Passivation Behavior of Iron in Concentrated LiBr Solutions Containing Molybdate and Nitrate at Elevated Temperature

Zairyo-to-Kankyo, 5495532006 Passivation Behavior of Iron in Concentrated LiBr Solutions Containing Molybdate and Nitrate at Elevated Temperature Hitoshi Yashiro, Masaru Kawata, Masahiko Itoh, Kenji Machizawa and Ken-ichi Shimizu Department of Chemical Engineering, Faculty of Engineering, Iwate University Hitachi Building Systems Co., Ltd. School of Chemistry, Faculty of Economics, Keio University Since the control of corrosion by the addition of inhibitors is one of the key technologies for the adsorption refrigeration systems, the passivation behavior of an iron electrode in the 17.3 mol/kg LiBr0.1 mol/kg LiOH solution containing molybdate and nitrate has been investigated at 428 K through electrochemical measurements and film analysis by radio frequency grow discharge optical emission spectroscopy (rf-gdoes). The results indicated that molybdate worked as a principal inhibitor in the system; molybdate was able to passivate iron without causing any pitting. However, the oxidizing power of molybdate was not sufficient to form a passive film on iron rapidly enough. The process could be assisted by the addition of nitrate; iron was passivated very quickly when the solution contained both molybdate and nitrate. Cares should be paid to that nitrate of higher concentrations could cause pitting especially when the concentration of molybdate was relatively low. The oxide films formed on iron in the presence of molybdate and nitrate were much thinner than those formed with only molybdate. Key words : adsorption refrigeration system, inhibitor, iron, corrosion, molybdate, nitrate, passive film, pitting Absorption refrigeration systems use highly concentrated LiBr solutions (up to about 65 mass ) as an absorbent of coolant (water) 1). The maximum temperature of the absorbent in a modern double stage generating machine is over 433 K. Carbon steel, the main structural material could be attacked severely unless adequate inhibitors are not added into the absorbent 2), 3). Among the inhibitors proposed so far, molybdate is regarded as the most preferable because of its high inhibition efficiency and low environmental hazard 4)7). Molybdate induces passivation of steel in an alkaline LiBr solution but does not cause pitting as chromate 8)10) and nitrate 2), 8) do. It was revealed that molybdenum oxide layer is formed on the outer side of the surface film on steel as a result of reduction of molybdate 5). Yashiro et al. 7) reported film formation kinetics on carbon steel in the concentrated LiBr solutions containing LiOH and Li 2 MoO 4. The thickness of the oxide layer increased in proportion to the cubic root of immersion time. However the passivation kinetics did not seem fast enough and hydrogen could evolve considerably until stable films are formed on steel. Therefore nitrate may be added into the absorbent as an assistant oxidizer especially at the start of operation. Mabuchi et al. 11) tested nitrate/molybdate mixture for inhibition of carbon steel in the concentrated LiBr solutions. The result indicated that the addition of nitrate decreased the amount of hydrogen evolved although the corrosion loss of carbon steel could increase slightly Presented at the Materials and Environments 2005Yokohama, 2005 435, Ueda, Morioka, 0208551 Japan 191, Kidamari-Higashidai, Tsuchiura, 3000027, Japan 411, Hiyoshi, Minato Kita-ku, Yokohama, 2238521, Japan depending on the concentration of nitrate. Thus, it is very important practically to control the initial passivation process of carbon steel in the absorption refrigeration systems. It is expected to be useful to monitor the corrosion potential of an iron electrode during the initial passivation period for the better understanding of a role of nitrate in the concentrated LiBr solution containing Li 2 MoO 4. The purpose of this research is to follow the passivation process of iron in the concentrated LiBr solutions containing molybdate and nitrate through measurements of corrosion potential and surface film analysis. We would note that all conditions are not identical as in commercial machines in the following laboratory experiments. Solutions are not dynamic but static. System is not evacuated but filled with argon. The ratio of solution volume to steel surface area is not same but much larger. Nevertheless, the following laboratory data would help us to understand the cooperative action of molybdate