Electrochemical Behavior of Alloy 825 at High-Temperature Pressurized Water–Chloride Solutions (30°C to 280°C)

Variations in the transpassive potential (E_t) and open circuit potential (OCP) of Alloy 825 were investigated at high-temperature pressurized water–Cl solutions. Using an experimental design strategy, the effects of temperature (T) (30°C to 280°C), Cl⁻ concentration (10 ppb to 1,000 ppb), and dissolved oxygen (DO) concentration (0 ppb to 200 ppb) on the electrochemical responses (E_t and OCP) of the alloy were analyzed by conducting only a short series of experiments. The optimum values of each factor was obtained from the designed matrix of the experiments and the constructed 3D response surface plots based on the most positive values of the E_t and OCP. The optimum experimental conditions in which the most positive E_t can be recorded was found as follows: T ≈ 30°C (significant), [Cl⁻] ≈ 10 ppb (significant), and [DO] ≈ 200 ppb (less significant). The most positive OCP values were recorded when the analyzed factors were as follows: T ≈ 120°C (significant), [DO] ≈ 200 ppb, and [Cl⁻] ≈ 10 ppb (less significant). Finally, the microscopic surface analysis of the alloy coupons revealed the great impact of temperature on the oxide film formation on Alloy 825.