Defect formation and anode/cathode relationships in underfilm corrosion of heterogeneous aluminium alloys

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Coating deterioration and underfilm corrosion of organically coated (lacquered) alloys and metals is a complicated process depending on many variables, environment, coating, and substrate properties. For heterogeneous aluminum alloys, the situation is much more complex than for steel, and the mechanistic details are not fully understood. These alloys have numerous intermetallic phases differing in their electrochemical properties among each other, and from the aluminum matrix. The latter does not support cathodic reactions very well, whereas some of the intermetallic phases do. The aim of this project is to elucidate the mechanistic details of coating failure on such alloys, especially with respect to the influence of intermetallic compound phases on the initial stages of underfilm corrosion, the role of galvanic coupling on the corrosion propagation, the occurrence of local anodes and cathodes, the role of corrosion products and the coating deterioration linked to the corrosion processes. One major method is Confocal Laser Scanning Microscopy for three-dimensional in-situ monitoring of corrosion metrology developing underneath the coating and - using ion-sensitive fluorescent dyes incorporated into the coating - chemical changes during delamination at the alloy surface and within the coating. In addition quartz crystal microbalance measurements for the study of coating properties, localized electrochemical impedance spectroscopy to delineate the usually complex impedance response seen in underfilm corrosion, and (surface) analytical techniques to study the role of corrosion products and copper replating will be applied. Based on the results, detailed models for coating failure on the aluminum alloys shall be developed.