Seawater Crevice Corrosion Resistance of Stainless Steels Coated with Silane and Antifouling Paint Systems

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. As discussed in an earlier project report documenting contract N00014-97-C-0216, specimens of several grades of stainless steel were extremely susceptible to crevice corrosion when partially coated with epoxy alone, or when top coated with antifouling paints of the ablative-Cu and elastomeric types. The use of zinc-rich primers is known to enhance resistance to crevice corrosion, but their use raises other issues. Of chief concern is the possibility of liquid metal embrittlement if the stainless steel were ever heated above the melting temperature for zinc. This current project addresses the alternate use of organic silane type coatings. Ideally, silane would eliminate the need for a zinc-rich primer and the epoxy barrier coating. Current test results show promise when silane is applied as an ultra-thin film on 6 Mo stainless steel. Additional testing is proposed to resolve potential problems when silane + ablative-Cu coated stainless steel is subjected to cathodic protection. Because of the apparent lack of insulation provided by the thin-film silane, cathodic disbondment of the topcoat has been observed. --Original contains color plates: All DTIC reproductions will be in black and white.