Regular Article
Anti-corrosion of amphoteric metal enhanced by MAO/corrosion inhibitor composite in acid, alkaline and salt solutions

https://doi.org/10.1016/j.jcis.2019.07.035Get rights and content

Abstract

An anticorrosive composite coating with enhanced corrosion resistance in acid, alkaline and salt solutions was fabricated by compounding micro- and nanoporous inorganic structure and organic corrosion inhibitor, which was used to improve the corrosion resistance of amphoteric metal and its oxides in various corrosive medium. Micro- and nanoporous structure was prepared by microarc oxidation (MAO) coatings on 2024 aluminium alloy, which was used both as the inorganic anticorrosion coating and the container for organic corrosion inhibitor (M16). Electrochemical impedance spectroscopy, Tafel plots and salt spray resistance were measured to research the anticorrosion performance of the MAO/M16 composite coating. Enhanced corrosion resistance was observed for the MAO/M16 coating compared to the MAO by itself. When the concentration of corrosion inhibitor M16 is at 2 w%, the best anticorrosive properties of the composite coating were obtained. Moreover, the MAO/M16 composite coating showed better corrosion-resistant performance than pure MAO coating and Al alloy substrate in the corrosion environment of 1 M HCl, 0.1 M NaOH and 3.5 w% NaCl solutions, respectively. The enhancement of corrosion resistance for MAO/M16 composite coating was achieved by a unique synergy between the microarc oxidation layer and the corrosion inhibitor. The composite coating indicates its promising applications in acid, alkaline and salt solutions environments and other harsh environments.

Introduction

Aluminum alloys have been widely used in aerospace, navigation, electronics industry and other engineering products due to the advantages of low density, high specific strength and good workability [1]. However, the further widespread applications are seriously limited by the disadvantage of its poor corrosion resistance owing to the amphoteric properties to be easily dissolved in acid or alkali media [2]. Although the surface of Al alloys can generate alumina film, but the oxide film is easy to be damaged in the application environment of seawater, majority acids and alkalis. Once the oxide film is destroyed, the corrosion will be more severe. Besides, aluminum and alumina are amphoteric metals and amphoteric oxides, which could be corroded in both acidic and alkaline environments. Therefore, it is necessary to protect aluminum alloys from corrosion in harsh environment. In order to improve the corrosion resistance, decorative property and functionality of aluminum alloys, surface treatments should be performed. There are many surface treatment technologies for aluminum alloys, including surface mechanical treatment, chemical treatment, electrochemical treatment, spraying [3], [4], [5], [6] and so on. Among them, a widely used method of surface treatment is anodization by chemical processing. Sulfuric acid anodic oxidation and chromic acid anodic oxidation are the most commonly applied in industry. But both sulfuric acid and chromic acid can cause different degrees of environmental pollution. In recent years, some new surface treatment technologies such as ion implantation, physical vapor deposition (PVD), chemical vapor deposition (CVD) and laser surface treatment have been developing rapidly; however, they suffer from issues such as complex operation, high cost and high energy consumption [7], [8], [9], [10]. On the other hand, the microarc oxidation (MAO) technology has received extensive attention of researchers due to the advantages of simple operation, low cost and low environmental pollution [11], [12], [13], which is a new technology with broad application prospect.

