The effect of heat treatment on the corrosion resistance of 440C stainless steel in 20% HNO₃ + 2.5% Na₂Cr₂O₇ solution

The effect of heat treatment on the corrosion resistance of 440C stainless steel was investigated in a 20% HNO₃ + 2.5% Na₂Cr₂O₇ solution using electrochemical noise (ECN) measurements, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) examinations. The noise resistance (Rn), which has been found to be inversely related to the localized corrosion rate, was measured to be 5.7E + 08 Ω-cm², 4.2E + 08 Ω-cm², and 3.7E + 04 Ω-cm² for the oil-quenched, air-quenched, and vacuum furnace cooled (VFC) samples, respectively, after 1200 s exposures. The Rn for all heat treat conditions stabilized within a range of 1.0E + 07 Ω-cm² to 3.2E + 08 Ω-cm² after 2 h exposures. The EIS response showed a polarization resistance (R_p) on the order of 6.6E + 04 Ω-cm², 5.3E + 04 Ω-cm², and 1.1E + 04 Ω-cm² for the oil-quenched, air-quenched, and VFC samples, respectively, after 2 h exposures. The EIS data are in good agreement with ECN data and indicate that after longer exposures, general corrosion mechanisms dominate and the corrosion rates are comparable. SEM examinations of specimens subjected to 1200 s exposures revealed that severity of pitting and intergranular corrosion damage was consistent with trends in the Rn data. Specifically, the electrochemical noise data as well as SEM examinations of specimens revealed a higher localized corrosion resistance of the hardened specimens during the early stages of passivation. This greater resistance to localized corrosion can be attributed to an increased stability of the natural passive film resulting from a higher concentration of chromium atoms in solution for the martensite phase.