Evaluation of the Performance of a Magnesium Rich, Non-Chromate Primer With/Without Topcoat on 2024-T351: The Role of Various Substrate Pretreatments

Many aerospace structures utilize precipitation age hardened aluminum alloys which heavily rely on the use of chromated primers and pretreatments to provide active corrosion protection. Carcinogenicity, high handling cost and lack of environmental safety have necessitated an accelerated phase-out of hexavalent chromium, concurrent with initiatives to find effective alternatives.

The sacrificial anode-based cathodic and barrier protection capabilities of a non-chromate magnesium rich primer (MgRP) with a non-film forming pretreatment, in both a topcoated and non-topcoated conditions is an emerging, promising corrosion mitigation strategy for precipitation age hardened aluminum alloy, 2024-T351. However, little is known about the effect of surface pretreatments upon which most coatings will be deposited. This work addresses how resistive surface pretreatments (such as chromate conversion coating (CCC), trivalent chromium pretreatment (TCP), non-chromate pretreatment (NCP), anodization without sealing (ANS), anodization with hexavalent chromium sealing (ACS) and anodized with trivalent chromium pretreatment (ATS)) affect the overall corrosion protection functions of MgRP-based systems. The effect of pretreatment properties on sacrificial protection function, barrier degradation, and scribe protection by MgRP was examined. In addition, alternate modes of corrosion protection by chemical species leaching from the pretreatment and primer were also investigated.

The pretreatment chemistry and thickness was characterized using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) while initial electrochemical properties were examined using electrochemical impedance spectroscopy (EIS). Anodization based pretreatment especially added a significantly resistive surface layer while conversion coatings have moderate barrier properties.

An accelerated electrochemical cycle test was adopted to evaluate the evolution of sacrificial protection function and barrier degradation of the coating in full immersion conditions. A finite full immersion exposure time was required for degradation of more electrically insulating pretreatment layers. The pretreatment degradation lowered the resistance between the MgRP and the 2024-T351 substrate and enabled delayed activation and triggered sacrificial anode-based cathodic protection by MgRP. In contrast, MgRP was galvanically coupled immediately and functioned as a sacrificial anode for the non-film forming (NFF) pretreatment.

This work has further illuminated and verified test methods which assess coating degradation and scribe protection that could be used in both the laboratory and the field. A suite of test methods was utilized to track the elemental Mg depletion, galvanic protection potential, barrier degradation, Mg corrosion products formation and corrosion volume loss at the scribe throughout exposure in field as well as laboratory accelerated life environments. In the case of systems without a topcoat, significant depletion of Mg pigment and coating degradation were observed in all environments but at different rates. In the case of NFF pretreated AA2024-T351 with MgRP, magnesium was galvanically coupled to AA2024-T351 immediately and was available for cathodic protection from the beginning of the exposure. In the case of trivalent chromium pretreatment (TCP) and other similar conversion coating pretreated AA2024-T351, initially there was limited galvanic coupling with the MgRP due to high pretreatment resistance. Upon prolonged exposure in full immersion, the global galvanic protection potential decreased to more negative potentials below the open circuit potential (OCP) of AA2024-T351 indicative of galvanic coupling. In anodized systems with chromate sealing (ACS), Mg pigment was not electrically connected to the AA2024-T351 until after long environmental exposure times because of the resistive nature of the pretreatment and improved sealing with increasing exposure time. The barrier properties of the MgRP pigmented coating also degraded with time at a higher rate in systems in the absence of topcoat. This was attributed to UV degradation of the pigmented coating resin and which was reduced by the UV resistant polyurethane topcoat. SEM/EDS characterization of the scribe after different ASTM B117/field exposure times indicated that the protective throwing power increased as a function of exposure time in all MgRP-based systems. Moreover, a secondary protection mode enabled by Mg(OH)2 redeposition was identified.

In addition, the galvanic throwing power of the MgRP was studied via the scanning vibrating electrode technique (SVET), which enabled the spatial analysis of net anodic and cathodic current densities for a MgRP on pretreated 2024-T351 coupled with a bare 2024-T351 scribe to be mapped. The effect of pretreatment resistance, coating to scribe area ratio and topcoat polymer properties on galvanic protection was elucidated. For NFF/MgRP, anodic current densities in the scribe indicative of local sites of pitting were lowered by 2-3 orders of magnitude. This was attributed to sacrificial anode-based cathodic prevention. TCP and ACS pretreated systems did not exhibit galvanic protection in lower coating to scribe area ratios. However, at higher coating to scribe area ratio, improved scribe protection with time was observed for these pretreatments. Alternate modes of corrosion protection attributed to chemical species leaching from the primer and pretreatments were elucidated as the cause of this effect.

The outcome of this research provided a scientific foundation for understanding how utilization of resistive pretreatments along with MgRP enables development of a multi-functional corrosion protection system which includes delayed cathodic/sacrificial protection of Mg, barrier protection by pretreatments/primer/topcoat and corrosion inhibition by chemical species leaching from the MgRP. The throwing power of the Mg in the primer was heavily limited by topcoat polymer properties in addition to high pretreatment resistances. The work performed herein suggests that a pretreatment with high resistance that doesn’t degrade significantly with time may not be suitable for use with MgRP because the substrate is decoupled from the sacrificial anode.