Effects of Cr Concentration on the Morphology and Corrosion Resistance of Ni-Cr Coatings Prepared by the Electrodeposition Process on 25CrMo4 Steel

Abstract

In the current study, Ni-Cr coatings with a various Cr concentration of 0.1 wt.% Cr, 0.2 wt.% Cr, 0.3 wt.% Cr and 0.4 wt.% Cr were performed on the 25CrMo4 low steel, using the electrodeposition process. The deposition of the Ni-Cr coatings was carried out at temperature of 30°C, direct current density of 2 A.dm-2 for 40 min, and a bath containing a Cr2(SO4)3 as source of chromium ions, Cr+. However, the effects of the Cr concentration on the morphology and the quality of the film were investigated. SEM, EDS and XRD analysis were used to characterize the Ni-Cr coatings. The SEM morphology analysis exhibited that the Ni-Cr with 0.2% Cr showed the lowest network of cracks and porosity with uniform and compact structure. The corrosion resistance of the Ni-Cr coatings has been investigated in 3.5 wt.% NaCl aqueous solution, using polarization curves and electrochemical impedance spectroscopy. According to the results, it was found that the variation of Cr con-tent in the Ni-Cr coating greatly affected the microstructure and the corrosion resistance of the coating. More-over, the coating developed with 0.2 wt.% of chromium (Cr) can give excellent results as to quality, morphology, and corrosion resistance in 3.5 wt.% NaCl environment. The results of polarization showed that 0.2 wt.% Cr layer had the more important resistance with an Ecorr of 0.488 mV, Icorr of-418 µm.cm-2 and Rp of 84 kΩ.cm-2. Therefore, EDX analysis confirmed that Chromium (Cr) oxides were formed, which contribute to its resistance.