Protection des métaux contre la corrosion
Protection des métaux contre la corrosionProtection des métaux contre la corrosion
Protection des métaux contre la corrosion
 
TRANSITION METALS
TRANSITION METALSTRANSITION METALS
TRANSITION METALS
 
TALAT Lecture 5203: Anodizing of Aluminium
TALAT Lecture 5203: Anodizing of AluminiumTALAT Lecture 5203: Anodizing of Aluminium
TALAT Lecture 5203: Anodizing of Aluminium
 

Similaire à Science of corrosion

Basics of corrosion_control
Basics of corrosion_controlBasics of corrosion_control
Basics of corrosion_controlchiragdeen777
 
Water chemistry and corrosion
Water chemistry and corrosionWater chemistry and corrosion
Water chemistry and corrosionVishnu Thumma
 
Corrosion of Steel in Concrete
Corrosion of Steel in ConcreteCorrosion of Steel in Concrete
Corrosion of Steel in ConcretePiyush Verma
 
CE336-09-Corrosion.PPT
CE336-09-Corrosion.PPTCE336-09-Corrosion.PPT
CE336-09-Corrosion.PPTakramAwad2
 
EC-CDT-Unit-2; Corrosion and types of corrosion
EC-CDT-Unit-2; Corrosion and types of corrosionEC-CDT-Unit-2; Corrosion and types of corrosion
EC-CDT-Unit-2; Corrosion and types of corrosionb23me005
 
Corrosion and its Control.ppt
Corrosion and its Control.pptCorrosion and its Control.ppt
Corrosion and its Control.pptakramAwad2
 
2-Corrosion-Its-Control.ppt
2-Corrosion-Its-Control.ppt2-Corrosion-Its-Control.ppt
2-Corrosion-Its-Control.pptShararehShojaei1
 
Mechanisms in Aqueous Solution for Corrosion of Metal Alloy
Mechanisms in Aqueous Solution for Corrosion of Metal AlloyMechanisms in Aqueous Solution for Corrosion of Metal Alloy
Mechanisms in Aqueous Solution for Corrosion of Metal AlloyLuís Rita
 
vnd.openxmlformats-officedocument.presentationml.presentation&rendition=1.pptx
vnd.openxmlformats-officedocument.presentationml.presentation&rendition=1.pptxvnd.openxmlformats-officedocument.presentationml.presentation&rendition=1.pptx
vnd.openxmlformats-officedocument.presentationml.presentation&rendition=1.pptxVictus4
 

Similaire à Science of corrosion (20)

Basics of corrosion_control
Basics of corrosion_controlBasics of corrosion_control
Basics of corrosion_control
 
CORROSION ENGINEERING.pptx
CORROSION ENGINEERING.pptxCORROSION ENGINEERING.pptx
CORROSION ENGINEERING.pptx
 
Water chemistry and corrosion
Water chemistry and corrosionWater chemistry and corrosion
Water chemistry and corrosion
 
Corrosion
CorrosionCorrosion
Corrosion
 
Corrosion & Mechanism of Dry corrosion
Corrosion & Mechanism of Dry corrosionCorrosion & Mechanism of Dry corrosion
Corrosion & Mechanism of Dry corrosion
 
Corrosion of Steel in Concrete
Corrosion of Steel in ConcreteCorrosion of Steel in Concrete
Corrosion of Steel in Concrete
 
CE336-09-Corrosion.PPT
CE336-09-Corrosion.PPTCE336-09-Corrosion.PPT
CE336-09-Corrosion.PPT
 
EC-CDT-Unit-2; Corrosion and types of corrosion
EC-CDT-Unit-2; Corrosion and types of corrosionEC-CDT-Unit-2; Corrosion and types of corrosion
EC-CDT-Unit-2; Corrosion and types of corrosion
 
Corrosion
CorrosionCorrosion
Corrosion
 
2-Corrosion-Its-Control.ppt
2-Corrosion-Its-Control.ppt2-Corrosion-Its-Control.ppt
2-Corrosion-Its-Control.ppt
 
Corrosion and its Control.ppt
Corrosion and its Control.pptCorrosion and its Control.ppt
Corrosion and its Control.ppt
 
2-Corrosion-Its-Control.ppt
2-Corrosion-Its-Control.ppt2-Corrosion-Its-Control.ppt
2-Corrosion-Its-Control.ppt
 
Corrosion.pptx
Corrosion.pptxCorrosion.pptx
Corrosion.pptx
 
Corrosion
CorrosionCorrosion
Corrosion
 
Corrosion
CorrosionCorrosion
Corrosion
 
Mechanisms in Aqueous Solution for Corrosion of Metal Alloy
Mechanisms in Aqueous Solution for Corrosion of Metal AlloyMechanisms in Aqueous Solution for Corrosion of Metal Alloy
Mechanisms in Aqueous Solution for Corrosion of Metal Alloy
 
Types of corrosion
Types of corrosionTypes of corrosion
Types of corrosion
 
vnd.openxmlformats-officedocument.presentationml.presentation&rendition=1.pptx
vnd.openxmlformats-officedocument.presentationml.presentation&rendition=1.pptxvnd.openxmlformats-officedocument.presentationml.presentation&rendition=1.pptx
vnd.openxmlformats-officedocument.presentationml.presentation&rendition=1.pptx
 
