Effect of Sulfate-Reducing Bacteria and Iron-Oxidizing Bacteria on the

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Effect of Sulfate-Reducing Bacteria and Iron-Oxidizing Bacteria on the Rate of Corrosion of an Aluminum Alloy in a Central AirConditioning Cooling Water System Hongfang Liu,* Bijuan Zheng, Dandan Xu, Chaoyang Fu,* and Yi Luo Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ABSTRACT: The microbiologically influenced corrosion behaviors of the 3A21 aluminum alloy in Wuhan (Hubei Province, China) municipal potable water containing wild-type total culturable bacteria (TCB) at different temperatures were investigated by mass loss and surface morphology analysis. When the water was inoculated with sulfate-reducing bacteria (SRB), laboratory results showed that SRB inhibited the mass loss-based rate of corrosion of the Al alloy coupons at 30 °C but accelerated the corrosion rate by 2- and 4.6-fold at 45 and 60 °C, respectively. It was also found that the presence of aerobic iron-oxidizing bacteria (IB) significantly reduced the corrosion rate at 45 °C. The average corrosion rate in water inoculated with enriched TCB was the highest, and it decreased at higher temperatures. At 60 °C, SRB were found to be the dominant bacteria in the water with TCB and in the water with IB.

1. INTRODUCTION Aluminum alloys have very important applications in the aerospace, architecture, transportation, and manufacturing industries,1−4 because of their low density and excellent mechanical properties. Unfortunately, Al is a chemically reactive metal that can be corroded despite the usually protective oxide film on its surface.5,6 Many studies focused on the corrosion protection, scale inhibition, and corrosion monitoring in cooling water recirculation systems,7−11 which are frequently contaminated with microorganisms from the air and aqueous media. Microorganisms have the potential to initiate, facilitate, and accelerate the corrosion reactions, known as microbiologically influenced corrosion (MIC).12,13 In some cases, microorganisms can offer protection from MIC.14,15 Microorganisms also cause biofouling that increases heat transfer resistance leading to losses of cooling capability.16,17 Microorganisms influence corrosion processes by several mechanisms such as the utilization of extracellular electrons from elemental metal oxidation for the reduction of an oxidant intracellularlly and the secretion of corrosive metabolites, including organic acids.18 Sulfate-reducing bacteria (SRB), which use sulfur compounds such as sulfate, sulfite, thiosulfate, and elemental sulfur itself as electron acceptors, are most frequently associated with MIC.19,20 Our previous study found that SRB caused severe corrosion damage to aluminum AA2024 coupons.19 MIC behaviors of aluminum alloys in the presence of other bacteria have also been investigated by other researchers. Bacillus cereus has been found to induce pitting corrosion on AA2024.21,22 Juzeliu̅nas et al. found that Aspergillus niger inhibited corrosion of Al probably because it promoted passivity at sites of localized corrosion by plugging the micropores and microcracks.23 Iron-oxidizing bacteria (IB) and SRB are prevalent in cooling water systems.24−26 Total culturable bacteria (TCB) is an important parameter for water quality in cooling water systems. © 2014 American Chemical Society

Rao et al. suggested that IB and SRB were responsible for the corrosion of carbon steel in the cooling water system of a nuclear test reactor,24 with a mean corrosion rate against the carbon steel coupons of ∼0.06 mm year−1.25 Rao et al. further characterized the bacterial diversity and counted the populations of IB, SRB, and TCB in the cooling water system of a fast breeder test reactor.26 The cooling water temperature affects the effectiveness of corrosion inhibitors and corrosion rates.27,28 In the case of MIC, it also influences the microorganism varieties as well as biocide efficacies. In this study, the effects of SRB, IB, and TCB on the MIC of the 3A21 aluminum alloy, which is widely used in cooling water systems, in Wuhan (Hubei Province, China) municipal potable water were investigated at different temperatures.

