Appl Microbiol Biotechnol DOI 10.1007/s00253-012-4389-1
ENVIRONMENTAL BIOTECHNOLOGY
Effect of W–TiO2 composite to control microbiologically influenced corrosion on galvanized steel Rubina Basheer & G. Ganga & R. Krishna Chandran & G. M. Nair & Meena B. Nair & S. M. A. Shibli
Received: 19 June 2012 / Revised: 14 August 2012 / Accepted: 24 August 2012 # Springer-Verlag 2012
Abstract Microorganisms tend to colonize on solid metal/ alloy surface in natural environment leading to loss of utility. Microbiologically influenced corrosion or biocorrosion usually increases the corrosion rate of steel articles due to the presence of bacteria that accelerates the anodic and/or cathodic corrosion reaction rate without any significant change in the corrosion mechanism. An attempt was made in the present study to protect hot-dip galvanized steel from such attack of biocorrosion by means of chemically modifying the zinc coating. W–TiO2 composite was synthesized and incorporated into the zinc bath during the hot-dipping process. The surface morphology and elemental composition of the hot-dip galvanized coupons were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The antifouling characteristics of the coatings were analyzed in three different solutions including distilled water, seawater, and seawater containing biofilm scrapings under immersed conditions. Apart from electrochemical studies, the biocidal effect of the composite was evaluated by analyzing the extent of bacterial growth due to the presence and absence of the composite based on the analysis of total extracellular polymeric substance and total biomass using microtiter plate assay. The biofilm-forming bacteria formed on the surface of the coatings was cultured on Zobell Marine Agar plates and studied. The composite was found
R. Basheer : G. Ganga : R. K. Chandran : G. M. Nair Inter University Centre for Genomics and Gene Technology, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala 695 581, India M. B. Nair : S. M. A. Shibli (*) Department of Chemistry, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala 695 581, India e-mail: smashibli@yahoo.com
to be effective in controlling the growth of bacteria and formation of biofilm thereafter. Keywords Corrosion . Hot-dip galvanization . Steel . Zinc . W–TiO2 composite . Biofilm . Biocorrosion
Introduction The physicochemical interactions between a metallic material and its environments can lead to corrosion. The interaction of bacteria with metal surface results in the formation of biofilms in a process known as biofouling. A biofilm can be defined as a surface attached (sessile) community of microorganisms growing embedded in a self-produced matrix of extracellular polymeric substances (EPS). Bacteria colonizing on a surface produce EPS that will glue the cells to the surface and eventually form the biofilm matrix. Generally, EPS are composed of polysaccharides but may also contain proteins, nucleic acids, and polymeric lipophilic compounds. In terms of weight and volume, EPS represents the major structural component of biofilms, being responsible for the interaction of microbes with each other as well as with interfaces (Flemming 2002; Neu et al. 2001). The primary colonizers of inanimate underwater surfaces are bacteria, which creates a favorable environment in the form of biofilm for the attachment of algae and the invertebrates like barnacles and other invertebrate larvae. Such an association creates a complex local environment on the surface of the metal, thereby enhancing the rate of corrosion of the metal surface exposed, leading to biofouling. Traditionally, it has been assumed that the interaction of bacteria with metal surfaces always causes increased corrosion rates (Ameer et al. 2011; Mansfeld 2007). Microbial activity within the biofilms formed on the surface of metals can affect the kinetics of cathodic and/or anodic reactions (Jones and Amy 2002) and can also considerably modify the