Corrosion
Corrosion  Corrosion
Corrosion
 
Corrosion
CorrosionCorrosion
Corrosion
 

Plus de Narayana Reddy Palakollu

Plus de Narayana Reddy Palakollu (13)

1 s2.0-s1570023203002794-main
1 s2.0-s1570023203002794-main1 s2.0-s1570023203002794-main
1 s2.0-s1570023203002794-main
 
1 s2.0-s0731708500002375-main
1 s2.0-s0731708500002375-main1 s2.0-s0731708500002375-main
1 s2.0-s0731708500002375-main
 
1 s2.0-s0039914097002452-main
1 s2.0-s0039914097002452-main1 s2.0-s0039914097002452-main
1 s2.0-s0039914097002452-main
 
1 s2.0-s0022072896048048-main
1 s2.0-s0022072896048048-main1 s2.0-s0022072896048048-main
1 s2.0-s0022072896048048-main
 
elective determination of L-dopa in the presence of uric acid and ascorbic ac...
elective determination of L-dopa in the presence of uric acid and ascorbic ac...elective determination of L-dopa in the presence of uric acid and ascorbic ac...
elective determination of L-dopa in the presence of uric acid and ascorbic ac...
 
1 s2.0-s0731708505002189-main
1 s2.0-s0731708505002189-main1 s2.0-s0731708505002189-main
1 s2.0-s0731708505002189-main
 
R 13 regulations b tech ist year syllabi of all branches 12 sept 13
R 13 regulations b tech ist year syllabi of all branches 12 sept 13R 13 regulations b tech ist year syllabi of all branches 12 sept 13
R 13 regulations b tech ist year syllabi of all branches 12 sept 13
 
Engineering Chemistry syllabus
Engineering Chemistry syllabusEngineering Chemistry syllabus
Engineering Chemistry syllabus
 
WATER
WATERWATER
WATER
 
Syllabus
SyllabusSyllabus
Syllabus
 
Appscj unior lecturerspreviouspaperswithexplanations
Appscj unior lecturerspreviouspaperswithexplanationsAppscj unior lecturerspreviouspaperswithexplanations
Appscj unior lecturerspreviouspaperswithexplanations
 
I b.tech -main exams time tables
I b.tech -main exams time tablesI b.tech -main exams time tables
I b.tech -main exams time tables
 
I b.tech -main exams time tables
I b.tech -main exams time tablesI b.tech -main exams time tables
I b.tech -main exams time tables
 

Dernier

In - Vivo and In - Vitro Correlation.pptx
In - Vivo and In - Vitro Correlation.pptxIn - Vivo and In - Vitro Correlation.pptx
In - Vivo and In - Vitro Correlation.pptxAditiChauhan701637
 
AUDIENCE THEORY -- FANDOM -- JENKINS.pptx
AUDIENCE THEORY -- FANDOM -- JENKINS.pptxAUDIENCE THEORY -- FANDOM -- JENKINS.pptx
AUDIENCE THEORY -- FANDOM -- JENKINS.pptxiammrhaywood
 
The Singapore Teaching Practice document
The Singapore Teaching Practice documentThe Singapore Teaching Practice document
The Singapore Teaching Practice documentXsasf Sfdfasd
 
CAULIFLOWER BREEDING 1 Parmar pptx
CAULIFLOWER BREEDING 1 Parmar pptxCAULIFLOWER BREEDING 1 Parmar pptx
CAULIFLOWER BREEDING 1 Parmar pptxSaurabhParmar42
 
Human-AI Co-Creation of Worked Examples for Programming Classes
Human-AI Co-Creation of Worked Examples for Programming ClassesHuman-AI Co-Creation of Worked Examples for Programming Classes
Human-AI Co-Creation of Worked Examples for Programming ClassesMohammad Hassany
 
3.21.24 The Origins of Black Power.pptx
3.21.24  The Origins of Black Power.pptx3.21.24  The Origins of Black Power.pptx
3.21.24 The Origins of Black Power.pptxmary850239
 
The Stolen Bacillus by Herbert George Wells
The Stolen Bacillus by Herbert George WellsThe Stolen Bacillus by Herbert George Wells
The Stolen Bacillus by Herbert George WellsEugene Lysak
 
2024.03.23 What do successful readers do - Sandy Millin for PARK.pptx
2024.03.23 What do successful readers do - Sandy Millin for PARK.pptx2024.03.23 What do successful readers do - Sandy Millin for PARK.pptx
2024.03.23 What do successful readers do - Sandy Millin for PARK.pptxSandy Millin
 
How to Add Existing Field in One2Many Tree View in Odoo 17
How to Add Existing Field in One2Many Tree View in Odoo 17How to Add Existing Field in One2Many Tree View in Odoo 17
How to Add Existing Field in One2Many Tree View in Odoo 17Celine George
 
How to Make a Field read-only in Odoo 17
How to Make a Field read-only in Odoo 17How to Make a Field read-only in Odoo 17
How to Make a Field read-only in Odoo 17Celine George
 
