“Carboxylatation” coating on zinc: A chemical conversion in organized molecular systems containing carboxylic acid

Chemical conversion coatings on zinc surface based on hydrophobic carboxylic acid were developed and studied in this work. The long carbon chain carboxylic acid as heptanoic acid was solubilized or dispersed in different organized molecular systems (OMS) with the help of surfactant (Brij® 58, C16E20, HLB = 15.8). In a first step, the phase diagram Brij®58/HC7/water was established at 20 °C to determine the different one-phase or several phase domains, such as microemulsion, liquid crystal and emulsion. Several techniques were employed to characterize the phases and the localisation of the carboxylic acid in the OMS phase. he coatings, named “carboxylatation”, were performed on galvanized steels in the OMS bath containing an oxidizing agent. After the degreasing and etching steps, the zinc substrate is oxidized into Zn2+ cations which precipitate in zinc carboxylate crystals. Coating morphology and composition were characterized by SEM and XRD. Then, stationary electrochemical techniques and climatic chamber tests was used to characterize the protective properties of the different coatings. sults show that the stability of the OMS is necessary to assure a good “solubilization” or dispersion of heptanoic acid in water. Nevertheless, carboxylic acid should be easily liberated from the OMS to react on the surface. In our case, the best choice is the use of a milky “pure” emulsion that is thermodynamically unstable, but sufficiently stable in time to assure a good dispersion and an easy transport and liberation of carboxylic acid on the zinc surface.