Effects of the Different Solid Deposits on the Corrosion Behavior of Pure Fe in Water Vapor at 500°C

A comprehensive corrosion investigation of pure Fe in an environment of solid sodium salt deposit (i.e., NaCl or Na2SO4) with mixtures of H2O and O2 at 500°C was conducted by mass gain measurement, X-ray diffraction (XRD), scanning electron microscope (SEM), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results showed that corrosion rates were accelerated with solid NaCl or Na2SO4 deposit due to their reaction with the formed protective scale of Fe2O3 and subsequently resulted in its breakdown. The corrosion rate of pure Fe with solid NaCl is higher than that with solid Na2SO4 because of the lower activation energy (Ea) for chemical reaction of Fe in solid NaCl+H2O+O2 (i.e., 140.5 kJ/mol) than that in solid Na2SO4+H2O+O2 (i.e., 200.9 kJ/mol). Notably, the electrochemical corrosion rate of pure Fe with solid NaCl deposit, 1.16 × 10−4 A/cm2, was a little lower than that with solid Na2SO4 deposit.


Introduction
Corrosion of metal materials is severe in the environment with solid salt deposit and dry or wet O 2 at medium and high temperatures [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18], especially for turbine blades in planes or ships and power boilers. Due to their excellent mechanical properties, low cost, and ease of machining, pure Fe and its alloys are the popular materials that were investigated in solid alkali chloride deposit in dry or wet O 2 [1][2][3][4][5][6][7]. Most researchers [2,5,6] thought that the solid NaCl could react with Fe 2 O 3 to generate Cl 2 via the reaction, 2NaCl + Fe 2 O 3 + 1/2O 2 = Na 2 Fe 2 O 4 + Cl 2 . Then, Cl 2 could react with Fe to form FeCl(s), i.e., Cl 2 + Fe = FeCl 2 ðsÞ. As such, FeCl 2 (s) would continuously evaporate under low vapor pressure at high temperature. The FeCl 2 vapor would diffuse outward through cracks and pores of the scale. Finally, the FeCl 2 vapor would react with O 2 to form Fe 3 O 4 and/or Fe 2 O 3 when they met during the process. Apparently, the solid NaCl could react with the oxidation scale that formed in dry or wet air at medium and high tempera-tures and lead to the breakdown of the protective scale to accelerate the corrosion rate of materials. Folkenson et al. [7] proposed a new mechanism as follows. Solid KCl reacts with O 2 and H 2 O to generate chloride ions (i.e., 2KCl + 1/2 O 2 + H 2 O + 2e − ⟶ 2KOH ðadsÞ + 2Cl − ðadsÞ) and subsequently react with iron ions to form FeCl 2 (s). Cao et al. [3] postulated a hypothesis of "dynamic water film" in which H 2 O molecules were continuously being absorbed on and evaporated from the surface of the material. The electrochemical corrosion might occur in the dynamic water film, which accelerates the corrosion of metal [3]. Shu et al. [10] used impedance spectroscopy to investigate the corrosion mechanisms of pure Fe and pure Cr with solid NaCl deposit in water vapor at 600°C. According to the analysis of the resistance and capacitance of corrosion scale, the electrochemical corrosion was proved to occur in the corrosion environments. After that, Tang et al. [1] investigated the interaction between chemical reactions and electrochemical reactions of pure Fe in this corrosion environment. The chemical reactions and electrochemical reactions follow "ce mechanism," in which Fe and Fe 2 O 3 first react chemically with NaCl, water vapor, and oxygen to generate HCl (g). Then, the HCl (g) reacts with pure Fe electrochemically via a one-electron electrochemical reduction to form H 2 .
