Zinc soap: a new 'green' conversion treatment for zinc

摘要

The objective of this study is to develop a new surface treatment of zinc, based on metallicrnsoap, which could be an environmentally friendly alternative to the usual conversionrntreatments of galvanized steel products for a temporary protection. This treatment requires thernuse of both n-alkyl carboxylic acid (heptanoic acid for example) extracted from vegetable oilsrnand an oxidizing agent. As in the phosphating process, the surface treatment is carried out inrnthree steps: degreasing, oxidation of the zinc substrate into Zn2+ cation, then precipitation ofrnzinc carboxylate crystals.rnCoating can be realized in a solution containing a chemical oxidant and heptanoic acid,rnsolubilized in a water-ethanol solution. In the present study we have tried to remove bothrncosolvant and oxidizing agent using electrochemical oxidation of heptanoic acid – sodiumrnhydroxide mixtures, having different pH values. For that, the electrochemical behaviour ofrnzinc in heptanoic acid solution at different pH was studied. The analysis of the intensitypotentialrncurves allowed us determination of the oxidation potential for which a protectiverncoating is obtained in these media. According to the pH and the oxidation potential, therndeposit growth in few seconds and two products can be present on the samples: zincrnheptanoate and zinc hydroxyheptanoate. EIS, stationary electrochemical techniques andrnclimatic chamber tests were carried out to characterize the protective properties of theserndifferent coatings.rnFinally, the results are very promising and this process appears to be a versatile "green"rnconversion treatment of zinc. It easily allows deposition of both hydrophobic and corrosionrnresistant zinc carboxylate coatings. This kind of treatment does not require cosolvant andrnchemical oxidant, which improves the biodegradability of the bath treatment.