Simultaneous Measurements of Polarization Resistance and Hydrogen Permeation Current of Iron in an Aqueous NaCl Droplet

Abstract

In this study, a Kelvin probe (KP) technique combined with the Devanathan-Stachurski electrochemical hydrogen permeation method was applied for simultaneous measurements of polarization resistance and hydrogen permeation current for iron to clarify the hydrogen uptake mechanism during drying of an NaCl droplet. The reciprocal of the polarization resistance, which is an index of the corrosion rate, the hydrogen permeation current, and the corrosion potential under the droplet were successfully measured. The corrosion potential decreased, and the hydrogen permeation current increased, after the NaCl droplet had been applied to the iron surface. The reciprocal of the corrosion resistance increased gradually during the drying of the droplet with increasing the corroded areas on the iron. The hydrogen permeation current decayed with the shift in the corrosion potential toward the noble side during the drying stage, before the droplet dried up completely. The hydrogen permeation current mainly followed the change in the corrosion potential. The hydrogen uptake mechanism of iron during corrosion is discussed in detail based on the corrosion potential, corrosion rate and hydrogen permeation behavior.

1. Introduction

In recent years, the strength of steel materials has been increased, facilitating resource and energy conservation. However, this increase in strength has led to a concurrent increase in the susceptibility of steel to hydrogen embrittlement (HE).^1,2) HE in steel is caused by absorption of atomic hydrogen. One source of hydrogen is aqueous corrosion, with hydrogen atoms produced by the hydrogen evolution reaction (HER).^3,4) Hydrogen uptake into steel during aqueous corrosion follows the HER, which is a cathodic reduction reaction. Therefore, the driving force and rate of the HER are important for clarifying hydrogen uptake behavior in steel, because these are associated with the content of hydrogen uptake in the steel.

The corrosion potential (E_corr) and environmental pH are the critical drivers of the HER. Figure 1 presents an electrode potential-pH diagram of iron (Fe) in water at 298 K. The red line in Fig. 1 is the equilibrium line for the HER, and the solid blue is a typical E_corr for Fe in a natural environment. The E_corr is mainly determined by both the rate of anodic dissolution of Fe and the oxygen reduction reaction (ORR) in an ambient environment. In this case, the driving force of the HER is the potential difference between the E_corr and the point on the red line at the environmental pH. The E_corr and environmental pH dominate the HER on Fe. As shown in Fig. 1, the driving force of the HER increases with decreasing E_corr and/or environmental pH. Therefore, the mechanism of hydrogen uptake into steel materials during aqueous corrosion should be clarified to facilitate methods for mitigating the HE.

Fig. 1.

Schematic drawing of the driving force for the HER on the Potential-pH diagram of Fe at 298 K. (Online version in color.)

Most steel materials are exposed to atmospheric environments and suffer from atmospheric corrosion. Atmospheric corrosion of steel proceeds under a thin water layer created by rain and dew condensation, and the rate of corrosion is associated with the water layer thickness in addition to environmental factors such as temperature, relative humidity (RH), and salt deposition. During atmospheric corrosion, the diffusion flux of the dissolved oxygen can be enhanced, because the diffusion layer is likely to become thinner than under conditions of natural convection.⁵⁾ This can also lead to a positive shift in the E_corr and enhancement of the corrosion rate (i_corr) of the steel under atmospheric environments.⁶⁾ However, this positive shift in the E_corr can lower the driving force for the HER. Additionally, since there is a very small amount of liquid in a thin water film, the solution chemistry during atmospheric corrosion can be changed relatively easily by a small amount of evolved ions. For example, during anodic dissolution, an increase in the i_corr of Fe can promote a decrease in pH due to the hydrolysis of the evolved Fe ions,⁷⁾ leading to an increase in the driving force for the HER. Therefore, to clarify the hydrogen uptake behavior of steel materials under atmospheric conditions, the E_corr and i_corr for Fe during atmospheric corrosion should be investigated in relation to the hydrogen uptake behavior.

Some researchers have applied the Kelvin probe (KP) technique to investigate the hydrogen uptake behavior of steel during corrosion.⁸⁾ Evers et al. successfully detected the change in surface potential caused by hydrogen uptake during steel corrosion, and similar experiments have been conducted by other researches.^9,10,11,12) We have successfully conducted simultaneous measurements of the E_corr and hydrogen permeation current (i_per) during drying of an NaCl droplet by combining the KP technique^6,13,14,15) with the Devanathan-Stachurski (DS) method.^{16,17,18,19,20,21)} The results showed that the E_corr was lowered with the onset of corrosion, and that a i_per was induced during the drying. The accumulation of iron rust also affects the hydrogen uptake behavior during atmospheric corrosion of steels.²¹⁾ However, there are few reports of simultaneous measurement of the i_corr and hydrogen uptake behavior using the KP technique.