5 charts on South Africa as a source country for international student recrui...
5 charts on South Africa as a source country for international student recrui...5 charts on South Africa as a source country for international student recrui...
5 charts on South Africa as a source country for international student recrui...CaraSkikne1
 
How to Add a many2many Relational Field in Odoo 17
How to Add a many2many Relational Field in Odoo 17How to Add a many2many Relational Field in Odoo 17
How to Add a many2many Relational Field in Odoo 17Celine George
 
UKCGE Parental Leave Discussion March 2024
UKCGE Parental Leave Discussion March 2024UKCGE Parental Leave Discussion March 2024
UKCGE Parental Leave Discussion March 2024UKCGE
 
Clinical Pharmacy Introduction to Clinical Pharmacy, Concept of clinical pptx
Clinical Pharmacy  Introduction to Clinical Pharmacy, Concept of clinical pptxClinical Pharmacy  Introduction to Clinical Pharmacy, Concept of clinical pptx
Clinical Pharmacy Introduction to Clinical Pharmacy, Concept of clinical pptxraviapr7
 
General views of Histopathology and step
General views of Histopathology and stepGeneral views of Histopathology and step
General views of Histopathology and stepobaje godwin sunday
 
How to Use api.constrains ( ) in Odoo 17
How to Use api.constrains ( ) in Odoo 17How to Use api.constrains ( ) in Odoo 17
How to Use api.constrains ( ) in Odoo 17Celine George
 
How to Solve Singleton Error in the Odoo 17
How to Solve Singleton Error in the  Odoo 17How to Solve Singleton Error in the  Odoo 17
How to Solve Singleton Error in the Odoo 17Celine George
 
Diploma in Nursing Admission Test Question Solution 2023.pdf
Diploma in Nursing Admission Test Question Solution 2023.pdfDiploma in Nursing Admission Test Question Solution 2023.pdf
Diploma in Nursing Admission Test Question Solution 2023.pdfMohonDas
 
Philosophy of Education and Educational Philosophy
Philosophy of Education  and Educational PhilosophyPhilosophy of Education  and Educational Philosophy
Philosophy of Education and Educational PhilosophyShuvankar Madhu
 

Dernier (20)

In - Vivo and In - Vitro Correlation.pptx
In - Vivo and In - Vitro Correlation.pptxIn - Vivo and In - Vitro Correlation.pptx
In - Vivo and In - Vitro Correlation.pptx
 
AUDIENCE THEORY -- FANDOM -- JENKINS.pptx
AUDIENCE THEORY -- FANDOM -- JENKINS.pptxAUDIENCE THEORY -- FANDOM -- JENKINS.pptx
AUDIENCE THEORY -- FANDOM -- JENKINS.pptx
 
The Singapore Teaching Practice document
The Singapore Teaching Practice documentThe Singapore Teaching Practice document
The Singapore Teaching Practice document
 
CAULIFLOWER BREEDING 1 Parmar pptx
CAULIFLOWER BREEDING 1 Parmar pptxCAULIFLOWER BREEDING 1 Parmar pptx
CAULIFLOWER BREEDING 1 Parmar pptx
 
Human-AI Co-Creation of Worked Examples for Programming Classes
Human-AI Co-Creation of Worked Examples for Programming ClassesHuman-AI Co-Creation of Worked Examples for Programming Classes
Human-AI Co-Creation of Worked Examples for Programming Classes
 
Finals of Kant get Marx 2.0 : a general politics quiz
Finals of Kant get Marx 2.0 : a general politics quizFinals of Kant get Marx 2.0 : a general politics quiz
Finals of Kant get Marx 2.0 : a general politics quiz
 
3.21.24 The Origins of Black Power.pptx
3.21.24  The Origins of Black Power.pptx3.21.24  The Origins of Black Power.pptx
3.21.24 The Origins of Black Power.pptx
 
The Stolen Bacillus by Herbert George Wells
The Stolen Bacillus by Herbert George WellsThe Stolen Bacillus by Herbert George Wells
The Stolen Bacillus by Herbert George Wells
 
2024.03.23 What do successful readers do - Sandy Millin for PARK.pptx
2024.03.23 What do successful readers do - Sandy Millin for PARK.pptx2024.03.23 What do successful readers do - Sandy Millin for PARK.pptx
2024.03.23 What do successful readers do - Sandy Millin for PARK.pptx
 
How to Add Existing Field in One2Many Tree View in Odoo 17
How to Add Existing Field in One2Many Tree View in Odoo 17How to Add Existing Field in One2Many Tree View in Odoo 17
How to Add Existing Field in One2Many Tree View in Odoo 17
 
How to Make a Field read-only in Odoo 17
How to Make a Field read-only in Odoo 17How to Make a Field read-only in Odoo 17
How to Make a Field read-only in Odoo 17
 
5 charts on South Africa as a source country for international student recrui...
5 charts on South Africa as a source country for international student recrui...5 charts on South Africa as a source country for international student recrui...
5 charts on South Africa as a source country for international student recrui...
 