and nitrate, which is really effective in commercial machines. Commercially available Fe rod (99.5, 3 mm) was used after polishing down to 6/0 of SiC paper. The Fe rod was covered with a heat-shrinkable PTFE tube leaving the top of 20 mm. The Fe plate (99.95, 30301 mm) specimen for the purpose of films analysis was further polished and finished using alumina powder of 0.06m. The specimens were degreased by ultrasonic cleaning in acetone. Commercial reagents of LiBrH 2 O, LiOH, Li 2 MoO 4 and LiNO 3 were dissolved into distilled water on the bases of molality. The concentrations of LiBr of 17.3 mol/kg and LiOH of 0.1 mol/kg were common to all test solutions. Fig. 1 shows the experimental apparatus for electro-

Zairyo-to-Kankyo Fig. 1 Schematic illustration of experimental apparatus for potentiometric measurements. (a) thermocouple, (b) PTFE cell, (c) Hg/Hg 2 Cl 2, (d) KCl solution (sat), (e) test solution at room temperature, (f) potentiometer, (g) Fe electrode, (h) test solution, (i) argon, (j) cooling water. chemical tests. The cell was made of PTFE with its inner volume of about 250 cm 3. The cell was filled with 100 cm 3 of test solution and Ar was passed through the solution. After two hours of deaeration, the cell was heated to 428 K, at which the solution started boiling. In order to monitor the potential of the iron electrode, a saturated calomel electrode (SCE) was used as an external reference electrode via triple junction : Hg/Hg 2 Cl 2 /KCl(sat)//KCl(sat) //LiBr(17.3 mol/kg)//test solution/fe. The thermal junction potential across the bridge was not evaluated and the potential of the Fe electrode is presented as measured. When anodic polarization behavior of the iron electrode was evaluated, Pt counter electrode was added into the system. Anodic polarization of the Fe electrode was started within ten minutes after the solution temperature reached 428 K. The scan rate was 1 mv/s. Iron plates were exposed in the deaerated test solution of about 20 cm 3 and heated to 433 K in a closed steel tube lined with PTFE for given period of time. Then the plate was washed with water, dried with a stream of Ar and analyzed by radio frequency grow discharge optical emission spectroscopy (rf-gdoes : HORIBA JY5000RF) 12). Fig. 2 Time-variation of the potential of Fe electrode in 17.3 mol/kg LiBr0.1 mol/kg LiOH with different concentrations of Li 2 MoO 4 at 428 K. Concentration of Li 2 MoO 4 : 0(a), 10 4 (b), 10 3 (c), 10 2 mol/kg(d). Fig. 2 shows the time variation of the corrosion potential of the Fe electrode in the LiBr solution with different concentrations of Li 2 MoO 4. The measurement was started just after the solution temperature reached 428 K (t0). When the concentration of molybdate was 110 2 mol/kg(line (d) in Fig. 2), the corrosion potential rose to 620 mv in 10 h and became stable. When the concentration of molybdate was 110 3 mol/kg(line (c) in Fig. 2), it took about 40 h for the Fe electrode to rise in potential. The concentration of molybdate of the order of 10 3 mol/kg is regarded as acceptable for steady running of commercial adsorption refrigerators 5). However, the initial passivation process does not seem fast enough at this concentration because the evolution of hydrogen could deteriorate the performance of the refrigerator. When the concentration of molybdate was 10 4 mol/kg(line (b) in Fig. 2), the corrosion potential stayed around 800 mv for the initial 30 h. The corrosion potential was almost same as in the case of the Fe electrode in the solution without molybdate (line (a) in Fig. 2). This indicates that the concentration of molybdate under 10 3 mol/kg is ineffective for the self-passivation of iron. In order to clarify the situation of the Fe electrode at each potential, polarization curves were measured just after the solution temperature reached 428 K. The result is shown in Fig. 3. The polarization curves indicate that the molybdate is essential for passivation. In the presence of molybdate even at the concentration of 110 4 mol/kg (line (b) in Fig. 3), the Fe electrode showed clear passive region between 600 and 450 mv. This suggests that the principal role of molybdate is to adsorb on Fe surfaces lowering the active peak rather than to oxidize it. The potential of the Fe electrode after long time exposure in Fig. 3 Anodic polarization curves of Fe electrode in 17.3 mol/kg LiBr0.1 mol/kg LiOH with different concentrations of Li 2 MoO 4 at 428 K. Concentration of Li 2 MoO 4 : 0(a), 10 4 (b), 10 3 (c), 10 2 mol/kg(d).

