Developments Of Environmentally Friendly Chromate-Free Conversion Coatings On Zinc Based Matirials

Zinc or zinc alloy coatings on steels applied by electrodeposition or hot dipping are wildly used to give protections for steels. The passivation process of zinc is a usual treatment for the galvanized steels. In conventional practice, a very popular way to reduce the corrosion rate of zinc was the use of chemical conversion layers based on hexavalent chromium ions, which can increase the passivation tendency of the zinc. This procedure is quite effective also for improving the adhesion of organic coatings deposited on the zinc surface, but there is a serious environmental problem associated with the use of Cr6+ in the formation of chromate conversion films for galvanized steels because of the high toxicity and the strong carcinogensis of the Cr6+ ions. So it is necessary to investigate alternative methods for corrosion protection for the use of chromium salts, which are now restricted by environmental protection legislation all over the world.In this paper, environmentally friendly chromate-free conversion coatings will be developed on zinc surface in comparison with traditional chromate treatments. Molybdate, titanium salt and cerium salt were selected as inorganic passivators, and silane coupling agents (SCA) include KH-792, KH-560, KH-570 and BTSE were selected as organic passivators. What is more, compound passivation between organic and inorganic passivators was investigated. The results showed that the cerium based conversion film was relatively better than molybdate or titanium based conversion film, and the compound effect between organic and inorganic passivators could increase the corrosion resistance to some extent. Finally, a new conception, "self-healing efficiency" was proposed to evaluate the self-healing abilities of films. Here are the main works we done.(1) Molybdate passivation system was investigated. The effect of the phosphate radical, sulfate radical or nitrate radical in conversion solution were studied, and the result showed that molybdophosphoric heteropoly acid was formed by phosphoric acid and molybdate, yet sulfate radical or nitrate radical exhibit no such performance. Moreover, some additives such as tannic acid or phytic acid were confirmed that they can increase the corrosion resistance as well as the ornament of the passivation films. Finally, the mechanism of the forming of molybdophosphoric heteropoly film was discussed.(2) As a kind of inorganic passivator, titanium trichloride, titanium tetrachloride, titanyl sulfate and titanium sulfate were investigated, and the contributions of each constituent in conversion solution were stuied. The result showed that the conversion film formed on zinc by the relatively cheap titanium sulfate was no less than titanium trichloride or titanyl sulfate, and generally, conversion coatings form by titanium salt exceeded that of molybdate.(3) Two kinds of quite different cerium based passivation methods were applied to form conversion coatings on zinc, one is carried out in an acidic conversion solution with a very fast passivation procedure less than 1 min, the other is carried out in a neutral conversion solution with a very long passivation procedure more than 1 week. The result showed that the conversion film formed in the acidic solution compare unfavourably with the one formed in the neutral solution. However, the passivation time of the latter method is too long to be applied industrially. Several feasible solutions to this problem therefore were studied to reduce the passivation time, and aeration was found to be a relatively better method.(4) In this part, silane coupling agents were used as passivators, and four kinds of SCA include KH-792, KH-560, KH-570 and BTSE were investigated befor and after they compound with three kinds of inorganic passivators include molybdate, titanium salt and cerium salt. The compound effect between SCA and inorganic passivators changed the microstructural aspects of the films, and in the meantime, increased the corrosion resistance of the conversion films. But which worth the whistle is that the corrosion resistance of the film raised by the compound effect depends largely on the sort of SCA and inorganic passivators as well as the mixture ratio between them.(5) The self healing ability of chromium include and chromium free conversion coating was investigated, and a new conception, "self-healing efficiency" was proposed to evaluate the self-healing abilities of films. The result showed that the self-healing efficiency of all chromium free conversion coating was far below the conventional Cr6+ conversion film.