Sacrificial Anode Based Cathodic Prevention Using Mg, Al, and MgAl Metal Rich Primers Applied to AA 7075-T651: An Assessment of Coating Performance and Substrate Protection

Age-hardenable aluminum alloys are vital to aerospace applications due to their high specific strength; as such, their corrosion resistance is critical to ensuring long-term sustainable use. To date, the aerospace industry has focused on legacy Al-Mg, Al-Cu-Mg, and Al-Zn-Mg-Cu alloys. These alloys are often prone to intergranular corrosion (IGC) as a result of their alloying composition, aging process, and environmental conditions. It is well established that individual microstructural features, such as constituent particles and precipitates, establish local anodes and cathodes that lead to micro-galvanic corrosion. Aerospace aluminum alloys are prone to exposure to coastal and marine environments increasing IG/IG-SCC susceptibility. The challenge is to mitigate this degradation process by imposing sacrificial cathode prevention (SCP). SCP is a strategy which involves creating a new galvanic couple below the electrode potential of the susceptible metallurgical phases.
Metal-rich primers (MRP) are a class of active corrosion protection coatings containing sacrificial metallic pigments that are more electrochemically active than the underlying substrate. They inhibit the corrosion of the substrate by providing sacrificial anode-based cathodic protection. Metal-rich primers provide corrosion protection in three ways, galvanic protection, chemical inhibition, and barrier effect. Various accelerated laboratory tests and electrochemical methods have been employed to assess the corrosion performance of metal-rich coatings, such as corrosion potential measurements, electrochemical impedance spectroscopy (EIS), galvanic corrosion, full immersion testing, hydrogen evolution testing, polarizability testing, ASTM B117 accelerated exposure testing, and high-fidelity testing in the form of scanning vibrating electrode technique (SVET) testing. Historically, the indoor laboratory accelerated cabinet style corrosion tests, in the form of ASTM B117, have been utilized to characterize the corrosion performance of age-hardenable Al alloys in various environments over a wide range of exposure times.
The objective of the proposed work is twofold. The mechanisms of substrate protection were elucidated and efforts were made to develop an assessment methodology for evaluating the corrosion protection performance and environmental cracking mitigation of MgRP, AlRP, and a composite MgAlRP applied to AA7075-T651. The degree of protection was quantified by interrogating corrosion potential suppression as well as reduced galvanically coupled potential afforded by the MRP relative to the electrode potential of susceptible grain boundary phases such as MgZn2. Characterization of corrosion products formed during electrochemical testing and accelerated environmental testing was conducted with X-ray diffraction (XRD) and Raman spectroscopy in plan-view. In conjunction, backscatter electron imaging (BSI) and elemental distribution maps using energy dispersive spectroscopy (EDS) were used to visualize the progression of damage through the coating cross-section indicated by various markers such as oxidation in the coating and corrosion product penetration into the substrate. Suppression was directly assessed by comparing pigmented coatings to unprotected substrate. In addition, the evaluation of corrosion performance in MRP coatings was not confined to a single electrochemical test but rather a suite of electrochemical testing was needed to evaluate the intricate and elusive aspects pertaining to corrosion performance in MRP coatings. For instance, insight into the mechanisms of corrosion protection and coating performance was made possible with in situ pH measurements collected during galvanic couple testing and the use of chemical stability modeling. This showed that the multiple pigment chemistries available in the composite MgAlRP coating give rise pH changes during dissolution at the reacting electrode interface which shift the stability of species away from the formation of corrosion products and into a region of stable Mg2+ and Al3+ allowing for additional pathways for coating utilization via the activation of Al-5wt%Zn pigment resulting in enhanced corrosion performance. This performance was not noticed in the MgRP as the coating was comprised of a single pigment chemistry, pure Mg, which is susceptible to rapid dissolution limiting the duration of protection offered to the AA 7075-T651. The Al pigment in the MgAlRP was chemically identical to the Al pigment within the AlRP which was determined to be ineffective in providing sacrificial anode-based cathodic protection and operated as a cathode opposite to the intended galvanic coupling with AA 7075-T651. This showed a significant departure from conventional testing which predominantly relies on accelerated environmental testing in the form of ASTM B117. This alone was not capable of discerning between the capacity to which an MRP coating was able to operate as an effective form of sacrificial anode-based cathodic protection or the mechanisms of protection.
The key scientific contributions produced in this thesis showed that the composite MgAlRP outperforms both the MgRP and AlRP in terms of sacrificial anode-based cathodic protection and was able to polarize AA 7075-T651 below the corrosion potential of AA 7075-T651 and the pitting potential of MgZn2, maintained anodic potentials throughout galvanic coupling, suppressed localized corrosion on bare AA 7075-T651, provided scribe protection, and output a greater amount of charge. All of the qualities observed in the composite MgAlRP were a result of the pigment additions of Mg + Al in a single coating, which represents a major technological achievement. This gives insight into future studies and coating design criteria which may consider the alloying of pigment, integration of multiple pigment chemistries (pure or alloyed) into a single coating, and effective electrochemical analysis techniques to evaluate performance.