Specialized Corrosion Summary
Pitting corrosion occurs where the anodic site becomes fixed in a small area and
the formation of holes in an otherwise unaffected area takes place.
Crevice corrosion is a type of pitting corrosion that occurs specifically within
the low flow region of a crevice.
Stress corrosion cracking (SCC) is a type of intergranular attack corrosion that
occurs at the grain boundaries under tensile stress.
Pitting corrosion requires two conditions to occur, low flow and areas of both
high and low oxygen concentration. When these conditions are met a
differential aeration cell is established which produces an electron flow from the
point of low oxygen concentration to the area of high oxygen concentration.
The difference in oxygen concentration is usually due to a low flow condition.
Pitting corrosion is a hazard due to the possible rapid penetration of the metal
with little overall loss of mass. Pitting corrosion is minimized by:
Avoiding stagnant conditions
Using the correct metals and alloys that are less susceptible to the
Avoiding agents in the medium that cause pitting
Designing the system and components such that no crevices are present
Stress corrosion cracking occurs when three conditions are met; the alloy is
susceptible to stress corrosion cracking, the alloy is exposed to specific
environment, and the alloy is in a stressed condition.
Chemisorption is the formation of a monomolecular layer of a compound
between the metal's surface atoms. This layer separates the metal's atoms
thereby weakening the metal and allowing any existing defects to propagate
when a stress is applied.
The important information of this chapter is summarized below.