Oxygen and ph microprofiles above corroding mild steel covered with a biofilm

O2 and pH microprofiles were measured above corroding mild steel covered with a biofilm. The pH in the anodic areas (tubercles) ranged from 5 to 7 and was always 9.45 at the surface of the cathodic areas. After 1 month of biofilm development, O2 was depleted at the anodic area but could reach the cathodic surface where it was reduced. Consequently, differential O2 concentration cells were the driving force for corrosion. The O2 microprofiles indicated that O2 was consumed in the tubercles, probably by microbial activity, while O2 was reduced electrochemically in the cathodic areas. It was concluded that O2 transfer to the cathodic surface was the rate limiting step for the corrosion process.