Polarization Curves of Carbon Steel in Concentrated LiBr Solutions Containing LiOH and Li₂MoO₄ at Different Temperatures after Short Immersion

Abstract

We have tried to measure polarization curves of SS400 carbon steel in concentrated LiBr solutions over 393 K, and investigated the influence of temperature as well as concentrations of LiOH and Li₂MoO₄ on its corrosion behavior. Test solutions were 65 mass% (mass% is replaced by % hereafter) LiBr solutions containing 0 to 0.2% LiOH and 0 to 0.03% Li₂MoO₄. Test temperatures were 393 and 438 K. The test solutions were deaerated. The specimen was immersed in the test solution for a short time of 0.3 ks and was subjected to measurement of polarization curve in the same solution. As a result, the followings were obtained: The anodic current density in 65% LiBr solution without LiOH increased monotonically with a rise in a potential, and the relation was maintained regardless of Li₂MoO₄ addition and temperature change. In 65% LiBr solutions with LiOH up to 0.2% at 393 and 438 K, the anodic polarization curves showed active dissolution and passivation. When 0.05% LiOH was added to the 65% LiBr solution, the corrosion potential negatively shifted, and the potential was maintained regardless of more addition of LiOH. As a LiOH concentration increased, a pitting potential was raised. The polarization curves at 393 and 438 K showed almost no change regardless of addition of 0.03% Li₂MoO₄, meaning that Li₂MoO₄ had almost no effect on corrosion inhibition to the specimen for the short immersion. The corrosion rate at 393 K was approximately 0.3 A·m⁻² regardless of the addition of LiOH nor Li₂MoO₄. Whereas, the corrosion rate at 438 K slightly decreased with increasing LiOH concentration, regardless of the addition of Li₂MoO₄. Cathodic current density in the solution with 0.2% LiOH and 0.03% Li₂MoO₄ increased with a rise in a temperature on the basis of Arrhenius relation. It is thought that insufficient effect of LiOH and Li₂MoO₄ on corrosion inhibition was observed because of a short immersion time of 0.3 ks before measurement of polarization curves.

Fig. 4 Polarization curves of the specimen in the solutions of 65% LiBr + x% LiOH (x = 0, 0.05, 0.1, 0.2) at 393 K.

1. Introduction

An absorption heat pump is an equipment capable of producing massive chilled water so that the heat pump is widely used for air conditioning of larger buildings and district heating and cooling. One of the features of the heat pump is to use water and LiBr solution as working media. The heat pump consists of an evaporator, an absorber, a condenser and a generator. In the evaporator, liquid water as refrigerant evaporates under a low pressure to take heat from tubes inside which the water is used for air conditioning flow. Water vapor moves to the absorber to be absorbed in concentrated LiBr solution. A concentration of the LiBr solution then decreases, meaning that the absorption efficiency of the solution diminishes. Therefore, the diluted LiBr solution is transported to the generator and boiled to remove the water out of the solution and to increase its concentration. Thereafter, the evacuated water is condensed in the condenser to be re-used as the refrigerant, while the re-concentrated solution is returned to the absorber again to absorb water vapor generating in the evaporator. In the heat pump, the generator made of carbon steel must contact the high-temperature (max. 437 K) and high-concentration LiBr solution (max. 65 mass%) that is highly corrosive to the steel.

To suppress the corrosion of the carbon steel for the generator, several countermeasures are proposed:

1. (i)    alkalization of the LiBr solution to obtain stable passive film on the steel,
2. (ii)    addition of an adequate corrosion inhibitor to the LiBr solution.

Some researches on the corrosion of carbon steel in the concentrated LiBr solution were carried out so far under the limited operation conditions of the absorption heat pump. For example, Tanno et al.^1,2) evaluated corrosion weight loss for carbon steel specimens in LiBr solutions containing LiOH and some oxoates such as molybdate, chromate, nitrate and tungstate as corrosion inhibitors. On the other hand, some attempts have been conducted to evaluate a corrosion rate of the carbon steel in the LiBr solution, and most of them^3,4) have investigated it in terms of corrosion weight loss of the steel after immersion in the LiBr solution for the period longer than 180 ks (= 50 h). This conventional corrosion evaluation method can reveal the practical behavior of the corrosion, but insufficiently discuss the mechanism of the corrosion because of the lack of electrochemical information such as contribution of anodic and cathodic processes.

