  High purity magnesium and a Mg-Al 5wt% Zn metal rich primer (MRP) were
compared for their ability to suppress intergranular corrosion (IGC) and
intergranular stress corrosion cracking (IG-SCC) in peak aged AA 7075-T651 by
sacrificial anode-based cathodic prevention. Tests were conducted in 0.6 M NaCl
solution under full immersion. These evaluations considered the ability of the
primer to attain an intermediate negative open circuit potential (OCP) such
that the galvanic couple potential with bare aluminum alloy (AA) 7075-T651
resided below a range of potentials where IGC is prevalent. The ability of the
primer to achieve an OCP negative enough that the AA 7075-T651 could be
protected by sacrificial anode-based cathodic prevention and the ability to
sustain this function over time were evaluated as a first step by utilizing a
NaCl solution. The primers consisted of epoxy resins embedded with either (1)
Mg flake pigments (MgRP) or (2) Mg flake pigments and spherical Al-5 wt.% Zn
together as a composite (MgAlRP). MgRP was an effective coating for cathodic
protection but dispensed less anodic charge than the composite MgAlRP.
Cross-sectional analysis demonstrated that some Mg flakes dissolved while
uniform surface oxidation occurred on the remaining Mg flakes which led to
impaired activation. The composite MgAlRP maintained a suitably negative OCP
over time, remained activated, dispensed high anodic charge, and remained an
anode in zero resistance ammeter testing. Chemical stability modeling and zero
resistance ammeter testing suggest that Mg corrosion elevates the pH which
dissolved aluminum oxides and hydroxide thereby activates the Al-5wt.% Zn
pigments, thereby providing a primary (i.e. Mg corrosion) and secondary process
to enable superior (activation of Al-5wt%Zn) sacrificial anode-based cathodic
protection.
