Corrosion behaviour of S43035 ferritic stainless steel in hot sulphate/chloride solution

The corrosion resistance of S43035 ferritic stainless steel at elevated temperatures of 308 K, 328 K, 348 K and 378 K was studied through potentiodynamic polarization test and optical microscopy analysis in 2 M H2SO4 at 0%, 1%, 3.5% and 6% NaCl. Results show that increase in temperature and NaCl generally hinders the formation of the passive film. Changes in corrosion rate from 308 K to 378 K at 0% NaCl was marginal. Metastable pitting was absent until 378 K due to increased electrolytic action of SO42− ions. In the presence of chlorides at 308 K metastable pitting is visible delaying the formation of stable passive film. Passivation behaviour was absent at temperatures above 308 K due to polarization similar to carbon steels and a strong decrease in hydrogen evolution over potential especially at 1% and 3.5% NaCl. Optical images showed mild deterioration at 378 K from 0% NaCl and the formation of corrosion pits at 308 K and 378 K from 6% NaCl. Negative enthalpy value at 0% NaCl implies exothermic nature of the steel corrosion reaction. Addition of chlorides changed the steel corrosion reaction to endothermic. Increase in NaCl concentration caused a decrease in activation energy and increase in entropy values due to decrease in passivation of the alloy and consequently increase in corrosion rate. Statistical analysis through ANOVA at confidence levels of 95% and 97.5% showed the strong influence of temperature variation on corrosion rate with F-values of 5.256 at 50.8% in contrast to the NaCl concentration whose value was below the significance level. Keywords: Corrosion, Ferritic, Sulfuric, Steel, NaCl, Passivatio