According to Stern,²²⁾ the i_corr of metals is associated with the polarization resistance (R_p), which corresponds to the slope of the potential-current response near the E_corr. Thus, the i_corr of metals can be evaluated by the measurement of the R_p.^23,24,25) Actually, R_p measurements are often conducted using electrochemical impedance spectroscopy (EIS). EIS is applicable to evaluate the i_corr under a thin electrolyte film.^26,27,28) In addition, a KP technique has been also applied to measure the polarization behavior of metals under a thin electrolyte layer.^29,30,31)

In this study, we applied the KP technique to simultaneously evaluate the i_corr and E_corr of Fe during drying of an NaCl droplet by combining with the Devanathan-Stachurski electrochemical permeation method (DS method). Additionally, the hydrogen uptake behavior of Fe was investigated together with the corrosion behavior.

2. Experimental Method

2.1. Sample

Figure 2 shows a schematic diagram of the sample used for the measurement of the R_p by the KP technique. The sample consisted of a 2-mm-thick iron plate (99.5% purity; Nilaco Corporation, Japan) and a glassy carbon plate (BAS Inc., Japan). Both of these materials were fitted with lead wires and then embedded in epoxy resin to form a coplanar surface. The gap between these materials was set to be 0.25 mm. Before the experiment, the sample was polished up to #1000 grit using waterproof abrasive SiC papers, and then washed in an ethanol (EtOH) bath. The exposed surface area of both materials was 3.6 cm² and the remaining surface was covered with Teflon tape.

Fig. 2.

Schematic diagram of the sample used for the polarization resistance measurement by the KP technique. (Online version in color.)

Figure 3 shows the sample used for the i_corr evaluation with measurement of the i_per of Fe during drying of an NaCl droplet. A half-moon shaped Fe plate and a glassy carbon plate, both 8 mm in diameter, were embedded in epoxy resin separated by a gap of 0.2 mm. These were the same materials as in Fig. 2, and the configuration of the Fe and glassy carbon in this sample was also similar to that shown in Fig. 2. However, the back side of the Fe, which was used as the hydrogen withdrawal side in the DS method, could be exposed to a solution to measure the i_per of Fe. The Fe plate on the hydrogen withdrawal side was electrochemically coated with 400-nm-thick palladium (Pd). The Pd coating was applied using a commercially available Pd bath (K-pure Pd; Kojima Chemical Co. Ltd., Japan) at 308 K and −15 mA/cm² for 58 s. The surface of the glassy carbon on the hydrogen withdrawal side was covered with rubber paint.

Fig. 3.

Schematic drawing of the sample used for the simultaneous measurement of the polarization resistance and the hydrogen permeation current. (Online version in color.)

2.2. KP System

Figure 4 shows a schematic diagram of the KP system used in this study. The KP system has been described in detail elsewhere.²⁰⁾ As shown in Fig. 4, a sample was placed horizontally under a Kelvin probe (KP), composed of a gold wire (99.5% purity; Nilaco Corporation, Japan) with a diameter of 1 mm. This diameter is associated with the spatial resolution of the potential measurement. The KP was driven at 237 Hz and the vibration amplitude was estimated to be 25 μm.

Fig. 4.

Schematic diagram of the KP system used for the simultaneous measurement of the polarization resistance and hydrogen permeation current. (Online version in color.)

In this study, the Volta potential differences between the sample and the KP were measured by the parasitic capacitance method.^32,33) The Volta potential difference was determined by the relationship between the applied bias voltage and amplitude of the output AC current. Finally, the Volta potential difference was converted to the E_corr with reference to an Ag/AgCl electrode (SSE; E_SSE = +0.197 V vs. SHE at 298 K) based on the method described by Stratmann et al.⁶⁾ The method is described in detail in our previous report.²⁰⁾

2.3. Validation of the R_p Measurement

A volume of 380-μL of 0.1-mol/dm³ NaCl was placed on the sample, which is shown in Fig. 2, under the KP. The initial water film thickness was approximately 1 mm, and the salt deposition amount was 5.8 g/m². During drying of the droplet, a constant current ranging from 20 μA/cm² to −20 μA/cm² was input in a stepwise manner between the Fe plate and glassy carbon, and the Volta potential difference between the Fe plate and KP was measured simultaneously. The Volta potential difference was measured at the center of the Fe plate, as indicated in Fig. 2. A multi-channel potentio/galvanostat (PS-08; Toho Technical Research Co., Ltd., Japan) was used to control the applied current. The Volta potential obtained in this experiment was converted to the electrode potential using the method mentioned above. From the relationship between the applied current and the electrode potential, the R_p was calculated.

