RESUMO
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.
RESUMO
Neutron reflectivity (NR) and X-ray reflectivity (XRR) were used to determine the structure and composition of trivalent chromium process (TCP) films on aluminum alloy AA-2024. Two deposition methods were developed: immersion and electroassisted (EA) deposition. The EA deposition method was designed to guarantee a well-defined film with a uniform structure and minimum contamination from Al and additives in the precursor solution. Quantitative NR and XRR analysis of the EA-TCP film confirmed linear growth as a function of deposition time. By analyzing both NR and XRR data on as-prepared and dried films, the film composition was determined to be Cr(2)O(3) x iH(2)O x x(ZrO(2) x jH(2)O) (i = 2.10 +/- 0.55, j = 1.60 +/- 0.45, and x = 0.85 +/- 0.14).
