Localised corrosion induced by a marine vibrio

Abstract

The corrision of mild steel in media with and without bacterial cultures was assessed using potentiostatic and potentiodynamic techniques and the production of biofilm on the metal surface was studied by scanning electron microscopy. Metal in a solution consisting of the inorganic components of Postgate's medium C was not passivated, but a passive surface was indiced by the addition of lactate, citrate, or phosphate. The breakdown potential ( E b of the passivated metal was most anodic for phosphate. No significant change in the electrochemical behaviour of the steel was seen when the formulation of Postgate's medium C was completed by the addition of yeast extract, but chloride, added to allow the growth of Vibrio alginolyticus, caused a reduction in the E b value. Vibrio alginolyticus reduced the E b value in Postgate's medium C from −0·37 to −0·43V, indicating its corrosive capacity. This value was reduced still further, to −0·60V, when sulphate-reducing bacteria were also present. Scanning electron microscopy showed the presence of colonies of V. alginolyticus on the metal surface. When cleaned, it was apparent that intense pitting had occurred beneath these colonies. It is suggested that V. alginolyticus may promote chemical or SRB-induced corrosion by removing a passive film from the metal, allowing aggressive species such as sulphides to affect the surface.