Electrooxidation of stainless steel AISI 304 in carbonate aqueous solution at pH8

The electrochemical behaviour of stainless steel AISI 304 (SS304) has been investigated in deaerated 0.1-1M NaHCO3 solutions at pH 8 using a rotating disc electrode. The polarization curves are characterized by a broad range of passivity at low potentials (-0.8 to 0.3 V), a depassivation region at 0.4 V vs SCE and, at high potentials (0.5 to 0.85 V), a passive region before oxygen evolution. In the low potential range, the SS304 electrode behaves like a Cr-rich metallic phase, and the dissolution of Fe2+ ions into the solution is hindered by the formation of a Cr2O3 layer. As the potential reaches 0.4 V, the oxidation-dissolution of Cr(III) oxide/hydroxide to CrO42- ions occurs, with the participation of bicarbonate/carbonate as a catalyst in the dissolution reaction. Since the chromium oxide/hydroxide dissolution and subsequent surface enrichment ofiron oxides occur, the applied potential, exposure time and oxidation charge have a considerable effect on the passive him properties. At high potentials, the presence of a passive him of iron oxides/hydroxides or oxyhydroxides plays a key role in the SS304 passivity with the presence of Fe(VI) species incorporated or adsorbed into the passive films. Colouration of the SS304 surface is observed in the second passive region. A him of a uniform gold colour formed on SS304, mild steel 1024 and iron in carbonate and.borate solutions at pH 8. The colour of the electrode surfaces remain unchanged in air and in solutions at positive potential but it disappears at open-circuit potential or is easily reduced in the first negative-going potential scan.