Corrosion of steel is inevitable in both production and daily life. The total economic loss caused by corrosion is estimated to be over 3% of GDP in each country annually (Koch et al., 2002). Accordingly, it is vital to conduct coating protection for steel. To date, Zinc and its alloy provide the most effective corrosion protection for steel (Maeda, 1996; Krylova, 2001). Galvanizing layer firstly serves as a barrier coating to prevent corrosive mediums such as Cl^– ions from penetrating into the matrix steel. This accumulation of the zinc corrosion products would further enhance the barrier action of the galvanizing layer. Furthermore, the galvanizing layer can act as a sacrificial anode, which zinc would preferentially corrode compared to the steel (Maaß et al., 2011; Schulz and Thiele, 2012). However, the above-mentioned HDGS has certain drawbacks, such as relatively inadequate protection performance to cope with the increasingly harsh natural environment. As an alternative, a new strategy involving the combination of the pretreatment layer and organic coating, called duplex coatings, have been proposed (Dallin et al., 2018; Svoboda et al., 2018). Therefore, the exploration of novel duplex coatings has become a hot issue.

Typically, the pretreatment layer is a chromate or phosphate coating, which is environmentally unfriendly. Hence, there has been increased interest in the design and improvement of “green” pretreatment technology (Van Schaftinghen et al., 2004). New pretreatment layers, such as silane film (Van Ooij et al., 2000), rare-earth conversion film (Tianlan and Ruilin, 2009), conductive polymer (Le et al., 2001), and self-assembled film (Yamamoto et al., 1993) etc., have been booming during the past years. In order to fully realize the potential of these new technologies, a suitable pretreatment layer must be precisely selected and evaluated with respect to their special physicochemical properties and synergistic electrochemical parameters. Nevertheless, there is little literature evaluating the lifetime of the duplex coatings. Based on the aforementioned facts, we are eager to summarize an overview of recent HDGS research from the following three aspects: (1) anticorrosion control of zinc coatings to meet HDGS applications, (2) emerging and state-of-the-art duplex coatings for HDGS, (3) the life evaluation of aforementioned coating systems. Finally, future developments of coating systems have been proposed.

Zinc is an amphoteric metal. There is a large number of research articles on the corrosion mechanism of zinc in neutral solution (Cachet et al., 2001, 2002; Dafydd et al., 2005). It is generally believed that zinc is transformed by two or three intermediate substances in the corrosion process. Cachet et al. (2001) used EIS to study the corrosion process of zinc in an aerated sulfate solution and discovered that inductive reactance at a low frequency tail appear in the impedance spectra. They believed that it was related to the transformation of three adsorbed particles formed in the corrosion process of zinc, namely Zn+ ad, Zn2+ ad, and ZnOH ad. Zn firstly loses an electron and becomes Zn+ ad. Then Zn goes through the following three conversion paths:

Keddam et al. (1992) found that the main corrosion products of zinc in chlorine-abundant environments were basic zinc chloride, basic zinc carbonate, and zinc oxide. Under air and sodium chloride existing neutral conditions, the corrosion reaction of zinc is:

As the corrosion proceeds, the following corrosion reactions will also occur in the anode region.

In neutral or alkaline environments, basic zinc chloride will precipitate on the surface of substrates to protect matrixes from corrosion, but in acidic environments it can be easily dissolved. Therefore, basic zinc chloride can be easily washed away by rain under exposure environments. This is a cause of zinc pitting (Cachet et al., 2002). Another cause of zinc pitting in the presence of Cl^– is the pH gradient. Cole et al. (2010) researched the corrosion products of seawater droplets interacting with zinc after short-term exposure. It was discovered that the oxide layer gradually thickened and began to significantly hinder the diffusion of ions, which would lead to the local acidification in the anode region. This could also cause pitting at the local defect site of acidification, which occurred along grain boundaries.

