Electrochemical Studies on the Film Formed by Direct Nitridation of AA2024 in a KNO3 Salt Bath at Low Temperature

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A new thermo-chemical, low-temperature nitridation method was successfully used to introduce interstitial nitrogen into the passive layer aluminum alloy 2024 to enhance corrosion resistance. The nitridation process is based on treating the aluminum samples in potassium nitrate salt in an ultra pure nitrogen atmosphere at 475 °C. Electrochemical linear polarization and cyclic voltammetry measurements, combined with characterization of surface morphology, were used to evaluate the coating performance after two weeks of immersion in 3.5% NaCl solution. Electrochemical testing and sample examination after immersion in NaCl solution indicated that the treatment improved the resistance of AA2024 to localized corrosion, but there did not appear to be a statistically significant change in general corrosion. The effect of the quenching after nitridation was also studied. It is shown through polarization resistance, Rp, measurements that quenching increases the Rp measurably, suggesting quenching provides improved coating properties.