An Environmental Trivalent Chromium Conversion Coating On Aluminium Alloy And Its Corrosion Properties

AA2024-T3is widely used in the industry by its superior performances such as good mechanical property and high strength. Nevertheless, AA2024is susceptible to corrosion due to the strengthening phases, thus chromate conversion coatings have long been used in the surface finishing process for AA2024. However, because of the toxic effects of hexavalent chromium compounds,an environmental friendly preparation to develop different kinds of new chemical conversion coatings without hexavalent chromium is required increasingly. Given this, the paper use polarization curve, electrochemical impedance spectroscopy (EIS) and scanning electron microscope(SEM) analysis to study the corrosion and protection of AA2024-T3. The main conclusions are as follows:(1) AA2024-T3corrosion behavior was much serious in neutral3.5wt%NaCl solution with the extension of corrosion time, the electrode surface erosion degree was also increased by the concentration of Cl-.(2) The corrosion behavior in neutral NaCl solution process could be divided into three stages. At the initial stage, there existed obvious capacitive feature in the high frequency range of the EIS. The charge transfer resistance Rt increased rapidly and the film capacitance Cdl moved in a negative direction. Due to the aggression of Cl-, it was promoted the dissolution of alloy matrix quickly and occurred pitting corrosion. During the second stage, inductive component in the lower frequency range appeared gradually, Rt increased slightly and Cdl reduced accordingly. It manifested that corrosion film provided part protection for alloy matrix. During the later stage, Rt and Cdi kept stable, the form of corrosion transited local pitting corrosion type to the total uniform corrosion. Corrosion dynamic modeling was deduced, it suggested that there existed a transition state of bivalent Al ion during the corrosion process in neutral NaCl solution.(3) The main optimal preparation conditions of trivalent chromium coatings process(TCP)were as follows:Cr2(SO4)35g/L, K2ZrF62g/L, additive A1g/L, temperature40℃, bath pH4.0.(4) The coating capacitance Cdi fitting by the EIS only showed small amplitude repeated shocks from0to30hours; what’s more, the reaction resistance Rt improved several times than the corrosion resistance of uncoated alloy matrix under the same corrosion system. Results showed that the coatings formed on AA2024-T3surface can provide better protection.