The Consumption Rule And Supplementation Method Of The Ti/Zr Conversion Solution Applied On AA 6063

Chromate conversion coatings?CCCs?have been traditionally used to protect aluminum alloy from corrosion.Unfortunately,the hexavalent chromate Cr?VI?in conversion solution is highly toxic and seriously harmful for human and environment.Therefore,the development of chromium-free conversion technology is an inexorable trend.The Ti-Zr conversion treatment,which is considered to be a promising candidate to replace chromate conversion treatment,has been extensively studied in recent years.Nowadays,the technical study about Ti-Zr conversion treatment has become mature.The group of Wenfang Li has developed a colored Ti-Zr treatment process on aluminum and/or magnesium alloys,which has a great prospect of industrial application.However,little studies were carried out to investigate the utility and maintenance of Ti-Zr conversion solution,which is an important work for the industrial promotion of Ti-Zr conversion treatment.The objective of this study is to investigate the consumption of the Ti-Zr conversion solution during the continuous utilization process,which can guide the replenishment of the conversion solution for continuous treatment.This study is based on the colored Ti-Zr conversion treatment process at room temperature,quick coating forming speed,which were developed by our group.The consumption law of Ti-Zr conversion solution was investigated by quantitative analysis methods and a replenishing method was concluded accordingly,which would provide a theoretical basis for the industrial application of the Ti-Zr conversion coating technique.The main results and conclusion are as follows:The detailed process of the experiment was shown as follows: Firstly,1 L Ti-Zr conversion solution was prepared.The coatings were continuously formed on the substrates according to a fixed solution/surface area ratio?144 cm²/L?.The concentration of each component in Ti-Zr conversion solution was analyzed by ICP-AES and UV.The results showed that the consumption of different coat-forming agents in Ti-Zr conversion solution for the first 28.8 dm² area was large.After that,the consumption of each component was almost the same for every 28.8 dm² area.The consumption was 0.09 g for fluorotitanic acid,0.01 g for fluorozirconic acid,0.05 g for oxidizing agent M and 0.05 g for tannic acid.Based on these,a continuous automatic supplementation method was used to recover the film-forming ability of the Ti-Zr conversion solution.The details of this method are as follows: a home-made A supplementation solution of 50 mL was added to the Ti-Zr conversion solution after the treating of the first 28.8 dm² area,then B supplementation solution of 50 mL was added for following batch of each 28.8 dm² area.Besides,during the continuous film formation process,the concentration of aluminum ions in conversion solution is increased with an increase rate of 0.03 g per 28.8 dm² area.The single factor experiment of aluminum ion showed that when the concentration of aluminum ion in conversion solution exceeding 90 mg/L,the comprehensive performance of the film gradually deteriorated.Thus,the concentration of aluminum ions in conversion solution should be controlled within 90 mg/L or less.When the concentration of aluminum ion in the conversion solution exceeded 90 mg/L,the sodium fluoride should be added to remove the aluminum ion.The formation mechanism of Ti-Zr converison coating was studied by UV,IR and XPS.The results showed that the conversion coating was mainly composed of metal oxides?TiO₂,ZrO₂,M₂O₅ and Al₂O₃?,metal fluorides?Zr F₄,AlF₃,etc?and metal organic complexes.The complexation reaction of tannic acid and metal ion was considered to be the reason for the rapid formation of the colored titanium-zirconium film.