Regulating corrosion reactions to enhance the anti-corrosion and self-healing abilities of PEO coating on magnesium
Graphical Abstract
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)2Mg(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.
Section snippets
Materials
Pure Mg (99.99% Pure) sheets (10 × 10 × 1 mm) were adopted as the substrate. Before modification, the pure Mg sheets were ground with 600 grit SiC paper, then ultrasonically cleaned in absolute ethyl alcohol and dried in air. For PEO coating preparation, sodium glycerophosphate (C3H7Na2O6P, Sinopharm Chemical Reagent, China) and potassium hydroxide (KOH, Sinopharm Chemical Reagent, China) were used. For PEO/MnOOH coating preparation, manganese chloride (MnCl2.4H2O, Sinopharm Chemical Reagent,
Preparation and surface properties
As shown in Fig. 1a2, the MnOOH films were easily prepared on PEO coating by a simple and low-cost immersion treatment in the Mn2+-containing solution. Magnesium oxide (MgO) phase in PEO coating results in that a large amount of OH– were produced near PEO coating in water. Exorbitant local alkalinity precipitate Mn2+ nearby the surface to form Mn(OH)2 (Eq. (4)). However, Mn(OH)2 is unstable and can be easily oxidized by O2 to form MnOOH (Eq. (5)). Thus, MnOOH layers were directly prepared on
Discussion
Mg materials with promising specific strength and specific stiffness are suitably applied in industry fields, but the poor anti-corrosion limits their widespread applications. PEO coating, a conventional barrier coating on Mg, is prone to cause local corrosion, and lacks of long-term protection due to the vicious cycle in its corrosion reactions. In this work, a novel active material amorphous MnOOH was constructed on the PEO coating, which could spontaneously participate the corrosion reaction
Conclusion
An amorphous MnOOH thin layer was prepared on PEO-treated Mg via a facile and low-cost preparation method of immersion treatment in Mn2+–containing solution. The prepared amorphous MnOOH layer regulated the corrosion reaction by changing the final surface structure and corrosion product of the PEO coating from a cracked and damaged Mg(OH)2 layer to a sheet-like LDH layer. This resulted in the highly improved anti-corrosion performance of the PEO coating. This property of the PEO/MnOOH coating
CRediT authorship contribution statement
Dongdong Zhang: Methodology, Investigation, Data curation, Writing – original draft, Formal analysis. Feng Peng: Conceptualization, Validation, Formal analysis, Writing – review & editing, Visualization. Jiajun Qiu: Conceptualization, Validation, Formal analysis, Writing – review & editing, Visualization. Ji Tan: Conceptualization, Validation, Formal analysis, Writing – review & editing, Visualization. Xianming Zhang: Formal analysis, Visualization. Shuhan Chen: Formal analysis, Visualization.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is financially supported by the National Natural Science Foundation of China (31771044, 51901239), Shanghai Committee of Science and Technology, China (19JC1415500 and 20S31901200), High-end Entrepreneurial and Innovative Teams of Ningbo High-level Talents Project (2018A-09-C), S&T Innovation 2025 Major Special Programme of Ningbo (2020Z095) and S&T Industrial Programme of Cixi (2019gy01).
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