|MadSci Network: Engineering|
I do not have any specific references but I have looked over several GM and Valeo painting procedures and talked with several painting engineers. I recommend that you call the company that manufactured your siding and ask for the laboratory. The chemists and engineers there have probably seen the phenomenom and if they haven't they will be excited about it. A search on IWON.COM for 'painting aluminum siding' and 'painting aluminum' gives many informative hits that are mostly anecdotal. Aluminum forms a hydrated oxide coating when exposed to air and moisture. Paint is usually non adherent to this coating and poorly adherent to alkaline treatments of this coating. The most effective surface treatments of aluminum are acid washes, phosphating or chromating treatments, acidic anodising [oxidation], and the method used by at least one siding manufacturer who uses several washing steps followed by a specific [probably acidic] primer and then the top coat. If the cleaning steps are compromised delamination of the paint can happen and poor cleaning is usually the cause [of these rare events according to one manufacturer]. Can there be an electro chemical cause? Yes. If the aluminum is anodic to a more noble metal such as iron or copper and there are electrolytic and electrical connections It is possible for the Al to corrode under the paint starting where the paint is breached and the metal exposed to water and air. When this happens it is usually obvious around the contact points but corrosion can happen at a distance if there is a electrolyte connection[liguid water]. If the Al is cathodic to a more active metal, only Magnesium comes to mind as a possibility, a galvanic reaction could reduce and destroy the adherent oxide layer and cause paint to peel. This would be noticed. An electric potential could substitute for the metals but the electrical and electrolytic connections would still have to exist. If the siding were subjected to an extremely high potential, say a lightning strike, dielectric breakdown could conceivably deatroy the adherent layer and cause the paint to falloff. Such a phenomenom would be similar to dielectric breakdown in a capacitor. Paint adherence problems are tough to diagnose at a distance and usually require considerable process and environmental study and surface chemical analysis to even arrive at a testable hypothesis let alone a complete solution. Talk to the siding manufacturer and their laboratory people.
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