Hydrophobic durability and anti-corrosion of plasma nitriding layer: Evolution mechanisms of corrosion behavior under variations in local hydrochemistry

Mechanisms for the effects of nitrogen on the durability of hydrophobic and corrosion resistance of modified layers prepared on the surface of AH32 steel by plasma nitriding have been investigated. It was found that plasma nitriding can significantly improve the hydrophobicity of AH32 steel. All the contact angles (CAs) of the AH32 steel with 1 h, 3 h and 5 h nitriding were higher than 120°, which had excellent hydrophobicity. The acidification degree of the solution within the nitriding layer’s micro-zone was effectively reduced due to the dissolution of nitrogen. The micro-nano structures of the nitriding layers corroded severely after 15 days of exposure to NaCl solution. When the nitriding time is extended to 5 h, the acidification degree of the solution was intensified due to the enhanced blocking effect, and the nitrogen in the nitriding layer saturated that cannot provide enough nitrogen ions to offset the acidification of the solution, so the durability decreased instead. In comparison, the modified layer with 3 h nitriding in this study had the best durability and long-term service protective effect on AH32 steel.


Introduction
AH32 steel has been widely used in the shipbuilding industry because of its excellent impact resistance, machinability and weldability due to the addition of alloying elements such as Mn, Si, Nb and V [1,2]. However, due to the high content of alloying elements, AH32 steel is susceptible to severe corrosion damage after longterm service in harsh marine environments containing multiple microorganisms and high salinity [3,4]. Although the surface coating is one of the most effective means to solve the poor corrosion resistance of metals, in the marine environment, high concentrations of chloride ions can easily damage the passive film on the metal surface and exacerbate its localized corrosion susceptibility [5][6][7]. Meanwhile, the biofilm formed on the metal surface by the covering of marine microorganisms can form gaps with the substrate. Consequently, oxygen concentration cells are formed, resulting in localized corrosion failure ultimately [8,9]. Therefore, to improve the corrosion resistance of metals in a marine environment with high salinity and a variety of microorganisms, it is imperative to prevent salt penetration and microorganisms adsorption on the metal surface.
A hydrophobic surface with excellent water repellency can effectively reduce the contact of corrosive media and microorganisms with the substrate material, which is one of the most effective means for improving metal corrosion resistance [10][11][12][13]. It is known that the construction of micro-nano structures of an appropriate size and the modification of low surface energy are the two most commonly used methods for preparing hydrophobic surfaces [14][15][16]. However, the micro-nano structures and low surface energy substances are easily damaged under the attack of mechanical damage, chemical reactions and bacterial contamination in the natural service environment [17][18][19]. As a result, the hydrophobicity of the surface has been reduced, while the anticorrosion effect has deteriorated.
The durability of the hydrophobic surface is thought to play a key role in limiting its wide range of applications. It is believed that the mechanical damage is one of the major challenge for hydrophobic coatings [20]. In marine environments, the hydrophobic coating may lose its hydrophobic and anti-corrosion effects due to the mechanical wear generated by wind waves, ocean currents and floats such as sand [21,22]. Moreover, electrochemical erosion is also a reason for the loss of hydrophobicity of coatings. Liu et al [23] prepared a longterm stable superhydrophobic coating on the surface of Mg-Li alloy by chemical etching and fluorination modification techniques. However, whether the coatings also have better durability properties in aggressive media such as chloride ions has not been elucidated. Ishizaki et al [24] prepared a superhydrophobic coating on the surface of AZ31 magnesium alloy by plasma-enhanced chemical vapour deposition. The coating exhibits good durability in both acidic and chloride ion environments within 24 h. However, the hydrophobicity after longer exposure times have not been clarified. It is believed that the micro-zones of hydrophobic structures on the metal surface and biofilm-covered areas are susceptible to localized corrosion attack in a chloride ion environment [25,26]. In addition, there is usually a long incubation period before localized corrosion occurs [27]. As a result, the hydrophobicity of the coating may not always be maintained at a high level during longterm service. The durability property of hydrophobic coatings may be severely reduced due to localized corrosion. In summary, improving the mechanical properties of the hydrophobic coating and lowering its localized corrosion susceptibility during service are effective ways to improve the hydrophobic durability of the coating.
