Plasma nitriding is widely used as a final operation to improve wear, fatigue and corrosion resistance of industrial parts. Nevertheless, corrosion resistance can be further increased by post-oxidizing or by manganese phosphate conversion coating. This paper considers the duplex treatment of plasma nitriding and manganese phosphating of 42CrMo4 steel surface, widely used for manufacturing of breech mechanism in the armament production. Plasma nitriding was carried out at 500 °C for 15 h in different nitriding atmosphere having hydrogen and nitrogen gas flow ratio with 3 to 1 (l/h) and of 1 to 3 (l/h). Plasma nitrided steel samples were manganese phosphated (without lubrication). The treated samples were characterized using metallographic techniques, microhardness, and NSS corrosion tests. Different compound layer thickness under different nitriding gas ratio was confirmed. The X-ray diffraction analysis (XRD) confirmed different volumes of ε-Fe2-3N, γ´-Fe4N nitrides in the compound layers and hureaulite Mn5 (PO3 (OH))2 (PO4)2 (H2O)4 in the manganese phosphate coatings. The visual and gravimetric evaluation was carried out and corrosion rates modified to weight gains were calculated for corrosion evaluation. After removing of corrosion products weight losses were measured and pitting factors were calculated. Duplex treatment (manganese phosphating after plasma nitriding) also displayed increased corrosion resistance and suppressed pitting propagation in comparison with the plasma nitrided steel samples and a potential to further increase of corrosion resistance of plasma nitrided structural steels.Keywords: Plasma nitriding, manganese phosphate, corrosion resistance
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