Lacquer adhesion to mechanically treated and conversion coated aluminium surfaces

Summary

The effects of surface pretreatment of aluminium on the adhesion of a chlorinated rubber lacquer have been examined using a relatively novel technique for direct generation and propagation of a blister near the interface between the lacquer and substrate. Assessment of the controlled growth of the blister allows ready differentiation of the poor performance of a mechanically treated substrate compared with a Chromate conversion procedure. This is believed to result from poor wetting of the mechanically treated adherend by the lacquer. Additionally, immersion of the lacquered substrates in sodium chloride solution for 8 days caused a further marked deterioration of the performance of the mechanically treated and coated substrate, which resulted from moisture uptake, through the lacquer to the metal surface, disrupting the bonding. Conversely, the chromate-conversion coated substrate appeared to improve after immersion in sodium chloride solution. The reasons for this behaviour remain unclear, but it is suggested that in situations of good adhesion, the test is influenced by changes in the mechanical properties of the lacquer coating.