Corrosion behaviour of stainless and carbon steels exposed to sulphate - reducing bacteria from industrial heat exchangers

Corrosion responses of some carbon steels and stainless steel in the presence of a mixed industrial culture of bacteria (referred to as SRB - sulphate - reducing bacteria) found in industrial heat exchangers was studied to recommend best alloys under this service condition. Cooling towers in three plants were studied for SRB presence, and two of the plants showed positive indication of SRB presence based on characteristic hydrogen sulphide odour, media blackening and biofilm presence relative to a control media. Mixed cultures obtained from the plant were thereafter grown in prepared media incubated at 35 degrees C for 18 days. Open circuit potential and potentio-dynamic polarisation studies in anaerobic conditions were done on the selected alloys in aqueous media with and without the grown SRB. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were then used to determine the corrosion morphology and corrosion products formation. The voltamograms show higher i(corr) for alloys under the SRB compared to the control media, indicating the SRB indeed increased the corrosion rates. The corrosion rates of the alloys were mainly affected by the SRB metabolic activity which produced S2- ions in the media. The surface analysis showed pitting on ASTM A106-B, localised corrosion on ASTM A516-70 and intergranular corrosion on ASTM A179. The EDS analysis supported the findings with higher weight percent of iron and sulphur on ASTM A516-70 due to the presence of iron sulphide. Iron sulphide precipitated on the surface of the specimens to form a passive film. The Tafel plot recorded a corrosion rate of 1.23 mm/year for ASTM A179, 1.08 mm/year for ASTM A516-70, 0.53 mm/year for ASTM A106-B and 0.91 mm/year for duplex stainless steel (2205) when exposed to SRB media. ASTM A516-70 carbon steel was chosen as a better suited material in this environment.