An electrochemical impedance study of Alloy 22 in NaCl brine at elevated temperature. I. Corrosion behavior

Alloy 22 in acidified, NaCl brines over wide ranges in temperature and composition is characterized by high interfacial impedance, thereby illustrating its high corrosion resistance. The impedance data are shown to transform correctly in the complex plane, via the Kramers-Kronig (K-K) relations, thereby demonstrating that the system conforms to the constraints of Linear Systems Theory (LST). The interfacial impedance (and the polarization resistance, R-p) initially increased with applied potential, within the passive range, due to the growth of the passive oxide layer. At a sufficiently high potential (V > 0.4 V-SHE), within the transpassive dissolution region, the interfacial impedance (polarization resistance) is observed to decrease with increasing voltage, due to layer thinning and/or oxidative ejection of cations from the barrier layer. The interfacial impedance data for Alloy 22 as a function of voltage can be accurately modeled using an electrical analog comprising two R//C sub-circuits in series, which in turn are in series with a R//constant phase element sub-circuit. Because this analog is in a minimum phase, passive circuit, it must satisfy the K-K transforms a priori, which confirms the compliance of the system with the constraints of LST. The rate of corrosion of the alloy was found not to depend significantly on pH (1 < pH < 8.1) or chloride concentration (0.001 < [Cl-] < 6.2 m, m=mol kg(-1) (H2O)), and was generally found to compare well with corrosion rates reported by other workers. (C) 2004 Published by Elsevier B.V.