Compared to the alkali chlorides, the corrosion mechanisms of metal/alloys with solid sulfate are lacking. However, the corrosion mechanisms are focused on molten sulfate. Recently, many studies have been carried out on the corrosion behavior of metals/alloys in a molten Na 2 SO 4 environment [19][20][21][22][23][24][25] and many corrosion mechanisms have been proposed. One of the well-known mechanisms is the sulfidation model [19,20], in which the formation of sulfides accelerates the corrosion. The other one is the acidic-basic fluxing [19,[21][22][23][24] mechanism, in which dissolution of the protective oxide scales, due to formation of basic Na 2 O, was considered the reason for the accelerated corrosion. Moreover, based on the electrochemical mechanism [25], corrosion was considered an electrochemical reaction in which the transfer of electrons accelerated the corrosion. Tang et al. [26] investigated the corrosion behavior of pure Fe under solid Na 2 SO 4 deposit in wet oxygen flow at 500°C. The results showed that the corrosion of Fe includes chemical corrosion and electrochemical corrosion. The chemical reaction and electrochemical reaction follows the "ce mechanism." Fe and Fe 2 O 3 first react chemically with Na 2 SO 4 , water vapor, and oxygen to generate H 2 SO 4 (g). And then, the H 2 SO 4 (g) reacts with pure Fe electrochemically via a one-electron electrochemical reduction to form H 2 . The coeffect between deposited solid Na 2 SO 4 and H 2 O+O 2 certainly exists and significantly accelerates the corrosion of pure Fe.
NaCl and Na 2 SO 4 are normal corrosive mediums. The corrosion of materials in solid salt environment depends on the anion of the salt [18,27]. However, the effects of the different solid deposits (NaCl and Na 2 SO 4 ) on the corrosion behavior of pure Fe in water vapor are still unclear. In this paper, the corrosion differences of pure Fe with solid NaCl and solid Na 2 SO 4 deposit in water vapor were comparatively studied to cognize the corrosion behaviors of materials in the corrosion profoundly.

Experimental
The pure Fe (99.9%) was used as experimental specimen. The metallography of the specimen is shown in Figure 1. The microstructure of pure Fe is ferrite. The maximum grain size is about 100 μm. Before the experiment, the sample was ground using silicon-carbide abrasive papers down to 1000 grit, degreased in acetone then ethanol, and dried in air before use. The NaCl and Na 2 SO 4 are of analytical purity (≥99.5%). The solid salt was deposited on the preheated Fe sample surface by repeatedly brushing and drying a saltsaturated solution. The mass of salt was about 4 mg/cm 2 . The temperature of the furnace was controlled at 500°C. H 2 O came from an 80°C water bath. Pure O 2 was passed through the glass bubbler with a flux of 200 ml/min.
The corrosion test was carried out in a thermal balance [2]. To prevent the H 2 O from condensing in the upper part of the thermal balance, a counterflow of N 2 was passed through the apparatus at 150 ml/min. After the furnace was heated to the desired temperature and the gas flow was stabilized, the specimen was quickly suspended into the furnace tube, and the test was started. All the measurements were carried out at ambient pressure. After the tests, the specimens were further examined by XRD and SEM.
A special three-electrode system was built for the electrochemical measurements in this particular environment [1]. To decrease the resistance of the solution and obtain a uniform electric field, the reference electrodes consisted of four platinum wires, each with a diameter 0.4 mm, and the counter electrode was a circular strip of platinum foil about 2 mm wide. All potential values in this paper were reported versus this platinum reference electrode. The Fe working electrode was a rod 10 mm long and 5 mm diameter. The three electrodes were placed in quartz tubes, which acted as insulators. All the gaps were sealed by high-temperature inorganic glue. The three-electrode system after solid NaCl and solid Na 2 SO 4 deposition was directly put into the furnace at the desired temperature and with water vapor for electrochemical measurements.
A PAR2273 Electrochemical Measurement System manufactured by EG&G was used for all electrochemical measurements, which also has the function to compensate the resistance between reference electrode and working electrode. In the galvanic corrosion measurement, the ratio of anodic area to cathodic area is 1 : 2. In the potentiodynamic polarization measurements, the measurements were carried out after 1000 s in the corrosion environment for obtaining an electrochemical stability and the scan rate was 1 mV/s. The resistance between reference and working electrodes was compensated during measurements according to the design of electrochemical system and testing work station. All measurements were repeated more than three times. Figure 2 shows the mass gain of pure Fe as a function of time at 500°C with and without solid NaCl or Na 2 SO 4 [26] in O 2 containing water vapor. As is seen from Figure 2, the corrosion of pure Fe is accelerated with solid NaCl or Na 2 SO 4 deposit. Compared to the case with solid Na 2 SO 4 deposit,  2 Scanning the corrosion rate of pure Fe with solid NaCl deposit is slightly higher at all-time duration.

