Determination of Iron Valence States Around Pits and the Influence of Fe(3+) on the Pitting Corrosion of 304 Stainless Steel

Potassium ferricyanide and potassium ferrocyanide were used to observe and monitor the pitting corrosion of 304 stainless steel (SS) at anodic polarization in situ. The results show that there are Fe(3+) ions around the corrosion pit when pitting occurs on 304 SS in NaCl aqueous solution. The effect of Fe(3+) surrounded pits on the pitting corrosion was also studied by testing the electrochemical behavior of 304 SS in different Fe(3+)/Fe(2+) solutions. The presence of Fe(3+) leads to the positive shift of corrosion potential and the increase of corrosion rate of 304 SS. There are two possible reasons for this phenomenon. On the one hand, Fe(3+) hydrolysis results in the decrease of pH value of solution. At the same iron ion concentration, the higher the Fe(3+) ion concentration, the lower the solution pH value. On the other hand, Fe(3+) may reduce on the electrode surface. The decrease of solution pH and the reduction of Fe(3+) resulted in the acceleration of the corrosion rate.