Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank

Transcription

1 Journal of Shipping and Ocean Engineering 4 (2014) D DAVID PUBLISHING Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank Ryuji Kojima 1, Michiaki Ikai 1, Jinsun Liao 2 and Yoichi Mori 2 1. Department of Marine Environment and Engine Systems, National Maritime Research Institute, Tokyo , Japan 2. Technology Development Division, Kurimoto Ltd., Osaka , Japan Abstract: Ozone is the principal active substances and usually employed in ballast water management systems. In the present study, the corrosion protective effect of ozone was conducted by immersion test and electrochemical techniques. It was found that corrosion protective effect was revealed in the range of 2.0 to 2.7 ppm of ozone concentration in seawater. The ratio of the rust area of specimen became 20% in that concentration region. The rusted area is strongly influenced by the ozone concentration and the flow rate determined by FEM (finite element method). Ozone has a good influence for ballast tanks, i.e., ozone can delay the rust of ballast tanks, provided that the suitable concentration of ozone is selected. In this case, ozone may stop the corrosion at the defects, if a part of the paint in ballast tank is peeled off. However, ozone may also promote the corrosion of steel when the ozone concentration is very high, e.g., 10 ppm. Attention should be paid to the ozone concentration, if we use ozone as an active substance for ballast water management systems. Key words: Ozone, corrosion protective effect, water ballast tank, coating film, electrochemical measurement, FEM analysis. 1. Introduction Ballast water provides stability and maneuverability to a ship. Usually ballast water is pumped into ballast tanks when a ship has delivered cargo to a port and is departing with less or no cargo. Large ships can carry millions of gallons of ballast water. However, discharging ballast water into another area means discharging the contained organisms that are new to its environment. The ballast water is taken from coastal port areas and transported inside the ship to the next port of call where the water may be discharged, along with all the surviving organisms. In this way, ballast water may introduce organisms that do not naturally belong there into the port of discharge. The species that do survive and establish populations are very hardy species that have the potential to cause major harm to ecology, economy or human health [1-4]. In order to prevent invasion into Corresponding author: Ryuji Kojima, Ph.D., research fields: environmental chemistry and photochemistry. kojima@nmri.go.jp. the environment, the treatment of ballast water before discharging from ships becomes necessary and important. There are various types of ballast water management systems using ozone and/or other active substances [5]. However, the use of ozone to the water ballast tank has some potential risks due to its strong oxidation property and promotion of corrosion to the steel of water ballast tank. Nonetheless, it has been reported that ozone has a corrosion protective effect on steel in seawater, and there is some relationship between corrosion protective effect and concentration of ozone in seawater [6, 7]. Furthermore, it is also reported that ozone has a corrosion protective effect on the steel dipped in circulated natural seawater because of the formation of thin coating film on the surface of steel specimens [8]. However, the specimen used in previous studies was small one (20 mm 9 mm 6 mm), while a water ballast tank consists of plates with larger surface area. It is not yet clear whether the corrosion protective effect of ozone can be attained for the