Results and Discussion
In our previous studies [1,13], it is found that the corrosion of pure Fe in both corrosion environments includes a chemical corrosion process and an electrochemical corrosion process, while the overall corrosion is dominated by the chemical corrosion process with a percentage of over 90%. Herein, we investigated the differences of chemical corrosion that is influenced by NaCl and Na 2 SO 4 .
To affect the chemical corrosion rate, there are two aspects: (a) the protection of scale on the surface of pure Fe. The compact and integrated scale can restrain the corrosion of substrate. (b) The activity of corrosion reactants. As it is known, the corrosion rate would increase with a decreasing active energy. The details of the effects are discussed as follows.
The scale includes solid salt deposition scale (NaCl or Na 2 SO 4 ) and corrosion scale on the surface of pure Fe. Figures 3(a) and 3(b) show the surface morphologies of solid NaCl and Na 2 SO 4 , respectively, before corrosion test. The results showed that both salt scales are loose and porous. However, the solid NaCl film was much looser and more porous than solid Na 2 SO 4 film, which led to an easy transport of H 2 O and O 2 to the interface of pure Fe and solid NaCl film, promoting the chemical corrosion process of pure Fe.
The corrosion of materials with solid salt deposit in water vapor is different with that in aqueous solution. The corrosion scale would stay on the surface of substrate, which should restrain the corrosion of substrate. Figure 4 shows the cross-sectional morphologies of pure Fe after 10 h corrosion at 500°C in NaCl + H 2 O + O 2 (Figure 4(a)) and Na 2 SO 4 + H 2 O + O 2 (Figure 4(b)) [26]. It indicated that the corrosion scale formed on the surface of pure Fe was loose and porous in both corrosion environments. A number of volatile species are formed in the corrosion process, which could contribute to the formation of the loose and porous corrosion scale [6]. As a matter of fact, some green deposits were observed on the tube inner surface of the furnace after many hours of experiments, confirming the formation of volatile species. However, the scale formed in solid NaCl + H 2 O + O 2 is looser and higher porosity than that formed in solid Na 2 SO 4 + H 2 O + O 2 [26]. This indicated that the reactants (H 2 O and O 2 ) could be easier to transport through the corrosion scale formed in the environment with NaCl. Eventually, it promotes the chemical corrosion process of pure Fe.
The  [27], the generation of Fe 2 O 3 or Fe 3 O 4 is closely relative to oxygen pressure. Fe 2 O 3 would be generated at a relatively high oxygen pressure, while Fe 3 O 4 would be generated at a relatively low oxygen pressure. From Figures 3 and 4, the NaCl scale is looser with higher porosity than Na 2 SO 4 scale; meanwhile, the corrosion scale formed in the case of NaCl + H 2 O + O 2 was also looser with higher porosity than that formed in the case of Na 2 SO 4 + H 2 O + O 2 . The oxygen could transport inward through the corrosion scale and solid NaCl scale easily. The oxygen pressure in the corrosion scale that formed in NaCl + H 2 O + O 2 is higher than that formed in Na 2 SO 4 + H 2 O + O 2 . This is the reason why the components of the corrosion scales in the two corrosion environments were different.
The corrosion mechanism of pure Fe in the two corrosion environments could be understood on the basis of the components, morphologies of the corrosion scales, and published research.
For the case of solid NaCl, firstly, NaCl reacts with Fe 2 O 3 and H 2 O to generate Na 2 Fe 2 O 4 and HCl [4].
The generated HCl could react with Fe to form FeCl 2 [4], meanwhile, HCl could also react with O 2 to form Cl 2 [27].
The Cl 2 could react with Fe to form FeCl 2 [28].
The generation of H 2 SO 4 and H 2 led to the formation of many holes and cracks in the scale (see Figure 4(b)).
