The inhibition of electron-transfer processes on aluminum alloy via chromate ion

Localized corrosion of aluminum alloys is problematic. The susceptibility to such corrosion is linked to the existence of intermetallic phases that generate galvanic couples on the alloy surface. Corrosion inhibition is accomplished by the use of inhibitors containing chromate ions (Cr VI) to form chromate conversion coatings on the alloy surface. The action of chromate ion is superior but due to the toxicity of chromate an impetus exists for its replacement with more benign alternatives. However, the mechanism of chromate inhibition is not well understood.;To gain insight into the inhibition mechanism of chromate, a separation of corrosion reactions approach was taken. Galvanic couples of pure metals and aluminum alloy 2024-T3 in various combinations were used to approximate the local corrosion processes taking place on the alloy surface. The samples were separated into separate chambers to allow for independent control of environmental conditions at the anode and cathode and the current between the electrode was measured with a zero-resistance ammeter. CrVI was shown to strongly inhibit cathodic reactions both in solution and as a pretreatment, indicating persistence of the effect. Addition of chromate resulted in a current transient attributed to CrVI reduction to Cr III prior to inhibition. Solution-phase chromate exhibited anodic inhibition of passive current but not at potentials positive of breakdown as explained with mixed potential theory.;Interaction of chromate with electrode surfaces was examined with voltammetry. Chromate was shown to inhibit many reduction processes on several electrode materials, including systems that require adsorption and outer sphere systems. This is achieved by the reduction of CrVI to CrIII and subsequent immobilization on the electrode surface. The surface layer is in near-monolayer quantities and the reduction reaction occurs at potentials well positive of in-field conditions experienced by aluminum alloy 2024-T3.;Replacements for chromate ion based inhibition were examined using information gained about the inhibition mechanism of chromate as a guide. Diethyldithiocarbamate and diazonium salt modification of various electrode surfaces was performed and examined by Raman spectroscopy. Voltammetry and galvanic couples were used to evaluate the replacement candidates. Specific interaction with intermetallic sites on AA 2024-T3 was observed.