Corrosion and protection

1. CORROSION
CONTROL
MATERIAL SELECTION
ALTERATION OF ENVIRONMENT
PROPER DESIGN
CATHODIC PROTECTION
ANODIC PROTECTION
COATINGS & WRAPPING
BY UMAIR AFTAB
Lecture No. 48-52

2. (1) MATERIAL SELECTION
(selection of proper material for a
particular corrosive service)
Metallic [metal and alloy]
Nonmetallic [rubbers (natural and synthetic),
plastics, ceramics, carbon and graphite, and
wood]

3. Metals and Alloys
No Environment Proper material
1 Nitric acid Stainless steels
2 Caustic Nickel and nickel
alloys
3 Hydrofluoric acid Monel (Ni-Cu)
4 Hot hydrochloric acid Hastelloys (Ni-Cr-
Mo) (Chlorimets)
5 Dilute sulfuric acid Lead

4. No Environment Proper material
6 Nonstaining atmospheric
exposure
Aluminium
7 Distilled water Tin
8 Hot strong oxidizing
solution
Titanium
9 Ultimate resistance Tantalum
10 Concentrated sulfuric
acid
Steel

5. E.g : Stainless Steels
Stainless steels are
iron base alloys that
contain a minimum
of approximately
11% Cr, the amount
needed to prevent
the formation of rust
in unpolluted
atmosphere.
wt.% Cr
Dissolutionrate,cm/sec

6. Alloying elements of stainless steel :
 Other than Ni, Cr and C, the following alloying elements
may also present in stainless steel: Mo, N, Si, Mn, Cu, Ti,
Nb, Ta and/or W.
 Main alloying elements (Cr, Ni and C):
1. Chromium
Minimum concentration of Cr in a
stainless steel is 12-14wt.%
Structure : BCC (ferrite forming element)
* Note that the affinity of Cr to form Cr-carbides is very
high. Chromium carbide formation along grain
boundaries may induce intergranular corrosion.

7. Binary diagram of Fe-Cr
Sigma phase
formation which is
initially formed at
grain boundaries has
to be avoided
because it will
increase hardness,
decrease ductility
and notch toughness
as well as reduce
corrosion resistance.

8. 2. Nickel
Structure: FCC (austenite forming element/stabilize
austenitic structure)
Added to produce austenitic or duplex stainless
steels. These materials possess excellent ductility,
formability and toughness as well as weld-ability.
Nickel improves mechanical properties of stainless
steels servicing at high temperatures.
Nickel increases aqueous corrosion resistance of
materials.

9. Anodic polarization curves of Cr, Ni and Fe in 1 N
H2SO4 solution

10. Influence of Cr on corrosion resistance of iron
base alloy

11. Influence of Ni on corrosion resistance of iron base alloy

12. 3. Carbon
Very strong austenite forming element (30x more
effective than Ni). I.e. if austenitic stainless steel
18Cr-8Ni contains ≤0.007%C, its structure will
convert to ferritic structure. However the
concentration of carbon is usually limited to ≤
0.08%C (normal stainless steels) and ≤0.03%C
(low carbon stainless steels to avoid sensitization
during welding).

13. Minor alloying elements :
 Manganese
Austenitic forming element. When necessary can be used to
substitute Ni. Concentration of Mn in stainless steel is usually
2-3%.
 Molybdenum
Ferritic forming element. Added to increase pitting corrosion
resistance of stainless steel (2-4%).
Molybdenum addition has to be followed by decreasing
chromium concentration (i.e. in 18-8SS has to be decreased
down to 16-18%) and increasing nickel concentration (i.e. has
to be increased up to 10-14%).
Improves mechanical properties of stainless steel at high
temperature. Increase aqueous corrosion resistance of material
exposed in reducing acid.

14.  Tungsten
Is added to increase the strength and toughness of
martensitic stainless steel.
 Nitrogen (up to 0.25%)
Stabilize austenitic structure. Increases strength and corrosion
resistance. Increases weld ability of duplex SS.
 Titanium, Niobium and Tantalum
To stabilize stainless steel by reducing susceptibility of the
material to intergranular corrosion. Ti addition > 5x%C.
Ta+Nb addition > 10x%C.

15.  Copper
Is added to increase corrosion resistance of stainless steel
exposed in environment containing sulfuric acid.
 Silicon
Reduce susceptibility of SS to pitting and crevice corrosion as
well as SCC.

16. Five basic types of stainless steels :
 Austenitic - Susceptible to SCC. Can be hardened by only
by cold working. Good toughness and formability, easily to
be welded and high corrosion resistance. Nonmagnetic
except after excess cold working due to martensitic
formation.
 Martensitic - Application: when high mechanical strength
and wear resistance combined with some degree of corrosion
resistance are required. Typical application include steam
turbine blades, valves body and seats, bolts and screws,
springs, knives, surgical instruments, and chemical
engineering equipment.
 Ferritic - Higher resistance to SCC than austenitic SS. Tend
to be notch sensitive and are susceptible to embrittlement
during welding. Not recommended for service above 3000
C
because they will loss their room temperature ductility.