How to Add a many2many Relational Field in Odoo 17
How to Add a many2many Relational Field in Odoo 17How to Add a many2many Relational Field in Odoo 17
How to Add a many2many Relational Field in Odoo 17
 
UKCGE Parental Leave Discussion March 2024
UKCGE Parental Leave Discussion March 2024UKCGE Parental Leave Discussion March 2024
UKCGE Parental Leave Discussion March 2024
 
Clinical Pharmacy Introduction to Clinical Pharmacy, Concept of clinical pptx
Clinical Pharmacy  Introduction to Clinical Pharmacy, Concept of clinical pptxClinical Pharmacy  Introduction to Clinical Pharmacy, Concept of clinical pptx
Clinical Pharmacy Introduction to Clinical Pharmacy, Concept of clinical pptx
 
General views of Histopathology and step
General views of Histopathology and stepGeneral views of Histopathology and step
General views of Histopathology and step
 
How to Use api.constrains ( ) in Odoo 17
How to Use api.constrains ( ) in Odoo 17How to Use api.constrains ( ) in Odoo 17
How to Use api.constrains ( ) in Odoo 17
 
How to Solve Singleton Error in the Odoo 17
How to Solve Singleton Error in the  Odoo 17How to Solve Singleton Error in the  Odoo 17
How to Solve Singleton Error in the Odoo 17
 
Diploma in Nursing Admission Test Question Solution 2023.pdf
Diploma in Nursing Admission Test Question Solution 2023.pdfDiploma in Nursing Admission Test Question Solution 2023.pdf
Diploma in Nursing Admission Test Question Solution 2023.pdf
 
Philosophy of Education and Educational Philosophy
Philosophy of Education  and Educational PhilosophyPhilosophy of Education  and Educational Philosophy
Philosophy of Education and Educational Philosophy
 