the presence of molybdate higher that 10 3 mol/kg corresponds to passive range. On the other hand, the corrosion potential of 800 mv indicates that the Fe electrode is in the active region. Fig. 2 indicated that the Fe electrode can be passivated more quickly by higher concentration of molybdate. However the concentration of molybdate in commercial machines can not be maintained as high as 10 2 mol/kg because some parts running at lower temperatures limit the solubility of it. Another possible method for quick passivation of iron would be supplementary addition of a stronger oxidizer like nitrate. Fig. 4 shows the time variation of the corrosion potential of the Fe electrode in the LiBr solution containing 10 2 mol/kg Li 2 MoO 4 and LiNO 3 of different concentrations. With the help of nitrate of 10 3 mol/kg, the Fe electrode passivated very quickly and the corrosion potential was very stable over a long period of time. Similar experiment was made in the LiBr solution containing 10 3 mol/kg Li 2 MoO 4 and LiNO 3 of different concentrations as shown in Fig. 5. The corrosion potential of the Fe electrode became nobler by more than 100 mv in the presence of nitrate. The only difference between Fig. 4 and 5 is that the corrosion potential was a little instable in the solution with lower concentration of molybdate. In Fig. 6, anodic polarization curves measured in the LiBr solution containing 10 3 mol/kg Li 2 MoO 4 and LiNO 3 of different concentrations are presented. The curve in the presence of 10 2 mol/kg LiNO 3 started from about 570 mv and showed the lowest passive current. This indicates that the Fe electrode was already passivated when the solution temperature became 428 K in this solution. But a sharp increase in current was observed within 100 mv of anodic polarization. The specimen was pitted after the extensive anodic polarization in all solutions. Thus, the quick passivation by nitrate risks pitting. The effect of nitrate was further examined in the LiBr solution containing 10 4 mol/kg Li 2 MoO 4 and the result is shown in Fig. 7. The addition of nitrate was effective for quick passivation but the corrosion potential was not as stable as in the case of higher concentrations of molybdate. It can be seen from Fig. 6 that the Fe electrode suffers from pitting above 500 mv. The corrosion potential of the Fe electrode in Fig. 7 was just around the pitting potential. Actually shallow pits were observed in the solution with 10 4 mol/kg Li 2 MoO 4 and 510 3 mol/kg LiNO 3. Thus the instability of corrosion potential in Fig. 7 can be attributed to pitting 11). Because the fluctuation of corrosion Fig. 4 electrode in 17.3 mol/kg LiBr0.1 mol/kg LiOH10 2 LiNO 3 : 0(a), 10 3 (b), 510 3 (c), 10 2 mol/kg(d). Fig. 6 Anodic polarization curves of Fe electrode in 17.3 mol/kg LiBr0.1 mol/kg LiOH10 3 mol/kg Li 2 MoO 4 with different concentrations of LiNO 3 at 428 K. Concentration of LiNO 3 : 0(a), 10 3 (b), 510 3 (c), 10 2 mol/kg(d). Fig. 5 electrode in 17.3 mol/kg LiBr0.1 mol/kg LiOH10 3 LiNO 3 : 0(a), 10 3 (b), 510 3 (c), 10 2 mol/kg(d). Fig. 7 electrode in 17.3 mol/kg LiBr0.1 mol/kg LiOH10 4 LiNO 3 : 0(a), 10 3 (b), 510 3 (c), 10 2 mol/kg(d).

Zairyo-to-Kankyo potential tended to cease with time, most of them were regarded as repassivating ones. But it should be noted that this was the case of the electrode with a small area (about 2 cm 2 ). A larger cathode area might enable pits to grow more deeply. Fig. 8 shows the results of GDOES depth analyses for passive films formed on Fe plates after different exposure time in the LiBr solution containing 10 3 mol/kg Li 2 MoO 4 at 433 K. The specimen surface after 20 h exposure looked almost as polished. The depth profile revealed that the iron oxide layer was as thin as air formed one and there was no molybdenum oxide layer. The corrosion potential value after 20 h in Fig. 2 indicates that the Fe plate was still in active state. After 80 h where the Fe plate was expected to have passivated from Fig. 2, the surface was covered with oxide layers of molybdenum and iron. The surface of the specimen turned purplish black. The film after 200 h exposure increased further in its thickness. The result indicates that the film continued to grow for long period of time in this solution. The typical film formation reaction in the presence of molybdate can be expressed as : 3Fe4HMoO 4 Fe 3 O 4 4MoO 2 4OH (1) This reaction is competitive to the hydrogen evolution reaction like : 3Fe4H 2 O Fe 3 O 4 4H 2 (2) It should be noted that the reaction (2) does not really lead Fe to passivation although water is estimated to be stronger oxidizer than molybdate. It is estimated that hydrogen evolved during the period where corrosion potential lay below 700 mv in Fig. 2. It takes rather long time for molybdate to form thick film by which water is prohibited to access the Fe surface. Thus the only fault of molybdate in this system is too slow passivation kinetics because evolution of hydrogen decreases the performance of refrigerator and increases the risk of hydrogen embrittlement. Fig. 9 shows the results of experiments carried out similarly but in the presence of nitrate. The film thickness showed almost no increase after 80 h. The thickness after 200 h was less than half of that formed in the absence of nitrate. The results together with the corrosion potential in Fig. 5 indicate that Fe attained to steady passive state quickly in the presence of nitrate. One example of the chemical reactions according to which nitrate accelerates the passivation of Fe is 15Fe8NO 3 4H 2 O 5Fe 3 O 4 4N 2 8OH (3) N 2 may not the only reduction product but other species like NH 3 or NO 2 could also form. The reaction (2) can be inhibited effectively by nitrate because it is much stronger Fig. 8 rf-gdoes depth profiles or corrosion products on iron after (a) 20, (b) 80 and (c) 200 h exposure in 17.3 mol/kg LiBr0.1 mol/kg LiOH10 3 mol/kg Li 2 MoO 4 solution at 433 K. Fig. 9 rf-gdoes depth profiles or corrosion products on iron after (a) 80 and (b) 200 h exposure in 17.3 mol/kg LiBr 0.1 mol/kg LiOH10 3 mol/kg Li 2 MoO 4 10 3 mol/kg LiNO 3 solution at 433 K.