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chemistry of any protective layers, leading to either acceleration or inhibition of corrosion (Little and Ray 2002; Ornek et al. 2002a,b). The main types of bacteria associated with metals in terrestrial and aquatic habitats are sulfate reducing bacteria (SRB), sulfate-oxidizing bacteria, ironoxidizing bacteria, manganese-oxidizing bacteria, and bacteria secreting organic acids and slime (Beech and Sunner 2004). von Wolzogen Kuehr in 1923 proposed the so-called cathodic depolarization mechanism, which assumes that the SRB remove atomic hydrogen from the iron surface, which causes accelerated corrosion of iron (Mansfeld 2007). When exposed to seawater media containing toxic metals and chemicals, such as Cd(II), Cu(II), Pb(II), Zn(II), Al(III), Cr (III), glutaraldehyde, and phenol, the SRB in the biofilm aggregated into clusters and increased the production of EPS (Fang et al. 2002). In the present work, hot-dip galvanizing technique, where by zinc is applied on the surface of steel, was adopted to prevent the corrosion of mild steel in seawater. Certain metals/metal oxides are used to enhance the antifouling characteristics of hot-dip galvanized coatings. Generally, metal oxides play an important role in the corrosion protection of mild steel. Various metal oxides such as ZnO, ZrO2, and TiO2 have been used as oxide barrier coatings on galvanized steel substrate (Hamid et al. 2010; Shibli and Francis 2008; Shibli and Francis 2011a). The use of TiO2 as a photo catalyst for the decomposition of organic compounds and microbial organisms including viruses, bacteria, and cancer cells has been reported (Blake et al. 1999; Kangwansupamonkon et al. 2009). Shieh et al. (2006) have reported an antibacterial performance of TiO2 against Escherichia coli that could reach 99.99 % bacterial reduction under activation by visible light. It has been reported that the incorporation of TiO2 increases the antifouling characteristics of hot-dip zinc coatings (Shibli and Francis 2011b). There are several literature reports for the modification of TiO2 surface with metals, such as Pt, Fe, Ag, Au, and Pd. This technique is considered as a promising tool to enhance the photo catalytic activity of TiO2 and to increase the quantum yield (Li and Li 2002; Sakthivel et al. 2004; Wodka et al. 2010). In the present work, TiO2 surface was modified using tungsten because of its high density, hardness, very high melting temperature, relatively high radiation opacity, and good thermal conductivity combined with very low thermal expansion (German et al. 2006). The effect of W–TiO2 composite on bacteriologically enhanced corrosion of hot-dip galvanized coupons was discussed in this paper. This was studied through X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive Xray spectroscopy (EDS), open circuit potential (OCP), optical microscopy, and biological parameters like bacterial growth characteristics, biomass estimation and difference in the EPS formation.
Materials and methods Preparation of tungsten wetted TiO2 composite by chemical reduction method The precursor materials used were sodium tungstate, Na2WO4⋅2H2O (Nice, India; assay, 96.0 %), titanium dioxide, TiO2 (CDH, India; assay, 99.5 %), and hydrazine hydrate (Ottokemi, India; assay, 80.0 %). A 1 M solution of sodium tungstate was prepared. To one part by weight of this standard solution, three parts by weight of TiO2 was added. The mixture was then stirred well using a magnetic stirrer and heated in a temperature range of 70–80 °C for 6 h. The resultant paste was dried in an oven and transferred to alkaline hydrazine hydrate (100 mL) taken in a breaker, and the whole mass was kept in a water bath (80 °C) with constant stirring for about 3–4 h. The resultant product was filtered and then dried in an oven. The dried product was powdered, and one portion of it was heated at 800 °C in order to ensure that no changes occur at this high temperature because it had to be used at molten zinc bath. The nature of the phases and the crystallite size of the powder before and after heating at 800 °C were determined using an X-ray powder diffractometer using Cu Kα radiation (λ01.5405 Å). Antifouling characteristics of W–TiO2 composite The ability of W–TiO2 to act as an antifouling agent was assessed microbiologically. For this, seawater along with samples of biofilm formed under boat hulls was collected aseptically from the Vizhinham harbor, Thiruvananthapuram, Kerala, India. The microbial samples collected were then isolated through plating methods in Zobell Marine Agar. EPS production by consortium of marine bacteria in the presence of the composite was checked against various concentrations (1–4 %) of the composite. After incubation of 24 h at 25 °C, the amount of total carbohydrates in the EPS produced was assessed by phenol-sulfuric acid method (Dubois et al. 1956) as a means of assessing extent of biofilm formation. Selection of substrate and pretreatment methods Mild steel is the material mostly used for construction as well as for other commercial processes because it is commercially affordable and possesses good mechanical strength. It is generally subjected to hot-dip galvanizing process because it is the substrate most suitable for the zinc alloying process, i.e., the hot dipping process. Other substrate will not undergo the hot-dip galvanizing process efficiently. In the present work, mild steel coupons of a dimension of 3.5 × 2.5 × 0.1 cm 2 having the elemental
