Types of corrosions

1. Uniform Corrosion
 This corrosion is also called General
Corrosion.
 Effect produced by most direct chemical
attacks.
 This is common form of corrosion.
 This type of corrosion is first seen as a general
dulling of the surface and, if allowed to
continue; the surface becomes rough.

How to prevent uniform
corrosion?
Uniform corrosion or general corrosion can be prevented through a number of
methods:
 Use thicker materials for corrosion allowance
 Use paints or metallic coatings such as plating,
galvanizing or anodizing
 Use Corrosion inhibitors or modifying the
environment

2. Galvanic Corrosion
 Also called as “dissimilar metal corrosion”
 Takes place when two metals are in physical
contact with each other and are immersed in a
conducting fluid.
 Corrosion damage induced when two dissimilar
materials are coupled in a corrosive electrolyte.

Examples
 Plate and screw of different electrical potentials
due to differences in processing
 Multiple component implant using different
metals for each component
 Copper and steel tubing are Joined in a domestic
water heater, the steel will corrode in the vicinity
of the junction

The relative nobility of a material can be predicted by measuring
its corrosion potential. The well known galvanic series lists the
relative nobility of certain materials in sea water. A small
anode/cathode area ratio is highly undesirable. In this case, the
galvanic current is concentrated onto a small anodic area. Rapid
thickness loss of the dissolving anode tends to occur under these
conditions. Galvanic corrosion problems should be solved by
designing to avoid these problems in the first place.
Galvanic corrosion between
stainless steel screw and Aluminium.
Galvanic corrosion between Steel
and Brass.

Note that the area ratio of the anode: cathode
is an important variable affecting the dissolution
current density (and hence corrosion rate)
pertaining to the anode. The area ratio is also
important when considering the relative amount
of current "available" from the cathodic reaction.
The following have been described as "main factors"
influencing galvanic corrosion rates in Skanaluminium's on-
line publication "Alubook - Lexical knowledge about
aluminium".
• Potential Difference between materials
• Cathode Efficiency
• Surface areas of connected materials (area ratio)
• Electrical resistance of the connection between the
materials and of the electrolyte.
Fig. Brass on Weathering Steel - rust forms in discrete
crystallites that are fine, red and diffusely reflecting, like
hematite. The massive re-crystallized layer is a shiny blue,
approaching the blue-black of secular hematite.

3. Differential Aeration Corrosion
 This type of corrosion takes place when a
metal is unevenly exposed to different
oxygen/air concentrations.
 The part which is exposed to less oxygen
undergoes corrosion.

Differential Aeration Corrosion
(Cont)
At the anode (less O2 concentration),
M ------------Mn+ + ne
At the cathode (more O2 concentration),
H2O + ½ O2 + 2e------ 2OH-

Examples
 Drop Corrosion:
It takes place when a drop of electrolyte is in
contact
with the metal surface. The metal surface covered
by
is in contact with lesser amount of air than the
uncovered metal surface. Thus, metal covered by
drop becomes anodic and corroded whereas the

Waterline Corrosion
 Waterline corrosion s a case of differential aeration corrosion, more prevalent in
cases such as ocean going ships, water storage steel tanks, etc., in which a portion
of the metal is always under water. The waterline corrosion takes place due to the
formation of differential oxygen concentration cells. The part of the metal below the
water line is exposed only to dissolved oxygen while the part above the water is
exposed to higher oxygen concentration of the atmosphere. Thus, part of the metal
below the water acts as anode and undergoes corrosion and part above the
waterline is free from corrosion. A distinct brown line is formed just below the water
line due to the deposition of rust.

4. Pitting Corrosion
 Pitting corrosion is a localized form of corrosive
attack that produces holes or small pits in a
metal.
 the bulk of the surface remains unattacked.
 Pitting is often found in situations where
resistance against general corrosion is
conferred by passive surface films.

4. Pitting Corrosion (cont…)
 Localized pitting attack is found where these
passive films have broken down.
 Pitting attack induced by microbial activity, such
as sulfate reducing bacteria (SRB) also
deserves special mention.

Pitting Corrosion (Images)
Corrosion Pits are the primary source of
leaks in water handling systems

5. Stress Corrosion
 This type of corrosion is observed in fabricated
articles which are subjected to various
mechanical operations
 Here mechanical operations refers Bending,
Hammering and annealing.
 This corrosion is usually unpredictable is
nature.

