Electrodeposition of engineering alloy coatings: Ph.D. thesis

Nickel based electrodeposited alloys were investigated with respect to their deposition process, heat treatment, hardness, corrosion resistance and combined wear-corrosion resistance. The investigated alloys were Ni-B, Ni-P and Ni-W, which are not fully developed for industrial utilisation at the moment. It was the intention of this study to investigate whether the mentioned alloy processes are able to substitute conventional deposition techniques for wear and corrosion resistance, namely Ni-P produced by electroless deposition and electrodeposited hard chromium. The considerations for substitution focussed on were increased deposition rates as well as improved corrosion and wear resistance.Some systems exhibited interesting deposition rates. Examples are 178 µm per hour of Ni-P(6), 85 µm per hour of Ni-P(15), 142 µm per hour of Ni-W(44) and 62 µm per hour of Ni-B(0.8) (weight percentages are written in brackets). Temperature and especially pH influenced the cathodic efficiency of the electrodeposition processes for Ni-W and Ni-P. Mass balance problems of the development alloy processes are identified.Heat treatment for one hour at approx. 350°C, 400°C and 600°C of electrodeposited Ni-B, Ni-P and Ni-W, respectively, resulted in hardness values of approx. 1000 HV0.1 in the case of Ni-P(6), approx. 1100 HV0.1 in the case of Ni-W(40-53) and approx. 1300 HV0.1 in the case of Ni-B(5). Cracks, which emerged during electrodeposition and heat treatment, were observed on Ni-W and Ni-B.The corrosion properties were investigated in neutral environments at 30°C by means of electrochemical methods. None of the electrodeposited nickel alloys exhibited unambiguous passivity with or without the presence of chloride. Therefore these layers are likely to be degraded by exposure to strong oxidants and high temperatures. Electrodeposited Ni-P(15) in the as plated state exhibited low anodic current densities in 1 M NaCl at 30°C at potentials where electrodeposited chromium and nickel were degraded by localised corrosion. The phosphorus content and heat treatment state influenced the anodic properties of electrodeposited Ni-P. Ni-W and Ni-B were in general less corrosion resistant compared to Ni-P.Degradation of hardened electrodeposited Ni-P(6-12), hard chromium and stainless steel (AISI 316) by both wear and corrosion as experienced in a slurry test showed that the wear contribution was predominant. The total wastage of Ni-P and stainless steel were almost similar, whereas hard chromium exhibited a six-fold decrease in the total wastage rate.