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composition of 0.090 % carbon, 0.340 % manganese, 0.036 % phosphorus, 0.048 % silicon and 0.029 % aluminium and remaining iron was used. The substrate was abraded with 100 grit emery paper, degreased using 5 % NaOH solution and then etched in 8 % HCl solution for 20 min at room temperature to ensure that the substrate was free from any superficial oxide layer. The coupons were then fluxed with 30 % NH4Cl solution for 30 min at 50±1 °C to avoid any further oxidation of the surface and to enhance the adhesion of molten metal onto the substrate. The galvanizing process Commercially available pure zinc (99.95 wt%) was used for the present work. The required quantity of zinc was melted in a graphite crucible kept at 450±10 °C in a muffle furnace. A required quantity of the prepared W–TiO2 composite was added into the molten zinc bath and stirred well using silicon carbide rod. The coupons were preheated to a temperature of 200±10 °C and then dipped into the molten bath for 10–15 s. The process parameters were fixed based on the performance of the coating prepared under varying experimental conditions. The excess zinc on the coupons was removed by blowing hot air while withdrawing the strips from the bath. Different compositions of W–TiO2 composite incorporated hot-dip zinc coatings were developed. At the preliminary stage, different stages of coupons, as a function of different W–TiO2 content, were prepared and subjected to different analysis and evaluation. Based on the electrochemical performance of the galvanized coupons, the optimum amount of composite added into the bath was fixed as 0.2 %. Pure zinc coating and 0.2 % W–TiO2 composite incorporated coating were used for the entire study. Morphological characterization of the coatings
aeration and sunlight. Temperature was maintained at 25 °C. Previously weighed metal strips were then immersed in triplicates in each trough such that equal surface area (10 cm2) of each coupon remains dipped in water. The experimental setup is kept for 20 days. Open circuit potential and pH measurement OCP of a metal signifies its tendency to corrode, and changes in its potential with time without the application of an external current can be related to the nature of the metal surface. It is a sensitive measurement to detect various stages of dissolution of a coating during long-term immersion tests. In this method, metal coupons were the working electrode, saturated calomel electrode was the reference electrode, and the three solutions were the medium for the measurements. OCP of the coupons (using Aplab digital multimeter model 1089) and pH of the solutions (using Eutech pH Tutor) were recorded periodically for a period of 20 days and plotted as a function of time to understand the changes that take place on metal coupons and solutions respectively. Screening of biofilm formation and biofouling activity by bacteria After 20 days of incubation, the metal coupons were recovered and washed thoroughly with sterile distilled water to remove any corroded debris and loosely attached bacterial cells. One of the two sides of the coupons was then swabbed and inoculated onto Zobell Marine Agar (Hi Media) and incubated for 24 h at 25 °C. Other side of the metal coupons was microscopically evaluated using an optical microscope (Olympuz SZ61, Taiwan) at a magnification of ×4.5. Individual bacteria were isolated from the consortia that formed the biofilm on the coupons. Five microliters of overnight-
Microstructure of the coatings was evaluated using a scanning electron microscope JEOL 6390 LV equipped with an energy dispersive X-ray spectroscope, JEOL 2200. The surface morphology and particle distribution in coatings were compared using SEM images. The grain size of the coatings was also compared using SEM images. The elemental composition of the coatings was analyzed using EDS patterns. Biological and electrochemical assay of antifouling characteristics of the coatings Immersion of metal coupons in water for biofilm formation Three sets of experimental set up, viz., distilled water as control, seawater, and seawater with biofilm scrapings were maintained in wide-mouthed glass troughs to ensure proper
Fig. 1 X-ray diffraction patterns of W–TiO2 composite a without heating and b after heating at 800 °C
Appl Microbiol Biotechnol Table 1 Antifouling activity of W–TiO2 composite at various concentrations