2 328 Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank plates of larger area, and suitable ozone concentration when a corrosion protective effect emerges. In order to investigate the influences of ozone concentration and flow rate of ozonated seawater on the corrosion protection of a water ballast tank, the author designed a special experimental module consisting of a steel panel and a plastic board, which were separated by a rub gasket, so that the gap between the steel panel and plastic board was 2-3 mm. The ozonated seawater was flowed through the gap between the steel panel and plastic board. The corrosion test was also investigated from the ratio of rusted area to the whole panel area in immersion test when ozone concentration is properly controlled. The relationship between rusted area and water flow pattern was also investigated by FEM (finite element method). On the other hand, corrosion protective effects of ozone in higher concentration were not yet investigated by electrochemical measurement. Therefore, the authors also investigated electrochemical measurement on the steels immersed in seawater with ozone concentration of 10 ppm. In the present paper, the above issues are reported. chemical compositions of KA-32 are given in Table 1. The material of steel panel is SS 400, mild steel for general structure specified by JIS (Japanese Industrial Specification) [10]. In order to make an experimental module for corrosion test, the set of a steel panel and two acrylic boards were used as shown in Fig. 2. The steel panel and the acrylic board were assembled as shown in Fig. 3, and a rub gasket was inserted between the steel panel and the acrylic board. The thickness of gasket is approximately 3 mm, so that the gap between the steel panel and the acrylic board was about 2-3 mm. Natural seawater of 200 liters was circulated from down side to upper side of the experimental module. 2.2 Corrosion Test Experimental Setup and Flow Analysis by FEM The experimental setup is schematically illustrated in 2. Experimental Details 2.1 Materials and Preparation There are two kinds of specimens. One is small coupon for electrochemical measurement and the analysis of cross-section of the specimen with a SEM (scanning electron microscope) (JSM-6380LA) in detail. The dimension of the small coupon was 20 mm 9 mm 6 mm, as shown in Fig. 1. The other is panel, and the dimension of the panels was 393 mm 161 mm 1.0mm, as shown in Fig. 2. The material of small coupons is KA-32 steel, which is used for the construction of water ballast tanks and graded by Class NK standards [9]. The Fig. 1 Schematic illustration (left side) and photograph (right side) of small specimen for electrochemical measurement. Fig. 2 Photograph of steel panel and acrylic board. Table 1 Chemical compositions of coupon. Chemical composition (wt.%) C Si Mn P S Cu Cr Ni Mo Al Ti < 0.18 < < 0.04 < 0.04 < 0.35 < 0.20 < 0.40 < 0.08 <0.015 < 0.02

3 Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank 329 Fig. 3 Photograph of experimental module for corrosion test using steel plate with acrylic panel. Fig. 4 Fig. 5 Schematic diagram of experimental setup. The appearance of the experimental apparatus. Fig. 4, and the appearance of the experimental apparatus is shown in Fig. 5. The experimental apparatus consists of four units, and each unit contains an immersion tank and a control tank. Natural seawater of 200 liters was circulated in each unit. Natural seawater was sampled to keep the salinity at 3%, and stored by a plastic tank of 3 tonnages. The natural seawater for the tests was exchanged every 1 month. In the case of experimental module, the experimental apparatus consists of a control tank and an experimental module, as shown in Fig. 3. Ozonated seawater was prepared by directly injecting ozone into the natural seawater with a continuous seawater ozone gas generator (Z-2AH, Z-5WH, Kofloc Japan). Flow on the surface of steel panels has been simulated using ANSYS Fluent programs of FEM [11]. A clearance gap between test panels and acrylic boards was set from 2 to 4 mm as calculation condition. A velocity of fluid was set from m/s to 1.88 m/s. Inlet of calculation model was at down side, and outlet was at upper side, respectively Seawater Temperature Dependence of Corrosion Behavior Ships navigate the tropical area in some cases, and the temperature of water ballast tank during voyage may be increased due to sunshine. Therefore, it is important to investigate seawater temperature dependence of corrosion behavior of steel. In the present study, the temperature of seawater was ranged at 20 C, 35 C, and 47 C. The exposure period was about 2 months. Silicon rubber heaters were used to keep the temperature of seawater. After immersion test, the specimens were cleaned and weighted, and the corrosion rate was calculated from the weights before and after immersion tests The Relationship between the Test Period and the Thickness of Corrosion Product Layer The corrosion test of small coupon was conducted by fixing coupons in plastic pipes of 25 mm in diameter, which is connecting the control tank (as shown in Figs. 4 and 5). In order to investigate the relationship between the test period and the thickness of corrosion product layer, the exposure period was