According to the morphologies shown in Figures 3 and 4, the more porous NaCl scale and corrosion scale formed in     [1,26]. For the ce mechanism, the relationship between phase angle and frequency could be given as Equation (9) [28].
where Φ is used for representing for phase angle, k 1 and k 2 for chemical reaction rate constants, ω for angular frequency, D for diffusion coefficient, k h for apparent heterogeneous rate constant, f for activity coefficient, α for charge transfer coefficient, n for number of electrons transferred, E d:c: for applied d. c. potential, E r 1/2 for reversible half-wave potential, and F, R , and T for their conventional electrochemical meanings. The value of k 1 can be calculated using Equation (9). Figure 6 shows the relationship between frequency and phase angles of pure Fe in solid NaCl + H 2 O + O 2 at 500°C. The plots have a maximum. It suggests that the corrosion mechanism of pure Fe in the two corrosion environments involves the interaction of the chemical and the electrochemical reactions, which is similar with pure Fe in solid Na 2 SO 4 + H 2 O + O 2 [26]. The calculated values of k 1 for pure Fe in solid NaCl + H 2 O + O 2 and solid Na 2 SO 4 + H 2 O + O 2 corrosion environments are 0.230 sec -1 and 0.031 sec -1 , respectively. Therefore, the chemical corrosion rate of pure Fe in solid NaCl + H 2 O + O 2 is higher than that in solid Na 2 SO 4 + H 2 O + O 2 because of its higher chemical reaction rate constant in the case with solid NaCl.
Chemical reaction rate is closely related with the activation energy. The lower the activation energy is, the higher the chemical reaction rate is. According to the logarithmic Arrhenius equation, the rate constant (k) dependence of temperature (T) is given by the relationship (g) [29]: where k is used for representing for the rate constant, A for a temperature-independent constant (often called the frequency factor), T for the absolute temperature, R for the universal gas constant, and E a for the activation energy. According to Equation (15) [1,26]. The potentiodynamic polarization plot of pure Fe in solid NaCl + H 2 O + O 2 at 500°C is shown in Figure 8. The anodic current densities of pure Fe in both two corrosion environments increase linearly with anodic potential increasing in the active polarization zone, which can be attributed to active dissolution in the aqueous environment, because the loose and porous corrosion scale could   [26], respectively. The amount of Fe corroded by electrochemical reaction was obtained using Faraday's rule. After the calculation, the chemical reaction rates of pure Fe in the two corrosion environments within 1 h are 0.036 g/h/cm 2 and 0.041 g/h/cm 2 , respectively. It must illustrate that the calculation time herein is in one hour, because the potentiodynamic polarization measurements were carried out within one hour, and there is no significantly variety of the electrochemical corrosion rate in one hour. This was proved by presented authors used an electrochemical instrument named CMB 1510B (based on weak polarization theory) manufactured by State Key Laboratory for Corrosion and Protection, to measure the electrochemical corrosion rate every 4 minutes during the whole corrosion reaction. The electrochemical corrosion rate of pure Fe in solid NaCl + H 2 O + O 2 is slightly lower than that in solid Na 2 SO 4 + H 2 O + O 2 .
As is well-known, charge transfer is the fundamental characteristic of the electrochemical reaction [30,31]. The corrosion scale and solid salt scale are the key influence factors for electrochemical reaction rate. The corrosion scale of pure Fe formed in solid NaCl + H 2 O + O 2 is looser and more porous than those formed in solid Na 2 SO 4 + H 2 O + O 2 (see Figure 4), and the solid NaCl scale is also looser and more porous than solid Na 2 SO 4 scale (see Figure 3).  [32], which also inhibit the electrochemical corrosion rate of pure Fe in solid NaCl + H 2 O + O 2 corrosion environment.

Conclusion
The corrosion rate of the pure Fe is significantly accelerated under a NaCl or Na 2 SO 4 deposit in an atmosphere of H 2 O + O 2 at 500°C. Both the salts of NaCl and Na 2 SO 4 could react with Fe 2 O 3 to result in a breakdown of the protective scale and subsequently accelerate the corrosion rate of pure Fe.
Compared to the case in solid Na 2 SO 4 + H 2 O + O 2 , the corrosion rate of pure Fe is much higher in solid NaCl + H 2 O + O 2 . The activation energy (E a ) for chemical reaction of pure Fe in solid Na 2 SO 4 + H 2 O + O 2 is 200.9 kJ/mol, which is higher than that of pure Fe in solid NaCl + H 2 O + O 2 .
The percentage contribution of the electrochemical reactions in total corrosion is insignificant. It was also found that the electrochemical corrosion rate of pure Fe with solid NaCl deposit was 1:16 × 10 −4 A/cm 2 , which was a little lower than that with solid Na 2 SO 4 deposit.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.