oxidizer than water. It is interesting to note that the formation of molybdenum oxide layer was not inhibited completely by nitrate. The molybdate could be adsorbed on the locally broken sites of iron oxide layer to repair them. The recurrence may have built the thin oxide layer of molybdenum. Fig. 7 indicated that the corrosion potential became stabilized with time. This may correspond to the film stabilization period by molybdate. When the concentration of molybdate is very low, iron oxide layer quickly formed with the aid of nitrate can not be stabilized and could fall to pitting. Because commercial refrigeration adsorption systems require proper use and control of corrosion inhibitors, the roles of molybdate and nitrate in the early stage of passivation of iron in the 17.3 mol/kg LiBr0.1 mol/kg LiOH solution were investigated at 428 K. The conclusions are: 1) Anodic polarization curves for an iron electrode indicated that molybdate (higher than 10 4 mol/kg) is essential for effective passivation. 2) The measurements of corrosion potential indicated that the higher the concentration of molybdate, the faster the iron electrode passivated. However, the passivation kinetics was not rapid enough with respect to the competitive hydrogen evolution reaction. 3) The addition of nitrate together with molybdate is quite effective for quick passivation of the iron electrode. However, the strong oxidizing power of nitrate could cause pitting especially when the concentration of molybdate is not sufficient (below 10 3 mol/kg). 4) The film analysis indicated that the oxide films continued to grow slowly on an iron electrode in the absence nitrate while the growth stopped soon in the presence of nitrate. 1) K. Tanno, M. Itoh, M. Aizawa, K. Mabuchi and H. Yashiro, Zairyo-to-Kankyo,, 236 (2003). 2) K. Tanno, M Itoh, T. Takahashi, H. Yashiro and N. Kumagai, Corros. Sci.,, 1441 (1993). 3) H. Yashiro, A. Sai, N. Kumagai, K. Tanno and K. Mabuchi, Zairyo-to-Kankyo,, 369 (1999). 4) M. Itoh, A. Minato, M. Aizawa and K. Tanno, Boshoku- Gjjutsu (Presently Zairyo-to-Kankyo),, 504 (1984). 5) M. Itoh, H. Midorikawa, M. Izumiya, M. Aizawa and K. Tanno, Boshoku-Gijutsu (Presently Zairyo-to-Kankyo),, 298 (1990). 6) K. Tanno, M Itoh, H. Sekiya, H. Yashiro and N. Kumagai, Corros. Sci.,, 1453 (1993). 7) H. Yashiro, A. Sai, N. Kumagai, K. Tanno and K. Mabuchi, Zairyo-to-Kankyo,, 288 (2000). 8) M. Itoh, M. Aizawa and K. Tanno, Boshoku-Gjjutsu (Presently Zairyo-to-Kankyo),, 142 (1987). 9) Y. Kojima and S. Tsujikawa, Zairyo-to-Kankyo,, 49 (1998). 10) Y. Kojima and S. Tsujikawa, Zairyo-to-Kankyo,, 117 (1998). 11) K. Mabuchi, T. Kikuchi, H. Midorikawa and M. Aizawa, Zairyo-to-Kankyo,, 526 (1996). 12) K. Shimizu, H. Habazaki, P. Skeldon and G.. F. Thompson, Surf. Interface Anal.,, 564 (2003). Manuscript received August 21, 2006; in final form October 14, 2006 428K 17.3 mol/kg LiBr0.1 mol/kg LiOH rf-gdoes