The effect of ozone on the corrosion behavior of Ni-Cr-Mo alloys in artificial seawater

The effects of ozonated seawater on the corrosion behavior of nickel-chromium-molybdenum alloys C-276, C-22, 625 and 59 (UNS N10276, N06022, N06625, and N06059) were studied and the results were compared with those obtained for aerated solutions. Corrosion rates and crevice corrosion information obtained from plate samples at intervals of 2, 4, 8, 16, 26, and 47 weeks were compared with electrochemical measurements of corrosion potential, linear polarization resistance (LPR), and cyclic polarization curves from concurrently immersed wire samples. It was observed that samples in ozonated seawater exhibited higher corrosion rates (5-34 mm/yr), as measured by both LPR and weight loss measurements, than those exposed to aerated seawater (<6 mm/yr). These relatively low corrosion rates were accompanied by the precipitation of voluminous amounts of black flocculent corrosion product in the ozonated solution, identified as hydrated nickel chlorates formed from dissolved nickel ions in solution. The only adherent corrosion product observed on samples in ozonated solutions is characterized by the presence of a thin oxide film exhibiting interference coloration, which varied from alloy to alloy, and with time of exposure. Creviced samples of alloy C-276 in ozonated seawater showed preferential corrosion at the interface between creviced and uncreviced surfaces, with the creviced surface being completely protected, in contrast to slight pitting damage observed in the crevice region of samples exposed Co aerated seawater. Cyclic polarization curves of alloys C-276 and C-22 also suggest increased crevice corrosion susceptibility in ozonated seawater, as indicated by the presence of a hysteresis loop and an increase in the passive current density compared to those observed in aerated seawater. Alloy 690 (UNS N06690) showed severe crevice corrosion in ozonated artificial seawater, indicating the importance of molybdenum as an alloying addition under ozonated conditions.