Microarc oxidation technology has been developed from the anodic oxidation technology and has broken the limitations of the anodic oxidation voltage parameters [14], [15], [16]. During the MAO process, a layer of hard ceramic film is generated on the aluminum alloys surface in situ during the coexistence process of chemical oxidation, electrochemical oxidation and plasma oxidation. The adhesion between the ceramic coating produced by microarc oxidation and the substrate is metallurgical combination. Therefore, the MAO coating has prominent corrosion resistance, wear resistance and high temperature resistance and is widely used in aviation, aerospace, mechanical, electronic and medical systems [17], [18], [19]. However, due to the discharge under the high temperature and high voltage, a lot of micropores and even cracks are formed in microarc oxidation ceramic layer. These pores and cracks could become the channels for corrosive media penetration and would cause corrosion defects in an environment invisible to the naked eye. On the other hand, the porous structure of the MAO layer also forms a mechanical loading effect for other materials [20]. In order to enhance corrosion resistance of aluminum alloys, composite coatings based on MAO coatings have been extensively studied, such as hydrophobic coatings [20], [21], hard coatings [22], [23], [24], carbon-base coatings [25], [26] and electroless deposition [27]. There are noticeable improvements on performance of the composite coatings. Besides, some researches were also performed to improve corrosion resistance of aluminum alloys in the corrosive environments of acids, alkalis and seawater, respectively. Specifically, inorganic SiO2 coatings on aluminum were adopted to enhance anticorrosion performance of aluminum alloys in the corrosive environment of acids [28]. Yanyo and Holmes-Farley have reported that a graded ORMOCER coating on aluminum alloy could provide effective anticorrosion performance to the aluminum substrate in the corrosive environment of alkalis [29]. Some studies have found that modified organic natural product could be used for corrosion protection of aluminum alloys in the corrosive environment of seawater [30]. In addition, aluminum corrosion protection could also be achieved by hydrophobic coatings on aluminum alloy [31]. However, the above methods provided limited improvements in the corrosion resistance of aluminum alloy in the environments of acid, alkali and seawater. And, there are few researches that could simultaneously provide effective corrosion protection for aluminum alloys in three corrosive environments with acid, alkali and seawater. Corrosion inhibitors are chemicals that prevent or slow metal corrosion and they are used in the corrosion environment in the appropriate concentration range and form [32], [33], [34], [35]. Organic corrosion inhibitors would form a solid adsorption film on the metal surface mainly by physical adsorption and chemical covalent bond. Thus, metal corrosion is inhibited by isolating the corrosion media from the metal. Among variety of organic corrosion inhibitors, imidazole derivative corrosion inhibitor is a kind of environmentally friendly corrosion inhibitor and is widely used in the engineering industry [36], [37], [38], [39]. Conventional corrosion inhibitors are placed in the usage environment or in organic coatings, which both have some defects. For instance, excessive amount of corrosion inhibitor could change the physical and chemical properties of the coating or could cause chemical pollution to the surrounding environment. And very little amount of corrosion inhibitor would not result in effective corrosion protection. For that reason, the porous structure of MAO layer is tend to load and capture corrosion inhibitor and a small amount of the inhibitor could produce a significant increase in the corrosion protection by the adsorption effect of MAO coating [40], [41]. Meanwhile, the synergy between corrosion inhibitor and MAO coatings would effectively enhance the anti-corrosion capability of aluminum alloys. However, it is still a challenge to design a novel anti-corrosion strategy for aluminum alloy in the harsh corrosive environments of acids, alkalis and seawater.

In this paper, we report a new strategy for the corrosion resistance of 2024 aluminium alloy with porous MAO membrane as the anticorrosion inhibitor container to enhance its corrosion resistance in acid, alkaline and salt solutions. Firstly, a imidazole derivative corrosion inhibitor was successfully synthesized, which was called M16 for short with effective anticorrosion property for Al alloy in harsh environments. Secondly, the micro- and nanoporous MAO membrane was prepared on aluminum alloy surface by microarc oxidation technology, which was used as the inorganic anticorrosion film and the inhibitor container. The MAO/corrosion inhibitor composite coating was then produced by a simple dip-coating method. Besides, the concentration of corrosion inhibitor for MAO/M16 composite coating was studied to obtain the optimized composition concentration for optimum anticorrosion performance. The anticorrosion performance of the composite coating, MAO coating and pure aluminum alloy under the corrosive environments of acids, alkalis and seawater were also studied by electrochemical methods, respectively. The corresponding results indicate that the composite coating possesses efficient corrosion resistance in three corrosive environments: acid, alkali and seawater.

Section snippets

Preparation of MAO coatings

The substrate material were rectangular samples (40 mm × 25 mm × 4 mm) of 2024 Al alloy. Prior to the MAO processes, the samples were polished with 400, 1200 and 2000 grit abrasive papers and were ultrasonically cleaned in acetone to degrease. The homemade experimental equipment for the MAO treatment was a 20 KW pulsed bipolar electrical source with a water-cooled system of stainless-steel spiral pipe also as the counter electrode. The Al alloy specimen was the anode. Due to the water-cooled

Results and discussion

The preparation process of the MAO/M16 composite coating is illustrated as follows. Firstly, the MAO coating was prepared by the microarc oxidation treatment of Al alloy to create some micro- and nanopores on the top surface. Then, the M16 corrosion inhibitor was deposited into the pores of the MAO coating via dip coating method. Fig. 1 presents SEM images of the porous MAO coating before and after deposition of M16 corrosion inhibitor. Fig. 1a and b shows the surface morphologies of the MAO

Conclusions

In summary, we have demonstrated a simple way to prepare MAO/M16 composite anticorrosion coating by microarc oxidation of aluminum alloy and dip coating process. The composite anticorrosion coating has good corrosion resistance in the environment of HCl, NaOH and NaCl solutions. The improvement of the corrosion resistance of aluminum alloy substrate is obtained by the synergism of barrier effect of MAO coating, the hydrophobic layer provided by the M16 corrosion inhibitor and the inhibiting

Acknowledgements

Thanks for the financial support of the NSFC (No. 51722510, 21573259), the outstanding youth fund of Gansu Province (1606RJDA31), Qingdao science and technology plan application foundation research project (17-1-1-70-JCH) and the “Hundred Talents Program” of Chinese Academy of Sciences (D. Wang).

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