The Study of Green Trivalent Chromium Conversion Coating Process on Aluminum Alloys

• Main Authors: Wei-Kun Chen, 程偉堃
• Other Authors: Ming-Der Ger
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/45675065694208991081

博士 === 國防大學中正理工學院 === 國防科學研究所 === 99 === The main purpose of this study is to develop the trivalent chromium conversion coatings with excellent anticorrosive performance on aluminum alloys at room temperature. To identify the formation mechanism of trivalent chromium conversion coatings, the selection of additives in conversion solution, analyzing in coating formation process, and the inspection of coating properties are performed and discussed in this study. Several materials examination and electrochemical measurement instruments, such as TEM, SEM, XPS, SIMS, LP, and OCP, are used to analyze the characteristics of conversion coatings. The results demonstrate that the difference in acid radical of the trivalent chromium conversion solution would cause the variations of formation mechanism and structure of trivalent chromium conversion coatings. The LP analysis shows that the trivalent chromium conversion coatings are incomplete as a result of the insufficiency in the driving force of conversion reaction when the conversion bath contains only one type of trivalent chromate. Therefore, the trivalent chromium conversion coatings produce in this case cannot inhibit the corrosion of substrates. The addition of accelerant to the conversion bath considerably promotes the oxidation reaction of aluminum alloys, which results in the elevation of pH at the interface between the solution and substrate, and increases the concentration of hydroxyl. Afterward, plenty of hydroxides and oxides deposit on the surface of aluminum alloys. According to the SEM observation, the surface morphology of conversion coatings produced with accelerant is different from without accelerant. The coatings formed in the solution with NaF shows a sheet-like surface structure. The addition of K2ZrF6 to the conversion bath results in a lot of cracks on the coatings. Nevertheless, the surface morphology of the coating formed in the solution with NaBF4 is similar to that without NaBF4. The XPS depth profiles of the coatings indicate that the addition of NaF and K2ZrF6 to the conversion bath would improve the capacity of scale growth and change the composition of coatings. However, none of coatings is formed on aluminum alloys, while NaBF4 are added to the conversion bath, indicating that this agent would not advance the oxidation of aluminum alloys. The results of LP curve show that the corrosion current density of conversion coatings obviously decreases when the coatings formed in the solution with NaF and K2ZrF6. However, the addition of NaBF4 would not improve the anticorrosive performance of aluminum alloys. The study of composition effects on the formation mechanism of trivalent chromium conversion coatings represents that the fluorine ions would be liberated as a result of the heterolysis cleavage of K2ZrF6 during the coating formation processes. Afterward, the fluorine ions promote the dissolution of aluminum alloys and result in the formation of conversion coatings. Apparently, the K2ZrF6 acts as the accelerant and major agent in the conversion reaction. However, the dissolution rate would be higher than the growth rate of coatings when aluminum alloys are conversed in the bath without Cr2(SO4)3. From the microstructure and composition analyses, the Al-Cr-Zr-F compound is existed in conversion coatings when the conversion bath contains with chromium, which would stabilize the formation reaction. Moreover, the addition of NaBF4 to conversion bath would activate the aluminum alloys, and facilitate the electrolyte penetrate though the conversion coatings and react with the aluminum alloys continuously. For the investigation of coating formation mechanism, the OCP results demonstrate that the potential change obviously as a result of the conversion coatings with a porous structure or diffusion channels at the initial conversion period of 0~300 s. However, the anodic reaction is restricted due to the conversion coatings converting into a dense and uniform structure when the conversion time exceeds 300 s. The SIMS depth profiles of conversion coatings show that the coating composition is changed by dissolution reaction when the conversion time exceeds 600 s. The cross-sectional TEM image indicates that the cavities and cracks are generated in the coatings when the conversion reaction proceeds for a long time. These defects would decrease the anticorrosive performance of conversion coatings. The result of SST indicates that the Cr-oxides are still existed in the coatings even if the coatings suffer the attack of corrosive agents. A fresh trivalent chromium conversion coating would be formed during the salt test, which mainly comprises the sustained Cr-oxides with new dissolute aluminum ions. Nevertheless, the anticorrosive performance of the coating without Cr-oxides is worse, and the dissolution reaction of the coatings would occur during the coatings suffering the attack of chlorine ions (Cl-).