Formation of Composite Coatings of Layered Double Hydroxide Particles and Hydroxide Gel by Electrophoretic Deposition on Mg Alloy and Corrosion Resistance

Abstract

To improve the corrosion resistance of Mg alloys, composite coatings of layered double hydroxide intercalated with carbonate (LDH-CO 3 , hydrotalcite) particles and magnesium and aluminum hydroxide gel were formed on AZ31 Mg alloy by electrophoretic deposition (EPD) using the LDH-CO 3 suspension with Mg(NO 3 ) 2 and Al(NO 3 ) 3 in total of 1.25125 mmol/L. Wet-dry cyclic corrosion tests were performed for the coated specimens after ultrasonication. LDH-CO 3 particle-rich layer and magnesium and aluminum hydroxide gel-rich layer almost alternately piled up to form a composite layer. The hydroxide gel adhered the particles together and to the substrate. At the bottom of the composite layer, flake-shape LDH-structured substance densely covered the substrate surface. The coverage and thickness of the composite layer increased from sub-micrometers to 30 µm with an increase of Mg and Al ion concentrations of the suspension. By ultrasonication, parts of the composite layer delaminated to expose the LDH-structured substance layer, while the coverage by the remaining composite layer increased with an increase of Mg and Al ion concentrations. The LDH-CO 3 particles and hydroxide gel composite coatings prevented corrosion initiation even in the areas where the LDH-structured substance layer was exposed.

Corrosion Protection Behavior of Aluminate Ion-Loaded Layered Double Hydroxide Coating on AZ31 Magnesium Alloy

“…7b) are asymmetrical owing to a shoulder peak on the lower angle side which is attributed to LDHNO 3 as reported in our previous work. 29 Formation of LDHNO 3 is also suggested by the result that it was formed in the Mg(NO 3 ) 2 and Al(NO 3 ) 3 electrolyte without LDH particles by electrodeposition as shown in Fig. S8.…”

“…6j) as previously reported. 29 Such a preferential deposition of the gel-like Mg-Al-DH at the bottom of the LDHAlO 2 coating is not significant on the cross-sectional image, but the formation of the thin dense inner layer is suggested by the surface observation (Figs. 6i and 6g).…”

“…Formation of LDHAlO 2 and Mg-Al-DH composite coating.-The composite coating of LDHAlO 2 or LDHCO 3 particles and Mg-Al-DH was formed by the EP-co-D in the similar manner to the previous work. 29 The LDHAlO 2 coating was coarser than the LDHCO 3 coating as shown in Figs. 6i and 6j.…”

“…[26][27][28] We therefore employed the EP-co-D method and developed composite coatings of LDHCO 3 particles and magnesium and aluminum double hydroxide (Mg-Al-DH) on Mg-3Al-1Zn (AZ31) alloy. 29 Another finding of this previous study was that LDHNO 3 formed the bottom layer in the EP-co-D coating. Another finding was that submicron to micron pores were formed in the EPco-D layer by H 2 gas generated by the electrolysis of water in the 80 vol% ethanol-20 vol% water mixed electrolyte.…”

“…Another finding was that submicron to micron pores were formed in the EPco-D layer by H 2 gas generated by the electrolysis of water in the 80 vol% ethanol-20 vol% water mixed electrolyte. 29 Recently, the water content of the electrolyte was decreased to 2.4 vol% to reduce the pore formation. 30 The purpose of this study was to develop a corrosion inhibitorloaded LDH coating on Mg alloys.…”