Nitriding can improve the surface mechanical properties of metals, and the nitrogen element in the nitriding layer can change the hydrochemistry environment of the micro-zone near the nitriding layer. Ultimately, the localized corrosion behaviors of the coating and substrate are affected [28][29][30]. In the previous studies [31,32], we found that the dissolution of nitrogen from the coating can decrease the concentration of hydrogen ions within the solution, and thus the pH value of the solution is increased. This can reduce the acidification of the solution within the localized area and mitigate the localized corrosion of the coatings. Conventional nitriding processes generally have higher nitriding temperatures and longer nitriding times. During this process, the alloying elements of the substrate are segregated. This can also lead to a drastic reduction in the corrosion resistance of the substrate materials [33]. Plasma nitriding technology allows nitriding at lower temperatures and with higher nitriding efficiency. This can reduce the impact of the nitriding process on substrate properties. Therefore, the preparation of a modified layer with high hydrophobic durability and corrosion resistance using the plasma nitriding method is expected to be one of the effective means to solve the poor durability of conventional hydrophobic surfaces. However, very little work has been done to study the effects of plasma nitriding on the durability property of hydrophobic and anti-corrosion coatings on metal surface and the related mechanisms have not been clarified. Therefore, the purpose of the present work was to investigate the microstructures, morphologies, surface roughness, chemical compositions, hydrophobicity and corrosion resistance of the nitriding layer prepared by plasma nitriding on the surface of AH32 steel, using atomic force microscope (AFM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), surface roughness tester, x-ray diffraction (XRD), contact angle meter (CAM), polarization curves and electrochemical impedance spectroscopy (EIS). The influence of nitriding time on the durability of hydrophobic and anticorrosion properties of AH32 steel were carefully studied and the related mechanisms are also discussed.

Materials and specimens
The chemical composition of the AH32 steel used in the present work is listed in table 1. Specimens were cut from a plate with a size of 20 mm × 20 mm × 5 mm. The specimens were mechanically abraded with emery paper up to 2000 grit successively and polished to a mirror finish with metallographic polishing paste. Prior to plasma nitriding, the specimens were ultrasonically washed in acetone and anhydrous ethanol in sequence.

Experimental procedures
The nitriding layers were prepared by a plasma-enhanced nitriding system which was described detailed in the present work [34], the schematic diagram of the system is in figure 1. The specimens were fixed on the sample holder, and then the vacuum was created using a mechanical pump and a molecular pump. Make a highpressured cleaning of the specimen surface with argon gas. The plasma nitriding was carried out for 1 h, 3 h and 5 h after adjusting the argon (used to sputter the substrate) flow to 140 sccm (ml/min), nitrogen (used to prepare the nitriding layer) flow to 10 sccm, voltage to −300 V and temperature to 380°C respectively. And detailed information on the nitriding process has been described in the previous works [31,34]. The specimens were cleaned and dried carefully after the nitriding was finished. The surface morphologies of the nitriding layers were observed by AFM (CSPM5500), SEM with EDS (SIGMA) and the phase analyses of the nitriding layers by different nitriding times were performed by XRD analyzer (D/Max 2400). The surface roughness of the nitriding layer was measured by a stylus-type surface roughness meter and the contact angle (CA) of the nitriding layer was measured with a micro-sampler (1 μl Micro injector).