For comparison, the E_corr and R_p for a corroding steel were measured in a bulk solution by a conventional three-electrode electrochemical method. A carbon steel sample (SM400B) was immersed in a 0.1-mol/dm³ NaCl solution. During the immersion, a constant current ranging from 20 μA/cm² to −20 μA/cm² was applied in a stepwise manner between the steel and a platinum (Pt) counter electrode, and the electrode potential of the steel was measured against the SSE. The value of R_p was also calculated from the relationship between the electrode potential and the applied current.

2.4. Simultaneous Measurements of R_p and i_per

As shown in Fig. 4, the cell for the DS method was set in the KP system. The sample shown in Fig. 3 was fixed to the DS cell so that the hydrogen withdrawal side surface was in contact with the NaOH solution. In the DS cell, an iridium (Ir) wire (0.5 mm in diameter) and a Pt wire (0.4 mm in diameter) were used as the reference and counter electrode, respectively. The compartment of the hydrogen withdrawal side of the DS cell was filled with a 0.2-mol/dm³ NaOH solution. The surface of the Fe plate on the hydrogen withdrawal side was polarized at +0.1 V with respect to the Ir wire. The following test was performed after the residual passive current decreased to less than 100 nA/cm².

A 50-μL droplet of a 0.1-mol/dm³ NaCl solution was applied to the hydrogen entry side. The initial water film thickness was approximately 1 mm, and the amount of salt deposition was 5.8 g/m². The NaCl droplet was dried at 298 K at a RH of lower than 20%. During drying of the droplet, the i_per and Volta potential difference between the Fe plate and KP were measured simultaneously. The Volta potential difference was measured at the center of the Fe plate, as indicated in Fig. 3. The R_p of the Fe plate was measured intermittently as described in section 2.3.

3. Results

3.1. Polarization Resistance Evaluation

Figure 5 shows the relationships between the electrode potentials and applied currents sampled during the drying of a 0.1-mol/dm³ NaCl droplet. The sampling times for each relationship are shown in Fig. 5: although the plots were scattered somewhat, almost linear relationships between the electrode potential and applied current were observed. The value of R_p was obtained by linear approximation of the relationship between the electrode potential and the applied current.

Fig. 5.

Typical examples for potential-current relationships measured for Fe during the drying of an NaCl droplet by the KP technique.

Figure 6 shows the changes in E_corr and R_p for Fe during drying of an NaCl droplet. For comparison, the value of E_corr and R_p for the carbon steel measured during the immersion in a 0.1-mol/dm³ NaCl solution by a conventional three-electrode method are also plotted in Fig. 6. Error bars for the values of R_p represent standard deviation. During drying of the droplets, the E_corr of the Fe gradually shifted to the less noble potential. Compared to the case where carbon steel was immersed in the NaCl solution, the E_corr of the carbon steel showed a similar transient. The reciprocal of the R_p of Fe, R_p⁻¹, during drying of the NaCl droplet was estimated to be a constant of approximately 0.4 kΩ⁻¹·cm⁻², which was almost the same as that measured for the carbon steel under immersion conditions. This indicates that the KP technique is reliable for evaluating the R_p of Fe during the drying of the NaCl droplet.

Fig. 6.

Changes in (a) the corrosion potential, E_corr and (b) the reciprocal polarization resistance, R_p⁻¹ for Fe during drying of the NaCl droplet. The E_corr and R_p⁻¹ measured for carbon steel during immersion in 0.1-mol/dm³ NaCl by a conventional electrochemical three-electrode method were plotted in the figure. (Online version in color.)

