Effect of Pretreatments on Alloy 2024-T351 Corrosion Protection by Magnesium-Rich, Nonchromium Primer (MgRP): Laboratory Characterization in Full Immersion

The role of different surface pretreatments on the sacrificial anode-based cathodic, as well as barrier, protection mechanisms afforded by magnesium-rich, nonchromium primer (MgRP) with and without topcoat (TC) has been investigated. Conversion coatings (chromate conversion coating, trivalent chromium-based pretreatment, and nonchromium pretreatment) and anodized coatings (without sealing, with chromate sealing, and trivalent chromium pretreatment sealing) were compared to clean and desmutted surfaces and those with a nonfilm-forming surface pretreatment with and without MgRP/TC. In preliminary studies, pretreatment chemistry and thickness were characterized using x-ray photoelectron spectroscopy and scanning electron microscopy/energy dispersive spectroscopy, while electrical properties were examined with electrochemical impedance spectroscopy. Full immersion tests in 5 wt% NaCl were conducted to evaluate the degradation characteristics of the coating and the subsequent evolution of the sacrificial anode-based cathodic protection, as well as barrier protection attributes. Factors studied included the total amount of encapsulated Mg retained in the primer, the Mg self-corrosion rate, the galvanic corrosion protection of Alloy 2024-T351 (UNS A92024), and the barrier properties. Preliminary findings for MgRP with conversion coating pretreatment and without a TC indicated that a finite full immersion exposure time was required to break down the more electrically insulating pretreatment layers. This process lowered the resistance between the MgRP and the Alloy 2024-T351 substrate and enabled sacrificial anode-based cathodic protection afforded by the MgRP. When an anodized coating was utilized as a pretreatment, cathodic protection was severely mediated as a result of the inherently high and persistent electrical resistance of this pretreatment. In contrast, Mg was galvanically coupled immediately and functioned from the start as a sacrificial anode for the nonfilm-forming pretreatment and abrasion-only pretreatments. Mg pigment depletion was tracked with x-ray diffraction in all the previously mentioned systems.