The document discusses corrosion as an electrochemical reaction where metals deteriorate when reacting with their environment. It details various forms of corrosion such as uniform, galvanic, and pitting corrosion, along with mechanisms and preventive measures to protect metals. Key factors influencing corrosion include environmental conditions, material properties, and techniques like cathodic protection and coatings.

1. 1
Service Behaviour of Materials
Corrosion
METALS WANT TO CORRODE – they want to exist as oxide
compounds because oxides contain less energy and are more stable!!

2. What is corrosion?
• Electrochemical reaction involving an anode and a
cathode.
• Deterioration of a material because of reaction with the
environment.
• Corrosion is affected by the properties of both the metal
or alloy and the environment. The environmental
variables include:
– pH (acidity)
– Oxidizing power (potential)
– Temperature (heat transfer)
– Velocity (fluid flow)
– Concentration (solution constituents)

3. What is corrosion?
• Cost equals 3 – 5% of GNP/ year
• Corrosion of steel – the biggie
• Combination of the material and it’s environment
- Examples:
– No Problem:
• Lead in Water
• Aluminum in atmosphere
• Nickel in hydraulic fluid
– BAD:
• Steel in marine environment
• Cu in Ammonia
• Stainless steel in chloride (sea water)
• Lead in wine

4. 4
Electrochemical considerations
For metallic materials, the corrosion process is normally electrochemical, that is, a
chemical reaction in which there is transfer of electrons from one chemical species
to another. Metal atoms characteristically lose or give up electrons in what is
called an oxidation reaction.
The site at which oxidation takes place is called the anode; oxidation is sometimes
called an anodic reaction.

5. 5
The electrons generated from each metal atom that is oxidized must be transferred
to and become a part of another chemical species in what is termed a
reduction reaction. For example, some metals undergo corrosion in acid solutions,
which have a high concentration of hydrogen (H) ions; the H ions are reduced
as follows:
Reduction of
hydrogen ions in
an acid solution
Reduction reaction in an acid
solution containing dissolved
oxygen
Reduction reaction in a neutral
or basic solution containing
dissolved oxygen
Reduction of a multivalent
metal ion to a lower valence
state
Reduction of a metal ion to its
electrically neutral atom
(1)
(2)
(3)
(4)
(5)

6. 6
For example, consider zinc metal immersed in an acid solution containing H+ ions. At some
regions on the metal surface, zinc will experience oxidation or corrosion as illustrated in
Figure 1, and according to the reaction
Figure 1 The electrochemical reactions
associated with the corrosion of zinc in an acid solution.
Since zinc is a metal, and therefore a good electrical conductor, these electrons
may be transferred to an adjacent region at which the H ions are reduced
according to
(6)
(7)

7. 7
Another example is the oxidation or rusting of iron in water, which
contains dissolved oxygen. This process occurs in two steps; in the first,
Fe is oxidized to Fe2+ [as Fe(OH)2],
and, in the second stage, to Fe3+ [as Fe(OH)3] according to
The compound Fe(OH)3 is the all too familiar rust.
As a consequence of oxidation, the metal ions may either go into the
corroding solution as ions (reaction (6)), or they may form an insoluble
compound with nonmetallic elements as in reaction (9).
(8)
(9)

8. 8
Electrode potentials
Not all metallic materials oxidize to form ions with the same degree of ease.
Figure 2 An electrochemical cell consisting of iron and copper electrodes, each of
which is immersed in a 1 mole solution of its ion. Iron corrodes while copper
electrodeposits.
Cathode
Reduction
Anode
Oxidation

9. 9
Figure 3 An electrochemical cell consisting of iron and zinc
electrodes, each of which is immersed in a 1 mole solution of its ion.
The iron electrodeposits while the zinc corrodes.
Cathode
Reduction
Anode
Oxidation

10. 10

11. 11
• Uniform corrosion often occurs on steel producing a reddish brown
product that is called rust. The rate of corrosion decreased and it can
have a long service life with a uniform colour.
• However the rust staining is normally not acceptable on visible parts
of buildings.
Corrosion that occurs evenly over the metal surface.
Uniform corrosion

12. 12
This aluminium plate was subjected to acid and the
corrosion attack was reasonably uniform with some holes
formed in the plate.