Carbon dioxide is another key factor that affects the corrosion of zinc. Except for zinc hydroxide, carbonate of zinc is the most widely existing corrosion product of zinc. The main forms of the carbonate of zinc are ZnCO₃ and Zn₅(CO₃)₂(OH)₆ (Chen et al., 2006; Persson et al., 2007). The corrosion reactions are:

Basic zinc carbonate would be attached to the surface of substrates to prevent ions from diffusion and reduce the corrosion rate of substrates. It was found that the corrosion rate of zinc would increase with the increase of the concentration of CO₂ (Falk et al., 1998). In addition, a large number of SO₂, NO_x, and other pollutants in industrial environments will also significantly affect the corrosion of zinc. SO₂ is a major contributor to acid rain and is prone to cause uniform corrosion of zinc. In an environment where SO₂ coexists with NaCl, there would be a synergistic effect between them that would result in severe corrosion of zinc (Qu et al., 2002).

In addition, the corrosion mechanism of zinc in highly alkaline environments, especially in reinforced concrete, has been extensively researched. In the initial service period, the pores of concrete are mainly saturated calcium hydroxide solution. When the pH value is 12∼14 (Yeomans, 2004), the galvanized layer is basically in a passivation state. Among the factors leading to the corrosion of HDGS reinforcement, CaCO₃ generated from the neutralization reaction of CO₂ in the air with Ca(OH)₂ in the pore fluid of the concrete and Cl^– have the most serious impacts (Al-Mehthel et al., 2009; Manna, 2009). HDGS concrete is in a passivation state at saturated Ca(OH)₂ solution with a pH of 12∼13. The corrosion current density I_corr is close to the passivation critical value. When the pH exceeded 13.5, the corrosion current density I_corr would increase rapidly on account of the absence of protective film. When the pH is less than nine, the galvanizing film could directly produce a hydrogen evolution reaction and, thus, damage the coating’s structure.

When Cl^– enters the concrete environment, it destroys the primary corrosion product film (consisting of calcium zincate) covered on the galvanized layer, making the galvanized layer re-enter into an activation state. When the concentration of Cl^– is less than the critical value (0.4∼0.5 mol/L), the passivation film would be slightly pitted by Cl^– in local region of calcium zincate with weak protection, and another corrosion product basic zinc chloride could be generated in local micro-regions. In the dense area covered by calcium zincate, pitting of Cl^– will not occur. The local continuous basic zinc chloride formed by pitting, together with the large area dense calcium zincate, can still offer better protection to the galvanized layer. When the concentration of Cl^– is greater than or equal to the critical value, the degree of corrosion of the galvanized layer would increase, and the penetration of Cl^– to the passivation film will change from pitting corrosion to local corrosion. With the increase of Cl^– concentration, a large amount of loosely deposited basic zinc chloride was generated in the area where the protection of calcium zincate was relatively weak. The basic zinc chloride was not tightly bound to the surface of the galvanized layer and it was easy to break off. Cl^– would continuously infiltrate into the bottom of calcium zincate from the shed, and gradually destroy the dense membrane structure of calcium zincate, so as to expand the corrosion of the galvanized layer and destroy the matrix metal (Yeomans, 2004).

Currently, a relatively mature hot-dip galvanizing process technique for HDGS has been employed that could compete against cold galvanizing and zinc thermal spraying, which has enormous benefits over the latter. For instance, hot-dip galvanizing has the advantages of lower processing costs, comprehensive protection, less labor and time, and more durability and reliability (Schulz and Thiele, 2012). There are various factors that affect the properties of zinc coatings, such as the type of steel, surface preparation of steel before galvanizing, alloying additives of zinc baths, temperature and duration of the galvanizing process, and galvanizing post cooling methods (Chen et al., 1992; Strutzenberger and Faderl, 1998; Sere et al., 1999; Morimoto et al., 2002; Safaeirad et al., 2008; Tsai et al., 2010). Hence, there are also certain challenges, result from the demands for hot-dip galvanizing process applied for HDGS. These challenges include: looking for suitable alloying elements to improve the corrosion resistance of coatings, achieving an environmental-friendly galvanizing process, and realizing an excellent adhesion with a more uniform coating. Therefore, the discovery of an optimal hot-dip galvanizing process will be applied into the corrosion protection of steel. This hot topic will open up new paths for research collaborations between the fields of metallurgy and materials science.

