Ultrathin, Layered Polyamide and Polyimide Coatings on Aluminum

Ultrathin, passivating films are attractive for protecting metal surfaces without completely masking substrate properties. Alternating polyelectrolyte deposition of layered poly(acrylic acid)/poly(allylamine hydrochloride) (PAA/PAH) films on Al produces ultrathin coatings that protect Al from Cl--induced corrosion. Although the resistance of these films is minimal, they form a passivating layer on the surface oxide and increase oxide resistance by 2−3 orders of magnitude. Heating of PAA/PAH yields cross-linked polyamides that have film resistances of about 5 MΩ cm2, even when the film is only 10-nm thick. Although the resistance of these films is high, reductions in corrosion current due to cross-linked films are still primarily due to passivation of the surface oxide as oxide resistance is larger than film resistance. Layer-by-layer covalent deposition of Gantrez/poly(allylamine) films yields ultrathin amic acid-linked layers that can be imidized by heating. Impedances of Al electrodes coated with Gantrez/poly(allylamine) films depend on the number of deposited bilayers and increase by an order of magnitude after imidization. These results suggest that increases in film resistance play a role in oxide passivation by this system.