Study on the surface morphology and corrosion rates of electroplated coatings based on binary alloys of refractory metals after accelerated acid tests

Abstract

ABSTRACT The corrosion rates of Ni-W, Co-W, Ni-Mo, and Co-Mo electrodeposited coatings after two-hour immersion tests in HNO3, H2SO4 and HCl environments were studied by the weight loss method. The best corrosion resistance was revealed for Ni-W alloy, and the worst for Co-Mo. For all tested samples (36 in total), the maximal corrosion rate was observed in nitric acid, and the minimal one, with an exception of Ni-W, in hydrochloric acid. It was found that for the W-containing coatings corrosion evenly affected each individual crystallite of the alloy structure changing its shape and volume, and the corrosion of Mo alloy-based coatings showed mixed character. According to the Pourbaix diagram, for Mo alloys (Ni-Mo and Co-Mo) the lower corrosion resistance compared to the coatings containing W can be explained by a narrower region of MoO3 formation, as well as by their higher solubility in comparison with WO3.