Study on intermediate annealing-twice nitriding process and corrosion resistance of pure iron

To address the challenge of increasing the thickness of the nitriding layer, an intermediate annealing-twice nitriding process was applied to pure iron, and the corrosion resistance of the nitriding samples was investigated. After the intermediate annealing-twice nitriding treatment, the thickness of the compound layer in the sample increased by 84 % compared to once nitriding, reaching 48.14 μm, while the transition layer thickness increased by 57 % to 52.49 μm. The intermediate annealing process at 600 °C for 1 h yielded a superior twice nitriding effect. And the intermediate annealing-twice nitriding process resulted in a significantly thicker nitriding layer compared to simple twice nitriding. The phase transformation and thickening mechanism of the nitriding layer during the process were revealed by EBSD, XRD, and TEM. Electrochemical tests showed that the nitriding samples exhibited repeated passivation behavior, and the sample with a thicker compound layer demonstrated better corrosion resistance.