Effect of H2SO4 addition on the corrosion behaviour of AISI 304 austenitic stainless steel in methanol-HCl solution

Effect of sulphuric acid addition on the corrosion behaviour of AISI 304 austenitic stainless steel was investigated in different concentrations of HCl (0.001-1.0 M) in methanol using potentiostatic polarization, cyclic anodic polarization and open-circuit potential (OCP) measurements. In methanol-HCl solution, only active anodic dissolution occurred and the rate of corrosion increased with concentration of HCl. Scanning electron microscopic studies revealed grain boundary attack of the specimen. The addition of H2SO4 into methanol-HCl solutions with H2SO4 to HCl concentration ratio >= 10:1 stimulated the dissolution rate in the active region, but at higher potentials, facilitated passivation with occurrence of pitting on the steel surface. Increase in H2SO4 concentration enhanced passivity by broadening the range of passivity and making the pitting potential (E-pit) and the protection potentials (E-prot) nobler due to higher inherent water content of H2SO4. Simultaneously, the passivation current density (i(p)) also increased indicating the formation of a more defective passive film probably due to higher acidity of the solution.