Low-temperature and high-temperature corrosion behaviour of powder metallurgical duplex stainless steels

The porosity, that is inevitable in stainless steels manufactured by powder metallurgy (P/M), conditions their corrosion resistance and leads to notable differences between their behaviour and that of wrought stainless steels of similar composition. This work presents results about the corrosion behaviour at low and high-temperatures of three duplex stainless steels obtained from three different mixtures of prealloyed austenitic and ferritic powders (25%316L+75%434L, 50%316L+50%434L and 75%316L+25%434L). Specimens manufactured from 100% 316L and 100% 434L powders were also studied for comparison. Polarization curves in 3.5% NaCl solutions at room-temperature demonstrate that the corrosion rate of these materials increases as the austenite content in the base metal decreases. The high-temperature corrosion behaviour was studied in exposures to air at 600, 750 and 900degreesC. Results prove that the concentration of Ni in the base metal greatly decreases the oxidation resistance of sintered stainless steels, because this element changes the chemical nature and affects the protective properties of the formed oxides.