4 330 Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank designed as 1 week, 2 weeks, 1 month, 3 months, and 5 months. Most of tests were conducted at 30 C, however, some test was performed at 47 C to investigate the effect of temperature on corrosion behavior of steel at the ozone concentration of 2 ppm. In order to investigate the corrosion behavior of steel in higher ozone concentration, corrosion test was conducted at about 10 ppm due to lacking information about electrochemical measurement [6]. The ozone concentration in the ozonated seawater was measured regularly according to the indigo method [12] using ozone reagent and an ozone meter (Kasahara Chemical Instruments Co., Ltd., Japan). Both the natural seawater and ozonated seawater were circulated during the test Electrochemical Measurements Electrochemical measurement was conducted on small coupons immersed in ozonated seawater and natural seawater. During corrosion test, the open-circuit potential of specimens was monitored by an Ag/AgCl (with saturated KCl) reference electrode. After immersion test of the specimens, potentiodynamic polarization was performed using a potentiostat (HAB-151A, Hokuto Denko Co., Ltd., Japan, Solarton Model 1280C, Solarton analytical, UK) after exposure for days. The polarization scan was started from -100 mv relative to the open-circuit potential with a scan rate of 0.5 mv/s. The test was terminated when a corrosion current density exceeded the value of 10 ma/cm 2. All of the electrochemical experiments were performed in-situ in the flowing seawater at room temperature, with Ag/ AgCl (with saturated KCl) reference, Pt counter and the specimens as working electrode, respectively Corrosion Behavior of the Steel Panel The exposure period was 8-10 days. The ozone concentration was kept and ranged from 2.0 to 2.7 ppm. However, ozone concentration might become more than 3 ppm or less than 2 ppm in some case. In order to investigate the effect of internal flow on corrosion prevention of steel panel, three small current plates were inserted between the steel panel and the acrylic boards so that the flow direction of natural seawater could be controlled. The size of current plates is mm 9 mm 3 mm. Relationship between rusted area and ozone concentration was investigated. The evaluation of the corrosion was conducted using the rusted area of panels. 3. Results and Discussion 3.1 Temperature Dependence of Ozonated Seawater The corrosion test results in various temperatures are given in Fig. 6, in which the corrosion test results at 20 C, 35 C and 47 C are indicated. The weight ratio in Fig. 6 means the ratio of coupon weight after immersion test to the coupon weight before immersion test. It can be seen that ozone has a corrosion protective effect regardless of seawater temperature in this temperature range. Therefore, it is presumable that the corrosion of water ballast tank can be prevented during voyage in the tropical sea area. 3.2 Relationship between Immersion Period and Thickness of Corrosion Layer The corrosion layer of the coupons after test for 1 week, 2 weeks, 1 month, 3 months and 5 months was examined with SEM in detail. The preparation of specimens for SEM analysis is schematically shown in Fig. 1. The coupons were cut with a micro cutter. The cutting surface (i.e., the cross section of the coupon) Fig. 6 Corrosion test results at 20 C, 35 C, 47 C.

