| Chromate conversion coatings(CCCs) have been used for many years to protect metals from corrosion and to improve the adhesion of paints and lacquers. They are highly effective,but unfortunately the Cr(VI) chemicals used in the conversion treatment are now known to be carcinogenic. This toxicity necessitates a search for alternative treatments. Ti and/or Zr based conversion coatings have been regarded as one of the most prospective alternatives to chromate conversion coating, which had been used in industry for a long time. A fast and high efficient Ti-Zr based conversion process was prepared on the surface of 6063 aluminum alloy by immersion in a fluotitanate and fluozirconate acid based solution assisted with film-forming accelerator and organic compound. The chemical composition, growth behavior,formation mechanism and corrosion resistance of the colored Ti-Zr conversion coatings were analyzed in this thesis.The modified golden conversion coating has been prepared by adding organic and oxidant in the Ti and Zr treatment solution. The treatment time of the coating formation decreased from 20 min to 1 min. The optimal passivation technics was prepared by Orthogonal experiment and Single factor experiment as follow: H2TiF6 2.0 g/L, H2ZrF6 0.8g/L, oxidant 2.5 g/L, complexant 2.0 g/L, t 60 s, T 25~30 ℃.The colored Ti-Zr is a multi-components chemical conversion coating which is mainly composed of metal oxide(TiO2, ZrO2, M2O5, Al2O3), fluoride(AlF3 and ZrF4) and metal-organic complex. Oxidant take part in the formation of the coatings, and accelerated the whole oxidation reduction reaction, so as to speed up the formation process.With increasing treatment time, the cracks on the surface of colored Ti-Zr coating were increased gradually and spread finally over the entire surface of the conversion coating. The coatings treated for 30 s to 60 s were desirable from the surface observation.The silicon-rich intermetallics present in 6063 aluminum alloy are cathodic active region on which the deposition of the colored Ti-Zr coating occurs predominately. The growth process of the coating can be divided into two direction(vertical and lateral) : At the initial time metal oxidation were deposited predominantly on the silicon- rich intermetallics, then organic complex and metal oxidation and fluoride are co-deposited on surface(vertical); Withthe increase of immersion time, the coatings component were deposited in the region surrounding the silicon- rich intermetallics, and then covering the entire surface progressively(lateral).Adding oxidant and complexant simultaneusly causes a colored Ti-Zr conversion coating to form in 60 s, and the coating is compact, uniform, smooth and contains more oxide. In addition, as compared with the coating without any additives, the coating with oxidant and complexant as additives simutaneously achieves the anti-corrosion time, polarization resistance and impedance increases by more than 3, 3 and 150 times respectively, and a corrosion current density decreased 5/6, which indicates that the corrosion resistance is significant improved.With the increase of immersion time, the protective property of the conversion coating is improved from 10 s ~ 60 s and become into poor from 90 s ~ 240 s which resulted from the development of the cracks on the conversion coatings. The results of EIS Nyquist plots and fitted parameters for substrate and treated samples with different treated times reveal that the impedance values increase gradually with the treatment time, reaching peak value at the treatment time of 60 s. The impedance for the 60s-treated specimen(1500kΩ·cm2) is three orders of magnitude higher than that of the untreated substrate(2kΩ·cm2). This is attributed to the colored Ti-Zr coating functioned as a barrier layer, hindering the transport of the electron charge ions. And the coating with higher concentration of oxidant exhibits higher corrosion resistance. The impedance reached the peak value when the concentration of oxidant was2.5g/L, which was much higher than that of the aluminum alloy.The corrosion process of the colored Ti-Zr immersed in 3.5% NaCl solution is divided into three stages: At the early stage of the corrosion(within 72 h), dissolution of the colored Ti-Zr coating is mainly occurred; Corrosion medium(72 h~168 h), electrolyte solution diffuse into the gap in the micro-cracks on the coatings; At the last stage of the corrosion(360 h), the corrosion of aluminum alloy became the dominate reaction.The scratching corrosion test showed that the colored Ti-Zr had good self-sealing ability.And the new coatings on the scratches are mainly consist of M5+ oxide or hydrate oxide.The soluble metal hydrates and the insoluble coatings near the scratches formed a protective barrier on the scratch, which protects the aluminum alloy from corrosion. The soluble M5+ is migrated into the surface on the scratch from near or bottom of the coating. The soluble metalions in the coating near the scratch is reduced resulted in the performance of self-healing decreased. The higher content of soluble metal in the coating, which contains more soluble M5+can diffuse into the damage area, obtained from the higher concentration of oxidant in the conversion solution. This results in an excellent self-healing performance of the coating.According to the observation results, the model of the self-healing of colored Ti-Zr coating is put forward. |
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