Science of corrosion

  • 1. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry UNIT –II INTRODUCTION Metals have a natural tendency to revert back to combined states. During this process mostly, oxides are formed though in some cases sulphides, carbonates, subparts etc. may result due to presence of impurities. Any process of deterioration and loss of solid metallic material by chemical or electrochemical attack by its SCIENCE OF environment is called corrosion. Corrosion is the reverse process of metallurgy. Metal Corrosion(Oxidation) Metallic Compound + Energy Metallurgy(Reduction) CORROSION Eg: Rusting of iron when exposed to atmospheric conditions. Rust is hydrated oxide (Fe 2O3.xH2O). CORROSION It may be defined as “the process of gradual deterioration or disintegration or eating away of metal by a chemical or electrochemical reaction in its environment ”. P. V. NARAYANA REDDYM.Sc.,. Ph.D Rusting of Iron based on Electrochemical Theory of corrosion Eg: Yellow rust, actually corresponds to Fe2O 3. 3H2O on the surface of iron. Greenish layer formation on the surface of Copper Theory of corrosion There are three theories of corrosion: (i) Acid theory, (ii) dry or chemical corrosion and (iii)Electrochemical or wet corrosion. (i)Acid theory of corrosion This theory suggests that corrosion of a metal (iron) is due to the presence of acids surrounding it. According to this theory, iron is corroded by atmospheric carbon di- oxide, moisture and oxygen. The corrosion UNIT-II products are the mixture of Fe(HCO3)2, Fe(OH)CO3 and Fe(OH)3. The chemical reactions suggested are given Science of Corrosion: Definition, Types of corrosion: Dry Corrosion, (Direct below Chemical attack), Wet Corrosion, Theories of Corrosion and Mechanism, Electro Chemical Theory of Corrosion. Galvanic Series, Galvanic Corrosion, Concentration Cell Corrosion, Oxygen absorption type. Factors Influencing Corrosion. Control of Corrosion – Cathodic Protection – Sacrificial anode and Impressed Current. Uses of Inhibitors. Electro Plating, and Electro less plating (copper and nickel) This theory is supported by the analysis of rust that gives the test for CO32- ion. Further, the process of rusting is reduced by the presence of lime and caustic soda (these two can absorb CO2, thus reducing corrosion). 1 2 Narayana Reddy
  • 2. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry Types Of Corrosion: Two types of Corrosion occur base on the environment e.g., Ag, Au and Pt do not undergo oxidation Corrosion. 1.Dry (or) Chemical Corrosion c) Volatile that is oxide layer volatilizes after formation and as such leaves the underlying metal surface exposed further attack. This causes continuous which is excessive. e.g. Molybdenum oxide (MoO3) This type of Corrosion occurs mainly through the direct chemical action of atmospheric gasses such as O2, d) Porous that is the oxide layer formed having pores or cracks. In this case the atmospheric oxygen halogens, H2S, SO2, N2 and anhydrous inorganic liquid with metal surfaces in its immediate proximity. Three types passes through the pores or cracks of the underlying metal surface. This causes continuous corrosion till of chemical Corrosion are as follows.. complete conversion of metal into its oxide. Pilling Bedworth Rule: If volume of metal oxide on the surface of a metal is more than or equal to the volume of (i)Oxidation Corrosion: This is carried out by the direct action of oxygen low or high temperatures on metals metal, the oxide layer will be protective. For example, for Al2O3, Fe, Ni, ZnW, Cr. It will be non-protective if volume in absence of moisture at ordinary temperature metals or very slightly attacked. The Exceptions are Alkali metals of oxide is less than volume of metal. (The specific volume ratio of W is 3.6, Cr = 2.0, Ni = 1.6. Hence, the rate of and Alkaline earth metals. At high temperature all metals are oxidized. The exception is Ag, Au and Pt. corrosion is very less in tungsten.) It is called Pilling Bedworth rule. 2M 2Mn++2ne- (De-electronation) (ii) Corrosion by other gases:Gases such as Cl2, SO2, H2S, NOx : In dry atmosphere, these gases react with metal (Metal ion) and form corrosion products which may be protective or non-protective. Dry Cl2 reacts with Ag and forms AgCl nO2+2ne- 2nO2- (Electronation) which is a protective layer, while SnCl4 is volatile. In petroleum industries at high temperatures, H2S attacks steel (Oxide ion) forming FeS scale which is porous and in- terferes with normal 2M+ nO 2 2Mn+ + 2nO2- (Metal oxide) operations. Mechanism: At the surface of metal oxidation occurs and the resulting metal oxide scale forms a barrier which (iii) Liquid metal corrosion: In several industries, molten metal passes through metal- lic pipes and causes restricts further oxidation. For oxidation to continue either the metal must diffused outwards through the scale to corrosion due to dissolution or due to internal penetration. For example, liquid metal mercury dissolves most the surface or the oxygen must defuse inwards through the scale to the underlying metal. Both the cases are metals by forming amalgams, thereby corroding them. possible. But the outward diffusion of metal is generally more rapid than inward diffusion of oxygen since metal ion is appreciably smaller than the oxygen ion and hence more mobile. 2.Wet or Electrochemical Corrosion: Metal + Oxygen Metal oxide It is a common type of corrosion of metal in aqueous corrosive environment. This type of corrosion occurs when the metal comes in contact with a conducting liquid or when two dissimilar metals are immersed or dipped partly When oxidation starts, a thin layer of oxide is formed on the metal surface and the nature of this film in a solution. According to this theory, there is the formation of a galvanic cell on the surface of metals. Some parts decides further action. of the metal surface act as anode and rest act as cathode. If the film is, The chemical in the environment and humidity acts as an electrolyte. Oxidation of anodic part takes place and it a) stable, it behaves has a protective coating in nature e.g., the oxide films on Al, Pb, Cu, Pt etc., are stable results in corrosion at anode, while reduction takes place at cathode. The corrosion product is formed on the and therefore further oxidation correction of prohibited. surface of the metal between anode and cathode. b) Unstable that is the oxide layer formed decomposes back into metal and oxygen. So, oxidation Corrosion To understand the wet theory, let us take the example of corrosion of iron. Oxidation of metal takes place at is not possible. anode while the reduction process takes place at cathode. By taking rusting of iron as an example, the reaction can Metal oxide Metal + Oxygen be explained as that it may occur in two ways: 3 Narayana Reddy 4 Narayana Reddy
