Corrosion resistance of Mg-Al-LDH steam coating on AZ80 Mg alloy: Effects of citric acid pretreatment and intermetallic compounds

In this study, the effects of intermetallic compounds (Mg17Al12 and Al8Mn5) on the Mg-Al layered double hydroxide (LDH) formation mechanism and corrosion behavior of an in-situ LDH/Mg(OH)2 steam coatings on AZ80 Mg alloy were investigated. Citric acid (CA) was used to activate the alloy surface during the pretreatment process. The alloy was first pretreated with CA and then subjected to a hydrothermal process using ultrapure water to produce Mg-Al-LDH/Mg(OH)2 steam coating. The effect of different time of acid pretreatment on the activation of the intermetallic compounds was investigated. The microstructure and elemental composition of the obtained coatings were analyzed using FE-SEM, EDS, XRD and FT-IR. The corrosion resistance of the coated samples was evaluated using different techniques, i.e., potentiodynamic polarization (PDP), electrochemical impedance spectrum (EIS) and hydrogen evolution test. The results indicated that the CA pretreatment significantly influenced the activity of the alloy surface by exposing the intermetallic compounds. The surface area fraction of Mg17Al12 and Al8Mn5 phases on the surface of the alloy was significantly higher after the CA pretreatment, and thus promoted the growth of the subsequent Mg-Al-LDH coatings. The CA pretreatment for 30 s resulted in a denser and thicker LDH coating. Increase in the CA pretreatment time significantly led to the improvement in corrosion resistance of the coated AZ80 alloy. The corrosion current density of the coated alloy was lower by three orders of magnitude as compared to the uncoated alloy.