THE EFFECT OF DISSOLVED OZONE ON THE CORROSION BEHAVIOR OF CU-30NI AND TYPE-304L STAINLESS-STEEL IN 0.5 N NACL SOLUTIONS

Electrochemical experiments on the effect of dissolved ozone on the corrosion behavior of Cu-30Ni and type 304L (UNS S30403) stainless steel have been performed in 0.5 N NaCl solutions at room temperature. The experiments performed included measurements of the corrosion potential as a function of time and ozone concentrations, cyclic polarization experiments, iso-potential measurements of current densities and Auger electron spectroscopy studies of the chemical composition of the corrosion product films. The results of these experiments have shown that for both the Cu-based alloy and the stainless steel, the corrosion potential exhibits a marked shift to more noble values (approximately 300 mv) for ozone concentrations less the 0.2 to 0.3 mg/L. At higher ozone concentrations, the corrosion potential is virtually independent of the level of ozone dissolved in the solution. In addition to the shift in the corrosion potential, the presence of dissolved ozone resulted in a reduction in the corrosion rate for the Cu-30Ni alloy, as measured by a significant decrease in the current density at a constant applied potential. This improvement in the corrosion resistance appears to be related to a reduction in the corrosion product film thickness and a higher fraction of oxygen to chloride in the corrosion product film. For the stainless steel, on the other hand, Auger electron spectroscopy indicated no appreciable differences between the passive film produced in ozonated solutions versus those in unozonated solutions. However, the noble shift in the corrosion potential was accompanied by a parallel shift in the breakdown potential, suggesting that films produced in ozonated solutions are more resistant to the initiation of localized corrosion. This behavior is believed to be due to a larger driving force for passive film repair, in contrast to repassivation of growing pits, since the repassivation potential of the stainless steel is not perceptably changed by dissolved ozone.