Chelating agent-assisted in situ LDH growth on the surface of magnesium alloy

T N Shulha,K N Lapko,S V Lamaka,M L Zheludkevich,D C F Wieland,M Serdechnova

doi:10.1038/s41598-018-34751-7

T N Shulha, K N Lapko + Show 4 more

Open Access

https://doi.org/10.1038/s41598-018-34751-7

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Abstract

In situ formation of layered double hydroxides (LDH) on metallic surfaces has recently been considered a promising approach for protective conversion surface treatments for Al and Mg alloys. In the case of Mg-based substrates, the formation of LDH on the metal surface is normally performed in autoclave at high temperature (between 130 and 170 °C) and elevated pressure conditions. This hampers the industrial application of MgAl LDH to magnesium substrates. In this paper, the growth of MgAl LDH conversion coating directly on magnesium alloy AZ91 at ambient conditions (25 °C) or elevated temperatures is reported in carbonate free electrolyte for the first time. The direct LDH synthesis on Mg alloys is enabled by the presence of organic chelating agents (NTA and EDTA), which control the amount of free and/or hydroxyl bound Mg2+ and Al3+ in the solution. The application of the chelating agents help overcoming the typical technological limitations of direct LDH synthesis on Mg alloys. The selection of chelators and the optimization of the LDH treatment process are supported by the analysis of the thermodynamic chemical equilibria.

Highlights

Magnesium alloys have attracted a lot of attention nowadays, thanks to the low density in combination with reasonable mechanical properties
In comparison with aluminium alloys, layered double hydroxides (LDH) synthesis on magnesium causes a lot of problems because of the high reactivity of the substrate leading to Mg(OH)[2] formation
The investigation and optimization of LDH formation on the surface of AZ91 was performed in the solutions containing either salicylic acid, EDTA, or NTA

Summary

Introduction

Magnesium alloys have attracted a lot of attention nowadays, thanks to the low density in combination with reasonable mechanical properties. Even strong passive protection cannot reliably serve for the full expected service life, since service-induced defects lead to the damage of the barrier This can result in a strong localized corrosion, starting at the defects and propagating under the coating at the layer-metal interface. Nakamura and Kamiyama have used LDH-based coatings for corrosion protection of magnesium alloy (AZCa612 and AZ31 respectively)[25,26] In both works, the formation of LDH film was achieved by steam-coating treatment, and improvement in corrosion behaviour was demonstrated. The formation of MgAl LDH-carbonate was optimized for the treatment of magnesium alloys, covered with PEO (plasma electrolytic oxidation) layer[34,35,36,37,38]. MgAl LDH, formed on the PEO-treated surfaces, helped overcome this problem (sealing of pores and cracks took place)

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