Galvanic corrosion behaviour of electrodeposited zinc and Zn–Ni coatings coupled with aluminium alloys

Abstract

The galvanic corrosion characteristics of detached electrodeposited pure zinc and Zn–Ni alloy coatings have been investigated on coupling with two aerospace aluminium alloys. Zinc alloys containing up to 24 wt-% nickel were coupled with an Al–Cu alloy, 2014–T6, and an Al–Zn–Mg alloy, 7075–T6, under total immersion conditions in quiescent 600mM NaCl solution. It was found that zinc alloys containing 5–14 wt-% nickel corroded preferentially on coupling with the aluminium alloys and reduced the corrosion rates of the aluminium alloys below those found in the uncoupled state. While pure zinc and Zn–Ni alloys containing <5 wt-% nickel also corroded preferentially on coupling with the aluminium alloys, in such cases the aluminium alloys suffered an accelerated rate of corrosion over that found in the uncoupled state as a result of, it is suggested, the formation of strongly alkaline conditions at the aluminium alloy surface. For zinc alloys containing over 14 wt-% nickel, aluminium alloy corrosion was also accelerated owing to a reversal in the directton of current flow observed during coupling.