Reliability Engineering Snapshot TM

Corrosion - Case Study No. 98: Crevice Corrosion of Stainless Steel

Stainless steel is not immune to crevice corrosion attack. Given the right environment, it will look just as bad as carbon steel in no time at all.
As can be seen in the picture to the upper left, the welds are noticeably unaffected by the environment. Stainless steel filler material is usually higher in chromium and nickel content. The higher nickel and chromium content makes the weld more immune to the environment in general, and to the crevice corrosion attack in particular. The picture to the upper right shows a discolored region of the stainless steel where crevice corrosion has taken place. The white dotted line indicates where the product use to be, and there is still some product buildup visible.
Stainless steel has a harder time protecting against crevice corrosion. That's because stainless steel relies upon the formation of a protective and passive oxide layer. If product builds up on the surface before a protective film can develop, or if the product somehow interferes with the neutrality of the film itself, then corrosive attack will occur. This is because once the film is developed it does not remain in a passive state. Instead, the protective film is in a dynamic state where it is always repairing itself when the film is broken. It relies upon a source of oxygen to make the repair, either from the environment, or from the product itself. Starve the surface of oxygen, and the oxide layer will not form. Crevice corrosion will begin. The picture to the left shows a severe case of crevice corrosion attack. Once a region cannot form its protective oxide layer, it becomes more active, or anodic. The rest of the surface is more passive, so the two areas are electrochemically linked together like a battery. However, there is a disproportionate ratio here in that the surface area ratio between the very small anodic area to the rest of the vessel is enormous. Therefore, the crevice attack accelerates with respect to the rest of the environment in order to maintain an electromotive balance with the rest of the vessel. Throw some chloride salts in the solution for good measure and you have a high performance corrosion cell.

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