In situ Evolution of Trivalent Chromium Process Passive Film on Al in a Corrosive Aqueous Environment

In situ neutron reflectivity (NR) is used to observe the structure and evolution of a Trivalent Chromium Process (TCP) passive film on Al in a NaCl-D(2)O solution. Using a split liquid reflectivity cell we mimicked the corrosion process on the anodic sites in alloy AA 2024-T3 in a pitting scenario. The split cell separates the anodic and cathodic reactions, allowing NR observation of the corroding anodic surface under potential control. We observed the evolution of the TCP film on the Al anode and compared the degradation of the Al with and without TCP protection. When held at 100 mV above the open-circuit potential (OCP), unprotected aluminum dissolves at a rate of 120 Å/h. By contrast, TCP-coated Al is stable up to the pitting potential (200 mV above OCP). In the passive state D(2)O molecules penetrate the bulk TCP film by partially replacing the hydrate water. In spite of exchange of hydration water, the TCP film is stable and the underlying aluminum is fully protected. The passive character of the TCP film is due to a dense layer at the metal-TCP interface and/or to suppression of ion transport in the bulk film. As the pitting potential is approached the film swells and NaCl-D(2)O solution penetrates the TCP film. At this point, 50 vol % of the TCP film is occupied by bulk NaCl-D(2)O solution. Failure occurs by aluminum dissolution under the swollen TCP film as the imbibed solution contacts the Al metal. Further increase in potential leads to complete stripping of the TCP film.