Percentage composition of composite (%)
Optical density
0
0.364
1 2 3 4
0.259 0.182 0.146 0.141
grown cultures of the bacterial isolates as well as the consortia was inoculated onto 45 μL of Zobell Marine Broth (ZMB) in microtitre plates and incubated at 25 °C for 48 h. After incubation, the wells were washed with sterile physiological saline and fixed with 99.99 % ethanol for 10 min. Ethanol was then removed, and the attached bacterial cells were stained in 2 % crystal violet for 20 min. The plates were washed with distilled water, and the amount of attached cells was measured using an ELISA reader at 570 nm (Abdi-Ali et al. 2006; Peeters et al. 2008). Biological assay for extracellular polymeric substance produced by bacteria ZMB was also inoculated with the bacteria that formed the biofilm on metal coupons by dipping the coupons in ZMB for 1 h. Glycerol [3 % (v/v)] was added as extra carbon source, and incubation was done in a shaker incubator at Fig. 2 The SEM micrographs of a pure zinc coating and b 0.2 % W–TiO2 composite incorporated coating with magnifications ×1,000 and ×1,500
25 °C with 120 rpm for three consecutive days. The supernatant was then collected by centrifugation at 10,000 rpm for 10 min to collect cell-free extract containing EPS. An estimate of total carbohydrate in the supernatant was estimated using phenol-sulfuric acid test. Determination of self-corrosion rate After 20 days of exposure in three different solutions, the corroded coupons were washed with 10 % ammonium persulfate solution, dried, and weighed. Self-corrosion rate of these coupons were calculated from the difference in their weights and was plotted against composition of W–TiO2 composite. Self corrosion rate ¼ weight loss=ðsurface area timeÞg cm 2 h 1
Results Chemical composition and antifouling characteristics of W–TiO2 composite The phase structure and chemical composition of the prepared composite before and after heating at 800 °C were analyzed using the XRD patterns shown in Fig. 1. The sharp peaks revealed the crystalline nature of the composite. The peaks at 2 theta values of 27.54 and 36.19 corresponding to
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the presence of titanium tungsten oxide (Ti 54 W 46 O 2 ) (JCPDS 85-0270) and the peaks at 2 theta value 54.42 and 56.73 corresponding to the rutile phase of TiO2 (JCPDS 860147) and tungsten (JCPDS 04-0806), respectively, were also noted. The antifouling efficacy of the composite was also analyzed, and the enhanced efficacy with increase in concentration is evident from Table 1. Fig. 3 The energy dispersive spectrum of a pure zinc coating and b 0.2 % W–TiO2 composite incorporated coating
Microstructure of the coatings The surface morphology and elemental composition of the hot-dip galvanized coupons were examined by SEM-EDS analysis. The incorporation of the composite generally suppressed a massive surface finish of the coating leading to microstructural uniformity. The SEM images of the pure
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zinc coating and the one incorporated with W–TiO2 composite are shown in Fig. 2, at magnifications of ×1,000 and ×1,500. The micrograph of W–TiO2 incorporated galvanized coupon revealed a significant improvement on the morphology with the particles distributed uniformly throughout the surface (Fig. 2b). The elemental composition of the pure zinc coating and that of W–TiO2 incorporated coating was examined based on the EDS patterns. The EDS spectrum of the composite incorporated coating (Fig. 3b) revealed the presence of W, Ti, and O along with zinc on the top layers when compared with that of the pure zinc coating (Fig. 3a). Biological and electrochemical assay of antifouling characteristics of the composite The trend of shift in OCP and pH measurements The OCP decay curves of hot-dip galvanized coupons (pure zinc coating and W–TiO2 composite incorporated coating) immersed in different solutions (distilled water, seawater, and scrapings containing seawater) for a period of 20 days are shown in Fig. 4. Both the coupons exhibited a zigzag variation of OCP in distilled water. But in the case of seawater, there was a steady OCP variation during the initial stages of exposure. After 10 days of exposure, the W–TiO2 incorporated coating showed least anodic shift compared with pure zinc coating. But in the case of seawater containing scrapings, both the coatings exhibited a remarkable anodic shift after 10 days, but the extent of potential shift was comparatively less in the case of W–TiO2 composite incorporated coating.
