Corrosion handbook
06/21 7
1.2.2 Pitting corrosion
Pitting corrosion is a localized form of corrosion that leads to the creation of small
holes or “pits” in the metal (see Fig. 5). This form of corrosion is mainly found
on passive metals. Passive metals and alloys, such as aluminum, titanium and
stainless steel owe their corrosion resistance to a thin oxide layer of only a few
nanometers on their surface. The corrosion process starts with a local break-
down of the passive layer. Local corrosive attack can be initiated on stainless
steels, for example, by chloride ions.
Fig. 6 shows the most significant phases of this corrosion phenomenon on
stainless steel. Pitting corrosion can be quite problematic. Whereas uniform
corrosion can be seen clearly on the surface, pitting corrosion often appears
only as small pinholes. The amount of material removed below the pinholes is
generally unknown as hidden cavities may form, making this type of corrosion
more difficult to detect and predict. Technically, there is no reasonable way
to control pitting corrosion so it must be excluded from the outset via design
considerations and use of the right materials, engineering and quality control.
In addition, pitting corrosion can often be the starting point for more severe
forms of corrosion such as stress corrosion cracking (see section “Stress
corrosion cracking (SCC)” on page 10).
Fig. 5: Example of pitting corrosion on
a stainless steel product.
b. Local breakdown of passivation due
to attack of chloride ion.
c. Start of corrosion on the active
(not passive) part of the surface.
d. Anodic dissolution of iron in the pit,
oxygen reduction takes place
outside.
a. Stainless steel with passivated
surface.
Fig. 6: Phases of pitting corrosion on stainless steel.
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