The corrosion reaction occurring in a neutral NaCl solution is anodic Fe dissolution and cathodic ORR, as shown below:   

$$\text{Fe}\to {\text{Fe}}^{2+}+2{\text{e}}^{-}$$(1)

  

$${\text{O}}_{\text{2}}+{\text{2H}}_{\text{2}}\text{O}+4{\text{e}}^{-}\to 4{\text{OH}}^{-}$$(2)

The i_corr can be predicted by the Stern-Geary equation, as follows:²²⁾   

$$i_{\text{corr}}=\frac{k}{R_{\text{p}}}$$(3)

where k is a proportionality constant.

During the drying process, the value of R_p⁻¹ was approximately 0.4 kΩ⁻¹·cm⁻². When k was assumed to be 20 mV,³⁴⁾ the i_corr of Fe during drying was estimated to be approximately 8 μA/cm². In this study, the thickness of the droplet was 1 mm initially and about 0.5 mm at 120 min after the droplet was applied. This suggests that the corrosion reaction proceeded under a diffusion-controlled condition of the ORR, since the electrolyte layer thickness was thicker than the convection layer.^35,36) In this case, the i_corr of Fe can be estimated from the limiting current density of the ORR on the Fe. The value of i_corr estimated in this study (ca. 8 μA/cm²) was comparable to the diffusion-limiting current of the ORR.

3.2. Changes in R_p and i_per for Fe during Drying of an NaCl Droplet

Figure 7 shows changes in the (a) E_corr, (b) R_p⁻¹, i_corr, and (c) i_per for Fe during drying of a 0.1-mol/dm³ NaCl droplet. During the drying, R_p measurements were conducted using the KP technique. The i_corr in Fig. 7(b) was evaluated from R_p⁻¹ by assuming that k was 20 mV. The values of i_per were obtained by subtracting the residual current. The droplet was applied on the sample at the time indicated by the arrow in the graph. As shown in Fig. 7, the KP technique and DS method were successfully combined to simultaneously measure E_corr, R_p⁻¹, and i_per for Fe during drying of the droplets. This suggests that this combined technique can be utilized to investigate the hydrogen uptake mechanism of Fe in atmospheric environments.

Fig. 7.

Changes in (a) corrosion potential, E_corr, (b) reciprocal of corrosion resistance, R_p⁻¹ and estimated corrosion rate, i_corr, and (c) hydrogen permeation current, i_per for Fe during drying of the NaCl droplet.

When the droplet was applied, the value of E_corr decreased in the less noble direction, while i_per increased. The hydrogen diffusion coefficient, D_H for Fe was reported to be 1.1 ×10⁻⁵ cm² s⁻¹.³⁷⁾ According to the calculation of the breakthrough time, the i_per should increase within ca. 4 min after an NaCl droplet is placed on the Fe surface.¹⁶⁾ Therefore, it is reasonable that the i_per increased soon after the E_corr decreased in less noble direction as shown in Fig. 7. In addition, this indicates that the corrosion reaction promoted hydrogen uptake into Fe. After application of the NaCl droplet, no value of R_p⁻¹ was obtained for 0.5 h due to the relatively large initial change in E_corr. Then, R_p⁻¹ started to increase, while i_per gradually decreased during the drying process. After 1.5 h, when the droplet had dried up completely, E_corr was shifted drastically toward the noble potential and the i_per started to decay. These results indicate that the change in the i_per was associated mainly with the change in the E_corr during drying of the droplet.

To investigate the corrosion behavior in detail, we observed the changes in corrosion morphology of the Fe during drying of the NaCl droplet (Fig. 8). As shown by the white arrows in Fig. 8(b), about 20 min after the application of the droplet, corrosion product formation was observed on the small pits, indicating that the pits acted as anodes and that the corrosion progressed unevenly. Although this uneven corrosion may create a distribution of E_corr, the potential difference is small.³⁸⁾ Therefore, the E_corr measured in this study showed the trend in the potential transient of the whole Fe surface. A small amount of corrosion products was deposited around the pit, suggesting that anodic sites are limited in the early stage of drying. This result is consistent with the small i_corr at the beginning of the drying of the NaCl droplet. As the droplet dried further, the amount of corrosion products around the pits increased, which corresponded well with the increased the value of R_p⁻¹. As shown in Fig. 8(f), the corrosion products of Fe were observed relatively abundant in the vicinity of the gap. The artifact by polarization resistance measurements is considered to be negligible, since the current distribution by the small polarization is uniform under a relatively thick droplet.³⁹⁾ When the droplets dried, corrosion products and salts were finally deposited.

Fig. 8.

