What is Pitting Corrosion
Pitting Corrosion
Under certain specific conditions, particularly involving chlorides (such as
sodium chloride in sea water) and exacerbated by elevated temperatures, small
pits can form in the surface of the steel (Pitting Corrosion).
This type of corrosion affects in particular some stainless steels grades.
Dependent upon both the environment and the steel itself these small
pits may continue to grow, and if they do can lead to perforation, while the
majority of the steel surface may still be totally unaffected.
Environmental Factors for Pitting Corrosion
The severity of the environment is very largely dependent upon two factors. The chloride concentration resistance to this type of corrosion is usually described in terms of what % Cl- (or ppm Cl- ) and °C it can resist.
It should be noted that the most common grade of stainless steel, Type 304, may be considered susceptible to pitting corrosion in sea water (2% or 20,000 ppm chloride) above about 10°C, and even in low chloride content water may be susceptible at only slightly elevated temperatures.
A safe chloride level for warm ambient temperatures to prevent Pitting Corrosion is generally about 150ppm (150mg/l). Grade 316 is more resistant to Pitting Corrosion and is commonly used in ambient sea water, but can be attacked in crevices or if the temperature increases even slightly.
The velocity of the liquid is also significant; a stagnant solution is more likely to result in pitting corrosion and crevice corrosion attack, particularly if there are particles to settle out of the liquid.
Note that there may also be a problem of stress corrosion cracking if austenitic stainless steels are used in chloride containing water at temperatures over about 60°C
see also:
pitting crevice of stainless steel
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