Effect of SO42− and Cl− anionic attack on the localized corrosion resistance and morphology of 409 ferritic stainless steel

Abstract

Abstract The corrosion polarization behaviour and optical characterization of 409 ferritic stainless steel (SS409) in 0.25 M H2SO4 (at specific NaCl concentration), 0.375 M H2SO4 and 0.5 M H2SO4 was evaluated with potentiodynamic polarization technique, open circuit potential measurement and optical microscopy characterization. Results showed the SS409 corrosion rate is proportional to the concentration of SO42− and Cl− anions in the electrolyte. At 0.25 M H2SO4, SS409 corroded at a rate of 7.29 mm/y; addition of 0.125% NaCl accelerated the corrosion rate to 16.75 mm/y coupled with a corresponding increase in corrosion current density from 6.71 × 10−4 Acm2 to 3.08 × 10−3 Acm2. Further increase in Cl− anion concentration to 0.75% NaCl increased the corrosion rate to 19.49 mm/y. At 0.25 M H2SO4 the pitting current value is 1 × 10−4 A, this value increased to 1.03 × 10−4 A at 0.25 M H2SO4 (0.125% NaCl) and 8.40 × 10−4 A at 0.25 M H2SO4 (0.5% NaCl). Beyond 0.5% NaCl, passivation behavior was completely absent. Intergranular cracks, large macro-pits and numerous micro-pits were observed for SS409 from 0.25 M H2SO4 (0.75% NaCl) due to Cr depletion and dissolution of inclusions. SS409 from 0.5 M H2SO4 showed the presence of deepened intergranular cracks, shallow pits and a deteriorated surface due to the action of SO42− anions. The morphology of SS409 in 0.25 M H2SO4 + 0.125% NaCl showed slight deterioration in addition to enlarged micro-pits.