On the Erosion–Corrosion Behavior of Active Screen Plasma Nitrided St52 Steel

Abstract

In this work, samples of St52 steel were plasma nitrided using an iron screen, in an N₂: H₂ gas mixture ratio of 4 : 1, at 500 and 550°C for 5, 10 and 15 h. The X-ray diffraction and optical microscopy methods were used for structural characterization of the coatings. Results indicated that the coatings were composed of Fe_2–3N and Fe₄N phases growing at longer deposition times. Moreover, the Fe_2–3N phase was decomposed to Fe₄N after 10 h of plasma nitriding. The erosion–corrosion behavior of nitrided coatings and a bare substrate were studied in various impact angles: 30, 45, 60 and 90 degrees. Polarization curves of the coated and uncoated samples were recorded between–900 to 600 mV, as a function of the slurry impact angle. Results showed that an active screen plasma nitriding method significantly enhanced the erosion–corrosion resistance of the St52 steel. Moreover, an impact angle of 30° on the sample surface yielded a lower weightloss whereas increasing the impact angle up to 90° caused more weight-loss due to the brittle characteristic of iron nitride coatings. According to SEM micrographs, by increasing the impact angle up to 90°, the depth of the removed mass increased substantially.