OXYGEN AND PH MICROPROFILES ABOVE CORRODING MILD-STEEL COVERED WITH A BIOFILM

O-2 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, O-2 was depleted at the anodic area but could reach the cathodic surface where it was reduced. Consequently, differential O-2 concentration cells were the driving force for corrosion. The O-2 microprofiles indicated that O-2 was consumed in the tubercles, probably by microbial activity, while O-2 was reduced electrochemically in the cathodic areas. It was concluded that O-2 transfer to the cathodic surface was the rate limiting step for the corrosion process.