Investigating the Crevice Corrosion Behavior of Coated Stainless Steel in Seawater

摘要

Normally, stainless steel is utilized without any type of coating whatsoever. However, there are occasions where coatings may be contemplated. One of present interest to the U.S. Navy is that associated with the use of antifouling coatings on ship hulls fabricated of non-magnetic. austenitic stainless steel. Testing in natural seawater has demonstrated that coatings can protect susceptible stainless steel from barnacle related crevice corrosion and localized corrosion at weldments. However, coating defects and damage create new areas for crevice corrosion to initiate. The risk of serious crevice corrosion damage to the substrate increases with the amount of bare metal exposed. Test results demonstrated that just a few square inches of bare stainless steel is sufficient to support the corrosion reactions producing penetrations in excess of 1.5 mm in less than six months. Even 6% Mo alloys are susceptible. Localized corrosion can be prevented by the use of cathodic protection, as demonstrated by testing with sacrificial zinc anodes. However, the apparent degree of polarization associated with zinc anodes may contribute to blistering and disbonding of the coating. Test results indicate that good surface preparation, expectedly, enhances bonding of the coating to the substrate. This degree of preparation also creates more optimum conditions for crevice corrosion propagation at sites which initiate.