Regular ArticleLong-term corrosion protection for magnesium alloy by two-layer self-healing superamphiphobic coatings based on shape memory polymers and attapulgite
Graphical abstract
Introduction
Magnesium (Mg) alloy has excellent properties such as high strength-to-weight ratio, low density, high thermal conductivity, good electromagnetic shielding performance, good machinability and easy recyclability [1], [2]. Thus, Mg alloy has wide potential applications in various areas including aerospace, military weapons and equipment, automotive, rail industry and shipbuilding industry. However, Mg is chemically reactive and Mg alloy can be easily corroded, for the electronegative potential of Mg is low (−2.37 V) [3]. So, the practical applications of Mg alloy are seriously hindered [4].
Various approaches such as chemical etching [5], hydrothermal method [6], anodic oxidation [7], micro-arc oxidation [8], sol–gel [9] and electrochemical deposition [10] have been developed for the corrosion protection of Mg alloy. Among all the approaches, superhydrophobic and superamphiphobic coatings are recently receiving great interest because of their high repellency to water and various other liquids with low surface tension [11], [12], [13], [14], [15]. There is a thin layer of air at the solid–liquid interface and the liquids could easily roll off from the coatings. Thus, the solid–liquid contact area and contact time are evidently reduced. So, the coatings can inhibit corrosion of metals by efficiently blocking or at least delaying the permeation of corrosive liquids to metals. It was reported recently that the long-term anti-corrosion performance of superamphiphobic coatings is obviously better than that of superhydrophobic coatings with similar initial anti-corrosion performance [16]. Compared with superhydrophobic coatings, the preparation of superamphiphobic coatings on metal surfaces is more stringent [17], and their application in corrosion protection of Mg alloy is deficient.
Superamphiphobic coatings can be prepared by various methods such as chemical etching [18], self-assembly [19], chemical vapor deposition [20], 3D printing [21] and templating [22]. Long term stability is important for their application in corrosion protection of metals [23], [24]. Self-healing technology is one of the effective approaches to maintain long term stability of superhydrophobic and superamphiphobic coatings [25], [26], [27]. The traditional self-healing technique is to impregnate the coatings with healing reagents, which can automatically release and restore the superhydrophobicity once the coatings were damaged [28], [29]. On the other hand, thermoplastic shape memory polymers (SMP) have recently gained much attention in the field of anti-corrosion [30], [31], [32]. The thermoplastic SMP coatings could structurally heal in time to restore the anti-corrosion performance [33]. Thus, it is promising to develop long lasting anti-corrosion coatings by combining superhydrophobic or superamphiphobic coatings with SMP coatings [34], [35].
Here, we report an alternative strategy for long-term efficient corrosion protection for Mg alloy using self-healing superamphiphobic coatings. The superamphiphobic coatings are prepared by scraping the SMP emulsion containing ceresine wax microparticles and a corrosion inhibitor (1, 2, 3-benzotriazole, BTA) onto Mg alloy followed by spray-coating the fluorinated attapulgite (fluoroATP) suspension onto the SMP-BTA coating. The ceresine wax microparticles can improve self-healing capability of the SMP coating [36], [37]. BTA can delay the occurrence of corrosion before the damaged coatings are repaired [38], [39], [40], [41], [42], [43], [44], [45]. The two-layer self-healing superamphiphobic coatings, no matter intact or self-healed, show long-term efficient corrosion protection for Mg alloy owing to the synergistic effect of the SMP-BTA coating and the fluoroATP coating.
Section snippets
Materials
AZ31B Mg alloy plates (30 mm × 30 mm × 3 mm) were purchased from Dongguan Feitai Metal Products Co., China. The plates were polished with 400# SiC sandpaper, cleaned with acetone and ethanol by ultrasonication, washed with deionized water, and finally dried in air. Bisphenol A diglycidyl ether (≥85%), neopentyl glycol diglycidyl ether (>40%) and Jeffamine D230 (>98%) were purchased from Aladdin, China. BTA was purchased from Aike Reagent, China. Ceresine wax (>99%) was bought from Yousuo
Morphology and wettability of SMP-BTA/fluoroATP coatings
The SEM images of Mg alloy with the SMP-BTA, fluoroATP and SMP-BTA/fluoroATP coatings are shown in Fig. 1a–c. The SMP-BTA coating has a compact and smooth surface (Fig. 1a). The SMP-BTA coating is hydrophobic with a CA3.5 wt% NaCl of ~105° and oleophilic with a CAn-hexadecane of ~50°.
In contrast, the fluoroATP coating has a micro-/nanoporous structure formed by random deposition of the fluoroATP nanorods originated from ATP in the spray-coating process (Fig. 1b). The ATP nanorods are about 1 μm
Conclusions
In summary, we have successfully prepared two-layer self-healing superamphiphobic coatings by the combination of the SMP-BTA layer and the fluoroATP layer for long-term corrosion protection for Mg alloy. The SMP-BTA/fluoroATP coatings feature excellent superamphiphobicity, initial anti-corrosion performance, long-term anti-corrosion performance and self-healing performance, compared with previously reported superhydrophobic and superamphiphobic coatings for corrosion protection. Even the
CRediT authorship contribution statement
Jiaojiao Zhang: Conceptualization, Methodology, Writing - original draft. Jinfei Wei: Conceptualization, Methodology, Investigation, Writing - original draft. Bucheng Li: Methodology, Investigation. Xia Zhao: Methodology, Investigation. Junping Zhang: Conceptualization, Supervision, Writing - review & editing.
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.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51873220) and the Foundation for Innovation Groups of Basic Research in Gansu Province, China (17JR5RA306).
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These authors contributed equally.