13. 13
Galvanic corrosion
Corrosion involving two or more metals where the less active
metal increases the corrosion rate of the more active metal.
Anode (active) Cathode (less active)
Fe Cu
Zn Fe
Fe Stainless steel
Al Brass
The Al door frame is anodic
compared with the brass
door hinge. In a warm, humid
environment the door frame
was corroded locally by
galvanic corrosion.

14. 14
• The metal to be protected is connected to a more active metal that
corrodes first and protects the less active metal.
• This is called cathodic protection but there must be sufficient
electrolyte to work properly. Normally this requires immersion in an
aqueous solution.
• Zinc protects Fe. Fe protects Cu.
Galvanic corrosion can also be good
Galvanic protection of steel as
provided by a coating of zinc. A
standard technique used for
car bodies.

15. 15
• Galvanising (hot-dip galvanising) is normally used to describe the
zinc coating obtained by dipping steel into a molten bath of zinc.
(60-120 microns)
• Electroplating deposits a layer of metal onto steel from an
aqueous solution by discharging the ions. This is more expensive
than galvanising but can give a smooth and shiny appearance. (3-
20 microns)
• Diffusion coating involves heating the steel in a furnace with zinc
power at about 400 ℃ so that zinc atoms diffuse into the steel. (no
pure zinc layer but some protection)

16. 16
• Different levels of protection are provided by zinc coatings of
different thickness applied by different methods.
• Protection is proportional to the thickness of the zinc coating.
Galvanised Electroplated Diffusion coated

17. 17
The pipes and connectors of this
railing are galvanised.
The pipes on this railing are
discoloured by rust stains but the
connectors are not stained.
The protective coating of zinc on the
connectors is thicker.
Uniform corrosion of the zinc has
not exposed iron containing layers
on the connectors so there is no
rust staining. The corrosion
products of zinc are white in colour.

18. 18
The zinc coating on the hot dipped
galvanised pole provides good
protection to atmospheric corrosion.
The thinner electroplated zinc
coating on the brackets did not
provide sufficient protection against
atmospheric corrosion.

19. 19
Crevice corrosion
• Crevice corrosion occurs inside narrow gaps where a low oxygen
concentration develops.
• The metal at low oxygen positions becomes anodic and is
attacked.
• This can also happen under debris or deposits on the metal
surface.
low O2 content,
acting as anode
Schematic illustration of the mechanism of crevice
corrosion between two riveted sheets.

20. 20
• Crevice corrosion on the
underside of a stainless steel
nut and on the top surface that
was horizontal and had some
debris settled on it.
• Crevice corrosion underneath
the gasket of a stainless steel
holder for a filter in a piping
system

21. 21
The crevice formed around the cover
on this galvanised lamp post trapped
moisture and resulted in lower
oxygen levels in the narrow gap.
The result was first more rapid
corrosion of zinc in the crevice and
then rusting of the base steel.
Crevice corrosion is best controlled by eliminating crevices: sealing
or opening up the narrow gaps; selection of more resistant alloys.

22. 22
Pitting corrosion
Attack is limited to very small areas on the surface of a metal that is
generally resistant to corrosion in that environment.
Stainless steels, aluminium alloys and many other metals that form
protective surface films can, in specific environments, suffer from
pitting corrosion.
Isolated spots of rusting formed
on this sheet are common on
304 stainless steel.

23. 23
Isolated points of corrosion have occurred in this copper pipe
even though a protective film has formed over most of the surface.
This pitting corrosion can result in leakage and expensive repairs
even though only a small amount of metal has been removed.

24. 24
The mechanism for pitting is probably the same as for crevice
corrosion in that oxidation occurs within the pit itself, with
complementary reduction at the surface.
Control of pitting corrosion is best achieved by: keeping the metal
surface clean, choosing the correct alloy for the expected service
conditions.
Once pitting has become established, it is difficult to stop or to control
because the pit is contaminated and tends to develop its own local
environment.
Pitting corrosion

25. 25
Intergranular corrosion
Corrosion that attacks along the grain boundaries is called
intergranular corrosion.
The grain boundaries are more chemically active and may have a
more anodic composition than the bulk of the grain.
This is a microscopic form of galvanic corrosion.
In general, stainless steels usually contain more than 11wt% Cr in solid
solution, which will react with O2 and form a thin layer of oxide and
therefore prevent further corrosion. However, some stainless steels
can suffer from intergranular corrosion.