2. MATERIALS AND METHODS 2.1. Test Specimens and Culture Medium. The nominal compositions of the 3A21 aluminum alloy are listed in Table 1. The aluminum alloy coupons were mechanically cut into 50 Table 1. Nominal Compositions of the 3A21 Aluminum Alloy element

concentration (wt %)

Mn Fe Si Ti Mg Zn Al

1.0 0.7 0.6 0.15 0.05 0.03 balance

Received: Revised: Accepted: Published: 7840

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below 30 °C. Therefore, 30, 45, and 60 °C were chosen as incubation temperatures for testing. Because of the evaporation of water, fresh municipal potable water was added to the open bottles daily during the 14 day incubation period. The water concentration ratio during incubation was the total water amount in milliliters divided by 500 mL. 2.2. Surface Morphology Analysis. Microscope images of the biofilms on aluminum alloy coupon surfaces were obtained using a field emission scanning electron microscope (Sirion 200, FEI) equipped with energy-dispersive X-ray detectors for elemental analysis. The coupons covered with biofilms retrieved at the end of the incubation were first rinsed with a sterilized medium and immersed in a medium containing 2 wt % formaldehyde for 8 h to fix the biofilms. Then, the samples were dehydrated sequentially in a series of ethanol/water solutions (25, 50, 70, 85, 95, and 100%) and finally dried with N2 at room temperature. Epifluorescence microscopy was conducted using a microscope (DP71, Olympus). In this work, acridine orange was used as the fluorescent dye. The green excitation light irradiated the live bacterial cells.

mm × 13 mm plates with a thickness of 1.5 mm. Prior to all experiments, the coupons were polished using SiC grit paper up to grade 1000 and rinsed with pure acetone and then 95% (v/ v) ethanol. Wuhan municipal potable water was used as the basic culture medium in this study, and the water chemistry parameters are listed in Table 2. SRB were isolated from a Table 2. Water Chemistry Parameters of Wuhan Municipal Potable Water pH [Cl−] (mg L−1) [SO42−] (mg L−1) [HCO3−] (mg L−1) [Na+] (mg L−1) [K+] (mg L−1) [Mg2+] (mg L−1) [Ca2+] (mg L−1) [Sr2+] (mg L−1) [Ba2+] (mg L−1) [Fe3+] (mg L−1) CODcr (mg L−1) turbidity (NTU) TCB (no. of cells mL−1) total hardness (mg L−1) total dissolved salts (mg L−1) water quality type

7.46 15.19 37.86 115.3 10.11 2.38 7.68 42.73 0.17 0.03 0.1 81.67 1.254 10 138.3 231 Na2SO4

3. RESULTS AND DISCUSSION 3.1. Rate of Corrosion of the Al Alloy Incubated in Municipal Potable Water. Table 2 shows that the Wuhan

central air-conditioning cooling water system and enriched in Postgate culture medium.29 The isolated IB were cultured in an enrichment medium, and TCB were enriched in a universal culture medium. The chemical compositions of IB enrichment and the universal culture media are listed in Tables 3 and 4, respectively. The SRB and IB populations were obtained using the most probable number method. The amount of TCB was obtained using the plate counting method. Table 3. IB Enrichment Medium element

concentration (g L−1)

sodium citrate ferric citrate ammonium citrate MgSO4·7H2O pH

8.0 2.0 0.13 0.4 7.0

Figure 1. Rates of corrosion of Al alloy coupons after exposure to municipal potable water for 14 days at different temperatures.

municipal potable water was rich in organic compounds and contained a significant amount of sulfate (37.86 mg L−1). Bacteria multiplied in the air-conditioning cooling water system using the water over operation for 6 months. The population of TCB was 1.0 × 107 cells mL−1, which far exceeded the national standard of 5.0 × 105 cells mL−1 for heterotrophic bacteria in open recirculation systems. The water concentration ratios were 1.5, 3, and 5.75 at 30, 45, and 60 °C, respectively. The mass losses of Al alloy coupons incubated in municipal potable water for 14 days at 30, 45, and 60 °C were used to calculate the corrosion rates presented in Figure 1. In this group, the corrosion rate decreased with an increase in temperature. The corrosion rate at 45 °C was 76% of that at 30 °C, and the corrosion rate at 60 °C was only 49% of that at 30 °C. As shown in Figure 2, there were plenty of mineral deposits on the coupon surface after incubation for 14 days. Such scale formation usually leads to a loss of heat transfer efficiency and even water flow blockage. In addition, a pit with a surface

Table 4. Universal Medium for Total Culturable Bacteria element

concentration (g L−1)

beef extract peptone glucose NaCl pH

3.0 10.0 10.0 5.0 7.0

Rates of corrosion of Al alloy coupons were measured by the mass loss method. UV-sterilized 3A21 aluminum alloy coupons were immersed in four groups of glass bottles containing 500 mL of municipal potable water, 450 mL of municipal potable water and a 50 mL SRB seed culture, 450 mL of municipal potable water and a 50 mL IB seed culture, or 450 mL of municipal potable water and a 50 mL enriched TCB seed culture. The temperature range for cooling water systems is typically from 5 to 60 °C, and corrosion is usually negligible 7841

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Figure 2. SEM and EDX of the Al alloy surface after exposure to municipal potable water for 14 days at 45 °C.

