Comparing three strategies for surface treatment of Mg coated by hexamethylene diamine tetra methylene phosphonic acid for corrosion protection

Manufacturing Mg and Mg-based alloys with controllable degradation rate has always been a challenge. Surface modification is one of the best ways to acquire protection against corrosion. Previously, many approaches such as phosphating (Na3PO4), fluoridation (HF), and alkalization (NaOH) treatments have been used to regulate the corrosion rate. Here, we compare the corrosion rates of 3-surface modified Mg samples coated with hexamethylene diamine tetra methylene phosphonic (HMDTMPA). The protection of those different layers was ranked in the order: phosphate > alkaline > fluoridation treatment. The chemistry of those chemical conversion layers was discussed in the light of scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The mechanism of tailoring a protective film of HMDTMPA on phosphated Mg was discussed according to the ability of P to bridge the substrate with the coating.