Furthermore, with a view to better corrosion control of the novel hot-dip galvanizing process, the following aspects should be taken into consideration to discover more appropriate hot-dip galvanizing process for HDGS. As demonstrated in Figure 1, the common corrosion control strategy for matching hot-dip galvanizing process to HDGS should cover the following issues: (1) the optimal selection of a suitable matrix steel, (2) the appropriate modification of the alloy composition in zinc bath, (3) suitable temperature and time of hot-dip galvanizing, (4) the adoption of passivation process. First, a suitable matrix steel should be precisely chosen. The anticipated galvanizing coating after zinc immersion can be realized when cooperating with a proper type of steel. In fact, the microstructure and mechanical property of prepared galvanized coatings would be significantly influenced by the chemical composition of matrix steel (Breval and Rachlitz, 1988; Lin, 1999; Vourlias et al., 2004; Vincent et al., 2006; Maaß et al., 2011; Schulz and Thiele, 2012; Kania and Liberski, 2014; Pokorny et al., 2016). The content of carbon and silicon in the matrix played a guiding role on the coating properties, meanwhile the component of manganese, phosphorus, and sulfur had relatively little impact on it. For example, different forms of carbon element would have different effects on the reaction between Fe and Zn. When carbon was in the form of spherical pearlite and lamellar pearlite, it will accelerate the Fe-Zn reaction. However, the reaction velocity of Fe-Zn would not be influenced as C element existed as tempered martensite or graphite. Besides, the higher the content of C element, the faster the Fe-Zn reaction. The acceleration of the Fe-Zn reaction would reduce mechanical properties of the galvanizing coating. Therefore, low carbon steels are often chosen as matrix steel. Moreover, the presence of Si element can accelerate the dissolution rate of Fe in a zinc bath. Steel containing a low content of Si could obtain a more dense resistance coating after the process of galvanizing. The higher content of Si element in steel will lead to the Sandelin effect, making ζ phase (brittle phase) grow intensely and form a gray coating with poor adhesion, which would significantly affect the appearance, microstructure, and properties of the galvanizing coating. That is to say, the exploration of a suitable chemical composition of steel will be the first step in the control of hot-dip galvanizing process. Second, modification of the alloy composition in zinc bath for a prescribed matrix steel plays an essential role in galvanizing optimization (Fasoyinu and Weinberg, 1990; Pavlidou et al., 2005; Asgari et al., 2007; Pistofidis et al., 2007; Culcasi et al., 2009). Particularly, the melting point, viscosity, and surface tension of zinc bath can be modified by some certain alloying elements, so that the coating thickness, surface gloss, and mechanical strength of HDGS can be tuned for the better. In consequence, modifying alloy compositions by introducing different alloying elements, such as Al, Mg, or Ni, can enhance the corrosion resistance of the galvanizing coating. Besides, addition of rare earth elements or Sb can also improve the fluidity of zinc bath and reduce the amount of zinc adhesion on the surface of HDGS. Sb has a relatively poor solubility in zinc bath, which will precipitate as Sb₃Zn₄. Thus, galvanic cells of Sb₃Zn₄/Zn will be formed. An adherent coating is deposited with the addition of Sb through barrier and galvanic protection (Peng et al., 2020). Third, an appropriate temperature and time for hot-dip galvanizing is another key factor to optimize or regulate the as-prepared galvanizing coating to improve the hot-dip galvanizing process (Fasoyinu and Weinberg, 1990; Asgari et al., 2007; Culcasi et al., 2009). Whether the temperature of zinc bath is too high and too low, it will have adverse effects on the coating properties. As for the hot-dip galvanizing time, it is necessary to shorten the dip time under the premise of ensuring a certain coating thickness, which could decrease the thickness of ζ phase and improve the plasticity of gal izing coating. Fourth, an effective passivation process is also beneficial to optimize the galvanizing coating. Chromate passivation technology, on account of its simple process, low cost, and good film adhesion, is a classic process typically used to improve the corrosion resistance of HDGS (Van Schaftinghen et al., 2004). Unfortunately, chromate coating is highly carcinogenic and toxic. Therefore, for safety and environmental reasons, some low-toxic or non-toxic treatments have also been developed in recent years. The optimal galvanizing coating originating from such chromate passivation enriches the large family of hot-dip galvanizing process and fulfills different demands for HDGS.

