Investigation of electroless nickel undercoat for duplex nickel chromium electrodeposition onto aluminium and its alloys

Abstract

The study investigates the deposition of a multilayer coating onto two aluminium alloys: Al 1050 and Al 6061. Of particular interest has been the treatment of the surface of both these samples for deposition of metallic coatings through a zincate based immersion process. The zinc immersion layer chemically displaces the oxide film on Al 1050 and Al 6061 and then zinc is deposited followed by nickel and chromium electrodeposition. Superior adhesion was associated with uniform, thin and fine grained deposits from Bondal solution which exhibited rapid and complete coverage of both aluminium samples. The metallurgical characteristics of the aluminium alloys and the processing sequence developed had a significant influence on the growth and morphology of the deposits from the Bondal solution and affected subsequent adhesion of electroplated nickel and chromium. This engendered the treated surface with sufficient catalytic nature to be able to receive a subsequent electroless nickel layer together with finishing layers of electrolytic nickel and chromium respectively. Due to the favourable physical properties of aluminium and its alloys (density, strength to weight ratio), there is a growing demand for nickel/chromium coated aluminium components for automotive and other applications. The most common method currently is to directly electroplate nickel onto the aluminium substrate. However, this can lead to problems with components having complex geometry in that chemical attack on the pretreated aluminium can occur in low current density areas before the substrate can be completely covered with nickel due to the acidic nature and high temperature of the nickel electroplating process. One way of preventing this current density related problem is to use an electroless nickel undercoat before the nickel/chromium deposit is applied. The mechanism by which this occurs is not fully understood and it is an objective of the current research to investigate the mechanism of nucleation of the electroless nickel layer on zincated aluminium. Results of X-ray photoelectron spectroscopy showed that the zincated layer was dissolved in the electroless nickel bath but zinc was detected below 10 nm from the top surface of the homogenous nickel phosphorus film, while aluminium and alloying elements from both alloys have diffused into the film. Hexavalent chromium electroplating from chromic acid is under pressure due to its health hazard and environmental problems and is subjected to increasingly stringent control and legislation. Health and safety considerations have prompted the electroplating industry to consider alternatives to coating processes that involve hexavalent chromium. It is a further objective of the research to compare the properties of nickel/chromium coated aluminium with top coats of chromium deposited from hexavalent and trivalent (chloride based) electrolytes. Duplex nickel and chromium electrodeposition were modified with an electroless nickel undercoat, thus four coatings were studied for the aluminium samples. Results obtained from the hardness measurements of Al 1050 and Al 6061 were compared for four coatings. Hardness tests profiles obtained for Al 1050 and Al 6061 show that deposits from trivalent chromium electrolytes were not as hard as hexavalent chromium deposits. However, the hardness of duplex nickel/chromium coatings was improved by use of an electroless nickel undercoat. Scratch adhesion tests were focused on the failure mode of coatings evaluated on the basis of scratch channels, frictional force and acoustic emission signals. The adhesion test showed buckling and chipping of the coatings with no sign of spallation or delamination. Four coatings exhibited a higher critical load for both aluminium alloy samples. This confirmed that failure occurred within the coatings, rather than adhesive failure at the coating/substrate interface. Results obtained from copper acetic acid salt spray and electrochemical corrosion tests for Al 1050 and Al 6061 exhibited excellent corrosion resistance. Scanning electron microscope images showed initiation and propagation of small pits which did not coalesce to form large and deep craters that could result in the eventual failure of the coatings. Micro discontinuous chromium deposits spread the corrosion current, thus improving corrosion performance. Icorr values obtained from linear polarization corrosion tests show higher values for duplex nickel/chromium coatings on Al 1050 than Al 6061. Icorr values show similar trends for duplex nickel/chromium coating on Al 1050 and Al 6061 modified with an electroless nickel undercoat. In conclusion results showed a significant improvement in the chromium electroplating characteristics of Al 1050 and Al 6061 with an electroless nickel undercoat.