Deposition and characterization of electroless Ni on fluoride free pretreated AZ91 magnesium alloy

Abstract

Poor corrosion resistance of magnesium alloys restricts their widespread applications in corrosive environments. The electroless nickel deposition is well established approach to combat the corrosion in various environments but limited due to the usage of various hazardous chemical species such as HF, fluorides and hexavalent chromium. The electroless nickel deposition on magnesium alloys is difficult due to the formation of poor oxide/hydroxide layer which may overcome by facilitating by the suitable surface pretreatment. An environment friendly pretreatment and a novel duplex-electroless nickel (ENi) plating methods on AZ91 magnesium alloy is developed and reported in the present paper. Two types of environment friendly baths, alkaline and acidic were formulated to pretreat the AZ91 magnesium alloy prior to electroless nickel plating. The duplex electroless Ni-P layer was successfully obtained to increase the corrosion resistance of AZ91 alloy; the first layer in alkaline pH (10.5) followed by another Ni-P layer either in alkaline (pH∼10.5) or acidic pH (5.0) bath. Morphology of the coating was characterized by scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDS). Corrosion resistance was characterized by using potentiodynamic and electrochemical impedance spectroscopy (EIS) in 0.5% NaCl solution. Results showed coating with the submicron size nodules produced at alkaline pH while amorphous at an acidic (pH∼5.0). A large increase in the corrosion resistance of AZ91 alloy was obtained with the duplex ENi layer using prior treatment in the phosphoric acid containing solution.