Zinc carboxylating: a new conversion treatment of zinc

Abstract This study is devoted to a new surface treatment of zinc, called “carboxylating”, which could be an environmentally friendly alternative to the usual conversion treatments. Carboxylating requires use of both n-alkyl carboxylic acid and peroxoborate as oxidizing agent, in a water–ethanol mixture. As in the phosphating process, the carboxylating one is carried out in four steps: the activation step, oxidation of the zinc substrate at an acidic pH, germination and growth of zinc carboxylate crystals. Influence of the activator content, treatment time, oxidizing agent concentration, and chain length of the carboxylic acid was successively examined. Carboxylates covered zinc samples were tested in corrosion conditions. The longer the carbon chain length, the more corrosion resistant the coating; its layered structure easily allows exchanging anions and a hydroxocarboxylate of zinc is stabilized during the corrosion test then delays the white rust formation.