26. 26
Stainless steel after welding Grain decohesion due to
intergranular corrosion
Intergranular corrosion in stainless steels after welding

27. 27
During the welding process of stainless steels, small precipitates
(particles of chromium carbides) form along the grain. Both the
chromium and the carbon must diffuse to the grain boundaries to form
the precipitates, which leaves a chromium-depleted zone adjacent to
the grain boundary. Consequently, this grain boundary region is now
highly susceptible to corrosion.
Mechanisms of intergranular
corrosion in stainless steels
after welding.
Cr23C6

28. 28
Stainless steels may be protected from intergranular corrosion
by the following measures:
(1) subjecting the sensitized material to a high-temperature
heat treatment in which all the chromium carbide particles are
redissolved;
(2) lowering the carbon content below 0.03 wt% so that
carbide formation is minimal;
(3) alloying the stainless steel with another metal such as
niobium or titanium, which has a greater tendency to form
carbides than does chromium so that the Cr remains in solid
solution.

29. 29
Selective leaching (dealloying)
• Corrosion when one alloying element is selectively removed from metal
alloys. The result is a weak and maybe porous material that can easily
fail.
• Removal of iron from grey cast iron leaves a weak and porous deposit
of rust and graphite flakes. This is called graphitic corrosion.

30. 30
Dezincification occurred in
this pipe at points around
the internal surface.
Removal of zinc from brass (copper/zinc alloy) leaves
a weak and porous deposit of copper. This is called
dezincification.

31. 31
Erosion-corrosion
Erosion–corrosion arises from the combined action of chemical attack
and mechanical abrasion or wear as a consequence of fluid motion.
Erosion–corrosion is commonly found in piping, especially at bends,
elbows, and abrupt changes in pipe diameter—positions where the
fluid changes direction or flow suddenly becomes turbulent.
The outside of the bend in
this pipe was more corroded
because of the action of the
water removing corrosion
product and exposing fresh
metal.

32. 32
Serious attack has occurred
on the leading edge of the
blades of these impellors
taken from nested pumps.
Impingement failure of an elbow that
was part of a steam condensate line.

33. 33
Stress corrosion
• Corrosion can be assisted by applied or internal stresses in a
component.
• Stress corrosion cracking is a result of a combination of corrosion
and applied or internal stresses. A crack can progress alternatively
through dissolution of its tip or mechanical propagation.
Cracking of this cold rolled brass strip
occurred because it was exposed to
small amounts of ammonia vapour.
This is a common problem for cold
worked brass. The internal residual
stresses were formed during the
rolling process.

34. 34
• Uniform Attack
Oxidation & reduction
reactions occur uniformly
over surfaces.
• Selective Leaching
Preferred corrosion of
one element/constituent
[e.g., Zn from brass (Cu-Zn)].
• Stress corrosion
Corrosion at crack tips
when a tensile stress
is present.
• Galvanic
Dissimilar metals are
physically joined in the
presence of an
electrolyte. The
more anodic metal
corrodes.
• Erosion-corrosion
Combined chemical attack and
mechanical wear (e.g., pipe
elbows).
Forms of corrosion
Forms
of
corrosion
• Crevice Narrow and
confined spaces.
Rivet holes
• Intergranular
Corrosion along
grain boundaries,
often where precip.
particles form.
attacked
zones
g.b.
prec.
• Pitting
Downward propagation
of small pits and holes.

35. 35
-- Use metals that passivate
- These metals form a thin,
adhering oxide layer that
slows corrosion.
• Lower the temperature (reduces rates of oxidation and
reduction)
Corrosion prevention
Metal (e.g., Al,
stainless steel)
Metal oxide
• Materials selection
-- Use metals that are relatively unreactive in the
corrosion environment -- e.g., Ni in basic solutions

36. 36
Using a sacrificial anode
steel
pipe
Mg
anode
Cu wire
e-
Earth
Mg2+
• Cathodic (or sacrificial) protection
-- Attach a more anodic material to the one to be protected.
steel
zinc
zinc
Zn2+
2e- 2e-
e.g., zinc-coated nail
Galvanized Steel
e.g., Mg Anode
Corrosion prevention
• Apply physical barriers -- e.g., films and coatings