Figure 4. Photos of Al alloy coupons after exposure to municipal potable water inoculated with SRB for 14 days at 45 (a) and 60 °C (b).

Figure 3. Rates of corrosion of Al alloy coupons after exposure to municipal potable water inoculated with SRB for 14 days at different temperatures.

3.2. Rate of Corrosion of the Al Alloy Incubated in Municipal Potable Water Inoculated with SRB. The rates of corrosion of the Al alloy coupons incubated in municipal potable water inoculated with SRB for 14 days are presented in Figure 3. Compared with the rates of corrosion for coupons

diameter of 1 mm appeared on the coupon surface, and a porous Al2O3 film was detected on the pit bottom. 7842

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Figure 8. SEM of the SRB biofilm on the Al alloy coupon after exposure to municipal potable water inoculated with IB for 14 days at 45 °C.

Figure 5. SEM of the SRB biofilm on the Al alloy coupon after exposure to municipal potable water inoculated with SRB for 14 days at 60 °C.

Figure 9. Rates of corrosion of Al alloy coupons after exposure to municipal potable water inoculated with TCB for 14 days at different temperatures.

Figure 6. Rates of corrosion of Al alloy coupons after exposure to municipal potable water inoculated with IB for 14 days at different temperatures.

Figure 7. Photos of Al alloy coupons after exposure to municipal potable water inoculated with IB for 14 days at 45 (a) and 60 °C (b).

Figure 10. Photos of Al alloy coupons after exposure to municipal potable water inoculated with TCB for 14 days at 45 (a) and 60 °C (b).

incubated in municipal potable water, the presence of SRB inhibited corrosion at 30 °C. Municipal potable water inoculated with SRB was still clear to the naked eye after incubation for 14 days. Only a thin layer of SRB biofilm appeared on each coupon surface at 45 °C in Figure 4a. At 30 °C, the rate of corrosion in the presence of SRB (0.02 mm year−1) was slower than that without SRB (0.08 mm year−1). Apparently, the SRB biofilm offered protection at 30 °C.30 At 45 and 60 °C, however, SRB enhanced the corrosion. Compared with the uninoculated control, the rate of corrosion

of the Al alloy in municipal potable water with SRB was 2 times (0.12 mm year−1 vs 0.06 mm year−1) and 4.3 times (0.17 mm year−1 vs 0.04 mm year−1) higher than those at 45 and 60 °C without SRB. In Figures 4b and 5, a mature and dense SRB biofilm appeared on the surface of the Al alloy coupons at 60 °C with a sessile SRB count of 1.0 × 106 cells cm−2. 3.3. Rate of Corrosion of the Al Alloy in Municipal Potable Water Inoculated with IB. As observed in Figure 6, the slowest rate of corrosion of the Al alloy coupons in municipal potable water inoculated with IB was 0.03 mm year−1 7843

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Figure 11. SEM and EDX analysis of the corrosion products on the Al alloy coupon after exposure to municipal potable water inoculated with TCB for 14 days at 45 (a) and 60 °C (b).

at 45 °C, which is lower than the corrosion rate without IB inoculation (control) of 0.06 mm year−1. One possible reason could be that the aerobic IB decreased the level of dissolved oxygen (DO). It is reported that local corrosion in the Al alloy can be promoted by O2 reduction on cathodic Fe-rich particles.31,32 The photos in Figure 7a and scanning electron microscope (SEM) in Figure 8 revealed IB colonies and corrosion tubercles on the coupon surfaces.24 Municipal potable water inoculated with IB was turbid after incubation for 14 days with an obvious layer of red sediments on the bottom, which was indicative of iron rust, providing the evidence of the existence of IB. Figure 6 also shows that the corrosion rate jumped to 0.18 mm year−1 at 60 °C, close to the corrosion rate with SRB inoculation of 0.17 mm year−1. Furthermore, the Al alloy coupons were covered with black biofilms at 60 °C as shown in Figure 7b. One possibility was that at 60 °C, native thermophilic SRB overtook IB.

