Fundamental studies on the corrosion behavior of weldments in marine microbial environments

Microbially influenced corrosion (MIC) deals with the effect that microorganisms can have on the normal corrosion rates of metals and alloys. This project involved gaining a better understanding of MIC in relation to weldments used in marine service. It was intended to establish MIC on a prototype weldment with a controlled set of conditions and microorganisms, use that set of conditions on a range of weldments in order to characterize their corrosion behavior, and develop some sort of ranking basis for weldments, according to their resistance to MIC. The project began with laboratory testing, in attempts to achieve MIC under controlled laboratory conditions. When the desired results were not achieved, test methods were altered to incorporate an exposure to a natural marine microbial environment. A range of weldments was selected to represent those used most in service, and included the following: three stainless alloy weldments, 304L/308L, 316L/316L, and AL-6XN/C-22; two carbon/low alloy steel weldments, HY-80/E10018 and HSLA-80/E10018; one nickel alloy weldment. Alloy 400/Alloy 400; one copper alloy weldment, 90-10 Cu-Ni/70-30 Cu-Ni; one aluminum alloy weldment, AL 5086/AL 5556; and one unalloyed titanium weldment. The natural exposure tests were conducted at an exposure site on the Roosevelt Inlet of the Delaware Bay, Lewes, DE. Control counterpart tests were conducted in the laboratory. Weldments underwent exposures in creviced and non-creviced conditions. Duration of all the tests was approximately one month, after which polarization resistance tests were performed on all weld coupons (in order to determine surface-average corrosion rates). Weld coupons were examined following the exposure in order to assess the corrosion behavior, with and without the presence of a microbial factor. What was generally observed was that corrosion occurred to greater extents in the Delaware Bay exposures than in the laboratory counterpart tests.