In Situ Growth Process of Mg-Al Hydrotalcite Conversion Film on AZ31 Mg Alloy

In this study,an environment-friendly hydrotalcite film has been deposited on AZ31 Mg alloy by a twostep technique.To improve conversion film technique and control film properties,batch studies have been carried out to address various process parameters such as immersion time,pH value,and temperature of the treatment solution.The morphologies and chemical composition were characterized by scanning electron microscopy(SEM) and energy dispersive X-ray spectroscopy(EDS).The corrosion resistance of the samples with the various final films was then further compared by polarization curves.It can be concluded from the results that,at low value of pH,temperature,or long deposition time,the precursor film is mainly composed of a cracked layer.The transformation of hydrotalcite film is largely influenced by the pH value.There are optimum values of pH,temperature of the coating bath and immersion time for the film formation process to achieve the best quality and corrosion resistance of the hydrotalcite film.The optimum process is as follows:the sample is first immersed in the pretreatment solution with a pH value of about 8 at a temperature of 60 ℃ for 30 min to form a precursor film and then this precursor film is immersed into the post treatment solution with a pH of 10.5 at 80 ℃ for 1.5 h to obtain the Mg—Al hydrotalcite conversion film.