The specimens were then exposed to the solution of 3.5 wt% NaCl at room temperature for 15 days. The electrochemical behaviors of the nitriding layer at different nitriding times were examined by an electrochemical workstation (Vertex. C.) during the exposure test. Three electrodes were used in the measurement system. The specimen as the working electrode, with the reference electrode and counter electrode being comprised of saturated calomel electrode and platinum plate. The open circuit potential (OCP) of each specimen was monitored for 30 min before the electrochemical test in order to ensure the system of the solution is stable. The EIS measurements of the nitriding layers at different exposure times were tested at the frequency spectrum ranging from 0.01 Hz to 1 × 10 6 Hz with an amplitude of 0.01 V. The Tafel polarization curves were tested at the current range from −200 mV to 400 mV relative to the OCP with a sweep rate of 1.6 mV s −1 [32]. After the exposure test, the specimens were cleaned and dried charily. And then, the morphologies, phase and surface roughness of different specimens were characterized. In addition, the corrosion weight loss rates of the AH32 steels with different nitriding times were also measured after the 15-day exposure test. To minimize errors, three parallel specimens were selected for measurement in each group. The corrosion products were cleaned by the solution which composed of 500 ml hydrochloric acid (HCl, ρ = 1.19 g ml −1 ), 3.5 g hexamethyltetramine and deionized water was configured into 1000 ml for 10 min at room temperature [35]. Figure 2 shows the static CAs of the nitriding layers at different nitriding times around the exposure test. The CA of each specimen was greater than 120°, which showed good hydrophobicity. In contrast, after the exposure test, the CAs of the nitriding layers were all decreased to some extent. It is found that the nitriding layers with 1 h and 5 h of nitriding had large variations in CAs, decreased by nearly 50°, while the CA of the nitriding layer with 3 h nitriding decreased the least, by only about 20°. It is indicated that the nitriding time had a significant effect on the hydrophobic durability of the nitriding layer on the surface of AH32 steel.  Figure 3 shows the SEM and AFM morphologies of the nitriding layers formed on the surface of AH32 steel with different nitriding times before and after the exposure test. It is found that all the surface morphologies of AH32 steels after plasma nitriding showed micro-nano structures that distributed a large number of nanoscale particles. And the heights of the micro-nano structures increased with the nitriding time (figures 3(a), (c) and (e)). That is, with increasing nitriding time, the surface roughness of AH32 steel increased. However, it is found  that the micro-nano structures of the nitriding layers were corrode severely after 15 days of exposure to NaCl solution. The heights of the micro-nano structures of the nitriding layers with 1 h and 5 h nitriding were decreased significantly (figures 3(b) and (f)), but the height reduction of the micro-nano structures by nitriding for 3 h was relatively smaller (figure 3(d)). It is also found from the results of EDS that the chemical composition of the nitriding layer and the corrosion product layer consists mainly of Fe, O and N. The N content in the nitriding layer increased with increasing nitriding time. However, the N content in the nitriding layer hardly differed for the 3 h and 5 h plasma nitriding. Moreover, the AH32 steel nitriding for 3 h had the lowest relative content of O in the corrosion products after the exposure test. Figure 4 shows the surface roughness of the nitriding layer formed on the surface of AH32 steel with different nitriding times before and after the exposure test. It is found that a significant difference in surface roughness of the nitriding layer under different nitriding times, and the longer the nitriding time before the exposure, the higher the surface roughness was. The surface roughness of the nitriding layer for 1 h nitriding was 182.5 nm, which is the lowest of the three nitriding layers. And the surface roughness of the nitriding layer with 3 h nitriding increase to 349.7 nm. By the time of 5 h nitriding, the roughness is 534.1 nm as the maximum. However, it is found that the reduction of the nitriding layer's roughness after exposure in NaCl solution was not linearly related to the nitriding time, the roughness reductions of 1 h and 5 h nitriding were more obvious than that of 3 h nitriding. This consistent with the result of the surface morphologies and the CA results. This further shown that after a prolonged exposure in NaCl solution, the hydrophobicity and micro-nano structures of the nitriding layer on the surface of AH32 steel were significantly influenced by the time of nitriding.  Figure 5 shows the XRD patterns of the nitriding layers formed on the surface of AH32 steel with different nitriding times. Characteristic peaks of Fe 3 N (at the position of 2θ = 43°) and Fe 2 N (at the position of 2θ = 38°a nd 40°) were detected in the nitriding layers after plasma nitriding. And the intensity of characteristic peaks of Fe 3 N and Fe 2 N in the nitriding layers after 5 h nitriding has no evident difference pared to that after 3 h nitriding, but the intensity of characteristic peaks of the two nitriding layers are both stronger than that after 1 h nitriding ( figure 5(a)). Moreover, it is found from the XRD patterns of the nitriding layers after the exposure test that characteristic peaks of corrosion products of Fe 3 O 4 (at the position of 2θ = 31°and 33°), Fe 2 O 3 (at the position of 2θ = 47°and 48°) and FeOOH (at the position of 2θ = 47°, 36°and 27°) were detected besides the original nitriding structure, and the intensity of characteristic peaks of corrosion products are significantly different with different nitriding times ( figure 5(b)). The nitriding layer of 3 h nitriding is exposed to the lower intensity of characteristic peak of corrosion products that indicates the lowest degree of corrosion at this condition. The degree of corrosion of AH32 steel after the exposure test at different nitriding times was inconsistent with the hydrophobic results before the exposure test. It is indicated that the hydrophobicity of the nitriding layers with different nitriding times changed significantly during prolonged exposure in the solution of 3.5 wt% NaCl, which can directly affect the durability of the nitriding layer. Therefore, further studies on the durability of nitriding layers at different nitriding times are needed.

