Electrochemical Study on Passive Behavior of Hastelloy C‐276 in Ammonium Chloride Solution With Sodium Thiosulphate
Shakir Hussain Koondhar, Muhammad Ali Siddiqui, H. M. Ahsen Ilyas, Di Taishen, Liu Huaizhu, Chen Liangchao, Jinping XiongABSTRACT
The corrosion behavior of C‐276 alloy in 5 M ammonium chloride solution containing different concentrations of sodium thiosulfate (0–0.4 M) was investigated using Potentiodynamic polarization, cyclic voltammetry, electrochemical impedance spectroscopy and Mott‐Schottky analysis. Sodium thiosulphate increasing shows the increasing corrosion current density i corr and passive current density, pitting potential also shift to lower values shows degradation of passive film. Cv analysis revealed anodic peak shifts to lower potentials with increasing thiosulfate concentration, suggested easier passive film transformation. EIS results showed a decrease in charge transfer resistance and an increase in capacitance, showed reduced film stability and increased surface heterogeneity. Mott‐Schottky analysis demonstrated both n‐type and p‐type semiconductor behavior, with increasing donor and acceptor densities confirmed higher defect concentration in the passive film.
The passive film is interpreted as a multi‐component oxide/hydroxide layer consisting of a Cr rich inner barrier layer and an outer hydroxide layer containing Mo and Ni. The corrosion mechanism is explained using the Point Defect Model (PDM), where defect generation, migration, and accumulation lead to passive film breakdown and localized corrosion. The synergistic effect of chloride and thiosulfate ions accelerates passive film degradation and enhances corrosion susceptibility of the alloy.