In situ growth of Mg–Al hydrotalcite conversion film on AZ31 magnesium alloy

► It is the first time that a Mg–Al hydrotalcite film is prepared on the Mg alloys of low Al content. ► The environmentally friendly two-step in situ growth method can shorten the formation time to 2 h. ► This film is very compact and uniform, which can provide effective protection to the AZ31 alloy. ► The film formation mechanism was also proposed based on the OCP measurement and surface analysis. In situ growth of Mg–Al hydrotalcite conversion film on AZ31 alloy has been developed by a two-step method. The characteristics of the films were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electronic microscope (SEM) observation, electrochemical and immersion tests. The film formation process was proposed based on the open circuit potential ( OCP) measurements and surface analysis. A precursor film with network cracks is first formed and then this film is transformed into a compact and uniform hydrotalcite (Mg 6Al 2(OH) 16CO 3·4H 2O) film after the post treatment. This dense Mg–Al hydrotalcite film can provide effective protection to the AZ31 alloy.