Corrosion Behavior of Mild Steel and Ferritic Stainless Steels in Oxalic Acid Solution

The corrosion behavior of mild steel (SS 41 [UNS⁽¹⁾ K02600]) and ferritic stainless steels (type 430 [UNS S43000], type 444 [UNS S44400]) in oxalic acid solutions was investigated by corrosion weight loss, DC polarization, and natural electrode potential (NEP) variations with time. The surfaces of the specimens were observed by scanning electron microscopy (SEM) and analyzed by x-ray photospectroscopy (XPS). In corrosion weight-loss tests at room temperature, the corrosion rate of SS 41 steel and type 430 stainless steel increased with increasing concentration of oxalic acid, but in the case of type 444 stainless steel, the weight loss was negligible. At the boiling point the corrosion rate of each steel increased with increasing oxalic acid concentrations. The corrosion rates of SS 41 steel and type 444 stainless steel were almost the same, and the value of type 430 stainless steel was greater than that of other steels. When the surfaces of type 430 and type 444 stainless steels were observed by SEM, they were considerably uneven. From the analysis by XPS, since the peaks corresponding to metallic Cr and Cr-oxides did not appear after etching, the corrosion of type 430 and type 444 stainless steels could be attributed to the dissolution of Cr. In the electrochemical measurements at room temperature, it was considered that in low concentration of oxalic acid, type 430 stainless steel was prevented by Cr, and type 444 stainless steel was prevented by Cr and Mo.