Effects of Temperature and Potential on the Passive Corrosion Properties of Alloys C22 and C276

Abstract

Potentiostatic polarization experiments were performed as a function of potential ͑200 to 700 mV Ag/AgCl ͒ and temperature ͑25-85°C͒ on the Ni-Cr-Mo alloys C22 and C276. X-ray photoelectron spectroscopy ͑XPS͒ and time-of-flight secondary ion mass spectrometry ͑TOF SIMS͒ 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 85°C, confirmed that the passive films on both alloys consisted of a Mo, Cr, and Ni oxide, with Cr present as Cr 3 ϩ 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.

The Kinetics of Nucleation of Metastable Pits on Metal Surfaces: The Point Defect Model and Its Optimization on Data Obtained on Stainless Steel, Carbon Steel, Iron, Aluminum and Alloy-22

“…It has been demonstrated previously [41][42][43] that the PDM can readily explain the observations of the passive state on Alloy-22. Characterization work 41,[44][45][46] has demonstrated that the passive film on Alloy-22 has a distinct layered structure with the inner layer primarily comprising Cr(III) oxide, so it is assumed in the optimization that point defective Cr 2 O 3 is the principal composition of the barrier layer. Fig.…”

Influence of Chromium and Molybdenum on the Corrosion of Nickel-Based Alloys

“…In a study comparing Alloy 625 to Alloy 276, Lillard [6] found that Alloy 276 had a lower passive current density, and was less susceptible to pitting. In a study on Alloy 276 and Alloy 22, Lloyd et al [24] found that Alloy 22 had a lower passive current density, and was less susceptible to pitting as well. This matches the theory regarding the passive current density; although, when comparing the components of the alloys, Alloy 276 has more Mo than both Alloy 22 and 625, and Alloy 625 has as much Cr as Alloy 22 and 6% more than Alloy 276.…”

Effect of Oxide Film Properties on the Kinetics of O2Reduction on Alloy C-22

“…A wide array of in-situ and ex-situ analytical techniques [1][2][3][4][5][6][7][8] has been employed to assess passive behavior and its influence on the corrosion performance of these alloys. The combination of high Cr and Mo contents leads to the formation of a Cr-dominated passive oxide film 2,3,[9][10][11] with a duplex film structure, 12 which provides resistance against oxidizing environments. 13 The nature and composition of these oxide films depend on alloy composition, potential, pH, temperature, and the composition of the environment in which they are formed.…”