17.  Duplex (austenitic + ferritic) – has enhanced resistance to
SCC with corrosion resistance performance similar to AISI
316 SS. Has higher tensile strengths than the austenitic
type, are slightly less easy to form and have weld ability
similar to the austenitic stainless steel. Can be considered as
combining many of the best features of both the austenitic
and ferritic types. Suffer a loss impact strength if held for
extended periods at high temperatures above 3000
C.
 Precipitation hardening - Have the highest strength but
require proper heat-treatment to develop the correct
combination of strength and corrosion resistance. To be
used for specialized application where high strength
together with good corrosion resistance is required.

18. Alteration of Environment
 Typical changes in medium are :
 Lowering temperature – but there are cases where
increasing T decreases attack. E.g hot, fresh or salt water
is raised to boiling T and result in decreasing O2
solubility with T.
 Decreasing velocity – exception ; metals & alloys that
passivate (e.g stainless steel) generally have better
resistance to flowing mediums than stagnant. Avoid very
high velocity because of erosion-corrosion effects.

19.  Removing oxygen or oxidizers – e.g boiler feedwater
was deaerated by passing it thru a large mass of scrap
steel. Modern practice – vacuum treatment, inert gas
sparging, or thru the use of oxygen scavengers. However,
not recommended for active-passive metals or alloys.
These materials require oxidizers to form protective oxide
films.
 Changing concentration – higher concentration of
acid has higher amount of active species (H ions).
However, for materials that exhibit passivity, effect is
normally negligible.

20. Environment factors affecting
corrosion design :
 Dust particles and man-made pollution – CO, NO,
methane, etc.
 Temperature – high T & high humidity accelerates
corrosion.
 Rainfall – excess washes corrosive materials and
debris but scarce may leave water droplets.
 Proximity to sea
 Air pollution – NaCl, SO2, sulfurous acid, etc.
 Humidity – cause condensation.

21. INHIBITOR
 IS A CHEMICAL SUBSTANCE THAT, WHEN
ADDED IN A SMALL CONCENTRATION TO
AN ENVIRONMENT, EFFECTIVELY
DECREASES THE CORROSION RATE
In the oil extraction and processing
industries inhibitors have always been
considered to be the first line of defense
against corrosion

22.  Inhibitors are chemicals that interact with a
metallic surface, or the environment this surface
is exposed, giving the surface a certain level of
protection.
 Inhibitors often work by adsorbing themselves
on the metallic surface by forming a film
 Inhibitors slow corrosion process by:
 - Increasing the anodic or cathodic polarization
behavior (Tafel slopes)
 - Reducing the movement or diffusion of ions to
the metallic surface
 - Increasing the electrical resistance to the
metallic surface

23. CLASSIFICATION OF INHIBITOR
 Passivating (anodic) inhibitors
 Cathodic inhibitors
 Organic inhibitors
 Precipitation inhibitors

24. Passivating (anodic) inhibitors
 Passivating inhibitors cause a large anodic shift
of the corrosion potential, forcing the metallic
surface into the passivation range.
 There are two types of passivating inhibitors:
- oxidizing anions, such as chromate, nitrite
and nitrate that can passivate steel in the
absence of oxygen.
- nonoxidizing ions, such as phosphate,
tungstate and molybdate that require the
presence of oxygen to passivate the steel

25. Cathodic Inhibitors
 Cathodic inhibitors either slow the cathodic
reaction itself or selectively precipitate on
cathodic areas to increase the surface
impedance and limit the diffusion of reducible
species to these area.
 Arsenic, antimony, calcium, zinc, and
magnesium are the cathodic inhibitors.

26. Organic Inhibitors
 Both anodic and cathodic effects are sometimes observed
in the presence of organic inhibitors, but as general rule,
organic inhibitors effect the entire surface of corroding
metal present in sufficient concentration.
 Organic inhibitors, usually designated as film forming,
protect the metal by forming hydrophobic film on the
metal surface. Their effectiveness depends on the
chemical composition, their molecular structures, and
their affinities for the metal surface.
 Because film formation is an adsorption process, the
temperature and pressure in the system is the important
factors.
 Organic inhibitors will adsorbed according to the ionic
charge of inhibitors and the charge of the surface.

27. Precipitation Inhibitors
 Precipitation-inducing inhibitors are film forming
compounds that have general action over the metal
surface, blocking both anodic and cathodic sites
indirectly.
 Precipitation inhibitors are compound that cause the
formation of precipitates on the surface of the metal,
thereby providing protective layer.
 Hard water that is high in calcium and magnesium is less
corrosive than soft water because of the tendency of the
salts in the hard water to precipitate on the surface of the
metal and form a protective film.
 The most common inhibitors in this category are the
silicates and the phosphates, i.e. sodium silicate is used
in many domestic softeners to prevent the occurrence of
rust water. In aerated hot water systems, sodium
silicates protect steel, copper and brass.

