Electrodeposition Of NiCoCr Protective Coatings

Electrodeposition is one of the most versitile approaches to deposite functional coatings on themerit of economic cost, fast deposition speed and easy processing, and therefore has been widelyapplied in various industries. Comparing to PVD, chemical deposition and thermal sprayingtechnology, electrodeposition also has the advantage to deposite metallic coatings on any curvedsurface with large aspect ratio, and thus frequently become popular in processing of nanomaterials,large scaled integrate circuits and biochemistry. On the other hand, Ni-Co-Cr alloy coatings havebeen identified to possess excellent protection to the alloy under highly severe environments such ashigh temperature vapor and supercritical water. As a result, electrodeposition of Ni-Co-Cr coatingsis the focus of this study.In this paper, the objectives of this study are to investigate the effect of the electrolyteparameters on the deposition of Ni-Co-Cr alloy coatings and assess the electrochemical behavior ofthe deposited coatings. The main factors that influence the electrodeposition include theconcentration of the main salts in the solution, complexing and wetting agents, buffer, pH value andtemperature of the electrolyte. The morphology, composition of the electrodeposited coatings werestudied as function of deposition time and current density via SEM and EDS. Through the study, anoptimal deposition processing with an ideal bath solution has been identified for the formation ofdesired Ni-Co-Cr alloy coating whose chemical composition consists of Ni30⁴0%, Co40⁵0%and Cr9¹5%in weight percentage. In addition, the electrochemical corrosion behavior ofelectrodeopsited Ni-Co, Ni-Cr and Ni-Co-Cr alloy coatings were investigated by weight loss test,potentiodynamic polarization technique and AC impedance, from which the effect of the coatingcomposition on corrosion resistance was rationalized.An ideal bath solution was identified enabling to electrodeposit the desired Ni-Co-Cr protectioncoating. The bath consists of NiSO₄·6H₂O160¹80g/L，NiCl₂·6H₂O45g/L，CoSO₄·7H₂O₂0³0g/L，CrCl₃·6H₂O15²5g/L，H₃BO₃25g/L，citric acid40⁵0g/L，and Sodium dodecylsulfate0.05g/L。The electrodeposition is processed in the above electrolyte with a pH value of2.5³.5under galvanostatic current density of50¹00mA/cm²at room temperature for5¹0min.The deposited coating thus achieved exhibits excellent corrosion resistance compared to the Ni-Coand Ni-Cr coatings, as testified from weight loss experiment, polarization and AC Impedance. Theas-deposited coating also shows very smooth, compact and shiny surface with good adhesion to theCu substrate, as assessed by bending test.