Corrosion Inhibition of TCP-Coated AA2024-T3 during Exposure to so2 Atmospheric Testing

High strength aluminum alloys (e.g., AA2024-T3) are widely used as aircraft structural components. The inclusion of two major alloying elements, Cu and Fe, make the metal more susceptible to localized damage in the form of pitting and intergranular corrosion. The susceptibility of the alloy to localized corrosion is due to the fact that galvanic cells form between the Cu- and Fe-rich intermetallic phases, sites that support relatively rapid oxygen reduction reaction kinetics, and the aluminum, which undergoes dissolution. The intermetallic phases are more noble than the surrounding aluminum matrix leading to the localized pitting or dissolution of the aluminum near the intermetallics. Inhibiting this localized corrosion is essential for achieving long performance lifetime of these structural components.

Conversion pretreatment coatings are one component of a multilayer coating system that is typically used to protect aerospace aluminum alloys from corrosion. The multi-layer coating system consists of a conversion coating, a primer and an organic topcoat. The focus of current research in our group is on the application of alternate, more environmentally-friendly conversion coatings that serve as suitable replacements for the currently-used chromate conversion coatings (CCC). The corrosion inhibition provided by the multilayer coating system depends on the chemical composition, physical structure and thickness and uniformity of the conversion coating. The need for some level of corrosion inhibition by the conversion coating arises when there is coating breakthrough, in other words a scratch through the coating system that exposes some bare metal. Conversion coating thicknesses are  generally in the 50-200 nm range and they provide corrosion inhibition, at least in part, by providing a barrier to oxygen and water/electrolyte accessibility to the metal surface.

The trivalent chromium process (TCP) conversion coating is an environmentally-friendly potential replacement for the currently used chromate conversion coating. TCP is regarded as one of the leading drop-in replacements for CCC. Prior work has shown that the TCP coating inhibits corrosion on aluminum alloys, in part, by serving as a barrier layer.

In this presentation, we report on the corrosion inhibition provided by a TCP conversion coating (Alodine T5900, Henkel Corp.) to degreased and deoxidized AA2024-T3 during atmospheric testing in SO₂. A moist SO₂ exposure test was used to mimic urban environment conditions. This test was carried out according to  ASTM  G87. The temperature was controlled at 40 ± 3 ºC by placing the chamber in an oven. One test cycle was 24 h during which time the specimens were continuously exposed to the SO₂/H₂O environment. Water  and SO₂ in the chamber atmosphere were replaced before each 24-h cycle. The whole test period was 14 cycles or 14 days. At the end of the test period, the specimens were removed and cleaned by rinsing with ultrapure water for approximately 30 s. The specimens were then air dried or dried under a stream of N₂ prior to any additional measurements. Two specimens were tested: uncoupled coated panels and galvanically-coupled coated specimens (stainless steel fasteners). The specimens were tested before and after the 14-day environmental test using electrochemical methods, SEM/EDX, Raman spectroscopy and optical profilometry.