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TMMOB Metalurji ve Malzeme Mühendisleri Odas ı Bildiriler Kitab ı 815 18. Uluslararas ı Metalurji ve Malzeme Kongresi | IMMC 2016 Comparison of Microbiologically Induced Corrosion Behavior of 316L Stainless Steel and Galvanized Steel as Cooling Tower Materials Tuba Ünsal¹, Simge Arkan¹, Esra İlhan-Sungur¹, Nurhan Cansever² ¹İstanbul University, ²Yıldız Technical University - Türkiye Abstract The aim of the study was to compare corrosion behavior of 316L stainless steel (SS) and galvanized steel (GS) in the presence of Desulfovibrio sp. by gravimetric and potentiodynamic polarization methods. The growth curves were obtained by the enumeration of Desulfovibrio sp.. Biofilm formation and corrosion products on the metal surfaces were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analyses. SEM images showed that Desulfovibrio sp. in the biofilm clustered into patches on the SS surface in contrast to that on the GS surfaces. It was determined that Desulfovibrio sp. leads to corrosion both of SS and GS. However, Desulfovibrio sp. showed more corrosive effect on the GS than SS. 1. Introduction Cooling tower is a heat exchanger which provides a water stream at lower temperatures to industrial systems through the release of waste heat to atmosphere [1]. The bacteria, fungi and algae that entry into the cooling systems by the water and the air in contact with water, led to the biofilm formation on the internal surfaces of cooling towers. The biofilm formation causes corrosion of various metal structures seriously in cooling towers such as circulation pipes, water basin and heat exchangers [2]. In order to prevent the damage of microorganisms to the coling tower material, corrosion-resistant metals are preferred. Especially, 316L stainless steel (SS) and galvanized steel (GS) are popular construction materials for cooling tower systems. SS is generally prefered in areas where water accumulates, such as the cold-water basin, owing to their higher corrosion resistances. The improved level of corrosion resistance in SS relates to the formation of a chromium oxide film layer (passivation layer) on the surface of the metal due to the chromium and nickel content, and the presence of molybdenum [3]. Galvanized steel is frequently used in the construction of cooling towers and tanks because of its resistance to corrosion and biofouling. The corrosion resistance of this metal has been attributed to the formation of a protective layer of Zn(OH) 2 [4]. However, cooling water conditions and the microbial activity of sulfate reducing bacteria (SRB) frequently isolated from the cooling water may cause severe corrosion resulting in structural failures of the SS and GS systems [5]. The type of metals can affect activities of microorganisms and hence corrosion behavior. To investigate this aspect of microbiologically induced corrosion (MIC), corrosion behaviors of SS and GS in the presence of Desulfovibrio sp. were investigated by gravimetric and potentiodynamic polarization methods. The growth curves were obtained by the enumeration of Desulfovibrio sp. using Postgate B medium over 720 h. Biofilm formation and corrosion products on the metal surfaces were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analysis. 2. Experimental Procedure 2.1. Microorganism This study was performed using pure cultures of the strain Desulfovibrio sp. isolated from cooling tower water by Ilhan-Sungur and Cotuk [6]. Pure cultures of Desulfovibrio sp. was grown in Postgate’s medium B (PB medium) at 30°C [7]. 2.2. Preparation of the SS and GS Coupons The nominal elemental composition (wt%) of the SS coupons used for the experiment was C 0.022, Cr 16.02, Ni 11.44, Mo 1.95, Mn 0.984, Si 0.382, P 0.035 and S 0.010. The coupons with dimensions of 25 x 25 x 1 mm were abraded through 240, 320, 400, 600 and 800-grit silicon carbide paper, polished with aluminum oxide, washed with sterile distilled water, degreased using acetone, and dried in a Pasteur oven at 70°C. The coupons were weighed, and the total surface area of each coupon was measured and then kept in a desiccator until use. The thickness of the zinc coating was 5 μm in the GS coupons used for the experiment. The dimensions of the coupons were 25 x 25 x 1 mm. The total surface area of each coupon was determined. The cut areas of all the coupons were coated with epoxy zinc phosphate primer (Moravia, Turkey) (grey) and then covered with epoxy finish coating (Moravia, Turkey) (black) to avoid the initiation of corrosion at these disturbed areas. The coupons were weighed and then kept in a desiccator until use. 2.3. Experimental conditions of lab-scaled test and control systems The experiments were carried out in two different systems such as lab-scaled test and control systems.
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  • TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