28.  The proper engg: design of equipments can be
important for corrosion prevention as a selection of
the proper materials.
 The engg: designer must be consider the materials
along with the necessary mechanical, electric &
thermal properties which are to be required.
 All these conditions must be balanced the
economical limitations.
DESIGNING

29. Design Considerations
 Wall thickness – allowance to accommodate for corrosion
effect.
 Avoid excessive mechanical stresses and stress
concentrations in components exposed to corrosive
mediums. Especially when using materials susceptible to
SCC.
 Avoid galvanic contact / electrical contact between dissimilar
metals to prevent galvanic corrosion.
 Avoid sharp bends in piping systems when high velocities
and/or solid in suspension are involved – erosion corrosion.
 Avoid crevices – e.g weld rather than rivet tanks and other
containers, proper trimming of gasket, etc.

30.  Avoid sharp corners – paint tends to be thinner at sharp
corners and often starts to fail.
 Provide for easy drainage (esp tanks) – avoid remaining
liquids collect at bottom. E.g steel is resistant against
concentrated sulfuric acid. But if remaining liquid is
exposed to air, acid tend to absorb moisture, resulting in
dilution and rapid attack occurs.
 Avoid hot spots during heat transfer operations – localized
heating and high corrosion rates. Hot spots also tend to
produce stresses – SCC failures.
 Design to exclude air – except for active-passive metals and
alloys coz they require O2 for protective films.
 Most general rule : AVOID HETEROGENEITY!!!

31.  Simply anodic protection is based on the
formation of protection film by external applied
anodic current.
 If carefully controlled anodic protection current is
applied to Ni ,Fe, Cr, Ti, and their alloys they are
passivated and the rate of metal dissolution is
decreased.
ANODIC PROTECTION

32. Three-electrode setup: (1) working
electrode; (2) auxiliary electrode;
(3) reference electrode

33.  How to protect a metal?
 To anodically protect a structure a
device is required that is
potentiostat.
 It is an electronic device that
maintain a metal at a constant
potential either respect to a
reference electrode.
WHAT IS A
POTENTIOSTAT?

34.  The figure shows that the potentiostat has the
3 terminals
 one connected to the tank another to the
auxiliary cathode and 3rd
to the reference
electrode.
 In operation the potentiostat remains
Constant potential b/w the tank & reference
cathode.
 The optimum potential for protection in
measured by electrochemical measurements.

35. OPERATIONS
 Carbon steel in concentrated sulfuric acid
exhibits solution potentials in the ACTIVE
CORROSION zone.
 An external source of direct current moves
the solution potential from the ACTIVE
CORROSION zone into the
PASSIVATION zone where corrosion
rates are an order of magnitude lower.

38. Anodic protection can decrease corrosion
rate substantially.
The primary advantages of anodic
protection are its applicability in extremely
corrosive environments and its low
current requirements.
Anodic protection has been most
extensively applied to protect equipment
used to store and handle sulfuric acid.
ADVANTAGES OF ANODIC
PROTECTION

39. Failure of electrical supply may be
hazardous because of depassivation.
The requirement for electrical current
makes it useless for protection in
organic liquid environment.
And also for components which are
not continuously immersed.
DISADVANTAGES OF ANODIC
PROTECTION

40.  Cathodic protection is a process used to
prevent the corrosion.
 Cathodic protection is a means whereby
cathodic polarity is imposed upon the whole
pile and its operation as an anode (with
consequent deterioration) is prevented.
CATHODIC PROTECTION

44. PRINCIPLES OF CATHODIC PROTECTION
 No net overall charge builds up on the metal as a result of corrosion
since the rate of the anodic and cathodic reactions are equal.
 Anodic reactions involve oxidation of metal to its ions, e.g. for steel
the following reactions occur.
Fe  Fe2+
+ 2e
 The cathodic process involves reductions & several reactions are
possible. In acidic water, where hydrogen ions (H+
) are plentiful, the
following reaction occurs.
2H+
+ 2e  H2
 In alkaline solutions, where hydrogen ions are rare, the reduction of
water will occur to yield alkali and hydrogen.
2H2O + 2e  H2 + 2OH-

45.  However, unless the water is deaerated
reduction of oxygen is the most likely
processed, again producing alkali at the surface
of the metal.
O2 + 2H2O + 4e  2OH-
 Corrosion thus occurs at the anode but not at
the cathode unless the metal of the cathode is
attacked by the alkali.
 The anode and cathode in a process may be on
two different metals connected together forming
a bimetallic couple.

