Corrosion behavior of WC–Co hardmetals in the oil-in-water emulsions containing sulfate reducing Citrobacter sp.

• A special SRB strain of Citrobacter sp. was isolated from O/W emulsion.
• The SRB degraded the emulsion and made it more aggressive to cobalt phase.
• The biofilms seriously affected the corrosion behavior of WC–30Co in various media.
• Either accelerated or inhibited corrosion occurred owing to microbial activities.

Many microbial issues have been widely proposed in the omnipresent metalworking fluids (MWFs). In this work, a particular sulfate reducing Citrobacter sp. strain was isolated from engaged oil-in-water emulsion, named as CK2. The occurrence of microbially influenced corrosion of WC–30Co hardmetals caused by CK2 in O/W emulsion and nutrient media was proved by electrochemical and micro-morphological analyses. Preferential cobalt loss determined the corrosion failure in biodegraded emulsions. CK2 made the emulsion more corrosive to hardmetals. Microbially influenced corrosion inhibition was observed in CK2-containing emulsion. In nutrient media, the corrosion rates of WC–30Co multiplied about 10 times due to microbial activities.