Characterization of Zr(IV)–Phosphonate Thin Films Which Inhibit O[sub 2] Reduction on AA2024-T3

Abstract

Zr͑IV͒-alkyl-phosphonate and Zr͑IV͒-aryl-phosphonate thin films of varying compositions were investigated for corrosion inhibition capabilities on AA2024-T3. A strong correlation between the alkyl chain length and the ability of these self-assembled systems to inhibit the O 2 reduction reaction at the alloy surface was found. In particular, modification using Zr͑IV͒-octadecylphosphonate initially inhibits O 2 reduction currents by more than 2 orders of magnitude in 0.1 M Na 2 SO 4 at a potential of Ϫ0.80 V vs a saturated calomel electrode and by greater than 95% after 5500 s under similar conditions. Scanning electrochemical microscopy reveals uniform inhibition of cathodic processes. Auger electron spectroscopy confirms the presence of Zr and P in the thin film. Infrared reflection-absorption spectroscopy indicates that the alkyl chains are in a liquidlike environment. Long-term testing of the Zr͑IV͒-octadecyl-phosphonate system in a sulfate-containing solution yields low levels of surface activation, as evidenced by small amounts of trenching adjacent to intermetallic particles and little change in near-surface Cu content.Aluminum alloys commonly used in the aerospace industry, such as AA2024-T3, have many advantageous mechanical characteristics due to the presence of intermetallic particles ͑IPs͒. However, these same particles, especially those rich in Cu, lead to an undesirable corrosion behavior. The reaction of AA2024-T3 with dioxygen is driven by the ability of Cu-rich IPs to behave cathodically of the bulk alloy, allowing the transfer of electrons needed to reduce dioxygen. 1-6 Under neutral and alkaline conditions, dioxygen reduction at IPs results in the production of hydroxide yielding a localized high pH, which can lead to the dissolution of the oxide near the IPs. 6,7 Once the dissolution of the surface oxide has progressed to a certain point, oxidation of Al in the underlying alloy occurs, resulting in the liberation of Al 3+ -containing species into solution or in the formation of a passive oxide layer. 7,8 Equations 1-3 show the redox reactions associated with these processes, which ultimately can lead to mechanical failureChromate conversion coatings ͑CCCs͒ have been widely used as a means of corrosion inhibition on Al alloys. CCCs prevent O 2 reduction through the formation of impermeable Cr oxide layers at the alloy surface which, under some conditions, are capable of selfhealing over damaged areas on the alloy surface. 9-12 Unfortunately, the positive aspects of CCCs are negated by the possible exposure to Cr͑VI͒, which has been shown to have deleterious health and environmental effects, especially during application and repair operations. Thus, environmentally benign corrosion inhibition systems are required. Primarily, the search for such systems has focused on surface coatings that contain Ce oxide and other metal oxides capable of oxygen reduction reaction ͑ORR͒ inhibition, 13-21 mixed Cephosphate coatings, 22-24 conducting polymers that passivate the alloy surface, 25-27 and self-assemble...

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