Electrodeposition on magnesium alloy diecasting
Magnesium alloy diecasting AZ91CC'~, AZ61C C'~ , AZ91HC ,~'~ and AZ71HC
were electroplated using different pretreatrrent sequences which
incorporated conventional zincate immersion processes. Satisfactory
peel adhesion in excess of 7. 7 KNrn -1 was achieved on AZ61CC using a
sequence which was designated canning. The conparatively low adhesion
achieved on the AZ91HC was due to its poor surface quality as cast.
Growth of deposits was nonitored using a strip-and-analysis technique
and the rrorphology of the various deposits were studied using scanning
electron microscopy. Different pretreatrrent 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 timecurves
and the rate of zinc deposition.
The role of fluoride in both the second stage solution and zinc
immersion stages of the canning pretreatrrent 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 fran the zincating solution.
H<:Jr...1ever, a zero adhesion value was indicated in both thennal cycling
and peel tests. The presence of fluoride in the immersion zinc
solution suppressed the rate of zinc deposition and affected the t.iIre
taken to reach equilibrium during potential-t.iIre determinations. A
rrechanism is suggested to explain the significance of fluoride
additions to the processing solutions.
pH and cx:::mposition 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
Although the minirru..un lE[vels of adhesion, using the Cannin~ sequence,
varied fran 7.72 KNm- for alloy AZ61CC to 1.54 KNm - 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 chrcmium 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 pretreatrrent sequences investigated, the Canning one was the
flOst premising with respect to peel adhesion and corrosion behaviour.