Properties of ketimine/acetoacetate coated aluminum substrates

Based on ketimine/acetoacetate coating technology, a new two-component high solids primer for aerospace use has been developed. The objective was to formulate a chromate-free primer that may be applied to all relevant aluminum surfaces, particularly after chromate-free pretreatment. During development, interesting indications of good adhesion and good corrosion resistance were found. For confirmation and in order to optimize the primer, research was focused on the interfacial performance of the coatings on typical aerospace substrates. In this paper, therefore, the nature of the substrates, including their pretreatments, is discussed with initial emphasis on the nitric acid pickling pretreatment, used in place of chromate pretreatments with their desirable properties. Although aging in relevant electrolytes of such pretreated aluminum leads to an increase of the thin oxide layer by further hydration normally influencing negatively interfacial performances, cross cut adhesion values of the primer, or the primer/topcoat, before and after aging in several electrolytes, were good on clad as well as on non-clad material. This was confirmed by microscopic morphology assessment during similar aging processes of the coating/substrate systems. The hydration of the nitric acid pickled layer was not found in the presence of primer, with or without topcoat. Using electron energy loss spectroscopy, indications of a distinct chemical bond between the primer and the nitric acid pickled aluminum were revealed. Electrochemical impedance measurements, determining film resistances of the various coated panels, also showed good responses when the primer was pigmented with a selected chromate-free inhibitor. Finally, in line with the previous indications and due to the special interfacial performance, in the presence of chromate-free pretreated substrates, filiform corrosion resistance with chromate alternative pigmentation approaches that of chromated primers.