Fig. 4 The OCP decay curves of galvanized coupons during long term immersion in a distilled water, b seawater, and c seawater scraping for biogrowth. Temperature: 25 °C (empty circle pure zinc coating, filled circle 0.2 wt% W–TiO2 composite incorporated coating)
The pH of distilled water, seawater, and seawater containing scrapings were monitored for a period of 20 days and are shown in Fig. 5. Variation in pH from alkalinity to acidity was noticed in both seawater and seawater containing scrapings (from pH9 to 6); this may be attributed to the acidic byproducts of bacterial metabolism. Screening of biofilm formation and biofouling activity by bacteria Bacterial swabs from the surface of metal coupons were plated and counted. In distilled water control, little or no growth occurred in both coupons (Fig. 6, - top). In the case of the coupons dipped in seawater and seawater with scrapings, profuse growth occurred on pure zinc coating, while only five colonies were observed on the W–TiO2 incorporated coupon (Fig. 6, middle and bottom). It revealed that the microbial attack was less in the case of W–TiO2 incorporated coating in both the cases. Microstructural evaluation of corroded coupons The optical micrographs showing the surface of pure zinc and W–TiO2 composite incorporated coatings after exposure to experimental solutions are shown in Fig. 7. From these figures, it was evident that the surface of both the coatings was not seriously affected by distilled water. But in the case of seawater and seawater containing scrapings, the microbial attack was least in W–TiO2 composite incorporated coating compared with pure zinc coating. From these three solutions, coupons in seawater containing scrapings undergo relatively high biocorrosion.
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the biofilm even though the biocidal effect was not that much significant on individual organisms (Figs. 8 and 9). The significance of variation among the individual batch on the extent of antifouling was not critical for all organisms.
pH
9
Determination of self-corrosion rate
7
5
0
5
10
15
The self-corrosion rate of galvanized coupons (pure zinc coating and W–TiO2 composite incorporated coating) immersed in different solutions (distilled water, seawater and scrapingcontaining seawater) is shown in Fig. 10. The self-corrosion rate of all the coupons was very low in distilled water. But in the case of seawater and seawater with biofilm scrapings, low corrosion rate was observed in the case of composite incorporated coating than pure zinc coating. No significance of variation on the corrosion rate among the individual batch was noted revealing high reproducibility on the results.
20
Time (days) Fig. 5 pH variation of solutions in which metal strips were immersed for biogrowth (circle distilled water, square seawater, triangle seawater scraping)
Characterization of EPS produced by bacteria The supernatant collected after centrifugation was filtered in 0.22-μm filters and analyzed for total carbohydrate content. After the analysis, it was understood that the culture containing W–TiO2 composite showed lower optical density (0.364) when compared to the culture in the absence of the composite (0.913).