    81518. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

    Comparison of Microbiologically Induced Corrosion Behavior of 316L Stainless Steel and Galvanized Steel as Cooling Tower Materials

    Tuba Ünsal¹, Simge Arkan¹, Esra İlhan-Sungur¹, Nurhan Cansever²

    ¹İstanbul University, ²Yıldız Technical University - Türkiye

    Abstract The aim of the study was to compare corrosion behavior of 316L stainless steel (SS) and galvanized steel (GS) in the presence of Desulfovibrio sp. by gravimetric and potentiodynamic polarization methods. The growth curves were obtained by the enumeration of Desulfovibrio sp.. Biofilm formation and corrosion products on the metal surfaces were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analyses. SEM images showed that Desulfovibrio sp. in the biofilm clustered into patches on the SS surface in contrast to that on the GS surfaces. It was determined that Desulfovibrio sp. leads to corrosion both of SS and GS. However, Desulfovibrio sp. showed more corrosive effect on the GS than SS. 1. Introduction Cooling tower is a heat exchanger which provides a water stream at lower temperatures to industrial systems through the release of waste heat to atmosphere [1]. The bacteria, fungi and algae that entry into the cooling systems by the water and the air in contact with water, led to the biofilm formation on the internal surfaces of cooling towers. The biofilm formation causes corrosion of various metal structures seriously in cooling towers such as circulation pipes, water basin and heat exchangers [2]. In order to prevent the damage of microorganisms to the coling tower material, corrosion-resistant metals are preferred. Especially, 316L stainless steel (SS) and galvanized steel (GS) are popular construction materials for cooling tower systems. SS is generally prefered in areas where water accumulates, such as the cold-water basin, owing to their higher corrosion resistances. The improved level of corrosion resistance in SS relates to the formation of a chromium oxide film layer (passivation layer) on the surface of the metal due to the chromium and nickel content, and the presence of molybdenum [3]. Galvanized steel is frequently used in the construction of cooling towers and tanks because of its resistance to corrosion and biofouling. The corrosion resistance of this metal has been attributed to the formation of a protective layer of Zn(OH)2 [4]. However, cooling water conditions and the microbial activity of sulfate reducing bacteria (SRB) frequently isolated from the cooling water may cause severe corrosion resulting in structural failures of the SS and GS systems [5].

    The type of metals can affect activities of microorganisms and hence corrosion behavior. To investigate this aspect of microbiologically induced corrosion (MIC), corrosion behaviors of SS and GS in the presence of Desulfovibrio sp. were investigated by gravimetric and potentiodynamic polarization methods. The growth curves were obtained by the enumeration of Desulfovibrio sp. using Postgate B medium over 720 h. Biofilm formation and corrosion products on the metal surfaces were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analysis. 2. Experimental Procedure 2.1. Microorganism This study was performed using pure cultures of the strain Desulfovibrio sp. isolated from cooling tower water by Ilhan-Sungur and Cotuk [6]. Pure cultures of Desulfovibrio sp. was grown in Postgate’s medium B (PB medium) at 30°C [7].

    2.2. Preparation of the SS and GS Coupons The nominal elemental composition (wt%) of the SS coupons used for the experiment was C 0.022, Cr 16.02, Ni 11.44, Mo 1.95, Mn 0.984, Si 0.382, P 0.035 and S 0.010. The coupons with dimensions of 25 x 25 x 1 mm were abraded through 240, 320, 400, 600 and 800-grit silicon carbide paper, polished with aluminum oxide, washed with sterile distilled water, degreased using acetone, and dried in a Pasteur oven at 70°C. The coupons were weighed, and the total surface area of each coupon was measured and then kept in a desiccator until use. The thickness of the zinc coating was 5 μm in the GS coupons used for the experiment. The dimensions of the coupons were 25 x 25 x 1 mm. The total surface area of each coupon was determined. The cut areas of all the coupons were coated with epoxy zinc phosphate primer (Moravia, Turkey) (grey) and then covered with epoxy finish coating (Moravia, Turkey) (black) to avoid the initiation of corrosion at these disturbed areas. The coupons were weighed and then kept in a desiccator until use. 2.3. Experimental conditions of lab-scaled test and control systems The experiments were carried out in two different systems such as lab-scaled test and control systems.

  • UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

    816 IMMC 2016 | 18th International Metallurgy & Materials Congress

    Postgate’s medium C (PC medium) was used in both systems [7]. SS and GS coupons were exposed to Desulfovibrio sp. cultures in separately during 8, 24, 72, 96, 168, 360 and 720 h in the lab-scaled test systems. Control systems containing sterile medium were set to work simultaneously with the test systems. The lab-scaled test systems were set up PC medium (1440 ml) was inoculated by appropriate volume (10%) of 1-day-old Desulfovibrio sp. culture resulting in an initial concentration of 25x106 per ml in the presence of SS and GS coupons. The culture in test systems were mixed with magnetic stirrer (150 rpm) and the experiments were carried out under anaerobic conditions in the glove box system at 30°C. The test coupons were removed at each sampling time for the enumeration of Desulfovibrio sp. and the determination of corrosion rate by the weight loss measurement method. In the control systems, weight loss measurement was carried out simultaneously with test systems. Biofilm formation and corrosion products on the metal surfaces were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analysis. 2.4. Enumeration of Desulfovibrio sp. Enumerations of the number of sessile and planktonic Desulfovibrio sp. were performed at each sampling time. Sessile and planktonic Desulfovibrio sp. counts were determined by the most-probable-number (MPN) technique using PB medium. MPN tubes were incubated in the dark at 30°C. In each inoculated tube, the growth of sulphate reducers was indicated by the formation of a black FeS precipitate and turbidity [7]. 2.5. Weight loss measurement The weight loss measurement was carried out with three coupons removed at each sampling time with SS and GS separately. Biofilm on the SS coupons were removed with sterile cotton swabs. Then corrosion products were wiped out completely by immersing the coupons 67% HNO3 and 0.6% HNO3 in an ultrasonic bath for 2-5 min and 10 s for SS and GS coupons, respectively. The surfaces of the exposed coupons were finally rinsed with distilled water, cleaned in 100% ethanol and dried in Pasteur oven at 70°C [8]. The difference between the initial and final weight was reported as weight loss both type of metals. The values of the corrosion rate were determined according to the ASTM standard G1-81 [9]. 2.6. Electrochemical measurements The electrochemical corrosion tests were performed with a computer-controlled testing device (Gamry-Interface1000, USA) by measuring the potentiodynamic polarization method. All electrochemical tests were carried out in a conventional electrochemical cell, with a carbon rod as the counter electrode, a saturated calomel electrode (SCE) as a reference electrode, and the prepared samples as the

    working electrode. The working solutions were mixed with a magnetic stirrer (150 rpm) and all of the measurements were performed at 30°C. Square-shaped SS and GS coupons with a top surface area of 1 cm2 were exposed to the Desulfovibrio sp. culture and sterile PC medium. While the potentiodynamic polarization curves of SS were obtained at a scanning rate of 1 mV/s between -800 mV and +1700 mV, the potentiodynamic polarization curves of GS were obtained at a scan rate of 1mV/s within ±500 mV compared to the corrosion potential (Ecorr). Electrochemical measurements were carried out at certain time intervals (8, 48, 96 and 168 h) during 168 h of exposure. 2.7. Surface analyses The biofilm formations and corrosion products on the coupons were analyzed by SEM and EDS at the end of the each sampling time. Coupons were fixed with 2.5% glutaraldehyde, followed by dehydration in a graded series of ethanol and air drying [10]. The dried samples were coated with a gold layer (30 nm) and imaged with an electron microscope (ZEISS (LS-10)). 3. Results and Discussion SS and GS coupons were exposed to Desulfovibrio sp. culture over 720 h in the lab-scaled test systems. Heterogenous, gelatinous and easily removable biofilm layers on the surfaces of the coupons were observed (Figure 1). However SEM images showed that Desulfovibrio sp. in the biofilm clustered into patches on the SS surface.

