Page 24 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

2 INTRODUCTION AND FORMS OF CORROSION

TABLE 1.1 Forms of Corrosion 1

1. General corrosion Uniform, quasi-uniform, nonuniform corrosion,
galvanic corrosion
2. Localized corrosion Pitting corrosion, crevice corrosion, filiform corrosion
3. Metallurgically influenced Intergranular corrosion, sensitization, exfoliation,
corrosion dealloying
4. Microbiologically influenced
corrosion
5. Mechanically assisted Wear corrosion, erosion–corrosion, corrosion fatigue
corrosion
6. Environmentally induced Stress-corrosion cracking; hydrogen damage,
cracking embrittlement; hydrogen-induced blistering;
high-temperature hydrogen attack; hot cracking,
hydride formation; liquid metal embrittlement; solid
metal-induced embrittlement
1 ASM Metals Handbook, Corrosion, Vol. 13, 9th ed., Craig and Pohlman, pp. 77–189.

carburization, hydrogen effects, and hot corrosion can be considered as types of gen-
eral corrosion (16) .
(1)
Liquid metals and molten salts at high temperatures lead to general corrosion .
Microelectrochemical cells result in uniform general corrosion. Uniform general cor-
rosion can be observed during chemical and electrochemical polishing and passivity
where anodic and cathodic sites are physically inseparable. A polished surface of a
pure active metal immersed in a natural medium (atmosphere) can suffer from gal-
vanic cells. Most of the time, the asperities act as anodes and the cavities as cathodes.
If these anodic and cathodic sites are mobile and change in a continuous dynamic
manner, uniform or quasi-uniform corrosion is observed. If some anodic sites persist
and are not covered by protective corrosion products, or do not passivate, localized
corrosion is observed (1).
Some macroelectrochemical cells can cause a uniform or near-uniform general
attack of certain regions. General uneven or quasi-uniform corrosion is observed in
natural environments. In some cases, uniform corrosion produces a somewhat rough
surface by the removal of a substantial amount of metal that either dissolves in the
environment or reacts with it to produce a loosely adherent, porous coating of corro-
sion products. After careful removal of rust formed because of general atmospheric
corrosion of steel, the surface reveals an undulated surface, indicating nonuniform
attack of different areas (1) as shown in Figure 1.1.
In natural atmospheres, the general corrosion of metals can be localized. The cor-
rosion morphology is dependent on the conductivity, ionic species, temperature of
the electrolyte, alloy composition, phases, and homogeneity in the microstructure of
the alloy, and differential oxygenation cell. The figure also shows high-temperature
attack that is generally uniform. It is also possible to observe subsurface corrosion
films within the matrix of the alloy because of the film formation at the interface of
certain microstructures in several alloys at high temperatures (3).