Discussion Biofouling generally refers to the adherence of micro- and macroorganisms on to the metal surfaces in marine and fresh water systems leading to the formation of fouled layers. Bacteria form biofilm with the aid of extracellular polymeric substances to gain attachment to the surfaces. The present
Screening of biofouling activity by microtiter plate assay Microtiter plate assay showed significant antifouling activity by the composite on the bacterial consortium that formed Fig. 6 Bacterial growth observed in Zobell Marine Agar plates. Biofilms were collected from the surface of galvanic coatings with composition a pure zinc and b 0.2 % W–TiO2 incorporated coatings immersed in distilled water, seawater, and seawater scraping
Distilled water
Sea water
Sea waterScraping
a
b
Appl Microbiol Biotechnol Fig. 7 The optical micrographs of a pure zinc coating and b W– TiO2 composite incorporated coating immersed in distilled water, seawater, and seawater scraping for 20 days at a magnification of ×4.5
study indicates that W–TiO2 incorporated zinc coating was effective in controlling the biofilm-forming capacity of bacteria, the initial stage of biofouling process. For this study, W–TiO2 composite was synthesized by chemical reduction control
W-TiO2
without composite
0.3
0.35
0.25
0.3
Absorbance
Absorbance
0.35
method, and its chemical composition and thermal stability were analyzed using XRD technique. It is evident from the XRD patterns that Ti54W46O2, rutile TiO2, and tungsten can be obtained by the chemical reduction of sodium tungstate and TiO2 using hydrazine hydrate. The presence of identical
0.2 0.15 0.1 0.05 0
0.25 0.2 0.15 0.1 0.05
1
2
3
4
5
6
7
8
9
10
Organisms
Fig. 8 Growth of individual bacterium under various conditions— control (uninoculated broth), in the presence of composite (W–TiO2), and without composite
0
control
without composite
W-TiO2
Fig. 9 Growth of consortium under various conditions—control (uninoculated broth), in the presence of composite (W–TiO2), and without composite
Self corrosion rate X 10 -5(g/cm2/h)
Appl Microbiol Biotechnol 5 Pure Zn
4
Zn+W-TiO2
3
2
1
0
Distilled water
Sea water
Sea waterscraping
Fig. 10 Comparison of the rate of corrosion of galvanized coupons after 20 days of immersion in three different solutions for biogrowth. (a pure zinc coating, b 0.2 wt% W–TiO2 composite incorporated coating)
peaks at 2 theta values in both the XRD patterns indicated that the composite was thermally stable up to 800 °C. The crystalline and chemical nature of the composite was thus identified, and also it was ensured that the composite would not undergo any change if it would be added into molten zinc bath. In this study, a bacterial consortium, comprising of 13 prominent organisms isolated from biofilm scrapings collected from the boat hulls, was initially used. But during subsequent subculturing, three of them failed to show considerable growth. The efficacy of this composite in antifouling was confirmed by inoculating the consortium in media containing various concentrations of the composite. As the percentage composition of composite increases, the optical density decreases revealed the better antifouling activity of W–TiO2 composite. The W–TiO2 composite, which is thermally stable and having high antifouling activity, was then added into molten zinc bath during hot-dip galvanization. The surface morphology and chemical composition of the zinc coatings were analyzed using SEM-EDS technique. From the SEM images, it was clear that the W–TiO2 particles were distributed uniformly throughout the surface and have uniform grain size. Tiny ridged spangles were also observed in the case of the composite incorporated coating. It has been reported that the TiO2 incorporated zinc coatings could have ridged spangles, and the size of the spangles would be larger than what observed in the case of pure zinccoated surface (Shibli et al. 2006). The coating incorporated with W–TiO2 exhibited more compact structure due to the formation of Fe–W–TiO2–Zn inner layers. Based on these observations, the structural improvement due to the incorporation of the W–TiO2 composite was attributed to the individual property of the composite along with suppression of the alloying reaction. The advantage of the present study using bacterial consortium obtained from boat hulls is that it represents the effect of the composite on the indigenous niche of biofilm
