ZnFe₂O₄/TiO₂ nanocomposite films for photocathodic protection of 304 stainless steel under visible light

Highlights

• Hydrothermal method for synthesizing ZnFe₂O₄ nanoparticles on TiO₂ NTs.

• The photoelectrochemical property of TiO₂ could greatly improve by doping ZnFe₂O₄.

• The ZnFe₂O₄/TiO₂ composite films provide a better photocathodic protection for 304SS.

Abstract

ZnFe₂O₄ sensitized TiO₂ nanotubes were fabricated via anodic oxidation and hydrothermal method. The ZnFe₂O₄/TiO₂ composite films with different concentrations were synthesized to explore the optimal photoelectrochemical property compared with pure TiO₂ nanotubes. The morphology and chemical composition of samples were characterized by scanning electron microscopy, energy-dispersive spectrometry, and X-ray diffraction. The performance of protection for 304 stainless steel was investigated by electrochemical tests, such as open circuit potential and photocurrent density. The results indicate that ZnFe₂O₄/TiO₂ composite films showed efficient photocathodic protection effect for 304 stainless steel under visible light, and the optimal specimen composite films shifted the corrosion potential of 304 stainless steel to −780 mV.

Introduction

Stainless steels are widely utilized in various fields in modern society due to their excellent anticorrosion property. However, its performance is related to the exposed environment, the localized corrosion may occur when immersed in a chloride-containing or moist environment [1]. A substantial amount of research has focused on improving the corrosion resistance of stainless steel, such as protective coatings [2], [3], corrosion inhibitors [4], [5]. The electrochemical protection method also serve as an efficient technique which has been largely applied in corrosion protection field, with the advantages of long life-service, large protection range and so on, but it usually results in energy and materials loss [6]. Thus, optimizing the electrochemical protection method is highly desired.

Since Yuan and Tsujikawa reported that the TiO₂ coatings could be applied for electrochemical photocathodic protection of metal [7], nanostructured materials based on TiO₂ have attracted considerable attention among anticorrosion researchers [8], [9]. The related research found that electrons generated in TiO₂ photoanode could transfer to the metal under illumination, resulting in a more negative potential value than its corrosion potential. Consequently, metal corrosion is prevented, which is the basic mechanism of the photocathodic protection of TiO₂ photoanodes for metals [10], [11], [12]. However, the wide band-gap of TiO₂ (Eg = 3.2 eV) limits the photogenerated electron excitation in the visible light. In addition, pure TiO₂ photoresponsive coatings can not provide effective photocathodic protection for metals under dark conditions [13], [14]. Therefore, various methods have been developed to sensitize the activity of TiO₂ photoelectrode to the visible light region, such as coupling semiconductor (CdTe [15], CdSe [16], MnS [17], NiSe₂ [18], NiO [19], and WO₃ [13], [20]), metals (Fe [21]), and nonmetal elements (N [22], S [23]) to protect the stainless steel.

Spine ZnFe₂O₄ with a relatively narrow band-gap (1.9 eV) has been widely investigated in the fields of solar transformation, photocatalysis [24], and hydrogen evolution [25] due to its visible light sensitivity and good photochemical stability [26], [27], [28]. It is found that the ZnFe₂O₄/TiO₂ coupling system exhibited superior optoelectronic properties compared with pure TiO₂ [29]. Rekhila et al. [30] reported that ZnFe₂O₄ prepared by nitrate route could be used as dispersed photons collector to sensitize and enhance the photoactivity of TiO₂ under visible light. Yin et al. [31] also demonstrated that double-layered films composed of ZnFe₂O₄ and TiO₂ can deliver enhanced photocurrent. These studies suggest that ZnFe₂O₄ can be a proper sensitizer for TiO₂, the band-gap difference between them may promote the separation of photoinduced charges and reduce their recombination [32], [33], [34], [35]. Therefore, it is of great significance to develop ZnFe₂O₄/TiO₂ nanocomposite films for the photocathodic protection.

In the present work, ZnFe₂O₄ nanoparticles were deposited on TiO₂ nanotubes (NTs) via hydrothermal method. The hydrothermal method can produce high temperature and high pressure due to the sealed condition of the autoclave, this phenomenon contributes in synthesizing crystals with high purity and various shapes [36], [37], [38]. The ZnFe₂O₄/TiO₂ nanocomposite films serve as photoanodes can provide outstanding photoelectrochemical performance for the photocathodic protection of 304 stainless steel (304SS).