User-Selectable Barrier, Sacrificial Anode, and Active Corrosion Inhibiting Properties of Al-Co-Ce Alloys for Coating Applications

Abstract

An Al-Co-Ce alloy system has been developed with three important protection abilities when deployed as a metal coating over high-strength aerospace alloys such as AA2024-T351 (UNS A92024). These alloy coatings can serve as a local corrosion barrier with tunable anodic properties, act as a sacrificial anode (i.e., its open-circuit potential [OCP] is ca. 20 mV to 300 mV below that of AA2024-T351), and supply soluble ions that function as corrosion inhibitors. These three functions are not mutually exclusive. However, optimization of each may not be achieved in the same way. The Al-Co-Ce alloy system due to its structural amorphicity and chemical composition, with electrochemical tunability based on solid solution Co and Ce composition. For example, the pitting potential (Epit) is more noble than the Epit of AA2024-T351 under acid, neutral, and alkaline conditions with pitting and repassivation potentials has demonstrated an excellent inherent resistance to corrosion optimized by Co content. The galvanic and chemical throwing powers of Al-Co-Ce metallic coatings under atmospheric conditions have also been investigated. Sacrificial cathodic prevention is demonstrated both experimentally and through modeling. Galvanic throwing power studies elucidated the conditions under which an Al-Co-Ce coating can protect a scratch exposing an AA2024-T351 substrate by polarization to a galvanic couple potential below the [Cl−]-ion-dependent Epit of AA2024-T351. The distance over which this condition can be achieved is termed the “galvanic throwing power” in this paper, and was modeled with numerical computations by mapping galvanic couple potential vs AA2024-T351 scratch position. The galvanic throwing power results indicated that substantial sacrificial cathodic prevention of AA2024-T351 scratches could be achieved with Al-Co-Ce alloy coatings. Performance was superior to conventional alclad coatings in terms of both extent of polarization and maximum scratch width protected. The behavior of the metallic coating can be tailored and provides the best protection (i.e., largest cathodic polarization of the widest scratches) when it contains a low-Co content, is exposed to either high- or low-pH solutions of low-chloride concentration, and when the exposed AA2024-T351 scratch exhibits slow cathodic kinetics. The characteristics of the chemical throwing power of the Al-Co-Ce alloy were also investigated by numerical computations and experimentation. When investigating the chemical throwing power, the galvanic properties were not ignored since they were used as boundary conditions to calculate release rates and to determine electric fields for ion migration. It was shown that alloy composition could be manipulated to enhance transport of inhibiting cations by both migration and diffusion. The conditions under which the Al-Co-Ce coating released and transported inhibitor ions to a scratch that exposes AA2024-T351 were elucidated. The criterion for protection is the achievement of a critical inhibitor concentration (specifically Ce3+) over the exposed AA2024-T351 within a few hours. Triggered release by acidification and alkalinity were also demonstrated. The results of this study confirm that Al-Co-Ce coatings can be tailored to optimize coating functionality by a combination of the three protection strategies. Moreover, many of the material parameters that govern optimization of each protection strategy and many of the physical conditions that enable its use are identified.