Free corrosion potential is one of the criteria to be used to assess the corrosion risks for metals. There is a correlation between increase of corrosion potentials of metal and the formation of a biological film on the material surface. This correlation is based on the assumption that microorganisms play a significant role in the increase of corrosion potentials of metal. The material used in test was 9030 type of steel, being used oil pipes steel and was provided from ERDEMIR. To determine the relation between microorganism enzymatic activity and free corrosion potential, the tests were carried under artificial sea water medium and hydrogen peroxide solution, glucose, gluconic acid and glucose oxidase and catalase enzymes were used as reagents. Results of these experiments indicated that increase in free corrosion potential is due to the combined effect of hydrogen peroxide and gluconic acid produced as a result of enzymatic activity of microorganism.