5 Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank 331 was observed by SEM. The SEM images of cross section of the coupon after immersion test in the ozonated seawater with various test periods are shown in Fig. 7. There is no layer before immersion test in Fig. 7a, but there is a very thin and compact corrosion layer on the surface of the coupons after the immersion test in the ozonated seawater, as shown in Figs. 5b-5f. The thickness of corrosion layer after the coupon is immersed for 1 week, 2 weeks, 1 month and 3 months is about 20 micro meters. The thickness of corrosion layer becomes a little larger after the coupon is immersed for 5 months as shown in Fig. 5f, and its value was 26.9 micro meters. It can be concluded that the thickness of corrosion layer is independent on immersion periods. 3.3 Electrochemical Measurement The open-circuit potential of the coupons in natural and ozonated seawater (concentration of ozone is 2 ppm and 10 ppm, respectively) is revealed in Figs. 8 and 9. For both the coupons in natural and ozonated seawaters, the potential decreased from onset of immersion test, and became comparatively stabilized after 2 days in 2 ppm and 15 days in 10 ppm, respectively. Many noises were observed in the open-circuit potential evolution of the coupon in ozonated seawater, probably due to the occurrence of localized corrosion induced by the injection of ozone. The stabilized potential of the coupon in ozonated seawater at 2 ppm and 10 ppm was -600 mv (vs. Ag/AgCl), nobler than that of the specimen in natural seawater, -625 mv and -660 mv (vs. Ag/AgCl). The potential value of the steel in the ozonated seawater with 10 ppm of ozone concentration was at the same level as that in the case of 2 ppm of ozone concentration exposed after 32 days. The potentiodynamic curves of the coupons immersed in natural and ozonated seawaters at 2 ppm and 10 ppm for days are exhibited in Figs. 10 and 11, respectively. Before potentiodynamic polarization test, the open-circuit potential (E corr ) of the specimens in natural and ozonated seawaters was -624 mv and -605 mv (vs. Ag/AgCl), respectively in the case of 2 ppm, as shown in Fig. 10, whereas, E corr in natural and ozonated seawater was -650 mv and -605 mv (vs. Ag/AgCl), respectively in the case of 10 ppm, as shown in Fig. 11. The anodic current density of the specimen in ozonated seawater at 2 ppm is obviously lower than that of the case in natural seawater. The anodic current density in the ozonated seawater at 10 ppm is higher than that in natural seawater. These results suggest that the anodic reaction Fig. 7 SEM images of corrosion layer at the ozone concentration of 2.0 ppm: (a) before immersion; (b) after 1 week; (c) after 2 weeks; (d) after 1 month; (e) after 3 months; (f) after 5 months.

6 332 Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank Fig. 8 Open-circuit potential evolution of specimens in ozonated seawater of 2 ppm and natural seawater without ozone. Fig. 9 Open-circuit potential evolution of specimens in ozonated seawater of 10 ppm and natural seawater without ozone. Fig. 10 Potentiodynamic polarization curves of specimens in ozonated seawater of 2 ppm and natural seawater without ozone. Fig. 11 Potentiodynamic polarization curves of specimens in ozonated seawater of 10 ppm and natural seawater without ozone. of corrosion of the steel in ozonated seawater of 2 ppm is depressed by the oxide film on the surface [8]. Fluctuation of current density was observed in Fig. 10. The reason for the fluctuation of current density in Fig. 10 is not fully understood at present, but it can be believed to be related with the oxide film on the surface of the specimen [8]. It can be said that ozone has a corrosion protective effect on steel when the zone concentration is 2 ppm, but no corrosion protective effect on steel when the ozone concentration is 10 ppm. 3.4 Relationship between Rusted Area of Steel Panel and Ozone Concentration The evaluation of the corrosion was conducted using the rusted area of panel. The results of corrosion test are shown in Fig. 12. The test period was 8-10 days. The ratio of rusted area to whole area of panel is presented in Fig. 13. It can be seen that corrosion of steel panel is strongly dependent on ozone concentration. The corrosion protective effect of ozone is comparatively sufficient when the ozone concentration is from 2.0 ppm to 2.7 ppm, and the ratio of rusted area to whole area of panel is about 20%. However, it is difficult to keep the ozone concentration at the determined value, so that the complete prevention of rust is difficult. In order to

7 Ozone concentration (ppm) Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank 333 Fig. 12 Aspects of test panels after corrosion tests at each concentration of ozonated seawater Fig. 13 The relationship between rusted area rate of test panels and ozone concentration Immersion period (day) Fig. 15 The transition of ozone concentration in the experiment. Fig. 14 Aspect of rusted area rate of 21.7% after exposure of ozonated seawater. Fig. 16 Aspect of rusted area rate of 39.8% after exposure of ozonated seawater.

