Electrochemical Evaluation of Al-5 wt% Zn Metal-Rich Primer for Protection of Al-Zn-Mg-Cu Alloy in NaCl

Abstract

An intact and X-scribed Al-5wt%Zn-rich primer (AlRP) without pretreatment or topcoat was evaluated for its ability to suppress potential-dependent intergranular corrosion and intergranular stress corrosion cracking of peak-aged AA7075A-T651 in NaCl salt fog and full immersion. The ability of the primer to provide sacrificial anode-based cathodic prevention of peak-aged AA7075-T651 substrate was evaluated both under the primer coating and at scratches. The AlRP evaluated consisted an epoxy-based resin embedded with spherical Al-5wt%Zn pigment particles. Performance was evaluated under full immersion in 0.6 M NaCl solution and compared to ASTM B117 salt spray exposure using two approaches. These consisted of the University of Virginia (UVA) cycle test on intact coatings and the full immersion galvanic couple testing on simulated scratched panels created when intact coatings form bimetal couples with bare AA7075-T651. Focus was placed on the ability of the AlRP to achieve a targeted intermediate galvanic couple potential near a “prevention” potential which suppresses stress corrosion crack growth, intermetallic particle corrosion as well as intergranular corrosion. The long-term (24-h) open-circuit potential (OCP) of AlRP-coated AA7075-T651 in 0.6 M NaCl indicated that the AlRP provided less than 100 mV of cathodic potential shift of the intact coating from its OCP in 0.6 M NaCl. Electrochemical cycle testing conducted at a potentiostatic hold of –0.95 VSCE demonstrates that the AlRP did not enable sacrificial anode-based cathodic protection as the coupled potential remained at the corrosion potential of bare AA7075-T651. Furthermore, the current observed throughout galvanic corrosion experiments coupling of AlRP to AA7075-T651 indicated the AlRP coating was a cathode in the bimetal galvanic couple. ASTM B117 salt spray exposure of the AlRP revealed oxidation of the AA7075-T651 substrate below the primer detected as a continually growing oxygen signal at the primer-substrate interface that did not arrest corrosion over the exposure period.