Electrodeposition on magnesium alloy diecasting

Abstract

Magnesium alloy diecasting AZ91CC, AZ61CC', AZ91HC and AZ71HC were electroplated using different pretreatment sequences which incorporated conventional zincate immersion processes. Satisfactory peel adhesion in excess of 7. 7 KNm -1 was achieved on AZ61CC using a sequence which was designated Canning. The comparatively low adhesion achieved on the AZ91HC was due to its poor surface quality as cast. Growth of deposits was monitored using a strip-and-analysis technique and the morphology of the various deposits were studied using scanning electron microscopy. Different pretreatment sequences resulted in different surface responses for the alloys but all alloys behaved in a similar manner in a particular sequence with regard to potential time-curves and the rate of zinc deposition. The role of fluoride in both the second stage solution and zinc immersion stages of the Canning pretreatment sequence was studied using techniques listed above and Auger electron spectroscopy. Complete coverage of the magnesium alloy surface with immersion zinc was achieved when fluoride was absent from the zincating solution. However, a zero adhesion value was indicated in both thermal cycling and peel tests. The presence of fluoride in the immersion zinc solution suppressed the rate of zinc deposition and affected the time taken to reach equilibrium during potential-time determinations. A mechanism is suggested to explain the significance of fluoride additions to the processing solutions. pH and composition of the zincating solution had a significant effect on the time taken to produce the step observed in the potential/time curves and hence equilibrium potential. Immersion zinc deposition occurred rapidly at first but then changed to a lower uniform rate at a point corresponding approximately to the step in the potential/time curve. Although the minimun levels of adhesion, using the Canning sequence, varied from 7.72 KNm-1 for alloy AZ61CC to 1.54 KNm-1 for alloy AZ91HC, all the alloys revealed ductile failure characteristics in the surface layer of the substrate after peel testing. Plated magnesium alloys exhibited good corrosion resistance when appropriately pretreated and overplated with adequate nickel chromium coatings. The immersion zinc layer was not preferentially attacked when pits penetrated to the coating/substrate interface. Hemispherical pits formed and attack on the substrate was severe. Of the pretreatment sequences investigated, the Canning one was the most premising with respect to peel adhesion and corrosion behaviour.