formers. Previously, similar works were carried out by Shibli and Francis (2011b) with Vibrio alginolyticusa as the test organism. Rickard et al. (2003) reported coexistence of diverse species of bacteria in natural settings of biofilm formation like oral cavities and drinking water supplies that exhibit fascinating universe of specific interspecies interactions. Furthermore, bacterial consortia showed a distinct pattern in the reduction of growth as well as biofilm formation in the presence of W–TiO2 as compared to individual organisms in microtiter plate assay. This could be due to the resistance of certain organisms to the composite when grown individually. However, it is difficult to explain the ecophysiology of a consortium of biofilm formers due to the complexity in biodiversity among the communities (Jiao et al. 2010). The pure and composite incorporated hot-dip galvanized coatings were immersed in three experimental solutions for a period of 20 days by measuring the OCP and pH consecutively. The bacterial consortia present in the seawater resulted in high microbial attack on the surface of galvanized coatings leading to high corrosion. During the initial stages of exposure, both the coatings exhibited high negative OCP values in the range of −1.10 to −1.05 V in both seawater and seawater with biofilm scrapings. This is due to the sacrificial action of η phase (pure zinc) by protecting the inner layers from biocorrosion during the exposure. As the time of exposure increases, the dissolution rate increases due to the attack of bacteria and the OCP values of the coupons shifted to more anodic region. During the course of exposure, the protective barrier layer formed on the composite incorporated coating minimizes the dissolution of zinc. The pure zinc coating exhibited remarkable anodic shift in the range of −0.85 to −0.80 V in seawater and −0.90 to −0.88 V in seawater with biofilm scraping. This is due to the high bacterial attack on the surface of pure zinc coating. The composite incorporated coating in both seawater and seawater with biofilm scraping showed least shift in OCP in the range of −1.05 to −1.00 V and −1.00 to −0.98 V due to its better antifouling characteristics. This was due to the less bacterial attack on the surface of W–TiO2 composite incorporated coatings. The pH measurement ensured that the solution became more acidic as the time of exposure increases due to the presence of acid-producing bacteria. Very high corrosion was observed in the case of pure zinc coating immersed in experimental solutions, during selfcorrosion rate analysis, due to high bacterial attack in the absence of W–TiO2. The self-corrosion rate measurements are in good agreement with OCP measurements. The surface morphology of the corroded samples was analyzed by optical micrography. The optical micrographs of pure zinc and W–TiO2 composite incorporated coating after 20 days of immersion in experimental solutions also revealed the
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antifouling activity of W–TiO2. The biofilm formed on the surface of coatings were swabbed and inoculated into Zobell Marine Agar plate, and the colony count was noted. The colony count was less in the case of composite incorporated coating compared with pure zinc coating. From these results, it was confirmed that the W–TiO2 composite caused remarkable improvement in the antifouling characteristics of galvanized coatings in different experimental solutions. It should be noted that at this concentration, the composite would pose negligible threat of toxicity to aquatic life/ ecosystem. Lower content of the composite led to lower performance against biocorrosion, while little higher content revealed a slight variation against biocorrosion. Higher content of TiO2 also led to poor mechanical stability of the coating. Moreover, it should be noted that higher concentration of TiO2 normally leads to toxicity. In particular, Ferin and Oberdörster have demonstrated that both anastase and rutile forms of TiO2 were toxic and that the retention time was long (half times of 51–53 days in the rat lung at low milligram doses) (Ferin and Oberdörster 1985; Gillian et al. 2007). The excellent antifouling characteristics of TiO2 incorporated zinc coating had been reported from our lab itself (Shibli et al. 2006; Shibli and Francis 2011b) and again reproduced the same during the preliminary studies of the present work. The present work had the objective of further improving the antifouling effect along with the advantage of utilizing the wetting effect that would be exerted by tungsten. Hence, the present study highlights the additional effect than the effect of TiO2 alone. In order to improve the wetting nature of TiO2, tungsten, which increases the wettability of the composite during the hot-dip process, was incorporated along with TiO2. The comparison of wetting nature of TiO2 and W–TiO2 composite to zinc bath was done through contact angle measurements. Acknowledgments The authors thank the Head of the Department of Chemistry and the Director of IUCGGT, University of Kerala for extending support to carry out the research work.
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