Changes in the corrosion morphology of Fe during drying of the NaCl droplet: (a) 0, (b) 20, (c) 40, (d) 60, (e) 80, and (f) 90 min. (Online version in color.)

4. Discussion

As shown in Fig. 7, hydrogen uptake into Fe was observed during the drying process of the NaCl droplet, together with a decrease in the value of E_corr. During drying of the droplet, the value of E_corr became slightly lower and i_corr gradually increased due to the onset of corrosion. However, the value of i_per remained at a magnitude of several tens of nA/cm² and did not change very much during iron corrosion, independent of the change in R_p⁻¹. This suggests that the evolution of the HER depends mainly on the E_corr of Fe in the early stage of drying, when the relatively thick electrolyte layer was formed.

As shown in Fig. 8, the corrosion reaction was initiated unevenly on the Fe surface in the early stage of the drying of the droplet, and it is considered that the iron dissolution reaction (Eq. (1)) occurred in the pits, while the ORR (Eq. (2)) occurred on the cathodic sites of the Fe surface around the pits. In the cathodic sites, as shown in Eq. (2), the pH will increase because of the formation of OH^– ions during the ORR. It has been reported that the pH quickly rises to about 12 during the ORR at the diffusion-controlled rate.^40,41,42) As shown in Fig. 1, at a pH of 12, the equilibrium potential for the HER can be estimated as −0.911 V (SSE). In this study, during the drying process of the NaCl droplet, the E_corr for Fe was approximately −0.4 V (SSE). In this case, the HER would not occur thermodynamically at the cathode sites, where the pH was highly alkaline.

On the other hand, in the anodic sites, Fe²⁺ ions were generated in the pits. The dissolved Fe²⁺ ions were further oxidized by dissolved oxygen to Fe³⁺ ions, which can cause a hydrolysis reaction (Eq. (4))⁴³⁾ resulting in a decrease of the pH in the pits.   

$${\text{Fe}}^{\text{3+}}+{\text{H}}_{\text{2}}\text{O}\rightleftarrows {\text{Fe(OH)}}^{\text{2+}}+{\text{H}}^{+}$$(4)

Assuming that the value of E_corr is −0.4 V (SSE) equal to the equilibrium potential for the HER, the environmental pH must be lower than approximately 3.4 for the HER to take place. As discussed in our previous study, during anodic dissolution of steel, the pH of the anodes was roughly estimated below 5.⁴²⁾ In addition, Kamimura et al. have reported that continuous anodic dissolution of Fe decreased in pH to 1.5 in NaCl solutions containing dissolved oxygen due to the hydrolysis of Fe³⁺ ions.⁴⁴⁾ Therefore, the HER was considered to occur in the pits as the anodic sites. Namely, pH decreases at the anodic sites promoted both the HER and hydrogen uptake. During drying, the increase in corrosion areas on the iron enhanced the i_per due to the increase in anodic sites.

In the last stage of the drying, the i_corr of Fe should increase due to enhancement of the ORR,^35,36) although our system could not measure the value of R_p just before the NaCl droplet dried up completely. This mechanism explains why the E_corr increases to a higher value in the last stage of drying, as shown in Fig. 7(a). In this stage, the i_per simply decayed regardless of the increase in the i_corr.

5. Conclusions

In this study, simultaneous measurements of the R_p and i_per of Fe were conducted during drying of an NaCl droplet using the KP technique and DS method and the hydrogen uptake behavior of Fe was investigated. The findings of this study are summarized below:

(1) By combining the KP technique and DS method, simultaneous measurements of E_corr, i_corr, and i_per of an Fe plate during drying of an NaCl droplet was achieved. The hydrogen uptake behavior of Fe during drying of the droplet was investigated in detail from the perspective of the changes in E_corr, i_corr, and i_per.

(2) The evolution of the HER depends mainly on the E_corr of Fe in the early stage of drying, when a relatively thick electrolyte layer is formed.

(3) Hydrogen uptake occurs at the anodic sites on the Fe during drying of the NaCl droplet. In the last stage of drying, the hydrogen uptake is inhibited due to the increase in E_corr.

Acknowledgements

This work was performed as a part of the research “Corrosion-induced Hydrogen Absorption to Steels” with a financial support by The Iron and Steel Institute of Japan. In addition, this work was also supported by JSPS KAKENHI Grant Number 26289268 and Iketani Science and Technology Foundation Grant Number 0291035-A.

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