Understanding corrosion modes of P/M ferritic stainless steels encountered in automotive applications

The corrosion properties of powder metallurgy (P/M) 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 scan rate of 0.5mV/s. In general, P/M alloys were seen to exhibit lower zero current potentials, or E(i=0), higher current densities, and a lesser degree of passivity than their wrought counterparts. The difference in zero current potential and current density values was attributed to a crevice corrosion mechanism within the pores of the P/M 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 P/M alloys. Intergranular corrosion tests carried out for 24 hours in boiling 6wt% CuSO₄ + 16wt% H₂SO₄ (per ASTM A763-93Z) were found to be too aggressive for all P/M specimens. A new intergranular test of 6wt% CuSO₄ + 0.5wt% H₂SO₄ was found to be useful in ascertaining a P/M specimen's degree of secondary compound formation, or "sinter sensitization". Finally, a heat treatment of 816°C (1500°F) for two hours was found to reduce the intergranular corrosion rate of P/M 434L specimens sintered at 1290°C (2350°F) by a factor of 77.