Our research group have investigated electrochemical behavior of a carbon steel in the concentrated LiBr solutions containing LiOH as alkaline agent and Li₂MoO₄ as inhibitor to elucidate the influence of environmental factors on the corrosion. The previous report⁵⁾ measured polarization curves of a carbon steel just after immersion in 65 mass% LiBr solutions at 393 and 438 K, and investigated the influence of LiOH concentration on corrosion behavior of the steel in the LiBr solutions. As a result, it was found that addition of LiOH induced active dissolution of the steel. In addition, influence of Li₂MoO₄ was investigated on the corrosion of the steel in 65 mass% LiBr + 0.2 mass% LiOH, and it was found that there was almost no influence on active dissolution of the steel. This extensive research aimed to realize influence of Li₂MoO₄ and temperature on the corrosion of the steel in the LiBr solutions with and without LiOH in more detail.

2. Experimental Procedure

The material used was SS400⁶⁾ carbon steel rod with a diameter of 6 mm. The rod was cut into a 20-mm-long piece, and a lead wire was soldered to one end of the piece to be a specimen (Fig. 1). The side surface of the specimen was immersed in 20 mass% di-ammonium hydrogen citrate solution at 353 K for 0.3 ks, dry-polished (to #6/0), rinsed with pure water, and dried rapidly by air blow. Then, the side surface was protected with polytetrafluoroethylene (PTFE) tape and a Fluorinated ethylene propylene (FEP) heat-shrinkable tube. Immediately before the polarization test, exposed surface of specimen was dry-polished (to #6/0).

Fig. 1

Schematic illustration for setup of specimen (mm).

Test solutions as described below were prepared by dissolving reagent grade chemicals of LiBr·H₂O, LiOH·H₂O and Li₂MoO₄ into deionized water at about 333 K^1,4) (hereafter, % represents mass%);

1. (i)    65% LiBr + x% LiOH (x = 0 to 0.2),
2. (ii)    65% LiBr + x% LiOH (x = 0 to 0.2) + y% Li₂MoO₄ (y = 0 and 0.03).

The test solution was poured into a sealed vessel with a rotary vacuum pump. The vessel was evacuated for 0.3 ks, and filled with nitrogen gas (99.99 vol%) until an atmospheric pressure. This operation was repeated three times to remove dissolved oxygen in the solution. After that, the solution was poured into the PTFE cell without any contact to air. The cell had a Pt counter electrode, an Ag/AgCl (3.3 kmol·m⁻³ KCl at ambient temperature) reference electrode, a thermo-couple, an inlet line of nitrogen gas, the gas outlet whose end was immersed in water, and a heater with a temperature-controlling unit. The test solution was subjected to nitrogen gas bubbling during the test. Thereafter, the solution was heated to the specified temperature by the heater. To avoid concentration of the solution, water vapor was collected by a condenser and returned to the test solution. After a temperature reached the specified value, the specimen for polarization test was set to the PTFE cell. The three electrodes including the specimen were connected to a potentiostat. First of all, corrosion potential with immersion time was measured until 0.3 ks. After it was confirmed that the potential is stable from 0.1 ks to 0.3 ks as found in the previous study,⁵⁾ a polarization curve was measured by potentiokinetic method with a potential sweep rate of 1.0 mV·s⁻¹. A cathodic polarization curve was firstly measured from the corrosion potential to −1.1 V_Ag/AgCl. After the measurement, the specimen was turned into open circuit condition. As a corrosion potential returned to the previous value, a cathodic polarization curve was measured again. If these polarization curves coincide with each other, an anodic polarization curve was measured from the corrosion potential to −0.25 V_Ag/AgCl. Hereafter, V_Ag/AgCl is simply represented as V.

3. Results

3.1 Influence of deaeration conditions

Figure 2 shows cathodic polarization curves of the specimen in 65% LiBr + 0.2% LiOH + 0.03% Li₂MoO₄ solution under the various deaeration conditions at 438 K. It was found that the corrosion potential was independent of the deaeration condition and around −0.7 V.

Fig. 2

Polarization curves of SS400 carbon steel specimen in 65% LiBr + 0.2% LiOH + 0.03% Li₂MoO₄ solution under the various deaeration conditions at 438 K.

