The relationship between alloying elements and biologically produced ennoblement in natural waters

A range of stainless steels, nickel-chromium and nickel-chromium-molybdenum alloys were exposed to coastal seawater from Mandapam (Indian Ocean) and freshwater from a perennial pond. Biofilms from both test waters produced an ennoblement of the open circuit potential (OCP) on all alloys as expected, which was slower but substantially larger in freshwater. In both waters an interesting relationship was perceived between the plateau OCP (E-max) and the mass percentage of the major alloying elements. In particular, iron exhibited strong positive correlations with E-max (r(2) >= 0.77; p < 0.0005), while the sum of chromium, nickel and molybdenum presented significant negative correlations (r(2) <= -0.81; p = 0.0002). Consistent with the regression analyses, Euclidean distance clustering yielded patterns where Inconel-600 and the nickel-chromium-molybdenum alloys had the smallest similarities of OCP with other alloys. The results emphatically reinforce a key role for surface passive films in the ennoblement phenomenon in natural waters.