In situ growth process of Mg–Fe layered double hydroxide conversion film on MgCa alloy

In this study, an environment-friendly layered double hydroxide (LDH) film has been deposited on MgCa alloy by a two-step technique. To improve the chemical conversion technique and control the film properties, batch studies have been carried out to address various process parameters such as pH value, treatment temperature and immersion time. The chemical composition was determined by X-ray diffractometry and energy dispersive X-ray spectroscopy. The morphology was characterized by scanning electron microscopy. The corrosion resistance of the samples with various films was compared by polarization curves and immersion test. It is found that the transformation duration of Mg–Fe LDH is long. Too high pH value and temperature has harmful effect on the purity of the film composition. The corrosion resistance of the films formed at low value of pH or high temperature or short treatment time is deteriorative. The optimum process is as follows: the sample is first immersed in the solution containing Fe3+/HCO3−/CO32− with a pH value of 5 at a temperature of 55 °C for 1 h to form a precursor film, and then this precursor film is immersed into the solution containing Fe3+/HCO3−/CO32− with a pH of 11 at 55 °C for 24 h to obtain the Mg–Fe LDH conversion film.