Spray Applied Amorphous/Nanocrystalline Aluminum Alloy Coatings as a Replacement for Aluminum Cladding

Abstract

A high density, low porosity Al-Co-Ce metallic coating has been successfully applied to an AA-2024-T351 substrate using a new PTS process in thicknesses ranging from 100-600 µm. The three proposed modes of corrosion protection brought about by coating (e.g., a localized corrosion barrier, a sacrificial anode to supply sacrificial cathode protection of any exposed AA 2024-T351, and active inhibitor release and protection of AA 2024-T351) have been verified to function and the results are promising. Although XRD has identified nanocrystalline peaks in the as-sprayed deposit, the best PTS applied coating samples have approached the barrier properties of fully amorphous melt spun ribbons of similar composition with regard to resistance to pitting. However, high surface area PTS coatings produced superior capabilities for active inhibitor release. IntroductionAluminum alloys are widely used in aerospace applications, where their high strength-toweight ratios make them ideal structural metals. They also have good atmospheric corrosion resistance as a result of the aluminum oxide films that form on the alloy surfaces. Unfortunately, the protective properties of these films may break down locally and allow extensive corrosion to propagate through the bulk material. A number of methods are used to achieve better corrosion protection of these structural aluminum alloys, including mechanical cladding, chromate conversion coatings, and the application of corrosion inhibiting paints which typically contain slightly soluble chromate pigments. Mechanical aluminum cladding, for instance, is widely used and provides good corrosion protection as a sacrificial anode by suppressing local pitting, however, lacks an active inhibitor and its corrosion resistance could be lost during surface repair. An additional factor to consider is that cladding is not field-replaceable. Field-replaceability is a benefit for many military and commercial applications where component removal is non-trivial and equipment downtime is very costly. Corrosion-inhibiting paints may provide soughtafter self-healing capability; however, these typically require the use of chromate-or lead-based pigments and, due to the toxicity of both lead and chromate, are being phased out from use based on environmental concerns. It is extremely desirable to develop a metal coating for high-strength aerospace alloys, such as AA 2024-T351, that can protect in three important ways: (i) serve as local corrosion barrier with respect to both uniform corrosion and local corrosion along with tunable corrosion barrier properties, (ii) act as a sacrificial anode (i.e., have an opencircuit potential (OCP) that is ca. 20-300 mV below that of AA 2024-T351), and (iii) 59 60 supply soluble ions that function as inhibitors to mitigate corrosion of high performance aluminum structures, especially at locations where the coating was scratched or removed. These three functions are not mutually exclusive. However, optimization of each in a spray applied coating may not be achieved in the same w...

Metallurgical and Physical Factors in Improving the Corrosion Resistance of Thermally Sprayed Semi-Amorphous Al-Co-Ce Coatings

“…Reduced substrate thermal loading enables short stand-off coating capability that, from an application standpoint is desirable because it allows for highly localized repair and the capability for coating complex geometries that are inaccessible to conventional cladding processes (20). Previous results have confirmed the ability of PTS to retain the coating's intended amorphous/nanocrystalline nature, as well as produce a high-density coating (25). Electrochemical testing has validated the ability of the asdeposited samples to deter corrosion as both an effective barrier coating, sacrificial anode and through the release of inhibitor ions.…”