| The rare-earth conversion coatings treatment technology is one of the most potential development directions of non-chromium conversion coatings treatment technology. However, researches on this technology are mainly focused on the aluminum and alloy, the zinc and alloy rare-earth conversion coatings technology has not been investigated systematically. The present study explores mechanisms of coating-formation and anti-corrosion on the basis of process researches, building up essential data of the rare-earth conversion coatings treatment technology.On the basis of the preliminary work, the rare-earth conversion coating was prepared on the surface of zinc coating using immersion method taking Ce(NO3)3·6H2O as main salt and H2O2 as the oxidant in the acidic atmosphere. Through theoretical calculations, it was shown that the pH values of zinc coating and liquid interface were up to 9.95, and could meet the depositions of Zn(OH)2, Ce(OH)3 and Ce(OH)4 at the interface, indicating the feasibility of process study. Process researches were investigated through combination method of orthogonal test and single factor experiments, using test time of the coating resistance to neutral salt spray as the evaluation standard, developing rare earth conversion coating process specification with characteristics of uniform bright appearance, resistance to neutral salt spray time up to 36 hr as following:Ce(NO3)3·6H2O 30g·L-1; H2O2 15ml·L-1, auxiliary film-forming promoting agent 4g·L-1; the promoter 0.5 g·L-1; H3BO3 2.5g·L-1, pH 2.0～3.0; solution temperature 15-45℃; immersion time 2-3 min; slot times 5-10 s.It is of great significance to the efficient formation of rare earth conversion coating with high corrosion resistance through the addition of oxidant and promoter of film-formation in the process formula. In the light of electrochemistry test, morphology analysis and dynamics data calculation, reaction mechanisms of oxidant and promoter of film-formation were elucidated respectively as following:micro-cathode reaction was speeded up by the addition of H2O2, resulting in the increase of pH for interface of micro-cathode region, which was favorable to the formation of metal surface hydroxides precipitation coating; the growth of conversion coating was realized by lowering the apparent activation energy for three stages of coating-formation reaction through the addition of promoter of coating-formation.According to the process formula developed, XPS and XRD analysis results of conversion coatings based on rare-earth, thermodynamics and dynamics analyses of film formation, and film-formation reaction mechanism simulated by quantum chemistry, the film-formation mechanism of rare earth conversion coating was concluded as following: due to the existence of crystal defects on the surface of zinc coating, which were a electrochemical uneven area with high energy, resulting in the formations of micro-batteries easily in the treatment liquid containing H2O2, taking place of reaction of zinc oxidation, H2O2 reduction and oxygen depolarization, the O in H2O2 and Zn on the galvanized zinc surfaces formed chemical bond through adsorption mode, and adsorbed on the surface of galvanized zinc in the form of OH- through the O-O bond rupture; O2 was adsorbed on the surface of zinc coating, giving rise to the breakage of O-H in H2O adsorbed on the surface of zinc coating and adsorbed in the form of HO2-, and furthermore reacting with the adsorbed H2O, occurring O-O bond rupture, and finally adsorbed in the form of OH- at the interface, resulting in massive production of OH- in the micro-cathode area of zinc coating interface. H2O2, as oxidative agent, could oxide Ce3+ to Ce4+ in treatment solution, therefore, when pH in micro cathode area reached a certain value, Zn(OH)2, Ce(OH)3 and Ce(OH)4 would deposit in areas with crystal defects, forming nucleus. With increasing of deposits, they diffused into around to form the basement coating; co-existing with dissolving of coating during the growth of the basement coating, growing up and covering the galvanization zinc surface finally, forming the rare earth conversion coating comprising of hydroxide deposition coating. Due to pH value of Ce4+ deposition is less than that of Ce3+, Ce4+ takes precedence over Ce3+ depositing on the interface, the valence of Ce in Ce conversion coatings is mainly +4. The hydroxide deposit would dehydrate to form stable oxide compounds and Ce3 oxidized to Ce4+ under the conditions of rare earth conversion coating being in dry medium or stocking process, forming composite coatings which were composed of ZnO, CeO2, Ce2O3, Zn(OH)2, Ce(OH)3 and Ce(OH)4. The growth of conversion coating was divided into three stages of the fast growth, slow growth and smooth growth. The apparent activation energy of the first stage was smaller, giving rise to the fast reaction of coating-forming; while the apparent activation energy of the second stage was biggest, resulting in the slower reaction of the coating-forming; the apparent activation energy of the third stage decreased, coating formation reaction and coating dissolving reaction tended to reach equilibrium in the competition, coating-formation reaction had lower activation energy; growth rates for various stages and the quality of conversion coating per unit area met the exponent law.Researches on the corrosion resistance mechanism of rare-earth conversion coating were based on investigations of coating corrosion resistance. By using immersion experiments, the neutral salt spray tests and electro-chemical testing methods, etc; comparison studies of corrosion resistance performances for rare earth conversion coatings, zinc coating and low-chromium conversion coating, it was shown that the rare earth conversion coating could significantly improve the corrosion resistance of zinc plating, whose corrosion resistance was better than that of low-chromium conversion coating. The corrosion resistance mechanism of rare-earth conversion coating was concluded by using microstructure analysis, XRD testing and calculations of corrosion kinetics data as follows: Rare-earth conversion coating was compact film which was formed by the accumulation of tiny fine particles without cracks and less defects, and the coverage of coating on the substrate of galvanized zinc layer, hindering O2 transmission and electronic transmission, inhibiting both reactions of anodic and cathodic of corrosions, lowering corrosion kinetics and protecting matrix from corrosion effectively, improving the corrosion resistance performance of the matrix. The occurrence of corrosion reaction of coating was due to the non-uniformity of the coating on chemistry and physics for microstructure, the apparent activation energy of conversion coating in 5% NaCl solution is greater than in 0.5% H2SO4 solution, showing that it was easier to occur corrosion for rare-earth conversion coating in 0.5% H2SO4 solution.The conversion coating must have good comprehensive performances when being used in the actual production. It was demonstrated through tests of adhesion, hardness and roughness of rare-earth conversion that rare-earth coating prepared under the optimized processing time, binding force and swelling stress between the coating and the substrate reaching equilibrium, having better binding effect, the best overall adhesion and the smallest surface roughness fluctuation, the film has characteristics of good uniformity, compactness and smooth. The formation of the conversion coating could enhance Vickers hardness of the galvanized zinc layer, which was favorable to improving the wear resistance of galvanized zinc parts, strength and service life.Zinc coating of rare earth conversion coating processing technology could be applied successfully to process Zn-Fe alloy coatings (iron content of 0.4-0.7%), rare earth conversion coating with characteristics of luminously smooth and good corrosion resistance could be prepared on the surface of Zn-Fe alloy coating, its corrosion resistance was similar to the low chromate conversion coating.Through present thesis research, it was shown that basic data of rare earth conversion coatings techniques could be built up, and a meaningful exploration for the replacement of the chromate treatment technology was carried out.
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