Emerging and state-of-the-art coating protection systems are normally made from an appropriate matrix steel with duplex coatings, generally consisting of a metallic zinc coating with a pretreatment layer followed by an organic coating. Up to now, plenty of novel and prospective duplex coatings have been discovered, modified, or further developed, which have certified their feasibility in corrosion resistance of HDGS. In the light of forms of the pretreatment layers, these duplex coatings cover silane films (Subramanian and Van Ooij, 1998; Van Ooij and Child, 1998; Van Ooij et al., 2000; van Ooij and Zhu, 2001), rare-earth conversion films, conductive polymers, and self-assembled films. A good deal of novel duplex coatings have been invented successfully by conducting different pretreatments on the matrix of HDGS. Table 1 lists four types of typical duplex coating systems for HDGS. Therefore, we will summarize these above-mentioned emerging and state-of-the-art duplex coatings in accordance with the category of their pretreatment technologies.

Hot-dip galvanized steel based on aforementioned coating systems is of great importance for various applications; consequently, these novel coating systems with optimized or anticipated properties are incessantly researched by materials or chemistry researchers. At the same time, the life evaluation of these duplex coating systems is quite tough but essential and also attracts the attention of a large number of researchers. The purpose of life evaluation of those coating systems for HDGS is to quantitatively assess the corrosion resistance of coatings and to determine whether the coating system fails to avoid greater losses, such as corrosion failure. Up to now, there has been no unified definition of the lifetime of duplex coating systems. There are three main opinions to judge the coating systems to reach its service life,: (1) any rust appeared on the surface of coated metal, (2) perforation of coated metal, (3) rust area of the coated metal reaching a predetermined value (Zhen, 2005). For the shortcoming of long field test cycle, indoor corrosion acceleration tests can effectively evaluate the service life of the coatings in a short time under certain environments. Therefore, a consensus has been reached that the life prediction of coated metals is usually obtained by summarizing the correlation of results of indoor corrosion acceleration tests and field tests among scientific researchers. In other words, it is vital to grasp the corrosion mechanism of the coated metal under the circumstance of indoor corrosion acceleration tests and put forward appropriate evaluation methods based on the corrosion mechanism. As to metal coated with single coating (metallic coatings or organic coatings), the accelerated corrosion evaluation method of metal has been proposed and recognized (such as various standards) and some of these standards also provide quantitative assessment criteria. For metallic coatings, galvanizing coating would be taken as an example. Service lifetime of galvanizing coating is traditionally referred to the time for the red rust area to reach 5% of the whole area of galvanizing layer in the situation of accelerated test. Zhang and Xu (2007) covered the correlation between accelerated corrosion tests and actual corrosion conditions, concentrating on the corrosion rate ratio of steel to zinc. “Zinc coating life predictor” was also invented based on numerous statistics on corrosion of steel and zinc. Fujita and Mizuno (2007) researched the perforation corrosion of zinc or steel coated with zinc alloy and evaluated the lifetime of various coated steel sheets based on different kinds of accelerated corrosion tests. They divided the perforation corrosion procedure into four periods. Stage 1: the whole surface of substrate steel is covered with zinc coating and no other than the zinc coating is corroded (τ₁). Stage 2: zinc coating is just lost fractionally, and this film continues to be corroded sacrificially to protect steel (τ₂). Stage 3: zinc coating is entirely lost and the steel starts to be corroded. Meanwhile, zinc corrosion products can inhibit the overall corrosion rate of galvanized steel (τ₃). Stage 4: zinc coating completely loses its protective effect and steel is corroded quickly (τ₄). The schematic diagram of corrosion process of galvanized steel was shown in Figure 6. So as to illuminate the connection between different accelerated corrosion tests and actual corrosion, they took (Zhang and Xu, 2007) as references to propose a concept named Perforation Corrosion Index (PCI) for the evaluation of the perforation corrosion of galvanized steel. Specific formula was as follows:

Where, V_Zinc and V_Steel were defined, respectively as follows:

d_coating and d_steel were, respectively the thickness of zinc coating and steel.