Discussion
The analyses of CAs (figure 2), morphologies (figure 3) and surface roughness (figure 4) in the present work suggest that the hydrophobicity of the nitriding layer on the surface of AH32 was affected by the different nitriding times. It is believed that the hydrophobicity of materials generally depends on their surface micro-nano structures [14], and the structures of the nitriding layers are mainly related to their growth process. It is assumed that the growth pattern of the nitriding layer generally follows the Stranski-Krastanov growth mechanism [36]. At the beginning of nitriding, the nitrogen ions were adsorbed on the metal surface, and a two-dimensional laminated nitriding layer of several atomic layers thicknesses was formed. Due to the lattice mismatch between the nitriding layer and the substrate metal, strain energy was accumulated within the nitriding layer. When the strain energy is released, it can stimulate insular growth in the nitriding layer. Island-like growth was easier to achieve when the strain energy in the nitriding layer was higher. So the heights of the micro-nano structures increased with the nitriding time, the more uneven the surface of the nitriding layer, the greater the surface roughness.
It is generally accepted that the hydrophobicity of the material surface produces a significant effect on its corrosion behavior [37,38]. The less likely the material is to be infiltrated by the corrosion medium because of the better hydrophobicity, so the corrosion resistance of metal materials gets better with the increase of CA. Therefore, the present work further researches the corrosion resistance of the different nitriding layers. Figure 6 shows the potentiodynamic polarization curves of the nitriding layers on the surface of AH32 steel with different nitriding times in the solution of 3.5 wt% NaCl before the exposure test. The fitting results of the potentiodynamic polarization curves in figure 6 are summarized in table 2. It is found that the corrosion resistance of the nitriding layers make differences with different nitriding times. With the increase of nitriding time, the corrosion potential of AH32 steel increases and the corrosion current density decreases. When the nitriding time was 5 h, the corrosion current density of the nitriding layer is 6.21 × 10 −7 A cm −2 which was reduced by two orders of magnitude compared with the nitriding layer with 1 h nitriding. The corrosion rates of the AH32 steel with nitriding time of 1 h, 3 h and 5 h were 3.41 × 10 −3 mm y −1 , 1.86 × 10 −4 mm y −1 and 1.71 × 10 −5 mm y −1 , respectively. The corrosion resistance of the nitriding layer was increased obviously with the increase of nitriding time. However, from the average corrosion weight loss results of different nitriding layers after 15 days of exposure test, the corrosion weight loss rate of AH32 steel with 3 h nitriding was the lowest, about 85.2 g m −2 a −1 . And the corrosion weight loss rates with the nitriding time of 1 h and 5 h are 194.6 g m −2 a −1 and 126.3 g m −2 a −1 , respectively. It can be found that the corrosion degrees of AH32 steel with different nitriding times after exposure tests differed from the corrosion resistance before exposure tests. If the nitriding time is further extended, the nitrogen in the nitriding layer tends to saturate. The nitriding layer cannot supply enough nitrogen atoms to mitigate the acidification of the micro-zone solution, and the hydrophobic resistance of the nitriding layer has decreased. This in turn leads to a decrease in corrosion resistance. This is consistent with the CA, roughness and XRD results.
