THE CORROSION BEHAVIOR OF FERRITIC STAINLESS-STEEL IN OXALIC-ACID SOLUTIONS

The corrosion behaviour of ferritic Type 430 and Type 444 stainless steels in oxalic acid solutions was investigated by measuring the corrosion weight loss, DC polarization, and natural electrode potential (NEP) variations with time. Elements dissolved in oxalic acid solution after weight loss test were analysed by atomic absorption analysis and by ultraviolet absorption spectroscopy. The stability constants for the formation of Fe2+ oxalate and Cr3+ oxalate complexes were obtained, and the relation between the stability and corrosion behaviour was also investigated from thermodynamic parameters. The atomic absorption analysis clarified that the amount of dissolved Cr was the same as that of corroded Cr, and hence Cr seemed to be completely dissolved into the solution. The ultraviolet-visible absorption spectroscopy analysis revealed the formation of [Fe(C2O4)3]4- and [Cr(C2O4)3]3- in the solutions after corrosion weight loss test. The formation of these complexes were also estimated from the values of successive stability constants. Thermodynamic parameters for the formation of Fe2- oxalate and Cr3+ oxalate were obtained from van't Hoff plot. The Gibbs free energy values (-AG) for formation of Fe2+ oxalate and Cr3+ oxalate became larger with increasing solution temperature. © 1990.