Through this formula, the PCI of accelerated corrosion tests would coincide with the actual corrosion in Detroit, proving the feasibility of PCI. Regrettably, it is not universal for various metals and different corrosion conditions.

As for the lifetime prediction of organic coatings, Mayne combined the Fick diffusion law with electrochemistry of coated steel plate to propose a life prediction formula of organic coating based on the “control theory of coating polarization resistance” (Maitland, 1962). The formula was as follows:

Where, L and l were, respectively the lifetime of coating and the depth of coating, D represented the diffusion coefficient of coating ions, ϕ was a constant, p_s and σ_n, respectively referred to the adhesion of coating and applied pressure on the surface of steel. Chuang (1997) established a mathematical model on the basis of coating bubbling, where the bubbling growth rate V is regarded as a function of torque, material properties and temperature. Finally, the life of the coating system was obtained:

Where, σ_f was the layered stress, W was the cross-sectional area of the coating, D_b δ_b was interfacial diffusivity, Ω, k, and T were constant, ν and E were, respectively PoissoN′s ratio and elastic modulus of coatings under wetting condition, h was the total thickness of coatings. Although the formula of organic coating life prediction was established, its accuracy was questionable. Due to the diversity of coatings and difference of corrosion environment, no general formula has been found that can accurately predict the service life of coatings so far.

Apart from the above-mentioned life prediction formula, EIS technology gradually becomes a powerful tool to predict organic coating life. EIS affords an in situ non-destructive measurement of the corrosion behavior of protective coating, which can reflect information about corrosion reaction, mass transfer, and charge transfer (Grundmeier et al., 2000; Vogelsang and Strunz, 2001). This technique is strengthened by the current response of the small sinusoidal perturbation of electrode potential in variations with perturbation frequency. Equivalent circuit methods are commonly utilized to analyze and interpret the EIS data. The equivalent circuit normally consists of electrical elements that simulate the electrode/solution interface. Resistance (R) and capacitors (C) are the most typically used electrical components. Under some circumstances, constant phase elements (CPE) instead of capacitors (C) were introduced to achieve satisfactory fittings. Warburg impedance is appropriate for diffusion process. Adopted equivalent circuit models should consider the following principles: (1) Electrical components used in equivalent circuits must have clear physical meanings. That is, they must correspond to physical properties of the system that can produce such electrical responses; (2) Equivalent circuits must generate spectra and parameters in the circuits have appropriate values. Although spectra are different from experimental spectra, acceptable errors are allowed; and (3) The proposed equivalent circuits should be as simple as possible. Cao and Zhang (2002) proposed six equivalent circuit models according to the characteristics of impedance spectra of organic coating systems in different immersion periods. For a wide variety of organic coating systems, three basic physical processes are common: (1) Dielectric relaxation process. This process is represented by coating capacitance C_c, which generally occurs in the high frequency region of impedance spectra; (2) Charge transfer process at metal/solution interface. This process is represented by charge transfer resistance R_ct and double layer capacitance C_dl, which occurs after the corrosive solution reaches the organic coating/metal interface; (3) Diffusion process. This process usually exists in the coating pores and occurs in the low frequency part of impedance spectra (Strunz, 2000).

