advanced metallurgy

1. Corrosion Metallurgy
Advanced Metallurgy
Presented by
Omar Mohamed Yousri
Ahmed Hassan Awad
Ain Shams University Design & Prod. Eng. Dep
Faculty of Engineering MDP 610

2. Introduction
Corrosion is a spontaneous electrochemical process and is defined
as degradation of material due to its reaction with the
environment.
To one degree or another, most materials experience some type
of interaction with the environment, a deterioration in mechanical
properties of the material occur, other physical properties or
appearance also.

3. Forms of corrosion
• The different forms of corrosion
• represent corrosion phenomena categorized according
to their appearance.
• The three categories used are:
– Group 1. Readily identifiable by ordinary visual
examination “Uniform – Pitting – Crevice – Galvanic”
Corrosion
– Group 2. May require supplementary means of
examination. “Erosion – Intergranual” Corrosion
– Group 3. Verification is usually required by microscopy
(optical, electron microscopy, etc.). “Selective leaching –
Stress corrosion cracking”

4. Uniform (or general) corrosion
• Uniform corrosion is characterized by corrosive attack
proceeding evenly over the entire surface area or a
large fraction of the total area. General thinning takes
place until failure.
• The breakdown of protective coating systems on
structures often leads to this form of corrosion.
• Surface corrosion can indicate a breakdown in the
protective coating system, however, and should be
examined closely for more advanced attack.
• If surface corrosion is permitted to continue, the
surface may become rough, and surface corrosion can
lead to more serious types of corrosion.

6. Pitting corrosion
• Pitting corrosion is a localized form of corrosion by which
cavities, or “holes,” are produced in the material.
• Pitting is considered to be more dangerous than uniform
corrosion damage because it is more difficult to detect or
predict it.
• Corrosion products often cover the pits. A small, narrow pit
with minimal overall metal loss can lead to the failure of an
entire engineering system.
• Pitting corrosion occurs when discrete areas of a material
undergo rapid attack while most of the adjacent surface
remains virtually unaffected.
• Apart from the localized loss of thickness, corrosion pits can
also be harmful by acting as stress risers.

8. Crevice corrosion
• Crevice corrosion is a localized form of corrosion usually
associated with a stagnant solution on the
microenvironmental level. Such stagnant
microenvironments tend to occur in crevices (shielded
areas) such as those formed under gaskets, washers,
insulation materials. Because oxygen diffusion into the
crevice is restricted.
• A differential aeration cell tends to be set up between
crevice (microenvironment) and the external surface (bulk
environment).
• Creation of highly corrosive microenvironmental conditions
in the crevice, conducive to further metal dissolution.

10. Galvanic Corrosion
• Galvanic corrosion occurs when dissimilar metallic materials are
brought into contact in the presence of an electrolyte.
• An electrochemical corrosion cell is set up due to differences in the
corrosion potentials of the dissimilar materials.
• The material with the more noble corrosion potential then becomes
the cathode of the corrosion cell, whereas the less noble material is
consumed by anodic dissolution.
• Effect of surface area ratio:
If aluminum rivets were
used on steel plates, the
rivets would corrode
extremely rapidly

11. • Corrosion rate increase with the increase in
potential difference.
• Corrosion current density strongly increased
with the decrease in anode surface area.

12. Selective leaching
• Selective leaching refers to the selective removal of
one element from an alloy by corrosion processes.
• A common example is the dezincification of
unstabilized brass, whereby a weakened, porous
copper structure is produced.
• The selective removal of zinc can proceed in a uniform
manner or on a localized (plug-type) scale.
• brass dissolves with Zn remaining in solution and Cu
replating out of the solution.
• Graphitization of gray cast iron, whereby a brittle
graphite skeleton remains following preferential iron
dissolution, is a further example of selective leaching.

13. Uniform layer dezincification in brass heat-exchanger tube
0.110 mm
(0.0042”)
Brass (Cu-30% zinc) exposed to room temperature salt solution for 80 days
Reprinted with permission of ASM International®. All rights reserved.
Outer tube
wall
Dezincified
metal

14. Erosion Corrosion
• Erosion corrosion is the cumulative damage induced by
electrochemical corrosion reactions and mechanical
effects from relative motion between the electrolyte
and the corroding surface.
• Erosion corrosion is defined as accelerated degradation
in the presence of this relative motion.
• The motion is usually one of high velocity, with
mechanical wear and abrasion effects.
• Grooves, rounded edges, and waves on the surface
usually indicating directionality characterize this form
of damage.
• Erosion corrosion is found in systems such as piping,
valves, pumps, nozzles, heat exchangers, turbine
blades.