46.  The principle of cathodic protection is in
connecting an external anode to the metal to be
protected and the passing of an electrical DC
current so that all areas of the metal surface
become cathodic and therefore do not corrode.
 In electro-chemical terms, the electrical potential
between the metal and the electrolyte solution
with which it is in contact is made more negative,
by supply of negative charged electrons, to a
value at which the corroding (anodic) reactions
are stifled and only cathodic reactions can takes
place.

47. METHODS OF APPLYING CATHODIC
PROTECTION
Cathodic protections can be achieved in
two ways.
(i) By “impressed” current.
(ii) By the use of galvanic (sacrificial) anodes.

50. IMPRESSED CURRENT
 The pipeline is buried in the ground, which receives
current from a DC power source through an auxiliary
inert electrode buried in the ground.
 In this process external anode is impressed onto the
cathode surface.
 The anodes required must have good electrical
conduction, low rate corrosion, good mechanical
properties, low cost, able to withstand high current
densities at their surfaces without forming resistive
barrier oxide layers, etc.

51.  Magnetite, graphite, high silicon iron (14-18%),
lead/ lead oxides, lead alloys, Platinised
materials (such as tantalum, niobium, titanium)
etc. are used as anodes.
 High driving voltage (up to 100 V) is applied thus
the anode can be placed remote from the
structure.
 Large areas of a structure can be protected from
a single anode.

53. GALVANIC
(SACRIFICIAL)ANODE
 Galvanic anode systems employ reactive metals
as auxiliary anodes that are directly electrically
connected to the steel to be protected.
 The difference between the natural potentials
between the anode and the steel, causes a
positive current to flow in the electrolyte from the
anode to the steel.
 Thus, the whole surface of the steel becomes
more negatively charged and become the
cathode.

54.  The metals commonly used as sacrificial
anodes are Aluminum, Zinc and Magnesium.
 These metals are alloyed to improve the long
term performance and dissolutions
characteristics.

55. ADVANTAGES:
 The main advantage of cathodic protection over
other forms of anti-corrosion treatments is that it
is applied simply by maintaining the DC circuit
and its effectiveness may be monitored
continuously.
 Cathodic protection is commonly applied to a
coated structure to provide corrosion control to
areas where the coating may be damaged.
 It may be applied to existing structures to
prolong their life.

56.  Another advantage is that by the use of
cathodic protection initially will avoid the need
to provide a “corrosion allowance” to thin
sections of structures that may be costly to
fabricate.
 It may be used to afford security where even
a small leak cannot be tolerate for reasons of
sfety of environments.

57. USES:
 Structures that are commonly protected by cathodic
protection are the exterior surfaces of:
(i) Pipelines.
(ii) Storage tank bases.
(iii) Steel sheets, tubular and foundation pilings.
(iv) Offshore platforms, floating & sub sea structures.
 Cathodic protection is also used to protect the internal
surfaces of:
(i) Large diameters pipelines.
(ii) Ship tanks (product & ballast).
(iii) Storage tanks (oil & water).
(iv) Water circulating systems.

58. Protective Coatings / Wrapping
 The main function of coating is to provided an effective
barrier .
 There are many methods by which we can coated the
materials, i.e.:
 Metallic coating
 Inorganic coating
 Organic coating
 Coatings may act as sacrificial anode or release substance
that inhibit corrosive attack on substrate.

59. Metal coatings :
Metallic coatings provide a layer that changes the surface
properties of the workpiece to those of the metal being
applied.
Noble – silver, copper, nickel, Cr, Sn, Pb on steel. Should
be free of pores/discontinuity because creates small anode-
large cathode leading to rapid attack at the damaged areas.
Sacrificial – Zn, Al, Cd on steel. Exposed substrate will be
cathodic & will be protected.
Application – hot dipping, flame spraying, cladding,
electroplating, vapor deposition, etc.

60. Inorganic coatings can be produced by chemical action,
with or without electrical assistance. The treatments change
the immediate surface layer of metal into a film of metallic
oxide or compound which has better corrosion resistance
than the natural oxide film .
 cement coatings, glass coatings, ceramic coatings,
chemical conversion coatings.
 Chemical conversion – anodizing, phosphatizing,
chromate.
Inorganic coating :

61. Coating liquid generally consists of solvent, resin and
pigment. The resin provides chemical and corrosion
resistance, and pigments may also have corrosion inhibition
functions.
 paints, lacquers, varnishes.
Organic coating :

62. Organic lining:
 The difference between a polymer coating and
lining is the difference in thickness, and therefore
in service condition and protection mechanism.
 A coating is thin less than 0.5mm and a lining
usually be around 3mm.
 A lining is thick enough to be a complete barrier
between the metal and environment but coating is
not.
 Lining is applied to the interior of the tank or
vessel while coating are applied at the exterior
surface.