    Figure 1. SEM micrographs of the biofilm formed on the SS (a) and GS (b) surfaces after 720 h of exposure in the Desulfovibrio sp. culture.

  • TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

    81718. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

    The growth curves of Desulfovibrio sp. in the biofilm and culture are shown in Figure 2. The Desulfovibrio sp. counts in the cultures were significantly higher than in the biofilms on the SS and GS (p< 0.01). Desulfovibrio sp. in the biofilm on SS and GS coupons completed the logarithmic phase after 72 h and then stayed in the stationary phase, and finally entered to the death phase after 168 h. The results showed that the durations of the growth phases of Desulfovibrio sp. were similar to each other.

    Figure 2. The growth curves of planktonic and sessile Desulfovibrio sp. in the presence of SS (a) and GS (b). The error bars represent the standard deviation of the mean. The weight loss values of SS and GS coupons showed that Desulfovibrio sp. caused an increase in weight losses compared to the control (p

  • UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

    818 IMMC 2016 | 18th International Metallurgy & Materials Congress

    surface was very likely composed of a ZnS deposit. It is possible that H2S enhances anodic dissolution of the zinc, resulting in insoluble ZnS corrosion products and cathodic hydrogen evolution [14].

    Figure 4. Polarization curves of the SS (a) and GS (b) coupons exposed to the sterile PC medium and Desulfovibrio sp. culture.

    Figure 5. SEM micrographs and EDS analysis of the corrosion products formed on the SS (a) and GS (b) coupons exposed to Desulfovibrio sp. after 720 h.

    4. Conclusions The experimental results are summarized as follows:

    1. The biofilm formed by Desulfovibrio sp. showed

    different morphology and polarization resistance according to the metals’ type.

    2. The durations of the growth phases of sessile Desulfovibrio sp. were similar on the SS and GS surfaces.

    3. Desulfovibrio sp. accelerated corrosion both of the SS and GS. However, the results indicate that GS is suffered corrosion by Desulfovibrio sp. more than SS.

    4. The corrosion rates both of SS and GS were not related with the Desulfovibrio sp. count.

    References [1] S.G. Choudhary, Hydrocarbon Processing, 77

    (1998) 91-102. [2] P.R. Puckorius, Water Corrosion Mechanism,

    ASHRAE Journal, 41 (1999) 57-61. [3] F. Morrison, Cooling Tower Institute, 29 (2008) 8-

    33. [4] X.G. Zhang, Corrosion and Electrochemistry of

    Zinc, Plenum Press, 1996, New York. [5] W.A. Hamilton, Annual Review of Microbiology,

    39 (1985) 195-217. [6] E. Ilhan-Sungur and A. Cotuk, Environmental

    Monitoring and Assessment, 104 (2005) 211-219. [7] J.R. Postgate, The Sulphate Reducing Bacteria,

    Cambridge University Press, 1984, Cambridge. [8] X. Sheng, Y.P. Ting and S.O. Pehkonen,

    Corrosion Science, 49 (2007) 2159-2176. [9] American Society for Testing And Material,

    Practice for preparing, cleaning and evaluating corrosion test specimens, In: Annual book of ASTM standards, Designation: G1-81, American Society for Testing Materials, 1986, Philadelphia.

    [10] C. Campanac, L. Pineau, A. Payard, G. Baziard-Mouysset and C. Roques, Antimicrobial Agents and Chemotherapy, 46 (2002) 1469-1474.

    [11] R. Javaherdashti, Microbiologically Influenced Corrosion: An Engineering Insight, Springer, 2008, London.

    [12] F.M. AlAbbas, R. Bhola, J.R. Spear, D.L. Olson and B. Mishra, International Journal of Electrochemical Science 8 (2013) 859-871.

    [13] H.A. Videla, Microbially induced corrosion: an updated overview, Ed. by H.W. Rossmoore, Biodeterioration and Biodegradation, 8. Elsevier Applied Science, 1991, London.

    [14] B.J. Little, P.A. Wagner and Z. Lewandowski, Corrosion, 294 (1998).


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