The surface of the non-alloyed carbon steel was subjected to thermo chemical modification by salt bath nitrocarburizing with or without post-oxidation based on the Tenifer procedure in order to improve mechanical properties, corrosion and wear resistance. Nitrocarburized layers were characterized by testing their basic properties (compound layer thickness, nitrocarburizing depth, surface hardness) according to current standards. Detailed estimation of the quality of the modified metal surface was performed by additional testing of chemical compositions by EDX and X-ray diffraction, microstructure, surface roughness and topography, and corrosion resistance. The corrosion resistance was investigated in a water solution of 5% NaCl using electrochemical polarization methods and electrochemical impedance spectroscopy. The corrosion properties of the treated and non treated samples are compared among themselves and with the properties of austenitic steel. The surface layer obtained after nitrocarburization treatment consists mainly of ε-Fe2-3N and γ'-Fe4N. The surface layer of the samples after post-oxidation consists also of ε-Fe2-3N and γ'-Fe4N, but contains additionally Fe3O4. The results obtained show that nitrocarburizing and post-oxidation treatments, based on the Tenifer procedure, result in better mechanical and corrosion properties of nonalloyed carbon steel. The corrosion properties of the thermo chemically modified nonalloyed carbon steel were improved and approach the corrosion properties of austenitic steel, known for its very good corrosion resistance.

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Effects of such thermochemical processes as carburising, nitriding and boriding on properties of maraging steel grades have been investigated. Despite certain improvements in the properties of the obtained surface, at present, carburising and boriding cannot be recommended owing to problems in the layer quality and difficulties in dimension control processes. Nitriding has proved to be the most suitable process and subsequent surfacing with a TiN layer on the previously nitrided substratum of maraging steel provided additional improvement in properties.

Nitriding increases surface hardness and improves wear resistance of stainless steels. However, nitriding can sometimes reduce their corrosion resistance. In this paper, the influence of nitriding on the corrosion resistance of martensitic stainless steel was investigated. Plasma nitriding at 440°C and 525°C and salt bath nitrocarburizing were carried out on X17CrNi16-2 stainless steel. Microhardness profiles of the obtained nitrided layers were examined. Phase composition analysis and quantitative depth profile analysis of the nitrided layers were preformed by X-ray diffraction (XRD) and glow-discharge optical emission spectrometry (GD-OES), respectively. Corrosion behaviour was evaluated by immersion test in 1% HCl, salt spray test in 5% NaCl and electrochemical corrosion tests in 3.5% NaCl aqueous solution. Results show that salt bath nitrocarburizing, as well as plasma nitriding at low temperature, increased microhardness without significantly reducing corrosion resistance. Plasma nitriding at a higher temperature increased the corrosion tendency of the X17CrNi16-2 steel.Keywords: Martensitic stainless steel / salt bath nitrocarburizing / plasma nitriding / corrosion resistance / X-ray diffraction Schlüsselwörter: Martensitischer Stahl / Salzbadnitrocarburieren / Plasmanitrieren / Korrosionsbeständigkeit / Röntgenbeugung