Corrosion-resistant Mg(OH)₂/Mg-Fe layered double hydroxide (LDH) composite films on magnesium alloy WE43

Highlights

• Electrodeposition of approximately 3.6 µm thick Mg-Fe LDH coatings have been deposited onto magnesium alloy WE43.

• The precursor Mg(OH)₂ layer assists the formation of compact LDH film.

• The morphology and structure of the coating are examined in FESEM and TEM.

• Electrodeposited Mg-Fe LDH films reveal an enhanced corrosion resistance in Hank's balanced salt solution.

Abstract

Background

A new synthesis approach was developed for Mg-Fe layered double hydroxide (LDH) film on oxidised magnesium (Mg) alloy WE43 surface by cathodic electrodeposition.

Methods

The synthesised Mg(OH)₂ surface layer provides essential divalent cations and a foundation for the direct growth of an Mg-Fe LDH film through the electrodeposition process with trivalent cations containing solution.

Findings

The cross-section analysis results disclosed that the Mg(OH)₂ layer altered the typical morphology of LDH film, leading to structured LDH nanoplates with a thickness of approximately 3.5 µm. The formation of structured LDH nanoplates resulted from the perpendicular arrangement of the ab-faces of the crystallites to the WE43 surface, as seen from Field Emission Scanning Electron Microscopy (FE-SEM) images. Electrochemical tests in Hank's Balanced Salt Solution (HBSS) report the E_corr value increased by 0.24 V and a decrease in i_corr by one order of magnitude of Mg(OH)₂/Mg-Fe LDH composite films against its substrate. Furthermore, the impedance modulus values of Mg(OH)₂/Mg-Fe LDH composite films can achieve a magnitude of three times greater than its substrate. These results indicate that the Mg-Fe LDH film can effectively capture the corrosion anions during corrosion through the anion-exchange process, making it a promising surface modification for Mg alloy.

Introduction

Magnesium (Mg) alloys have been of interest as orthopaedic implants since the end of the 19th century due to their degradability, biocompatibility, and similar mechanical properties to natural bone [1], [2], [3]. With the degradability properties of Mg alloys, secondary surgery can be avoided, which reduces the patient's pain and cost [4]. However, the rapid corrosion of Mg alloys and rise in local alkalinity at the early stage of healing has imposed detrimental effects on the application of biodegradable Mg alloys [5,6]. Various surface modifications have been explored to control the corrosion rate as this approach can effectively protect Mg alloys from corrosion without changing the bulk properties [7,8].

Studies regarding surface modification of Mg alloys have demonstrated that hydrothermal treatment [9,10], micro-arc oxidation (MAO) [11], apatite coatings [12,13], and conversion coatings [14,15], as potential approaches to improve corrosion protection and biocompatibility of Mg alloys. Although these methods can protect the Mg alloys, such coatings' porous structure would limit their protective ability once their coating integrity is damaged. Thus, several studies have focused on an active coating system, which reacts to physical and/or chemical damage, such as layered double hydroxide (LDH) [16,17]. As an active coating, LDH films can absorb Cl⁻ anions and release corrosion inhibitors to the corrosive environment when physical and/or chemical damage occurs.

The brucite-like structure of LDH consists of positively charged layers, with an interlayer region of exchangeable anions, allowing the entrapment of corrosive anions (i.e., chloride ions Cl⁻) [18,19]. This ability avoids the built-up of soluble chlorides in the coating as the concentration of inhibitors overwhelms the chlorides formations at the coating/surface interface [20,21]. In turn, such a process remarkably improves the corrosion resistance of the Mg alloys [22,23]. Notably, with their nanostructured surface, LDH enhances cell adhesion ability and proliferation, leading to the overall improvement of Mg alloys' biocompatibility [24], [25], [26].

Looking at the prospects of LDH as a protective film, various studies have been reported on the synthesis of LDH films to mitigate corrosion on Mg alloys [27], [28], [29]. The in-situ growth method includes co-precipitation, hydrothermal, and steam coating is a common approach for synthesising LDH films on Mg substrates [20,[30], [31], [32]]. Despite multiple works reported, there is still an ongoing discussion on the formation mechanism of in-situ growth of LDH films [29]. One of the hypotheses presented is that divalent metal hydroxides act as the precursor in forming LDH films with the substitution of Mg^2 +  in Mg(OH)₂ by trivalent cations. Peng et al. [33] showed that an Mg(OH)₂ layer by hydrothermal treatment synthesised on Mg alloy could act as a base for the growth of LDH film. Similar findings were reported by Zhang et al. [34] with the successful synthesis of Mg-Al LDH film on anodised AZ31 through the hydrothermal process. The authors showed that the LDH film has a dense structure and improve corrosion resistance.

On the contrary to the first hypothesis, the second hypothesis proposed involves the replacement of trivalent cations by divalent cations in the formation of LDH films with the trivalent as the precursor. Findings from Yang et al. [35] and Zhang et al. [25] reported compact LDH films with improved corrosion resistance are synthesised through the formation of trivalent hydroxide salts as the precursor. At present, the latter hypothesis is widely accepted due to the easier formation of trivalent metal hydroxides in the synthesis process. Nevertheless, the hypothesis on divalent metal hydroxides as a precursor is an attractive approach, as utilisation of Mg(OH)₂ layer as the precursor will be a novel method for LDH synthesis as all Mg alloys are known to form Mg(OH)₂ layer on their surface. Thus, in this work, we propose the synthesis of LDH film through electrodeposition technique that adopts the ion substitution theory.

Hitherto, most literature reports have studied Al-based LDH coatings on Mg alloys. Although Al-based LDH coatings are suitable for atmospheric applications, they are considered less desirable for biomedical applications due to the concentration of Al^3 +  ions and their potential neurotoxicity [36,37]. Given this limitation, other trivalent metal ions such as Fe^3 +  are considered suitable alternatives to Al^3 +  ions, as Fe is deemed biodegradable and biocompatible [25,26,38]. However, the idea of synthesising Mg-Fe LDH film on Mg alloy WE43 through electrodeposition for orthopaedic implant applications has yet to be investigated, although electrodeposition has been reported as a simple and easy to control method which is not limited by the size and shape of a matrix [39].

A review of past works highlights the research gap of using electrodeposition as a coating method for LDH film on WE43 alloy for implant applications. Thus, in the present work, we propose for the first time the synthesis of LDH film through electrodeposition technique on an oxidised layer. The combination of the oxidised layer with LDH will provide an extended period of protection for Mg alloy. In the present study, Mg-Fe LDH coatings were prepared on magnesium hydroxide, Mg(OH)₂ layer through electrodeposition at various applied potentials of −1.2 V_SCE to −1.6 V_SCE. The selection of different applied potentials assesses the effect on the surface morphology of LDH films and the corrosion resistance ability in the physiological environment.