Figure 3 shows anodic polarization curves of the specimen in 65% LiBr + 0.2% LiOH + 0.03% Li₂MoO₄ solution under the different deaeration conditions at 393 K. It can be seen in the figure that the anodic polarization curve in active dissolution region as well as the corrosion potential were almost no change with difference in the deaeration process. It is summarized from the results in Figs. 2 and 3 that the shapes of cathodic and anodic polarization curves including the corrosion potential are considered to be hardly influenced by the deaeration condition.

Fig. 3

Polarization curves of the specimen in 65% LiBr + 0.2% LiOH + 0.03% Li₂MoO₄ solution under the various deaeration conditions at 393 K.

3.2 Influences of LiOH concentration and temperature

Figure 4 shows polarization curves for the specimen in 65% LiBr solutions containing LiOH of different concentrations at 393 K. In the solution with addition of 0.2% LiOH, a corrosion potential was about −0.7 V. As a potential was scanned in positive direction, an anodic current density increased from the corrosion potential to about −0.6 V, and then decreased due to passivation. When a potential reached about −0.3 V, a current density increased rapidly like pit initiation. Although an anodic current density in active dissolution region was almost independent of a LiOH concentration, a minimum of the passive current density increased and the potential at which the current density rapidly increased in the passive region shifted lower as a LiOH concentration decreased. In the solution without LiOH, on the other hand, a corrosion potential was about −0.6 V, higher than that in the solutions with LiOH. An anodic current density increased monotonically with a rise in a potential, and then almost coincided with those in the solutions with 0.05 and 0.1% LiOH. A cathodic current density in the solution with 0.2% LiOH increased in accordance with a constant Tafel slope as a potential shifted lower until around −0.9 V, and when a potential was lowered to −1.1 V, a slope of the cathodic current density diminished. The shape of cathodic polarization curve below −0.9 V was almost independent of the LiOH concentration.

Fig. 4

Polarization curves of the specimen in the solutions of 65% LiBr + x% LiOH (x = 0, 0.05, 0.1, 0.2) at 393 K.

Figure 5 shows polarization curves for the specimen in 65% LiBr solutions containing LiOH of different concentrations at 438 K. A corrosion potential in the solution containing 0.2% LiOH was about −0.7 V, and was independent of temperature. There were active dissolution region, passive region and pitting-like region in a shape of anodic polarization curve, similar to that at 393 K. The potential at the active dissolution peak shifted from −0.60 V to around −0.65 V with a rise in a temperature. The curve clearly showed passive region followed by rapid current increase such as that at 393 K. The potential at the rapid current increase was lower than that at 393 K, and negatively shifted with a decrease in a LiOH concentration. In the solutions containing LiOH of 0.05 and 0.1%, the rapid increase in current density was observed around −0.6 V over which active dissolution became relatively mild. In the solution without LiOH, on the other hand, a corrosion potential was −0.6 V, higher than those in the solutions with LiOH. In addition, an anodic current density monotonically increased with a rise of potential. On the other hand, a cathodic current density increased in accordance with a constant slope as a potential lowered until around −0.8 V, and below the potential, a slope of the cathodic polarization curve diminished.

Fig. 5

Polarization curves of the specimen in the solutions of 65% LiBr + x% LiOH (x = 0, 0.05, 0.1, 0.2) at 438 K.

3.3 Influences of Li₂MoO₄ addition and temperature

Figure 6 shows polarization curves of the specimen in 65% LiBr solutions containing 0.03% Li₂MoO₄ and LiOH of different concentrations at 393 K. In the solutions containing LiOH, corrosion potentials were about −0.7 V, independent of Li₂MoO₄ addition as well as concentration of LiOH. There were active dissolution peaks at about −0.6 V in the anodic polarization curves, independent of a LiOH concentration and Li₂MoO₄ addition. In the solution without LiOH, a corrosion potential was near −0.5 V, higher than those in the solutions containing LiOH. An anodic current density increased monotonically with a rise of a potential, almost the same as the result without Li₂MoO₄. On the other hand, a cathodic current density increased in accordance with a constant slope as a potential lowered until −1.1 V. The slope was independent of LiOH concentration and slightly smaller than that without Li₂MoO₄.

Fig. 6

Polarization curves of the specimen in the solutions of 65% LiBr + x% LiOH (x = 0, 0.05, 0.1, 0.2) + 0.03% Li₂MoO₄ at 393 K.

