Electrochemical investigation of the "double-edged" effect of low-dose biocide and exogenous electron shuttle on microbial corrosion behavior of carbon steel and copper in enriched seawater

•Thirty ppm THPS inhibits planktonic cell proliferation, but not biofilm formation.•Riboflavin promotes microbial corrosion of carbon steel but not copper in low dose THPS medium.•Riboflavin enhances the energy uptake of SRB from carbon steel to weaken THPS.•Low level of THPS hardly inhibits MIC of carbon steel and copper under harsh biofilm conditions.•Biocide delivery requires consideration of the type of microbial corrosion of the protected metals.