  • 3. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry (i) evolution of hydrogen type (ii) absorption of oxygen type This type of corrosion causes displacement of hydrogen ions from the solution by metal ions. All metals above hydrogen in electrochemical series have a tendency to get dissolved in acidic solution with simultaneous At anode: oxidation occurs. At cathodic part, reduction reaction (electro nation) occurs. It does not affect the evolution of H2 gas. The anodes are large areas, whereas cathodes are small areas. cathode, since most metals cannot be further reduced. At cathodic part, the dissolved constituents in the conducting medium accepts the electrons forming ions (OH-, O2-). The metallic ions formed at anodic part and the Absorption of oxygen: - For example, rusting of iron in neutral aqueous solution of electrolytes in presence of ions formed at cathodic part diffuse towards each other through conducting medium and form a corrosion product atmospheric oxygen. Usually the surface of iron is coated with a thin film of iron oxide. If the film develops cracks, somewhere between anode and cathode. anodic areas are created on the surface. While the metal parts act as cathodes. It shows that anodes are small areas, while the rest metallic part forms large cathodes. Mechanism: Electrochemical corrosion involves flow of electrons between anode and cathode. The anodic reaction involves dissolution of metal liberating free electrons. M Mn+ + ne- The cathodic reaction consumes electrons with either evolution of hydrogen or absorption of oxygen which depends on the nature of corrosive environment. (i) Evolution of hydrogen: This type of corrosion occurs in acidic medium e.g., considering the metal Fe, anodic reaction is dissolution of iron as ferrous ions with liberation of electrons. At anode: Fe Fe2+ + 2e- (Oxidation) Fe Fe2+ + 2e- ( Oxidation) At cathode: The released electrons flow from anode to cathode through iron metal. ½ O2 + H2O + 2e- 2OH-(Reduction) Fe2+ + 2OH- Fe(OH)2 (a) If oxygen is in excess, ferrous hydroxide is easily oxidized to ferric hydroxide. 4Fe(OH)2 + O 2 + 2H2O 4Fe (OH)3 The product called yellow rust corresponds to Fe2O3. xH2O. The electrons released flow through the metal from anode to cathode, whereas H+ions of acidic solution are eliminated as hydrogen gas. Concentration Cell Corrosion : 2H++2e- H2 This type of corrosion is due to electrochemical attack on the metal surface exposed to an The overall reaction is electrolyte of varying concentrations or of varying aeration. The most common type of concentration cell corrosion Fe+2 H+ Fe2+ + H2 is the differential aeration corrosion which occurs when one part of metal is exposed to different air concentration from other part. This causes a difference in potential between the differently aerated areas. Experimentally it has 5 Narayana Reddy 6 Narayana Reddy
  • 4. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry been observed that poor oxygenated parts are anodic. Differential aeration of metal causes a flow of current called particular atmosphere, i.e. sea water. In galvanic series, oxidation potential of metals is arranged in the decreasing the differential current. order of activity of a series of metals. The series is towards the increasing noble nature. Differential Aeration Corrosion: It occurs when a metallic surface is partially immersed in an electrolyte and More anodic: Mg, Mg alloys, Zn, Al, Cd, Fe, Pb, Sn, Ni–Mo–Fe alloys) Brasses, Cu, Ni, Cr–steel alloy, Ag, partially exposed to air as shown in the figure below. Poorly oxygenated metallic part becomes anodic and Ti, Au, Pt towards noble nature. undergoes oxidation. Well oxygenated part becomes cathodic. At the cathode, O 2 takes up electrons to form OH- ions. Electrochemical series Galvanic series If a metal e.g., Zn is partially immersed in a dilute solution of a neutral salt e.g., NaCl and the solution is not agitated properly, then the parts above and adjacent to the waterline are strongly aerated and hence a. This series consists of metals and non- a. This series consist of metals and alloys. metals b. Position of pure metal and when present in become cathodic. Whereas parts immersed show a smaller oxygen concentration and become anodic. so there is a b. The position of a metal in this series is the form of alloy is different. difference of potential which causes flow of current between two differentially aerated areas of same metal. Zinc permanently fixed. c. It predicts the relative corrosion c. It predicts the relative displacement tendencies. will dissolve at anodic areas and oxygen will take up electrons at the cathodic areas forming hydroxyl ions. tendencies. d. Corrosion of metals and alloys is studied in d. Electrode potentials are measured by unpolluted sea water. Zn Zn2+ + 2e- (Oxidation) dipping pure metals in their salt solution of 1M concentration. ½ O2 + H2O + 2e- 2OH- (Reduction) Following are the facts about differential aeration corrosion: (a) Less oxygenated part is the anode. Therefore cracks serve as foci for corrosion. (b) Corrosion is accelerated under accumulation of dirt, scale or other contaminations. This restricts the access of oxygen resulting an anode to promise greater accumulation. The result is localized corrosion. (c) Metals exposed to aqueous media corrode under blocks of wood or glass which restricts the access of oxygen. GALVANIC SERIES Electrochemical reactions are predicted by electrochemical series. A metal having higher position can replace (reduce) other metals that have lower position in the series. For example, Zn + CuSO 4 ZnSO 4 + Cu that is, Zn + Cu++ Zn ++ + Cu Or in other words, zinc will corrode faster than copper. Some exceptions have been observed in this generalization. For example, Ti is less reactive than Ag. Galvanic series is the series of metals that is made keeping in view the process of corrosion of a metal in a 7 Narayana Reddy 8 Narayana Reddy