Figure 7 shows polarization curves of the specimen in 65% LiBr solutions containing 0.03% Li₂MoO₄ and LiOH of different concentrations at 438 K. In the solutions with LiOH, corrosion potentials and the potentials at the active dissolution peaks were about −0.7 V and about −0.6 V, respectively. Increase in a LiOH concentration diminished a minimum of passive current density and the potential at the rapid current increase, but the addition of Li₂MoO₄ and a temperature did not clearly affect the parameters. In the solution without LiOH, a corrosion potential was −0.6 V, higher than those with LiOH. An anodic current density increased monotonically with a rise of a potential. In the cathodic polarization curve, a current density increased in accordance with a constant slope as a potential lowered until around −1.1 V, and the slope was almost independent of LiOH concentration and temperature. In comparison with Fig. 5, significant effect of the Li₂MoO₄ on the trend of polarization curve was not observed.

Fig. 7

Polarization curves of the specimen in the solutions of 65% LiBr + x% LiOH (x = 0, 0.05, 0.1, 0.2) + 0.03% Li₂MoO₄ at 438 K.

3.4 Observation of the specimens after polarization curve measurements

The anodic polarization curves for the specimen in 65% LiBr solutions at 393 K are shown in Fig. 8. The curve (a) was measured in 65% LiBr solution without LiOH nor Li₂MoO₄. The curve (b) was measured in 65% LiBr + 0.2% LiOH + 0.03% Li₂MoO₄ solution from the corrosion potential to −0.6 V. The curve (c) was measured in the same solution as that for the curve (b) from the corrosion potential to −0.25 V. The photographs of the specimens obtained after the measurements are shown in Fig. 9. It is confirmed that pits occurred in the specimens (a) and (c), not (b). The facts indicate that the rapid increase in an anodic current density is caused by pitting corrosion. Therefore, the potential at rapid current increase is represented by pitting potential hereafter.

Fig. 8

Polarization curves of the specimen from corrosion potential to (a) −0.4 V in 65% LiBr solution, (b) −0.6 V and (c) −0.25 V in 65% LiBr + 0.2% LiOH + 0.03% Li₂MoO₄ at 393 K.

Fig. 9

Surfaces of the specimens after polarization until (a) −0.4 V in LiBr 65% solution, a (b) −0.6 V and (c) −0.25 V in 65% LiBr + 0.2% LiOH + 0.03% Li₂MoO₄ solution at 393 K.

Table 1 shows the pitting potentials of the specimen in the 65% LiBr solutions containing LiOH and Li₂MoO₄ at 393 and 438 K. It was noted in the table that the pitting potential shifted lower as a LiOH concentration decreased at each temperature, independent of a Li₂MoO₄ concentration, and shifted lower with a rise in a temperature.

Table 1 Pitting potential of the specimens in the 65% LiBr solutions containing LiOH and Li₂MoO₄ at 393 and 438 K.

4. Discussion

4.1 Effect of dissolved oxygen on polarization curve

As shown in Fig. 2, the shape of the cathodic polarization curve was unaffected by the solution pre-treatment processes such as the vacuum evacuation, the nitrogen bubbling and the air bubbling, although the air bubbling supplies oxygen sufficiently in the solution. It is considered that a dissolved oxygen concentration is very low because the solution was used at a quite large concentration (65%) of LiBr and at a quite high temperature (393 or 438 K) close to the boiling point of the solution (443 K). Therefore, the cathodic polarization curve may not be affected by oxygen reduction reaction. Except for the oxygen reduction reaction, the main reactions causing the measured cathodic current density may be the following hydrogen generation reactions.   

\begin{equation} \text{2H$^{+}$} + \text{2e$^{-}$} \to \text{H$_{2}$} \end{equation} (1)

  

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

There were some researches in which cathodic polarization curves of Pt were measured in acidic solutions with different pHs, and it was revealed that the curve relating eq. (2) appeared in lower potential than that relating eq. (1), and that an increase in pH induced a decrease in a cathodic current density relating eq. (1) but no change in that relating eq. (2).⁷⁾ The curves shown in Fig. 2 were measured under higher pH condition due to addition of LiOH and consisted of one Tafel line, so that the curves are considered to be caused by the reaction of eq. (2). Similarly, it was found in Fig. 3 that the anodic current density in active dissolution region was independent of the solution pre-treatment processes such as the nitrogen-bubbling and the vacuum evacuation. The facts suggest that dissolved oxygen exists too low to affect the anodic polarization curve when either of the two deaeration techniques was employed.

