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Corrosion Fundamentals

Crevice Corrosion

Crevice or contact corrosion is the corrosion produced at the region of contact of metals with metals or metals with nonmetals. It may occur at washers, under barnacles, at sand grains, under applied protective films, and at pockets formed by threaded joints. Whether or not stainless steels are free of pit nuclei, they are always susceptible to this kind of corrosion because a nucleus is not necessary.

Cleanliness, the proper use of sealants, and protective coatings are effective means of controlling this problem. Molybdenum-containing grades of stainless steel (e.g. 316 and 316L) have increased crevice corrosion resistance.

Crevice Corrosion of a Titanium Flange

The crevice corrosion shown above happened when an aerospace alloy (titanium - 6 aluminum - 4 vanadium) was used instead of a more corrosion-resistant grade of titanium. Special alloying additions are added to titanium to make alloys which are crevice corrosion resistant even at elevated temperatures.

Screws and fasteners have are common sources of crevice corrosion problems. The stainless steel screws shown below corroded in the moist atmosphere of a pleasure boat hull.

Screws Showing Crevice Corrosion Problems
(Courtesy of marinesurvey.com)

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