Development of a nanostructured film based on samarium (III)/polydopamine on the steel surface with superior anti-corrosion and water-repellency properties

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Abstract

Hypothesis

The application of various hydrophobic/superhydrophobic coatings on the surface of metals has become the hot topic of the recent studies. The corrosion protection effectiveness and environmental issues are two important factors that should be taken into consideration when developing advanced surface coatings. Recently, the rare-earth elements (i.e., samarium) and biopolymers (i.e., polydopamine) have attracted much attention in the metals' corrosion control field.

Experiments

The Sm(NO3)3 containing solution was sprayed to the steel (St-12) sheets. Then, the Sm-modified plates were post-modified by polydopamine biopolymers that were synthesized by the self-polymerization (using tris (hydroxymethyl) aminomethane as a buffer), and oxidant-induced (using CuSO4 as an oxidant) approaches. The structural analysis was carried out by different techniques such as contact angle (CA) test. Moreover, the electrochemical impedance spectroscopy (EIS) and polarization tests were performed to investigate the anti-corrosion performance of various samples.

Findings

The CA test results revealed that by applying the nanostructured Sm-based film, the surface of the metal becomes near superhydrophobic (CA > 140°). EIS results evidenced the significant impact of the post-treatment of the Sm-treated samples by polydopamine (PDA) nanoparticles (NPs) on its corrosion protection ability enhancement. Also, the polarization test results confirmed that all treatments could retard the corrosion of steel via a mixed-type inhibition mechanism.

Introduction

Corrosion, which is one of the destructive phenomena, results in enormous economic damages to the industries every year. There are many practical approaches, i.e., application of inhibitors and coatings, to increase the corrosion resistance of the metals [1],[2]. Steel is the most widely used metals in many industrial applications; however, it has a significant tendency to corrosion, and because of that, its application has been limited. Among the various methods applied for the corrosion control of metals, the ceramic-based conversion coatings have attracted more attention due to their simplicity in operation and cost-effectiveness. Besides, the conversion coatings (CCs) are used as the pre-treatment for metals to improve the adhesion of the organic coatings to the metals' surface [3], [4], [5], [6], [7].

For many years, the traditional chromate/phosphate-based CCs have been used as coatings for the pre-treatment of the metals. However, the recent usage of these CCs has been restricted by the environmental protection agency (EPA) because of the toxicity of the hexavalent chromium ions [5],[8],[9]. The rare-earth-based CCs have been introduced as a more promising alternative for chromate due to their non-toxicity and environmental compatibility [10], [11], [12]. Among these elements, cerium is the most extensively used element for this purpose because of its availability [13], [14], [15], [16]. There are also numerous studies on the application of the rare earth elements such as Nd [17],[18], Pr [19], La [20],[21], Gd [22], and Sm [23],[24] for development of the thin anti-corrosive CCs on the steel surface [25], [26], [27].

In recent years, the metals corrosion resistance improvement via application of the superhydrophobic films has drawn much attention [28], [29], [30], [31]. There are many industrial applications for the superhydrophobic coatings, i.e., anti-fouling paints [32], self-cleaning [33], and snow anti-sticking [34], films for window and antenna, anti-frosting metals' surface [35], anti-staining fabrics [36], anti-soiling coatings [37], oil-water separation [38], and so on. Many approaches, i.e., chemical etching [39], hydrothermal method [40], anodizing [41], electrodeposition [42], inorganic-organic sol-gel coatings [43], nanocomposite coatings [44], templating [45], direct application of the superhydrophobic compounds on the surfaces [46], or as the post-treatment [47], etc. [48],[49] have been used to obtain superhydrophobic surfaces. Pan and coworkers [50], developed superhydrophobic films via spraying a poly(methyl methacrylate) and waterproof silica nanoparticles mixture onto the steel surface. They also investigated the corrosion resistance of the film in the NaCl solution and revealed a significant reduction of the corrosion current density of steel. Cao et al. [51], increased the corrosion resistance of the brass subjected to the 3% NaCl solution by applying a polydopamine film on its surface. They also found that via the film functionalization with 1H, 1H, 2H, 2H perfluorodecanehiol, the surface becomes more superhydrophobic, leading to a better corrosion protection performance.

Dopamine, which is a hormone and neurotransmitter, can be readily oxidized in the alkaline environments. The oxidized form of dopamine is named polydopamine [52],. Polydopamine has been used for various applications i.e. drug delivery [53], photo-thermal treatment [54], energy [55], etc. [56], [57], [58], [59]. However, the polydopamine usage as a corrosion inhibitor has been rarely studied. For example, Habibiyan et al. [60], have examined the polydopamine effectiveness on the steel corrosion mitigation in the acidic solution. They evidenced that the polydopamine could effectively increase the corrosion resistance of steel. Besides, the polydopamine synthesized via the oxidant-induced polymerization approach in the presence of sodium periodate oxidant resulted in the highest inhibition efficiency.

In general, the rare-earth-based CCs have been proposed as the less toxic alternative for the chromate/phosphate-based CCs replacement. However, the rare-earth-based CCs include structural defects that weaken their corrosion protection performance in corrosive environments. The use of green biopolymers to improve the rare-earth-based CCs properties is one of the ideas that have not been well explored. On the other hand, the superhydrophobic surfaces have been designed for different applications such as corrosion control of the metals. Therefore, combining polydopamine biopolymer with samarium-based CC for the construction of a robust superhydrophobic anti-corrosion coating is the major novelty of this study.

In this research, polydopamine was synthesized by two approaches and used for the post-treatment of the Sm-based CC. The impact of two synthesis methods of polydopamine on the Sm-based CC properties has been studied by CA, scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDS), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), atomic force microscope (AFM) as well as EIS and potentiodynamic (Tafel) polarization tests. For this purpose, the samarium salt-based solution was sprayed on the steel surface and then through the self-polymerization (air-exposed), and the oxidant-assisted approaches the hydrophobic polydopamine films were deposited on the samarium-treated substrates.

Section snippets

Materials

The details of the test materials used for the construction of the superhydrophobic film are given in Table S1. The St-12-based steel panels with the chemical composition (wt%) of 0.10% C, 0.035% P, 0.45% Mn, 0.035% S, 0.007% N, and the rest of Fe were cut to the size of 12 cm × 4 cm.

Fabrication of nanostructured Sm-coating

The steel specimens' surfaces were deoxidized by sandpapers and then carefully degreased by acetone. The prepared samples were preheated to 65 °C. Then, the chemical treatment solution (Table 1) was continuously

The γs evaluation

The γs can be obtained by measuring the CA using pure water. Through the Neumann's formula and Young's equation [63], the γs of the substrate was calculated. The CA test results are presented in Fig. 2 and Table 3. Results depict that through the creation of the Sm-based coating on the metal surface, the γs value was significantly affected. According to the obtained results, it is evident that with the formation of the Sm-based layer, the CA increased, indicating that the surface becomes less

Conclusion

According to the previous studies, it can be understood that the hydrophobicity index of the metal surface can influence its corrosion resistance [28], [29], [30], [31],[39], [40], [41], [42], [43], [44], [45], [46], [47]. This study has shown that by chemical modification of the steel surface via the Sm-PDA, a rough anti-corrosion film with good superhydrophobicity nature can be formed on the steel surface. Some studies have used biopolymers to control metal corrosion [60],. Moreover, a

CRediT authorship contribution statement

Farshad Bahremand: Writing - review & editing. Taghi Shahrabi: Writing - review & editing. Bahram Ramezanzadeh: 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.

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