Electrochemicals corrosion tests of an AISI-SAE 4140 steel nitrided by post-discharge microwave plasma

Abstract

AISI-SAE 4140 low alloy steel was nitrided in post-discharge plasma microwave conditions at 820 K for 15 min, using a laboratory reactor. The microstructure and composition of the nitrided layer were analyzed. The corrosion resistance of untreated and nitrided samples was evaluated in several corrosion electrochemical tests such as polarization resistance, Tafel, anodic polarization, cyclic polarization and electrochemical noise, in a chloride environment. The open circuit potential shifted to noble potential, the low corrosion rate obtained by resistance to polarization (RP) and Tafel clearly indicate that microwave post-discharge process is adequate for corrosion protection of AISI-SAE 4140 steel. The untreated and nitrided pieces were characterized by X-ray diffraction (XRD) to identify the crystalline phases, and by scanning electron microscopy (SEM) to examine the morphology and thickness of the nitride layer.