Doctoral dissertation
Synergistic action of organic inhibitors to mitigate corrosion of aluminium, copper and aluminium-copper alloy in chloride solution
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Abstract
The inhibitory action of organic compounds, 2-mercaptobenzimidazole (MBI), octylphosphonic acid (OPA) and their binary combinations was studied on aluminium, copper and aluminium alloy (AA) 2024 in chloride environments by ex-situ conventional electrochemical methods and surface-analytical techniques. In addition, the influence of pre-treatment of the metal surface and the choice of solvent for liquid-phase deposition on adsorption of MBI and OPA was studied on individual metals, aluminium and copper. Although OPA is not an inhibitor of copper, it can synergistically boost corrosion inhibition of copper when added to MBI. It is believed that the porous straw-like morphology of the [Cu-Cl-MBI]n overlayer together with the formed Cu2O underlayer was responsible for the excellent corrosion protection of copper. Unambiguous evidence is presented that the chloride ions act simultaneously as a promoter of polymerized [Cu-Cl-MBI]n film formation and a reactant incorporated in the film. In contrast, a synergistic effect between MBI and OPA as corrosion inhibitors was not observed for AA2024.
Localized electrochemical methods such as SVET and SECM were employed to study in-situ electrochemical activity on Al/Cu galvanic couple and AA2024. Anodic and cathodic reactions of Al/Cu were significantly suppressed when the copper surface was treated with an MBI inhibitor associated with the isolation of copper metal from aluminium due to the inhibitory effect of MBI on copper. Treatment of Al/Cu with OPA was not as effective as with MBI. SECM imaging in substrate generation-tip collection mode revealed the local concentrations of hydrogen gas over Al metal which is associated with the pitting corrosion. Immersion of AA2024 in an OPA-containing solution caused significant localized corrosion, while no local electrochemical activity on AA2024 was detected in an MBI-containing solution, indicating that the MBI inhibitor was very effective against pitting corrosion. Furthermore, the chemical analysis employed at the cross-section of the Al2CuMg revealed the MBI layer on the porous copper surface which together with the remaining Mg indicates that MBI reduces the dissolution rate of dealloying of this phase, but also the rate of oxygen reduction on the copper remnant sites.
The novel approach used in the present work involves investigating the corrosion protection properties of mixed inhibitors for copper and aluminium substrates in chloride-containing solutions, which will serve as a benchmark for further investigation of the aluminium alloys with intermetallic particles containing Cu being the main culprits for localized corrosion.