The above results indicated that the nitriding layer's surface states and corrosion behaviors on the surface of AH32 steel had changed significantly during prolonged exposure to NaCl solution. It is thought that EIS enables in situ characterization of the electrode surface state and electrode processes. Figure 7 shows the EIS results of the nitriding layer on the surface of AH32 steel with different nitriding times exposed to 3.5 wt% NaCl solution during the exposure test. It is found that the impedance modulus (|Z|) results of AH32 steel all decreased gradually with the increase of exposure time, irrespective of the nitriding time. This indicated that corrosion occurred in all the different nitriding layers as the exposure progressed. Figure 8 shows the equivalent circuit used for the electrode process of the nitriding layers with different nitriding times by the EIS results of figure 7, where R 1 denotes the resistance of 3.5 wt% NaCl solution. R 2 denotes the resistance of the nitriding layer. C 1 denotes the capacitance of the nitriding layer. R 3 and C 2 are the resistance and capacitance of the AH32 substrate, respectively. The fitted parameters obtained from the equivalent circuit models are summarized in table 3. It is reported that by comparing the capacitance of various film layers, the pore sizes or the defect amounts within the film layers can be determined [39][40][41]. Therefore, in this study, the size of the micro-nano hydrophobic structures of different nitriding layers can be determined by comparing the value of C 1 . It is believed from the previous works [32,38] that the total amount of the micro-nano pores within the nitriding layer and the overall roughness of it were decreased when the C 1 value of the nitriding layer was increased. The C 1 value of the  nitriding layer with 1 h nitriding showed an order of magnitude increase by the second day of exposure. A significant decrease in the number and size of the micro-nano pores in the nitriding layer had occurred at this point. Combining the EIS and CA results, it shown that the hydrophobic durability of the AH32 steel with 1 h nitriding was poor and that the hydrophobic effect was already lost in less than two days of exposure to the NaCl solution and the substrate was subject to accelerated corrosion. Similarly, the C 1 value of the nitriding layer with 5 h nitriding changed significantly by the fifth day of exposure, indicating that the nitriding layer lost its hydrophobic effect at this point. However, the C 1 value of the nitriding layer with 3 h nitriding did not change significantly until the eleventh day of exposure. It was further demonstrated that the highest hydrophobic durability of AH32 steel was achieved at a nitriding time of 3 h. The hydrophobic surface of the material can improve its anti-corrosion performance has a great relationship with the voids between the micro-nano scale relief structure on the nitriding layer surface are occupied by air.  When the air against the droplet from infiltrating, it also prevents the corrosion solution from partially contacting the surface thus inhibiting corrosion to some extent [42,43]. However, once the micro-nano structure is changed, the material's hydrophobicity and corrosion behaviors will also change. Local hydrochemistry environment acidification near the micro-nano structures can destroy the hydrophobic structures, which, in turn, may affect the corrosion and hydrophobicity of the modified layers. In addition, the dissolution of nitrogen can mitigate the acidification of the micro-zone solution by forming NH 4 + , and reducing the corrosion of the substrate. Therefore, to study the corrosion damage behavior of hydrophobic surfaces during the exposure test, it is necessary first to elucidate the hydrochemical environment in local micro-regions. It is generally agreed that the transport of medium in solution relies heavily on diffusion, electromigration and liquid convection [44][45][46][47][48]. The convection of the liquid within an occlusion region can be ignored for this work, and the effects of diffusion and electromigration processes on the transport of the medium in the solution are mainly considered. The flux equation is given by [44] The rate of production or depletion of species i by chemical reaction is given by (1) and (2), the transport of medium in a dilute solution within the void of the occlusion zone follows the equation

Combining equations
R i can be obtained by converting the reaction current density in the specific chemical reactions which follows Faraday's laws, where l/A is the proportional coefficient of pore geometry size, the current density satisfies the equation where β is the Tafel slope of the cathodic reduction that follows the equation.
