Surface chemistry and corrosion behavior of aluminum-copper systems: Air-formed films to complex conversion coatings

Understanding the mechanism of corrosion inhibition by carcinogenic chromates is critical to the development of environmentally safe coatings containing benign chromate substitutes. An integrated approach to correlate the surface chemistry and corrosion behavior of a wide range of systems has been undertaken. Electrochemical behavior was studied by open circuit potential (OCP) measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Surface chemistry was studied using variable-angle X-ray photoelectron spectroscopy (VAXPS), X-ray absorption near edge spectroscopy (XANES), secondary ion mass spectroscopy (SIMS), infrared spectroscopy and synchrotron infrared micro spectroscopy (SIRMS) and Raman spectroscopy.;Using SIRMS, the ASTM recommended acetone degreasing was shown to initiate pitting of AA2024-T3 via photochemical formation of acetic acid. Due to the known tendency for photoreduction of Cr6+(3d0) following soft X-ray dosage during XPS, a novel method has been developed to prevent this reduction. This method yields, for the first time, an accurate determination of the Cr6+ content of a CCC.;The pretreatment of the alloy prior to conversion coating has been shown to have significant influence on the surface intermetallic distribution, composition and corrosion resistance of the initial oxide film and subsequent conversion coating. AlconoxRTM pretreatment was found to result in a highly protective surface film that inhibits the subsequent formation of CCC. The study also shows that coupling of the alloy to platinum during the bromate pretreatment increases the corrosion resistance of the subsequently formed CCC by over an order of magnitude due to reduction in surface copper content.;Adsorption of chromate ion on the passive oxide film formed on the metal surface was observed to induce fixed negative charges that inhibit chloride ingress on planar surfaces. While deprotonation of the aluminum hydroxide film by chromate was found to lead to corrosion resistant aluminum oxide, deprotonation induced by chloride ion resulted in the formation of a lower corrosion resistant aluminum oxyhydroxide.;CCCs are considered to have the unique ability to repassivate the surface following a mechanical damage of the corrosion protective coating. Direct evidence for repassivation of AA2024-T3 by chromates has been provided for the first time using SIRMS, OCP measurements, SIMS, Raman spectroscopy. For the first time, molybdate ions have been found to not only repassivate mechanical damage but at rates significantly faster than chromates and hence represent a potential candidate for benign coating formulation. (Abstract shortened by UMI.)...