Abstract
Weldments representative of a range of marine structural materials were exposed to a natural marine environment which was known from previous studies to induce microbially influenced corrosion (MIC). The natural environment was at a University of Delaware site on the Delaware Bay, Lewes, Delaware. Companion laboratory control tests were conducted at the University of Tennessee in 0.2 μm filtered Delaware Bay water and in synthetic seawater. The natural and control tests were conducted with weldments in both creviced and non-creviced conditions. Open-circuit potentials (OCPs) and corrosion rates (polarization-resistance measurements and microscopic examinations) were evaluated for all tests. The weldments studied were: 304L, 316L and AL-6XN stainless steels; HY-80 and HSLA-80 low-alloy steels; Alloy 400 Ni-Cu alloy; 90-10 Cu-Ni alloy; 5086 aluminum alloy; and unalloyed titanium. In the non-creviced condition, ennoblement of the OCP, to varying degrees, relative to the laboratory control tests, occurred for all weldments. Clearly, a microbial effect at the Delaware Bay site was responsible for this ennoblement (higher OCP values). For the creviced condition, in most cases, ennoblement did not occur. Rather, the OCPs in the natural microbial environment were less than those in the laboratory control environments -- a result that could be rationalized in terms of higher crevice-corrosion initiation rates in the natural microbial environment. On comparison of corrosion rates in the natural Delaware Bay water with those in the laboratory control tests, it was determined that the microbial influence was one of significant corrosion acceleration for the 304L, 316L, Alloy 400, and 90-10 Cu-Ni weldments, with Alloy 400 experiencing the greatest degree of acceleration. Corrosion acceleration also occurred for the low-alloy steel weldments, HY-80 and HSLA-80, but to a smaller degree. Conversely, the microbial influence resulted in corrosion inhibition for the 5086 aluminum alloy and titanium weldments. For the AL-6XN weldment, the microbial influence produced corrosion inhibition in the non-creviced condition, but corrosion acceleration in the creviced condition.