  • 5. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry Active (Anodic) 1. Mg a) Metal surface are not homogeneous. 2. Mg alloys 3. Zn b) External environment is not homogeneous. 4. Al c) Films are not perfectly uniform. 5. Cd 6. Al alloys d) Crystallography directions are not equal in the reactivity. 7. Mild steel 8. Cast Iron e) Environment is not uniform with respect to concentration. 9. high Ni cast Iron Pitting is usually the result of the breakdown or cracking of the protective film on a metal at specific points. This 10. Pb-Sn solder 11. Pb gives rise to the formation of small anodic and large cathodic areas. In process of correct environment this 12. Sn 13. Iconel produces corrosion current. 14. Ni-Mo-Fe alloys 15. Brass e.g., Stainless steel and aluminum show characteristic pitting on chloride solution. Pitting is caused by the presence 16. Monel 17. Silver solder of sand, dust scale and other extraneous impurities present on the metal surfaces. Because of differential amount of 18. Cu oxygen in contact with the metal, the small part (underneath the impurity) becomes the anodic areas and the 19. Ni 20. Cr stainless steel surrounding large parts become the cathodic areas. Intense corrosion takes place in the anodic areas underneath 21. 18-8 stainless steel 22. 18-8 Mo stainless steel the impurity. Once a small pit is generated, the rate of corrosion will be increased. 23. Ag 24. Ti 25. Graphite 26. Au Noble(Cathodic) 27. Pt Table: Galvanic series GALVANIC CORROSION : When two dissimilar metals are electrically connected and exposed to an electrolyte, the metal higher in electrochemical series undergoes corrosion. This type of corrosion is called Galvanic corrosion .e.g., Zinc (higher in electrochemical series) forms the anode and is attacked and gets dissolved; whereas copper (lower in Factors Influencing Corrosion: electrochemical series) acts as cathode. The rate and extent of corrosion depends on Mechanism: If the solution is acidic then corrosion occurs by hydrogen evolution process and if the solution is (i) Nature of the metal neutral or slightly alkaline in nature then corrosion occurs by oxygen absorption process. The electrons flow from (ii) Nature of corroding environment. the anodic metal to the cathodic metal. Nature of metal: Zn Zn2+ + 2e- (Oxidation) (a)Position in galvanic series: when 2 metals or alloys are in electrical contact in presence of an electrolyte the more Thus the corrosion is a localized accelerated attack resulting in the formation of pits, holes or active metal having higher position in the galvanic series undergoes corrosion. The greater is the difference in cavities. Pitting corrosion therefore results in the formation of pinholes, pits and cavities in the metal. The position, the faster is the corrosion. pitting corrosion may be due to following reasons: 9 Narayana Reddy 10 Narayana Reddy
  • 6. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry (b)Over voltage: reduction in overvoltage of the corroding metal accelerates the corrosion rate. E.g. Zn in 1N H2SO4 (c) Impurity of atmosphere: Pollutants like H2S, SO2, CO2 and acid vapours cause more pollution where they undergoes corrosion slowly because of high overvoltage of zinc metal (0.7 V) which reduces the effective potential dissolve. In sea water (salty in nature which acts as an electrolyte) corrosion rate increases. Some suspended to a small value. In presence of CuSO4 the corrosion rate of zinc is accelerated. particles are dissolved in humidity and form electrolyte which helps in corrosion. (c) Relative areas of anodic and cathodic parts: When 2 dissimilar metals are in contact, the corrosion of the anodic (d) pH Value: pH value means concentration of H+ (acidic nature). In acidic medium (pH less than 7), part is directly proportional to the ratio of areas of cathodic part and the anodic part. Corrosion is rapid and corrosion is faster. Also, in basic medium pH > 7, some metals such as Pb, Zn, Al, etc. form complexes and hence localized if anodic area is small, because the current density at a smaller anodic area is greater. they corrode. Pourbiax relation between pH of medium and potential of metal deals with the corrosion process and (d) Purity of Metal: Impurities in a metal generally cause heterogeneous state forming minute electrochemical cells it gives idea how to reduce corrosion. resulting corrosion of anodic part. E.g. Zinc metal with impurities Pb or Fe. Corrosion resistance of a metal may be Example: Zn corrodes minimum at pH 11, but at higher pH (more than 11) it corrodes faster. At pH 5.5, Al improved by increasing its purity. corrodes minimum. (e)Physical state of Metal: The rate of corrosion is influenced by physical state of the metal such as grain size, (e) Nature of ions present: Cu++ ions present in the vicinity of Fe, accelerate corrosion, while silicates present in the stress; orientation of crystals etc., The smaller the grain size of even in pure metal becomes the anode undergoing vicinity resist corrosion. corrosion. (f) Conductance effect: Due to presence of salts and water in earth, it is of con- ducting nature. More conductance (f) Nature of surface film: In aerated atmosphere, all metals get covered with a thin surface film of metal oxides. The leads to more stray current and hence fast corrosion. Dry sandy soil is less conducting and hence less corrosion, ratio of the volumes of metal oxides to the metal is known as specific volume ratio. Greater is this value lesser is the while mineralised clay soil is more conducting hence more corrosion occurs. oxidation corrosion rate. (g) Oxygen concentration cell: Oxygen is one of the important element responsible for corrosion. It forms oxides (g) Passivity of Metal: Passive metals are resistant to corrosion due to the formation of highly protective but very and hydroxides (in presence of H2O) on the surface of metal as corrosion product. Oxygen concentration cell thin film on the metal or alloy surface E.g. Corrosion resistance of stainless steel is due to passivity character of is formed on the surface of metal due to difference in oxygen concentration (iron rod half dipped in water corrodes Chromium present in it. due to this effect). Dipped portion will be anode and outer portion will be cathode. (h) Solubility of corrosion products: In electrochemical corrosion if the corrosion product is soluble in the corroding medium then corrosion is rapid. If the corrosion product is insoluble, then acts as barrier thereby suppressing further corrosion. (i) Volatility of corrosion products: If the corrosion product is volatile, then the underlying surface is exposed for Cathodic Protection : further attack. This causes rapid and continuous corrosion. E.g. MoO3 is volatile. The cathodic protection of metals is used to control corrosion metals where it is impractible to alter the (ii) Nature of corroding environment: nature of the corrosion medium. The principle involved in this method is to protect metals and alloys from (a)Temperature: As the temperature of environment is increased the reaction rate is increased thereby accelerating corrosion by making them completely cathodic. Since there will not be any anodic area on the metal, therefore corrosion. corrosion does not occurs. (b)Humidity of air: critical humidity is defined as the relative humidity above which the atmosphere corrosion rate The following are 2 types of cathodic protections. of metal increases sharply. The value of critical humidity depends on nature of metal and corrosion products. 1. Sacrificial anodic protection Corrosion of a metal is furnish in humid atmosphere because gases (CO2, O 2) and vapours present in atmosphere 2. Impressed current cathodic protection furnish water to the electrolyte essential to establish an electrochemical corrosion cell. The oxide film on the metal Sacrificial anodic protection: surface has the property to absorb moisture. In presence of this absorbed moisture, corrosion rate is enhanced. In this method, the metal structure can be protected from corrosion by connecting it with wire to a more Rain water may also wash away the oxide film from the metal surface. This leads to enhanced atmospheric attack. anodic metal. As this more active metal is sacrificed in the process of saving metal from corrosion, it is known as The exceptions are Cr, Al. 11 Narayana Reddy 12 Narayana Reddy