16. Stress corrosion cracking
• SCC is the cracking induced from the combined influence of
tensile stress and a corrosive medium.
• The required tensile stresses may be in the form of directly
applied stresses or residual stresses.
• Cold deformation and forming, welding, heat treatment,
machining, and grinding can introduce residual stresses.
• SCC usually occurs in certain specific alloy-environment-
stress combinations.
• Usually, most of the surface remains unattacked, but with
fine cracks penetrating into the material. In the
microstructure, these cracks can have an intergranular or a
transgranular morphology.
• SCC is classified as a catastrophic form of corrosion because
the detection of such fine cracks can be very difficult and
the damage not easily predicted.

17. SCC in aircraft
component

18. Intergranular corrosion
• Intergranular corrosion is localized attack along the 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 or specific phases precipitated on the
grain boundaries.
• Such precipitation can produce zones of reduced corrosion
resistance in the immediate vicinity.
• A classic example is the sensitization of stainless steels.
Chromium-rich grain boundary precipitates lead to a local
depletion of chromium immediately adjacent to these
precipitates, leaving these areas to corrosive attack.

20. Metallurgical Factors affecting
corrosion
• Material properties
• Effect of Alloy composition
• Microstructure
• Degree & type of deformation
• Heat treatment
• Passive layer formation

21. 1. Material properties
• Electrode potential is the major metallurgical
factor affecting corrosion rate.
• Electrode potential depends mainly on the
electrode chemical composition (alloy
composition), the presence of impurities (non
metallic inclusions), electronic structure,
crystal structure.

22. Electromotive Series
(emf series)
in reduction form
22

23. Metal crystal Structure
• BCC having an open structure while FCC & HCP having close
packed structure.
• FCC & HCP are more resistance to corrosion than BCC.
• As the structure become close packed, the corrosion
affinity decrease.

24. 2. Effect of alloy composition
Effect of Ni and Cr in carbon and alloy steel:

25. • As Cr % and Ni % in steel increase the exchange
current density decrease.
• The exchange current density represent corrosion
rate.
• So as Cr and Ni % increase corrosion rate
decrease.

26. 3. Microstructure
• The microstructure phases and micro
constitutional structure such as grain
boundary, second phase precipitations, phase
morphology greatly affect corrosion rate.
• It may form micro galvanic corrosion cells as a
result of the difference in the stability or free
energy. Where less stable (high energy) area
will be anode relative to more stable area
(lower energy) which act as cathode.

27. Grains are more stable than Grain boundary due to the
arrangement of atoms, so Grain boundaries are anode
and Grains are cathode. (anode corroded)

28. • Microstructure (micro galvanic cell between
different phases for example between ferrite and
cementite in carbon steel)
• Cementite is intermetallic compound which is
more stable than Ferrite solid solution
• Cementite is cathode while ferrite is anode.
Dark area is cementite while bright area is ferrite

29. 4. Degree & type of deformation
• Type of deformation:
a. Cold deformation: cold deformed structure is
less resistant to corrosion due to the presence of
large amounts residual internal stresses and large
amounts of structural defects.
b. Hot deformation: hot deformed structure is
relatively more stable and more corrosion
resistant due to the presence of recrystallized
structure which is free from internal residual
stresses and structural defects.

30. Degree of deformation:
• Structural defects and internal energy increased
with increasing degree of deformation.
• Corrosion resistant decreased with increasing the
degree of deformation.
A mechanically deformed metal or alloy can
experience galvanic corrosion due to differences in
atomic plane distortion and a high dislocation
density.

31. 5. Heat treatment
• Types of heat treatment:
1. Annealing (Having high phase stability with very low
thermal and internal stresses; very slow cooling rate)
“very low corrosion rate”
2. Normalizing (Relatively stable phase with low thermal and
internal stresses; slow cooling rate in air) “low corrosion
rate”
3. Hardening or quenching (very unstable phase with high
thermal & internal stresses; high cooling rate) “ very High
corrosion”
4. Aging or precipitation hardening (unstable phase with
relatively high thermal & internal stresses; high cooling
rate followed by tempering) “High corrosion rate”

32. 6. Passive layer formation
During corrosion of metals, metal oxide layer
formed at the surface of the metal.
• If metal oxide is stable; further corrosion of
metal is prevented by the formed metal oxide
(passive layer)
• If metal oxide is unstable; corrosion process
will continue (porous metal oxide – volatile
metal oxide)

33. Characteristics of self protective layer
(passive layer)
• Adherence
• Compacted
• Thermal & electrical
conductivity
• Good Mechanical
properties
• Formability