Regulating corrosion reactions to enhance the anti-corrosion and self-healing abilities of PEO coating on magnesium

Highlights

• An amorphous MnOOH film was prepared on PEO coating via a facile immersion treatment.

• MnOOH film could react with corrosion produced Mg²⁺ and OH^- to form protective LDH.

• MnOOH film could cut-off the vicious cycle in corrosion reactions of PEO coating.

• PEO/MnOOH coating had a certain self-healing ability for scratch and coated films.

Abstract

Herein, a novel smart chemical strategy for regulating the corrosion reaction to reinforce the plasma electrolytic oxidation (PEO) coating on magnesium by amorphous MnOOH is proposed. The MnOOH film could provide PEO coating with both physical shielding and chemical active healing abilities. During corrosion process, MnOOH spontaneously reacts with the corrosion-produced Mg²⁺ and OH^– to form layered double hydroxides, resulting in high improvement of the long-term anti-corrosion of PEO coating. Moreover, the PEO/MnOOH coating presented a certain self-healing ability for scratches, and could be used for repairing corrosion defects on further coated films.

Introduction

Rapid corrosion of magnesium (Mg) is a key obstacle that limits their widespread application [1]. Constructing a dense and stable physical barrier coating is a cost-effective and common method to minimize contact between the corrosive medium and Mg substrate to inhibit the corrosion of Mg [2], [3]. Plasma electrolytic oxidation (PEO) coatings are widely explored for the corrosion protection of Mg materials owing to their high tunability (structure, composition, and thickness) and high adhesive strength [4], [5]. However, the existence of micropores on the PEO coating and its fragile corrosion product Mg(OH)₂ phase have an adverse impact on its long-term protection properties. Doping additives (ions or particles) and sealing the micropores are the most commonly used strategies to enhance the anti-corrosion performance of PEO coatings [6]. In past studies, the additives of ions (e.g., F, Ce, and Cu) and insoluble nanoparticles (e.g., SiO₂, TiO₂, and carbon nanotubes) have been added to PEO coatings to reinforce their corrosion resistance [7], [8], [9], [10], [11], [12]. Beyond that, a second layer film (e.g., Ca-P and silane coatings) is commonly used to avoid the disadvantages of pores and cracks on PEO coatings [13], [14]. Nevertheless, localized corrosion still occurs easily in these physical barrier coatings [4], [5], [6]. Generally, most damaged coatings require cumbersome repair or result in premature coating failures. Hence, coating with the self-healing/repairing abilities is vital. It is therefore important to develop a smart strategy which can provide the coating with both physical shielding and chemical active healing abilities.

Corrosion reactions determine the final corrosion products and surface structure of the coatings. This critically affects the corrosion behaviors and anti-corrosion performance of PEO coatings [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. A common corrosion process of a PEO coating can be described by the following steps: (i) H₂O and Cl^– destroy the outer porous layer and inner dense layer to produce Mg²⁺ and OH^– (Eq. (1)); (ii) with an increase in corrosion and local alkalinity, the corrosion product Mg(OH)₂ is formed (Eq. (2)); and (iii) Mg(OH)₂ is converted to soluble MgCl₂ because it is easily broken by Cl^– (Eq. (3)). The corrosion reactions therefore form a vicious circle, which is the main reason for its local corrosion and premature failure. If we construct an active coating that can regulate the corrosion reaction to break the corrosion process, which may therefore be a feasible smart strategy for enhancing the self-protective ability of PEO coatings.

MgO + H₂O + Cl^– → Mg²⁺ + 2OH^– + Cl^–

Mg²⁺ + 2 OH^– → Mg(OH)₂

Mg(OH)₂ + Cl^– → Mg²⁺ + Cl^– +2OH^–

Layered double hydroxides (LDHs), a special type of hydroxide, have similar formation conditions but much better corrosion resistance compared to Mg(OH)₂ [29]. LDH is a species of clay that belongs to 2D materials with a unique lamellar structure that can block or resist Cl^–. LDH films has been widely researched for the corrosion protection on metals [30], [31]. According to the ion substitution theory, the formation process of LDH can be described as ion-substitute hydroxides [32]. Recently, Meng et al. reported that LDH can be synthesized using metal hydroxides as precursors at room temperature [33]. In other words, using hydroxides and their derivatives to construct a smart coating that may spontaneously participate in Eq. (2) to form LDH phase during corrosion process of PEO coating and break its corrosion circle.

In the present work, a novel active material of amorphous manganese oxyhydroxide (MnOOH) was prepared on PEO-treated Mg through an immersion treatment. The prepared MnOOH can regulate corrosion reactions of the PEO coating on Mg to form LDH phase during corrosion, and thus improving the long-term corrosion resistance of the PEO coating. Moreover, the low-temperature corrosion test suggested that the function of the MnOOH in regulating the corrosion reaction worked well over a wide temperature range. Furthermore, the PEO/MnOOH film presented a certain self-healing/repairing ability for scratches, and could be used for repairing corrosion defects on further coated films, such as polydopamine (PDA) and graphene oxide (GO) films.