4.2 Effects of additions of LiOH and Li₂MoO₄ on corrosion rate

Corrosion rates were estimated by extrapolation of the Tafel lines in the polarization curves shown in Figs. 4 to 7, and were plotted as functions of LiOH concentration in Fig. 10. The Tafel slopes around the corrosion potentials were characterized as follows: The cathodic Tafel slope in the solution without LiOH was gentler than that in the solution with LiOH, and the anodic Tafel slopes were quite similar value regardless of the addition of LiOH.

Fig. 10

Effect of concentration of LiOH on corrosion rate of the specimen in 65% LiBr + 0 or 0.03% Li₂MoO₄ solutions at 393 K and 438 K.

In the case of 393 K, the corrosion rates in the solutions without Li₂MoO₄ were around 0.3 A·m⁻², almost independent of a LiOH concentration.⁵⁾ Similarly, the corrosion rates in the solutions containing 0.03% Li₂MoO₄ were independent of the LiOH concentration, and were around 0.3 A·m⁻². In the case of 438 K, the corrosion rates in the solutions without Li₂MoO₄ slightly decreased with increasing LiOH concentration.⁵⁾ In addition, it was confirmed that the corrosion rate was constant with regardless of addition of 0.03% Li₂MoO₄ as an inhibitor.

It was found from the polarization curves shown in Figs. 4 to 7 that the solutions without LiOH made the specimen passivate, and that the solutions with LiOH at more than 0.05% made the specimen actively dissolve, like alkali corrosion. As mentioned above, the cathodic reaction in the solution with LiOH was considered to be the reduction reaction of water (eq. (2)), and the reaction seems to be independent of OH⁻ concentration. The assumption may be confirmed by the following explanation guided from Figs. 4 to 7. So far, in acidic solutions (not neutral nor basic solution), it is known that an anodic dissolution current density of iron increases with an increase in pH, and the phenomenon is explained by that the formation reaction of Fe(OH) or Fe(OH)⁺ intermediate is rate-determining step against the whole reaction.^8,9) However, the anodic polarization curves measured in this study showed that the anodic dissolution current density hardly increased with an increase in pH (cf. Fig. 4). The features of the anodic and the cathodic reactions may produce the corrosion rate which was independent of LiOH concentration at 393 K, as shown in Fig. 10. In comparison to that the corrosion rate at 438 K slightly decreased with an increase in LiOH concentration, the change may be negligible. The specimen immersed in the solution without LiOH was under the passive state, so that suppression of the corrosion is expected in the solution. However, the corrosion rate was larger than that in the solution with LiOH.

Although the cathodic polarization curve was line shape between −0.9 and −0.7 V in the solution with LiOH, the curve changed into slightly gentler line between −0.8 and −0.6 V in the solution without LiOH (cf. Fig. 4). The latter curve is considered to be caused by a reduction reaction of passive film formed on the specimen immersed in the solution for 0.3 ks, and then cathodic reaction may be enhanced. Although the mechanism to determine the corrosion rate in the solution without LiOH may be different from that in the solution with LiOH, a corrosion rate was independent of a LiOH concentration, as it happened. Addition of Li₂MoO₄ as an inhibitor had almost no inhibition effect to the corrosion. The reason is now being investigating.

4.3 Effects of additions of LiOH and Li₂MoO₄ on pitting corrosion

It is found from Figs. 8 and 9 that not only the active dissolution region but also the passive region and the pitting corrosion were observed in an anodic polarization curve in the solutions with addition of 0.2% LiOH. These regions were shown in Figs. 4 to 7. In addition, as shown in Table 1, a decrease in a LiOH concentration made a pitting potential lower and a minimum of the passivation current density increase, that is, pitting retardation lower.

