Interface strength and degradation of adhesively bonded porous aluminum oxides

For more than six decades, chromic acid anodizing has been the main step in the surface treatment of aluminum for adhesively bonded aircraft structures. Soon this process, known for producing a readily adherent oxide with an excellent corrosion resistance, will be banned by strict international environmental and health regulations. Replacing this traditional process in a high-demanding and high-risk industry such as aircraft construction requires an in-depth understanding of the underlying adhesion and degradation mechanisms at the oxide/resin interface resulting from alternative processes. The relationship between the anodizing conditions in sulfuric and mixtures of sulfuric and phosphoric acid electrolytes and the formation and durability of bonding under various environmental conditions was investigated. Scanning electron microscopy was used to characterize the oxide features. Selected specimens were studied with transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy to measure resin concentration within structurally different porous anodic oxide layers as a function of depth. Results show that there are two critical morphological aspects for strong and durable bonding. First, a minimum pore size is pivotal for the formation of a stable interface, as reflected by the initial peel strengths. Second, the increased surface roughness of the oxide/resin interface caused by extended chemical dissolution at higher temperature and higher phosphoric acid concentration is crucial to assure bond durability under water ingress. There is, however, an upper limit to the beneficial amount of anodic dissolution above which bonds are prone for corrosive degradation. Morphology is, however, not the only prerequisite for good bonding and bond performance also depends on the oxides' chemical composition. Surface coating: interface strength and degradationChromic acid anodizing has been the dominant electrochemical process used to create a thin aluminum oxide layer upon aluminum alloys. Such thin layers are critical to the corrosion protection of alloys that enable many of our daily expectations, including aerospace applications. However, such a chromic acid treatment is being forced to be soon phased out as a result of the associated environmental and health concerns. Now a team led by Arjan Mol from Delft University of Technology in the Netherlands, with co-workers from Denmark and Belgium, are working on alternative treatment processes and reveal the fundamental adhesion and degradation mechanism at the interface between the surface oxide and an accompanying resin. Aided by both imaging and spectroscopic characterization, this study provides fresh insights into the interplay between the anodizing conditions and the formation and durability of bond strength, showing that the morphology and chemistry of the surface oxide are the two factors that should be considered in the selection of chromium-free surface treatments.