Accelerated degradation of stainless-steel passive film by humic acid-mediated extracellular electron transfer

The influence of humic acid-mediated extracellular electron transfer (EET) of Shewanella putrefaciens CN32 in microbiologically influenced corrosion (MIC) on 304 stainless steel was investigated. Humic acid plays a key role in accelerating passive film degradation in the presence of S. putrefaciens CN32, with a concentration-dependent effect; but has no effect when alone. It facilitated the formation of larger biofilm and increased Fe and Cr dissolution, resulting in a thinner, weakened passive film and intensified pitting corrosion. In-situ scanning electrochemical microscopy demonstrated S. putrefaciens CN32 utilized humic acid to perform outward EET, injecting electrons into the passive film and accelerating MIC.