Sekine et al. (1992) discovered a linear relationship between f_θmax and R_c, in which R_c is the coating resistance and f_θmax refers to the frequency with which phase angle is at maximum. The relationship may work as a standard to judge the corrosion resistance of organic coatings, whereas f_θmax can be easily measured. Mahdavian and Attar (2006) analyzed and compared the differences in anti-corrosion performance of zinc phosphate coatings and zinc chromate coatings through the phase angle at 10 kHz. Haruyama and Sudo (1993) put forward a novel concept: breakpoint frequency f_b, a frequency where firstly the phase angle descends to 45°, and discovered a better correlation between delaminated areas of organic coating on the metal and breakpoint frequency. Mansfeld and Tsai (1991) put forward two new parameters: θ_min and f_θmin, which, respectively represent the minimum phase angle and corresponding frequency. It can be indicated that the θ_min and the proportion of f_b/f_θmin were all up to the stripping area between the as-prepared organic coatings and substrate metal. The above-mentioned methods all analyze high frequency regions of the impedance spectrum to predict the performance of organic coatings, which could avoid the measurements in low frequency regions of the impedance spectrum and develop fast lifetime evaluation for organic coatings. In addition, Zuo et al. (2008) found that the phase angle and coating resistance in the middle frequency domain from Bode plots show analogously descending tendencies. The changes of phase-angle at 10 Hz varying with the soaking time in 3.5% NaCl solution was in accordance with the variation of coating resistance, which could reflect the corrosion resistance of organic coating qualitatively. In the event that the phase angle at 10 Hz decreased into a range of 20∼40°, the electrolytes would thoroughly permeate into the organic coatings and electrochemical reactions underneath those protected coatings began to react quickly. Therefore, the phase angle at the frequency of 10 Hz could be applied for a quick evaluation of organic coating performance.

More recently, Hu et al. have studied the electrochemical impedance spectra of LY12 aluminum alloy/epoxy coating at distinct immersion stages in 3.5% NaCl solution (Hu et al., 2003; Zhang et al., 2004). They thought that the aging failure of organic coatings could be divided into four stages.

Stage 1: At the initial time of immersion, only one time constant appears on the electrochemical impedance spectrum, which shows an equivalent circuit of coating pore resistance (R_p) in parallel with coating capacitor (C_c), in which R_s is solution resistance. Figure 7 shows the equivalent mode A, Nyquist plots and Bode plots of the composite electrode immersed in 3.5% NaCl solution for 12 min.

Stage 2: When water, O_2, and other substances with larger diffusion coefficients pass through the coating and reach the substrate, the electrochemical interface of metal/electrolyte solution is established, and electrochemical reaction impedance start to appear. The equivalent circuit of this stage is shown in model B in Figure 8. The high frequency section of the impedance spectrum corresponds to the coating itself (C_cR_p), and low frequency regions of that corresponds to electrochemical reaction impedance (C_dlR_ct) at the metal/solution interface, in which C_dl refers to double-layer capacitance and R_ct is electrochemical reaction resistance.

Stage 3: Affected by the barrier of coating, the transmission of corrosion products on the metal surface is inhibited, resulting in the emergence of diffusion impedance Z_diff. The equivalent circuit of this stage is shown in model C in Figure 9. In addition, Figure 9 also shows the impedance complex plan of the electrode immersed in pure water for 48 h.

Stage 4: When the corrosive ions with small diffusion coefficients such as Cl^– ion reach the metal/coating interface through the coating, chemical reactions between Cl^– and the soluble corrosion products at the interface appear to generate salt film. Faraday impedance is the sum of salt film impedance (R_sfC_sf) and electrochemical dissolution impedance (R_ofC_of) of the alloy. The equivalent circuit of this stage is shown in model D in Figure 10. Figure 10 illustrates the appearance of double capacitance arcs by taking the impedance complex plan of coated metal immersed in 3.5% NaCl solution for 388 h as an example. It is considered that the high frequency capacitive reactance arc corresponds to the characteristic impedance of film forming reaction involving Cl^– ions on aluminum substrate surface, while the low frequency capacitive reactance arc corresponds to the anodic electrochemical reaction impedance of the surface of aluminum substrate.

Speaking of duplex coatings, so far there have not been any accelerated corrosion evaluation standard for composite coating systems, such as carbon steel coated with firstly a galvanizing coating and then an organic coating. It is inappropriate to evaluate the corrosion resistance of composite coatings by simply applying the evaluation criteria of single coatings. For example, some researchers (Bacon et al., 1948) discovered that when the electrochemical impedance modulus of organic coatings reached 10⁸ Ω cm² or above, organic coatings had better corrosion resistance. While the electrochemical impedance modulus of organic coatings was less than 10⁸ Ω⋅cm², their anticorrosion performance was relatively poor. Even when the electrochemical impedance modulus descended to 10⁶ Ω⋅cm², the coatings could be considered to have failed completely. But for HDGS, its impedance modulus is usually no more than 10⁶ Ω⋅cm². It cannot be indicated that the zinc coating has failed by impedance modulus alone and, in fact, the zinc coating may be intact. Therefore, this evaluation standard does not apply to metallic coating. That is, the evaluation standard for single coating is not suitable for composite coating.

