RESUMO
The beneficial action of the alloying elements Cr, Mo, W, N, B and Cu on the corrosion resistance of stainless steel has been attributed to the formation of protective passive surface film containing these elements. A study on the formation of such passive film with the help of oxyanions of the alloying elements has been undertaken. Chromate, molybdate, tungstate, nitrite and nitrate anions improve the corrosion resistance of 304 stainless steel in H3PO4/Cl- solution. NO-2/Cr2O72- and MoO42- are more effective in retarding the active corrosion, while the nitrogen species [NO2-, NO3-] are the most effective for pitting attack. The effect of binary and tertiary mixtures of these oxyanions has been examined. Some mixtures afford considerable protection for both active and pitting corrosion. In their presence, the corrosion resistance of the simple 304 steel may be compared with that of more expensive highly alloyed grades