Experimental studies in the effect of fluid flow upon repassivation of corroding iron and cobalt

Abstract

Too many unknowns and insufficient data exist for development of a comprehensive theory for repassivation under controlled flow conditions. From results of the first hypothesis which states that supersaturation of the solution at the anode surface may occur, a constant apparent supersaturation ratio for ferrous sulfate of about 3.5 for 1 M H2SO4 was calculated at the surface for passivation with tau < 0.5. For tau > 0.5, the calculated apparent supersaturation ratio approached the saturation concentration for ferrous sulfate at the electrode surface in acidic solutions. At low current densities (i less than or equal to is) where the saturation concentration for ferrous ions at the electrode surface was not obtained, passivation was not observed in acidic solutions. These results suggest that, in the systems studied, repassivation occurs only after achieving saturation of ferrous sulfate at the surface. In the second hypothesis, the ferrous sulfate film was assumed to precipitate at saturation of the solution at the anode surface, and to grow in thickness thereafter until passivation occurred. Experimental results follow the trend predicted by the second hypothesis for short passivation times, but large deviations occurred for tp> 0.1 seconds.