Corrosion modes in P/M ferritic stainless steels

The corrosion properties of powder metallurgy ferritic stainless steel alloys were investigated and compared to those of wrought 409Cb stainless steel. Anodic potentiodynamic polarization was performed on all specimens in 0.1M NaCl with a sean rate of 0.5mV/s. In general, the powder metallurgy alloys were seen to exhibit lower open circuit potentials, higher current densities, and a lower degree of possivity than their wrought counterparts. The differences in open circuit potential and current density values were attributed to a crevice corrosion mechanism within the pores of the powder metallurgy part and the formation of secondary compounds, such as chromium carbides, during sintering. Crevice corrosion within pores was also thought to degrade the passivity of the powder metallurgy alloys. Intergranular corrosion tests carried out for 24h in boiling 6w/o CuSO₄⁺ 16w/o h₂SO₄ were found to be too aggressive fo all the powder metallurgy specimens. A new intergranular test in 6w/o CuSO₄ + 0.5w/o H₂SO₄ was found to be useful in ascertaining the degree of secondary compound formation, or "sinter sensitization" in the powder metallurgy specimens. Finally, a heat treatment at 816°C(1500°F) for 2h was found to reduce the intergranular corrosion rate of the powder metallurgy 434L specimens sintered at 1290°C (2350°F) by a factor of 77.