8 334 Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank Ozone concentration (ppm) Immersion period (day) Fig. 17 The transition of ozone concentration in the experiment. Fig. 18 (a) Appearance diagrams of panels with three current plates; (b) corrosion test with three current plates; (c) corrosion test without control board. understand the influence of ozone concentration fluctuation on the corrosion protective effect, the ozone concentration was measured at frequent intervals for each corrosion test. It is found that for the test where corrosion protective effect of ozone was confirmed as shown in Fig. 14, the ozone concentration fluctuation range was comparatively narrow as shown in Fig. 15. When the ozone concentration fluctuation was large, the corrosion protective effect ozone was insufficient as shown in Fig. 16. In this case, the ozone concentration measurement result during the test is given in Fig. 17. The ozone concentration was sometimes over 3.0 ppm in this case. It can be concluded that the corrosion protective effect of ozone is not sufficient, when ozone concentration is more than 1.5 ppm or over 3.0 ppm. As shown in Fig. 18, three current plates were inserted between the steel panel and the acrylic board, so that the water flow direction could be changed and controlled. The current plates were set at inlet side. The appearance of steel panels after corrosion test with or without current plates was also shown in Fig. 18. There is no rusted area at the left side (indicate black circle in Fig. 18) of panel. This result suggests that the current plates can change the rusted area. 3.5 Simulation of Seawater Flow by FEM Contours of velocity magnitude and pathlines of seawater on panels were investigated using FEM. In the analysis, the evaluation of flow on the panel was conducted: (1) by changing the clearance gap between steel panel and acrylic board from 2.0 mm, 2.5 mm, 3.0 mm and 4.0 mm while keeping the flow rate of seawater at m/s at inlet side; (2) by changing the seawater flow rate at inlet side from m/s, m/s and 1.88 m/s while keeping the clearance gap between steel panel and acrylic board at 3.0 mm, in order to assess the compression effect of gasket in the experiment. As can be seen from Figs. 19 and 20, the results of calculation showed that there was no turbulence flow on the surface of steel panel (Fig. 19). However, higher flow rate of seawater may cause the turbulence, and this was observed when the flow rate was at 1.88 m/s (Fig. 20). Actually, the flow rate in the experiment was m/s, and there is no turbulent flow in this case. It was also found that the seawater flow is smooth flowing and vortex-free on the surface of steel panel, as shown in Fig. 21. By comparing Fig. 21 with Fig. 12, it can be observed that there is a relationship between the rusted area and seawater flow pathline. It can be concluded that the corrosion of steel panel is influenced by both the ozone concentration and seawater flow rate. This result shows good agreement with previous study [13]. Hence, the flow rate has a significant influence on the localized corrosion behavior of steel in the ozonated solution. In the case of laminar flow (flow rate < 0.2 m/sec.), increasing flow rate tends to stabilize the passive film on steels, and thus shifts the breakdown potential to a noble value.

9 Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank 335 Fig. 19 FEM calculation results by changing the clearance gap between steel panel and acrylic board from (a) 2.0 mm; (b) 2.5 mm; (c) 3.0 mm and (d) 4.0 mm at the rate of seawater at m/s at inlet side. Fig. 20 FEM calculation results by changing the seawater flow rate from (a) m/s; (b) m/s and (c) 1.88 m/s at the clearance gap of 3.0 mm between steel panel and acrylic board. Fig. 21 Calculation results of (a) contours of velocity magnitude and (b) pathlines of seawater on panel by FEM. Rate of seawater: m/s, clearance gap: 3.0 mm.