  • 7. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry sacrificial anode. The metals which are commonly used as sacrificial anodes are Mg, Zn, Al and their alloys. The Inhibitors are mainly classified into types important applications of this method are Anodic inhibitors: 1. Protection of underground cables and pipelines from soil corrosion. Anodic inhibitors are those which prevent the corrosive reaction occurring at anode by reacting with the ions of 2. Protection of ships and boat hulls from marine corrosion. the anode and forming insoluble precipitates. The precipitate thus absorbed on the surface of the metal and forms 3. Prevention of rusty water by inserting Mg sheets or rods into domestic water boilers or tanks. a protective coating resulting in reducing the corrosion rate. If insufficient inhibitors are used, it results in certain area unprotected leading to severe local attack. The anodic inhibitors used are phosphates, chromates, molybdates, alkalis, tungstates etc. Cathodic Inhibitors: The cathodic reaction occurs in acidic solution is the evolution of hydrogen given as 2 H+ + 2e-  H2 The diffusion of hydrogen ions in the acidic solution can be slowed by using organic inhibitors like a mines, Impressed current cathodic protection: mercaptanes, and heterocyclic nitrogen compounds etc. This organic inhibitor is absorbed at the metal surface and As the name implies, an impressed current is applied to convert the corroding metal from anode to cathode. reduces the corrosion rate. The applied current is in opposite direction since to nullify the corrosion current. This can be accomplished by The cathodic reaction occurs at the neutral solution is applying sufficient amount of direct current source like battery or rectifier to an anode like graphite, high silica H2O + ½ O 2 + 2e-  2OH- iron, stainless steel or platinum buried in the soil or immersed in the corrosion medium. And connected to the In this, the corrosion reaction rate can be controlled by reducing the diffusion of oxygen to the cathodic corroding metal structure which is to be protected as shown in the diagram below.. area or by removing oxygen from the corroding medium. For this, Na2SO3 is used to eliminate O2 and Mg, Zn or Ni salts are used to reduce the diffusion of O2 to the corroding area. These inhibitors react with OH- ions at the cathode forming a layer of insoluble hydroxides which are impermeable and hence reduce the diffusion of oxygen to the cathode area. ELECTROPLATING: Principle of Electroplating: This process involves coating of a thin layer of one metal over another metal by passing direct current through an electrolytic solution. The base metal to be plated is made of cathode whereas the anode is made of either coating metal itself or an inert material in the electrolytic cell. In impressed current cathodic protection, electrons are supplied from an external cell, so that the object Procedure: itself becomes cathodic and not oxidized. This type of cathodic protect ion has been applied to buried structures The article to be electroplated is first treated with organic solvent to remove oils, greases etc. then, it is made free such as tanks and pipelines, transmission line-towers, marine piers, laid-up ships etc. since, their operating and from surface scales, oxides, etc., by treating with dil. HCl or H2SO 4. The cleaned article is then made cathode of an maintenance costs are less, they are well suited for large structures and long term operations. electrolytic cell. The anode is either the coating metal itself or an inert material of good electrical conductivity. INHIBITORS: The electrolyte is a solution of a soluble salt of the coating metal. The electrolytic solution is kept in an A corrosion inhibitor is “a substance which when added in small quantities to the aqueous corrosive electroplating tank. The anode and cathode are dipped in the electrolytic solution. When direct current is passed, environment effectively decreases the corrosion of a metal”. coating-metal ions migrate to the cathode and get deposited there. Thus, a thin layer of coating-metal is obtained 13 Narayana Reddy 14 Narayana Reddy
  • 8. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry on the article, made as the cathode. For brighter and smooth deposits, favourable conditions such as low temperature, medium current-density and low metal-ion concentration are used. “Coming together is a beginning; keeping together is progress; working together is success.” ELECTROLESS PLATING: Definition: The process of producing a thin, uniform and hard deposit of metal on an activated substrate (Metal or non-metal) by using suitable soluble reducing agents without any electrical energy, and the driving force for the deposition is auto catalytic redox reactions. The reducing agent reduces the metallic ions to metal, which gets plated over the catalytically activated surface giving a uniform thin coating. Metal ions + Reducing agent Metal + Oxidised products Process: The process involves 1. Pretreatment or activation of work piece to be plated. 2. Preparation of bath composition. 1. Pretreatment or activation of work piece to be plate (i) Metals like Cu, Ag etc. are known as non-catalytic metals. Surface of such metals need activation. They are activated by using steel or iron pieces for initiating the reactions. “Time is the coin of your life. It is the only coin you have, and only you can determine how it will (ii) Non-metals like glasses, ceramic, plastics are activated by dipping in SnCl2, PdCl2 in HCl. This process be spent. Be careful lest you let other people spend it for you.” produces a thin film of palladium coating on non-metal surfaces which in turn causes the work piece to get activate for electroless plating. ΩΩΩΩΩΩ www.pvnreddy.blogspot.com 15 Narayana Reddy 16 Narayana Reddy