3.4. Rate of Corrosion of the Al Alloy in Municipal Potable Water Inoculated with TCB. At the three selected test temperatures, the average rates of corrosion of Al alloy coupons immersed in municipal potable water inoculated with TCB were the highest. As shown in Figure 9, the presence of TCB greatly promoted the corrosion of Al alloy coupons. The corrosion rate decreased with an increase in temperature, which inhibited the activities of mesophilic bacteria in the water. At 60 °C, the corrosion rate was 0.21 mm year−1, close to the value of 0.17 mm year−1 for the case with SRB inoculation. It was possible that SRB was the main contributor of MIC in TCB at 60 °C. As shown in Figure 10, several whitish nodules formed on the coupons taken from the municipal potable water inoculated with TCB at 45 °C. At 60 °C, dense black films were visible on the coupons taken from municipal potable water inoculated with TCB at 60 °C. They look similar to those with SRB inoculation in Figure 5. The black film is typically associated 7844

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Figure 12. Epifluorescent microscope photos of the Al alloy surfaces after exposure to municipal potable water (a) and municipal potable water inoculated with SRB (b), IB (c), and CB (d) for 14 days at 45 °C.

4. CONCLUSION The Wuhan municipal potable water was rich in organic compounds and sulfate. After water recirculation for 6 months in the air-conditioning cooling water operation, the population of TCB in the water reached 1.0 × 107 cells mL−1. The water concentration ratios were 1.5, 3, and 5.75 at 30, 45, and 60 °C, respectively. The rate of corrosion of the Al alloy in municipal potable water for 14 days decreased with higher temperatures. Porous Al2O3 was found on the pit bottom. Inoculation of SRB of the water inhibited the corrosion at 30 °C but accelerated the corrosion of the Al alloy by 2- and 4.6-fold at 45 and 60 °C, respectively. It was also found that sessile SRB cells (1.0 × 106 cells cm−2) attached to the surface of the Al alloy coupons at 60 °C. The presence of aerobic IB introduced into municipal potable water through inoculation resulted in the lowest rate of corrosion of the Al alloy coupons, 0.03 mm year−1 at 45 °C. However, the rate of corrosion of the Al alloy coupons increased to 0.18 mm year−1 at 60 °C. The average rate of corrosion of Al coupons after incubation in municipal potable water inoculated with TCB for 14 days was the highest, and it decreased with higher temperatures. At 60 °C, SRB were the dominant bacteria in municipal potable water inoculated with either TCB or IB. The epifluorescent microscope found that the Al alloy coupon in the case with TCB inoculation had the highest biofilm coverage among all cases at 45 °C.

with SRB biofilms that produce black FeS. The SEM images in Figure 11 show that a dense film shielded the surface of the Al coupon at 45 °C. It also showed a deep crevice that could be penetrated by a corrosive medium.33 The population of sessile TCB on the Al alloy surface was found to be 6.5 × 105 cells cm−2, among which the population of sessile SRB was only 3.5 × 10 cells cm−2. Therefore, the dominant bacteria in TCB at 45 °C were not SRB. The energy-dispersive X-ray (EDX) analysis showed that the white corrosion product was mainly Al2O3 (Figure 11). As mentioned above, at 60 °C, the biofilms, mainly consisting of SRB, flourished on the Al alloy surface incubated in municipal potable water inoculated with TCB. The population of sessile TCB on the Al alloy surface was 4.5 × 105 cells cm−2, among which the population of sessile SRB was found to be 4.0 × 105 cells cm−2, which amounted to 89.0% of TCB. 3.5. Epifluorescent Microscope Images of the Al Alloy Surfaces in the Three Test Systems at 45 °C. Epifluorescent microscope images of the Al alloy surfaces in Figure 12 were obtained after exposure to municipal potable water, municipal potable water inoculated with IB, and municipal potable water inoculated with TCB for 14 days at 45 °C. As shown in Figure 12a, only a part of the coupon surface was green, which indicated that the biofilm coverage was not complete without inoculation. On the contrary, the whole surface of the Al alloy surface in the case with TCB inoculation in Figure 12d was green, suggesting a complete bacterial biofilm coverage. This was consistent with the observations in Figures 10 and 11. The Al alloy coupon in the case with TCB inoculation was covered with more bacteria than in the case with SRB inoculation (Figure 12b) and also the case with IB inoculation (Figure 12c) at 45 °C.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 7845

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (51171067), the Natural Science Foundation of Shenzhen City (JC201005310696A), and GREE Electric Appliances, Inc., of Zhuhai, China. We acknowledge the support of the Analytical and Testing Center of the Huazhong University of Science and Technology. We thank Prof. Tingyue Gu (Ohio University, Athens, OH) for his support.

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