Effects of temperature and potential on the passive corrosion properties of alloys C22 and C276

Potentiostatic polarization experiments were performed as a function of potential (200 to 700 mV(Ag/AgCl)) and temperature (25-85degreesC) on the Ni-Cr-Mo alloys C22 and C276. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF SIMS) were used to determine the chemical composition and thickness of the films formed. The currents recorded as a function of applied potential were due to dissolution, with only minor increases in film thicknesses observed. Measured currents for C22 were lower than for C276 and decayed over the entire period of measurement at each temperature. Those on C276 more closely approached steady state. The temperature dependence of the currents on C22 was significantly lower than that on C276. Surface analyses, performed on specimens anodically treated at one potential but after a sequence of temperatures up to 85degreesC, confirmed that the passive films on both alloys consisted of a Mo, Cr, and Ni oxide, with Cr present as Cr3+ and Mo present in several oxidation states. The passive films on C22 showed a distinct layered structure, consisting of an inner layer rich in Cr and Ni, and an outer layer enhanced in Mo. By contrast, the oxide films on C276 did not show such a clear separation into layers, and the relative Cr content was much lower. The increase in oxide thickness with increasing anodic potential and the low temperature dependence of the passive current observed for C22 are consistent with an oxide dissolution rate which is low compared to the rate of creation of oxygen vacancies leading to film growth. The absence of a dependence of film thickness on potential and the higher temperature dependence of the passive currents on C276 are consistent with control of the overall anodic process by ion-transfer at the oxide/solution interface. (C) 2003 The Electrochemical Society.