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A number of studies have been reported on the use of plasma electrolytic carburising technology for surface hardening of different metals for higher corrosion and wear resistance resulted from this technique. However, very few have focused on the optimization of the pulsed nanocrystalline plasma electrolytic carburising process parameters. In this study, a design of experiment (DOE) technique, the Taguchi method, has been used to optimize the pulsed nanocrystalline plasma electrolytic carburising, not only for surface hardening but also for the corrosion protection of Ti-48Al-2Cr-2Nb (at%) titanium alloy by controlling the coating process's factors. The experimental design consisted of four factors (glycerol concentration, electrical conductivity of electrolyte, applied voltage and duration of process), each containing three levels. All experiments were done under constant frequency and duty cycle of rectangular shape pulse current method. Pure aluminum and titanium carbides were detected by XRD on the surface of the treated samples. Tafel polarization measurements were carried out to determine the corrosion resistance of the coated samples. The results were analyzed with related software. An analysis of the mean of signal-to-noise (S/N) ratio indicated that the corrosion resistance of pulsed plasma electrolytic carburized Ti-48Al-2Cr-2Nb (at%) alloy was influenced significantly by the levels in the Taguchi orthogonal array. The optimized coating parameters for corrosion resistance are 1200 g/l for glycerol concentration, 360 mS/cm for electrical conductivity of electrolyte, 400 V for applied voltage, 30 min for treatment time. The percentage of contribution for each factor was determined by the analysis of variance (ANOVA). The results showed that the applied voltage is the most significant factor affecting the corrosion resistance of the coatings.

Salt bath oxinitriding is a duplex surface treatment developed to improve tribological and corrosion properties of ferrous materials. In this research, gray cast iron samples were nitrided at the temperature range of 480°C–580°C, and then oxidized in an oxidative salt bath. The phase composition of surface layer was identified by X-ray diffraction. Using a microhardness tester, hardness of nitrided gray cast iron was measured. Corrosion behavior of treated (nitrided and oxinitrided) samples was evaluated using potentiodynamic polarization technique in 3.5% NaCl solution. XRD analyses indicate that the surface layer in nitrided and oxinitrided samples is composed of ε-iron nitride ( Fe 2-3 N ) and magnetite ( Fe 3 O 4), respectively. Results show that the corrosion resistance of gray cast iron can be improved up to 170%.