Examples
 Season Cracking
 Caustic embrittlement of mild steel

6. Corrosion Fatigue
 Corrosion Fatigue is a special case of stress
corrosion caused by the combined effects of
cyclic stress and corrosion.
 Control of corrosion fatigue can be
accomplished by either lowering the cyclic
stress or by corrosion control.

7. Crevice Corrosion
 Crevice corrosion is a localized form of corrosion
usually associated with a stagnant solution on the
micro-environmental level.
 Such stagnant microenvironments tend to occur in
crevices (shielded areas) such as those formed
under gaskets, washers, insulation material,
fastener heads, surface deposits, disbonded
coatings, threads, lap joints and clamps.

Crevice Corrosion (cont..)
 Occurs under gaskets, rivets and bolts,
between valve disks and seats.
 Well-known examples of such geometries
including flanges, gaskets, disbonded
linings/coatings, fasteners, lap joints and
surface deposits.

8. Intergranular Corrosion
 Intergranular corrosion refers to preferential (localized)
corrosion along grain boundaries.
 or immediately adjacent to grain boundaries, while the
bulk of the grains remain largely unaffected.
 This form of corrosion is usually associated with
chemical segregation effects (impurities have a
tendency to be enriched at grain boundaries) or specific
phases precipitated on the grain boundaries.

Intergranular Corrosion (cont…)
 This selective dissolution may lead to the
dislodgement of grains.
 Intergranular corrosion in sensitized stainless
steels and exfoliation in aluminum alloys
represent industrially significant examples of this
form of damage.

Intergranular Corrosion (Images)
FIG. Intergranular corrosion of a failed
aircraft component made of 7075-T6
aluminum (picture width = 500 mm)
Fig. Severe problem in the welding of stainless
steels, when it is termed weld decay.

9. Microbiological Corrosion
 Caused by microbes like bacteria, algae fungi
etc
 These microbes can be aerobic or anaerobic
 Aerobic bacteria decrease the concentration of
oxygen in the medium in contact with metal
surface.
 The main product of corrosion in anaerobic
corrosion is iron sulphide

10. Filiform Corrosion
 Occurs under painted or plated surfaces when
moisture permeates the coating.
 Filiform corrosion is also known as "underfilm
Corrosion" or "filamentary corrosion".
 Filiform corrosion can be visually recognized
without using a microscopy

Prevention methods for Filiform
Corrosion
 Filifrom Corrosion can be prevented with the
following methods:
 control the relative humidity
 use brittle coatings

Images
The mechanism of filiform corrosion is shown in the figure below.

11. Erosion Corrosion
 It is the result of relative movement between
the corrosive fluid and metal surface
 All types of equipments exposed to moving
fluids are subjected to erosion corrosion.
 Surface chemistry can play a role in erosion
corrosion due to mechanochemical effects.

Erosion Corrosion Examples
 Ship propellors
 Hydralic turbines
 Pump impellers
 Diesel engine cylinder

12. Soil Corrosion
 The two factors which promote corrosion ie..,
moisture and dissolved electrolytes are
present in soil, making it corrosive.
 Presence of micro-organisms in soil further
leads to corrosion of underground and
pipeline.

13. Hydrogen Damage
 Hydrogen Damage is the process by which
various metals, most importantly high-strength
steel, become brittle and fracture following
exposure to hydrogen.
 Hydrogen damage is often the result of
unintentional introduction of hydrogen into
susceptible metals during forming or finishing
operations and increases cracking in the material.
 This phenomenon was first described in 1875.

Divisions in Hydrogen Damage
Hydrogen damage can be divided into following
types
 Hydrogen Blistering
 Hydrogen embrittlement
 Decarburization
 Hydrogen attack

14. Fretting Corrosion
 Fretting refers to wear and sometimes
corrosion damage at the asperities of contact
surfaces
 This corrosion occurs at the interface between
contacting, highly loaded metal surfaces when
subjected to slight vibratory motions is know
an fretting corrosion.

Fretting Corrosion (cont..)
 This corrosion is most common in bearing
surfaces in machinery.
 he most common type of fretting is caused by
vibration.
 The protective film on the metal surfaces is
removed by the rubbing action and exposes
fresh, active metal to the corrosive action of
the atmosphere.

15. Selective Leaching
 Selective leaching, also called dealloying,
demetalification, parting and selective
corrosion
 Selective leaching is a corrosion process in
which one constituent of an alloy is
preferentially dissolved by the environment,
leaving the dealloyed metal weak and often
porous

Types of corrosions

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