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Study of the Preparation and Properties of the Trivalent Chromium Black Passivation with Non-cobalt Sulfate System

Author: ZhouLiQin
Tutor: KeChangMei
School: Wuhan University of Science and Technology
Course: Chemical processes
Keywords: zinc plating trivalent chromium black passivation system of non-cobalt corrosion resistance environmentally friendly
CLC: TG174.4
Type: Master's thesis
Year: 2011
Downloads: 138
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Abstract

Nowadays, the common technology corrosion protection for steel is galvanization followed by chemical conversion treatment. Unfortunately, the application of chromate passivation of zinc has been prohibited already and gradually at domestic and international, because of its toxicity, carcinogenic, disadvantage in man and the environment. Cr（Ⅲ） passivation possesses as good performance as Cr（Ⅵ） passivation, but the toxicity related to Cr（Ⅲ） passivation is only one percent of the latter one, thus Cr（Ⅲ） passivation will be the most acceptable technics instead of Cr（Ⅵ） passivation.Trivalent chromium black passivation technology as one of the trivalent chromium passivation process, which commonly used cobalt （or nickel） salts as the blackening agent. But under certain conditions, Co（Ⅱ） can be oxidized to Co（Ⅲ）,as Co（Ⅱ） is instability, which can win electronics from Cr（Ⅲ）, changing into a stable Co（Ⅱ）, leaving the Cr（Ⅲ） into Cr（Ⅵ）,in addition, the cobalt （or nickel） salts will cause permanent environmental pollution. Therefore, a new and environmentally friendly trivalent chromium black passivation technology is urgently needed, especially develop a new generation of black agents. The research project to explore a transition mental inorganic sulfide as the blackening agent, which replaced the existing nickel-cobalt salt as the black agents. This idea has been mentioned in the literatures, but basically had no one to try. In addition, the chromic acetate as the main passivation film salts, which is also nobody to try this method before.During the research of the non-cobalt salts passivation films, the corrosion resistance of passivation films is studied by Pb（Ac）2·9H2O drop test and weight lose method, compared with the optimal passivation without the passivation. The corrosion resistance of passivation films is analyzed by tafel polarization curves and electrochemical impedance spectroscopy measurement （EIS）.The composition structure and morphologies of coating are identified by electron microscope （SEM） and so on.In short, the passivation film based on potassium chloride galvanized obtained under above conditions was blackish and uniform. The passivation films can endure Pb（Ac）2·9H2O drop for 101s, the NSS results of the non-cobalt salts black chromium passivation film was 7 grade. The SEM indicated that the black passivation film forms on the galvanized steels are more homogeneous than the non-seal one and also had a thick black film. The XRD also showed that except the Zn crystal, other substances have appeared, they included CrPO4, CrOOH, Cr（OH）₃·3H₂O, Cr₂O₃ and Zn（OH）₂. The technological process is so simple that the unfamiliar operators can manipulate it. What’s more, the consumption of expenses is small during passivation process. Accordingly, it is a kind of perfect chromate-free passivation technique, which has promising prospect in industrial applications.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Corrosion control and protection > Metal surface protection technology
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