Studies on Pitting Corrosion of Stainless Steel (SS-304) by a Marine Strain of Sulfate Reducing Bacteria (Desulfovibrio vulgaris)

摘要

The effect of a marine isolate of sulfate reducing bacteria (SRB),strain Desulfovibrio vulgaris on pitting of SS-304 was investigated. The metal coupons were exposed to a semi-continuous culture of SRB for 600 h along with a control set. Open circuit potential (OCP) and sulfide production were monitored simultaneously. Pitting was observed both by optical and scanning electron microscopy (SEM). The growth curve of the SRB strain was standardised and monitored up to 96 h. The log phase of the growth was noticed from 12 h to 50 h and the decline phase started after 80 h of growth. During the growth phase,sulfide production ranged from 0.02 to 0.055 mmol L~(-1). Throughout the experimental period, OCP values ranged between -130 mV and -430 mV(SCE). The OCP showed two sharp peaks of -430 mV(SCE) each after 175 h and 425 h respectively. SEM pictures showed SRB cells embedded in the corrosion deposits and pits. Pitting ratio (pits cm~(-2)) after 300 h and 600 h of exposure to SRB culture was 5 and 13.33 cm~(-2) respectively. The study points out that the marine isolate of SRB could be the causative agent for the low redox potentials and also for the acceleration of pitting in SS-304. Sulfide production by SRB could have influenced the shift in potential to a more active value by decreasing the electroreduction charge on the metal surface. The decay of passive film due to sulfide production by SRB favours the initiation of localised corrosion process such as pitting.