However, by summarizing the above research on the lifetime evaluation of metal coatings and organic coatings, it was found that the failure process of two distinct kinds of coatings studied above are similar to each other to some extent, and both of their failure processes can be regarded as going through four stages. Based on these, we propose a novel method for evaluating the accelerated lifetime of composite coating system. There are synergies among the overall lifetime of the composite coatings (L_c) (Dallin et al., 2018) and the lifetime of organic coating (L_f) and the lifetime of zinc coating (L_z) for composite coating system: L_c = k (L_f + L_z), k = 1.5∼2.3. Therefore, the research of composite coating lifetime can be divided into two aspects: organic coating lifetime and galvanizing coating lifetime. For organic coatings, we can evaluate the failure process by the four stages proposed by Hu’s group and the appearance of double capacitance arcs marks the final lifetime of the organic coating. For galvanizing coatings, we not only need to judge the four stages of corrosion process, but also need to combine the potential reversal of steel and zinc to comprehensively judge the life of the galvanized layer. Due to the sacrificial anode protection ability of the galvanizing layer, even when the corrosive substances such as Cl^– pass through the galvanizing layer and reach the substrate, the carbon steel may not immediately corrode, which results in the time that the corrosive ions such as Cl^– reach the metal/coating interface not necessarily being the same as the time for the protection life of galvanizing coating. When the corrosion rate of galvanized steel increases rapidly, the open circuit potential of galvanized steel should reverse. While zinc loses the anodic protection of steel and the carbon steel substrate starts to corrode quickly, the corrosion rate of galvanized steel should increase rapidly, and the open circuit potential of galvanized steel will reverse. Specific experimental verification is still in progress and ready to be published.

Novel and high-efficiency corrosion resistant coating systems for HDGS are highly significant for their wide applications in things such as high-speed rail overhead line parts. They are important to, for example, galvanizing process control, paint coatings, and electrochemical protection; they ensure that the global HDGS field is at a leading position by applying various coating systems for their superior corrosion resistance. For the moment, novel coating systems still need to acquire corrosion resistance coatings with better adhesion property to the substrates on the basis of the types of HDGS, and need to enhance the corrosion resistance efficacy for the purpose of prolonging the service life of devices or fulfilling the applications of HDGS under more severe natural environments. Hence, plenty of novel and efficient coating systems are invented and reported as a brief summary in this light. Furthermore, valid tactics for the discovery of new coating systems, such as silane films, rare-earth conversion films, conductive polymers, and self-assembled films, have been illustrated by the latest reported literature.

The future developments of coating systems for HDGS would affect a number of fields. Firstly, the modifications of the present coating systems would be affected through, for example, the structural modification of silane films and composition optimization of rare-earth conversion films. Also affected would be the discovery of novel coating systems. To cope with the worsening environment, new-emerging coating systems with better improved corrosion resistance should be necessary for the future applications of HDGS. Thirdly, methodology research for the evaluation of these aforementioned coating systems would be influenced by any developments in coating systems for HDGS. Due to the lack of unified lifetime evaluation methods for these composite coating systems, it is desiderate to establish reasonable lifetime evaluation criteria. The establishment of evaluation methods will have a huge positive impact on production and life, asit will allow us to judge whether the coating systems will lose protection functioning to avoid greater losses. Therefore, it is essential to comprehensively understand the experimentally and theoretically intrinsic properties of the corrosion process for the HDGS composite coating systems in relation to the life evaluation of these coating systems and the design of preferable coating systems for HDGS.

This review is mainly accomplished by ZY under the guidances of HM and JH.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.