b a = - Table 3. Fitted parameters obtained from the equivalent circuit models in figure 7. Combining equations (4)-(6) and the boundary conditions are The general solution of equation (3) is where l 1 and l 2 are the two solutions of equation (3), q 1 and q 2 are the coefficients. The rest of the symbols are explained in the appendix. The constants and values of parameters used for the modeling are given in table 4. The solution's pH value variation at specific locations in the micro-zone with nitriding time can be calculated. Meanwhile, considering the effect of combining N + and H + to form NH 4 + on the pH value, the calculated results were corrected, and the correction factors are related to nitrogen content. Figure 9 shows the calculated results of the distributions of pH value within the pore of the nano-scale structure at different nitriding times. According to the potential-pH diagram of Fe-H 2 O and related studies [53,54], it is known that iron is susceptible to severe corrosion at room temperature when the pH of chlorine-containing solutions is around 3 to 3.5. The decrease rate of pH was fastest in the local micro-zone of AH32 steel with 1 h nitriding. By the next day, the pH had fallen below 3.5. However, the rate of pH decrease in the micro-zone was slowest when the nitriding time was 3 h, and it was not until after the seventh day that the pH was less than 3.5. This indicates that the AH32 steel with 3 h nitriding is exposed to NaCl solution for at least seven days before serious corrosion occurs. The results of the calculations and experiments also show that it is not the case that the longer the nitriding time, the greater the effect of the nitrogen atoms on the properties of the nitriding layer. Because the surface roughness was increased as the nitriding time increased, the occlusion effect brought about an increase in local corrosion susceptibility. However, due to the metal-nitride barrier effect, it is not the case that the longer the nitriding time, the higher the nitrogen content in the layer [55]. After a certain period of nitriding, the nitrogen content in the layer is saturated   [34,56]. Longer nitriding does not provide enough nitrogen atoms to neutralize the acidification of the microzone, so the degree of corrosion becomes more serious instead.
The above results and discussion in the present work suggest that nitriding layers with different nitriding times on AH32 steel influence their durability of hydrophobic and anti-corrosion behaviors. Figure 10 shows the schematics diagram of the influence mechanism of nitriding time on the hydrophobic and anti-corrosion properties of AH32 steel. The diffusion of oxygen from the bulk solution to the interior was obstructed by the occlusion zone formed by the micro-nano structures in the nitriding layer. As the increase of exposure time, the Fe 2+ accumulated within the occluded zone, which can result in positive charge in the internal solution. In order to maintain electrical neutrality, Clin the bulk solution migrate toward the occlusion zone under the influence of electrostatic forces. The hydrolysis of the metal chlorides generated H + , which acidified the solution in the occluded area, causing further corrosion. More Fe 2+ accumulated toward the gap opening, and large amounts of oxides were formed due to the relatively high oxygen concentration here. The occlusion effect was exacerbated by the formation of oxides at the openings. The nitriding layer and the substrate undergo severe corrosion damage under autocatalytic action, and the hydrophobic effect was diminished ( figure 10(a)). When the nitriding time was 3 h, although the depth of the occlusion zone was deeper, the acidification of the solution in the occlusion zone was reduced due to the dissolution of nitrogen ions during the corrosion process, which in turn reduced the local corrosion of AH32 steel, the hydrophobic durability of the nitriding layer was the highest ( figure 10(b)). However, when the nitriding time was 5 h, due to the nitrogen in the nitriding layer tended to saturate. The nitriding layer cannot provide enough nitrogen atoms to mitigate the micro-zone solution's acidification, and the nitriding layer's hydrophobic durability was reduced (figure 10(c)).

Conclusion
Effects of nitriding time on the hydrophobicity and durability of the nitriding layer formed by plasma nitriding on the surface of AH32 steel were investigated. The following conclusions could be drawn based on the present results.
(1)Plasma nitriding can significantly improve the hydrophobicity of AH32 steel. The CAs were increased with the prolongation of nitriding time. And the CAs of the AH32 steel with 1 h, 3 h and 5 h nitriding can each be higher than 120°.
(2)The degree of acidification of the solution within the micro-zone of the nitriding layer was effectively reduced due to the dissolution of nitrogen. The micro-nano structures of the nitriding layers were corrode severely after 15 days of exposure to NaCl solution. In comparison, the modified layer with 3 h nitriding had the lowest corrosion degree and the highest hydrophobic durability.
(3)When the nitriding time is extended to 5 h, the acidification degree of the solution was intensified due to the enhanced blocking effect, and the nitrogen in the nitriding layer tended to saturate that cannot provide enough nitrogen ions to offset the acidification of the solution, so the durability decreased instead. The modified layer with 3 h nitriding had the best durability and long-term service protective effect on AH32 steel.