Increasing wear and corrosion resistance of tool steel by anodic plasma electrolytic nitriding

This paper considers the effect of the anodic plasma electrolytic nitriding (PEN) in a solution of ammonium nitrate and ammonium chloride on wear and corrosion resistance of tool steel including its microhardness and surface roughness. An X-ray diffractometer and scanning electron microscopy were used to analyse the phase composition and structure of the modified surface. Dry wear test of the nitrided, hardened and untreated samples was carried out. The effect of the processing temperature on the corrosion resistance of the nitrided samples was examined by potentiodynamic polarization in the 3% solution of sodium chloride. The nitrate-based electrolyte is shown to provide the formation of the outer oxide layer and hardened sublayer containing martensite and retained austenite. The anodic PEN of steel for 5 min results in an increase in microhardness to 1280 HV50 and a decrease in wear rate by 5.5 times compared with the hardened sample. It has been established that the corrosion current density decreases from 21 mu A/cm(2) in untreated steel to 3.7 mu A/cm(2) after PEN of 700 degrees C.