Different forms of stainless steel corrosion

• General or uniform corrosion This is not really a category but corrosion can occur uniformly or locally. If the contamination is evenly distributed over the entire surface, this if often the result of choosing the wrong material. If, for example, SS 304 is used in a contaminated environment, a better choice would have been SS 316. As a general rule, we recommend SS 316 for outside use, but SS 304 may be adequate in some less contaminated surroundings. However, we would not be satisfied with “a 75% chance that no problems will arise”. Corrosion is frequently the result of external contamination: the material is fully intact but steel particles from outside stick to it and begin to rust.
• Local corrosion - this is generally more dangerous than general corrosion. The cause is often one of the forms below. And once the corrosion has appeared, it can move quickly and the material rusts through quite fast.
• Galvanic corrosion This results from the combined use of different materials such as aluminium and stainless steel. Depending on the conditions, it is the “weaker” or “less noble metal” that is the culprit, in this case the aluminium. But the conditions (outside in inside, aggressive or not) do play an important role (see further explanation below).
• Pit corrosion or pitting: conditions such as damage, contamination, chlorine, etc. cause the passivation layer to fail and this cannot be repaired. Once it starts, the corrosion process goes into overdrive.
• Crevice corrosion: the name says it all - corrosion that occurs in crevices. The absence of oxygen in the crevice can make it difficult for the self-healing passivation layer to form. This needs to be taken into account in the design phase (flanged connections) and during welding. In high risk circumstances there should be no hesitation to use root or double welds with minimal craters in the surface.
• Intercrystalline corrosion: conditions such as heating result in the chromium content being locally less than 10.5%, the minimum level needed for a passivation layer to form.   Without a strong passivation layer, there is insufficient corrosion resistance.
• Stress corrosion
• Finally, there is erosion of the material. This is not really corrosion in the sense we understand the word - rust formation. However under some conditions such as tanks, time and usage cause the material to wear away.

The above is a small sample of the different kinds of corrosion but do not let this put you off. It exists, but given normal usage and a good design, we can expect very few problems in practice.