10 336 Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank On the other hand, if the flow rate is higher than 0.4 m/s, turbulence flowing occurs, so that the passive film on steels becomes unstable, and the combination of high ozone concentration and significant flow rate may increase both the pitting and crevice corrosion susceptibilities of steels in chloride solution [13-15]. 4. Conclusions In the present study, the corrosion protective effect of ozone on steel was confirmed, as the ozone concentration was at the range from 2.0 ppm to 2.7 ppm for both small steel coupons and steel panels immersed in seawater. This corrosion protective effect of ozone was also observed at high temperature seawater. The ratio of rusted area to whole area of panel was about 20% when the ozone concentration was about 2.0 to 2.7 ppm. Flow analysis of FEM also suggests that the rusted area is strongly influenced by the flow rate on the panel. These results imply that ozone has a corrosion-protective effect for water ballast tank, i.e., ozone can delay the rust of water ballast tanks, provided that the suitable concentration of ozone is selected. In this case, ozone may prohibit the corrosion at the defects, if a part of the paint in water ballast tank is peeled off. However, ozone may also promote the corrosion of steel when the ozone concentration is much higher, e.g., 10 ppm [16]. Attention should be paid to the ozone concentration, if we use ozone as an active substance for ballast water management systems [17]. Acknowledgment This study was supported by JSPS (Japan Society for the Promotion of Science) Grants-in-Aid for Scientific Research (Grant No. JSPS KAKENHI ). References [1] Mountfort, D. O., Hay, C., Dodgshun, T., Buchanan, S., and Gibbs, W Oxygen Deprivation as a Treatment for Ships Ballast Water-Laboratory Studies and Evaluation. J. Mar. Env. Eng 5: [2] Tamburri, M. N., Wasson, K., and Matsuda, M Ballast Water Deoxygenation Can Prevent Aquatic Introductions while Reducing Ship Corrosion. Biological Conservation 103: [3] Zhou, X., Liu, X., Deng, S., and Bai, X Pilotscale Experiment of Using Hydroxyl Radical to Kill Tiny Organism in Ship Ballast Water. The Ocean Engineering 22: [4] Perrins, J. C., Cordell, J. R., Ferm, N. C., Grocock J. L., and Herwing, R. P Mesocosm Experiments for Evaluating the Biological Efficacy of Ozone Treatment of Marine Ballast Water. Marine Pollution Bulletin 52: [5] Lloyds Lloyd s Register Guide to Ballast Water Treatment Technology. Lloyds. Accessed February ads/2010/06/ballast-water-treatment-technology_feb pdf. [6] Ikai, M., Kojima, R., Liao, J., Mori, Y., Kishimoto, K., and Yao, M Influence of Ozone Injection on Corrosion Behavior of Steel in Ballast Water Treatment Systems. Presented at the 6th International Conference on Ballast Water Management (ICBWM 2012), Singapore. [7] Ikai, M., Liao, J., Kishimoto, K., and Yao, M Effect of Ozone Injection on Corrosion Orotection of Steel in Ballast Water Treatment Systems. Presented at the 5th International Conference on Ballast Water Management (ICBWM 2010), London, UK. [8] Liao, J., Kishimoto, K., Yao, M., Mori, Y., and Ikai, M Effect of Ozone on Corrosion Behavior of Mild Steel in Seawater. Corrosion Science 55: [9] ClassNK. Information of Grade K A32 Steel Plate. ClassNK. [10] JIS (Japanese Industrial Standard). Rolled steels for general structure, JIS G JIS. [11] ANSYS Inc. Product reference: ANSYS Academic Research, Release ANSYS Inc. [12] Bader, H., and Hoigne, J Determination of Ozone in Water by the Indigo Method. Water Research 15: [13] Brown, B. E., Lu, H. H., and Duquette, D. J Effect of Flow Rates on Localized Corrosion Behavior of 304 Stainless Steel in Ozonated 0.5 N NaCl. Corrosion 48: [14] Koike, K., Inoue, G., Takata T., and Fukuda, T Ozone Passivation Technique for Corrosive Gas Distribution System. Jpn. J. Appl. Phys. 36: [15] Cabrera, N., and Mott, N. F Theory of the Oxidation of Metals. Rep. Prog. Phys. 12 (1):

11 Influence of Ozone Injection on Corrosion Behavior of Steel in Water Ballast Tank 337 [16] Andersen, A. B., Dragsund, E., and Johannessen, B. O Ballast Water Treatment by Ozonation-Corrosion. DNV technical report of DNV. [17] Kojima, R., Ikai, M., Shibata, T., and Ueda, K Deterioration of Water Ballast Tank Coating Systems by Active Substances in Ballast Water Management Systems. Journal of Shipping and Ocean Engineering 4 (15):