In general, pitting corrosion, one of localized corrosion, of a metal is initiated by that halogen ions (Br⁻ in the present system) locally break the passive film on it. In addition, the pit propagates because of the two reasons: one is that the halogen ions with negative charge concentrate inside the pit due to attraction to the metal ions with positive charge. The other is that acidification takes place inside the pit due to hydrolysis of the metal ions. The two phenomena retard re-passivation of the metal. Zhang et al. investigated that effect of solution pH on susceptibility to stress corrosion cracking, one of localized corrosion, of stainless steel was investigated.¹⁰⁾ In the research, it was revealed that a susceptibility of the cracking was diminished as the solution pH increased from 7, and was explained that suppression of a decrease in pH due to the hydrolysis of metal ions and promotion of subsequent re-passivation of the metal may take place in the alkaline solution. It was also reported that an increase in buffer capacity of the alkaline solution induced suppression of the susceptibility of the cracking because of the same reason.¹⁰⁾ It is considered by the reason mentioned above that a pitting potential was lowered with a decrease in a LiOH concentration in the present research. It is noted that the corrosion potential of the specimen in the solution with LiOH at a concentration more than 0.05% is about −0.7 V, lower than the pitting potentials shown in Table 1, so that no pitting corrosion occurs during immersion in the solution with LiOH. Conversely, pitting corrosion occurred for the specimen in the solution without LiOH as shown in Figs. 8(a) and 9(a). The evidence strongly suggests that the pitting potential is lower than that in the solution with 0.05% LiOH and the corrosion potential is higher beyond the pitting potential due to passivation.

4.4 Effect of temperature on cathodic current density

As shown in Fig. 10, the corrosion rate at 438 K was larger than that at 393 K. However, the corrosion potentials at 393 K and 438 K were almost the same value. The findings suggest that both of the anodic and the cathodic current densities increase with a rise in a temperature.

Influence of temperature on cathodic current density was investigate in terms of Arrhenius plot. The cathodic polarization curves were measured in the LiBr solutions containing 0.2% LiOH with and without 0.03% Li₂MoO₄ at 353, 393 and 438 K, and the cathodic current densities were picked up at −0.8 V. The Arrhenius plots for the cathodic current density are shown in Fig. 11. It was found in the figure that the plots almost linearly lined up and that the lines almost coincided with each other, regardless of Li₂MoO₄ addition. Subsequently, activation energies obtain from slopes of the Arrhenius plots were 54 kJ·mol⁻¹.

Fig. 11

Arrhenius plot of the cathodic current density of the specimen in the 65% LiBr + 0.2% LiOH solutions with and without 0.03% Li₂MoO₄ at −0.8 V.

The summary of the research is as follows: In this field of the absorption heat pump, addition of LiOH aims alkalization of the concentrated LiBr solution to passivate the carbon steel and avoid corrosion, and addition of Li₂MoO₄ aims to make the passive film more stable by its oxidizing ability. However, the present results revealed that no addition of LiOH made the steel passivate and induced pitting corrosion. Furthermore, addition of LiOH made the steel not passivate but activate. Addition of Li₂MoO₄ did not affect the corrosion potential nor the shape of the cathodic polarization curve.

Tanno et al.⁴⁾ reported that the surface of the carbon steel after immersion for 720 ks was covered with a film and corrosion was suppressed. On the other hand, the present research measured the polarization curves of the specimen after immersion in the solution for a short time of 0.3 ks. Thus, the sufficient corrosion inhibition effects of LiOH and Li₂MoO₄ are expected after immersion for a long time, and the extended research should be conducted in a longer immersion time to understand the roles of LiOH and Li₂MoO₄ for suppression of the corrosion.

5. Conclusions

1. (1)    The anodic current density in 65% LiBr solution at 393 K and 438 K without LiOH increased monotonically with a rise in a potential, regardless of the addition of Li₂MoO₄.
2. (2)    The anodic polarization curves in 65% LiBr solutions with LiOH up to 0.2% at 393 and 438 K had the active dissolution and the passivation potential regions. The corrosion potential was lower than that in the solution without LiOH.
3. (3)    The polarization curves in 65% LiBr + 0.03% Li₂MoO₄ solutions with LiOH up to 0.2% at 393 and 438 K were quite similar to those in the solutions without Li₂MoO₄, meaning that Li₂MoO₄ demonstrated almost no effect for corrosion inhibition.
4. (4)    In the range of LiOH concentration from 0 to 0.2%, the corrosion rate at 393 K was approximately 0.3 A·m⁻², regardless of the addition of Li₂MoO₄. On the contrary, the corrosion rate at 438 K slightly decreased with increasing LiOH concentration, regardless of the addition of Li₂MoO₄.
5. (5)    Insufficient effects of LiOH and Li₂MoO₄ on corrosion inhibition were observed in the polarization curves of the carbon steel after immersion in the solutions for a short time of 0.3 ks. It is considered that the inhibition effect is expected after immersion for a long time over 720 ks, so that an extended research after a long immersion was needed.

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