Biosusceptibility evaluations of protective sealants used in aircraft fuel tanks

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 12, June 2005, pp Biosusceptibility evaluations of protective sealants used in aircraft fuel tanks R B Srivastava, Meenu Awasthi, M C Upreti & G N Mathur Defence Materials & Stores Research & Development Establishment, Kanpur , India Received 22 July 2004; accepted 14 March 2005 The microbial attack on coatings used in aircraft fuel tanks has important implications because such coatings or sealants must remain intact for effective corrosion protection. In general, all the polysulphide based sealants under test were found to be very effective against individual cultures, viz. sulphate reducing bacteria () and Aureobasidium pullulans. However, when the cultures were introduced collectively (viz. mixed and combination), growth of microorganisms resulted in degradation of coatings to a greater extent. Analysis of electrochemical data reveals that sealant RDL- can be used as a broad-spectrum sealant as it was found effective not only against single cultures but also when cultures were introduced collectively. IPC Code: C21D1/72, C23F11/00 Aluminium and its alloys particularly offer high resistance to environmental factors. In spite of their high resistance, these alloys are prone to biocorrosion 1. Growth of organisms at the interface between water condensates and the fuel in tanks and radiators of jet aircrafts has been reported to promote corrosion of aluminium alloy, used as structural component, in these applications 2,3. In the presence of condensed water, organisms may utilize fuel hydrocarbon as sole carbon source for growth 4. When microorganisms colonize to form layer or biofilm on the surface of the fuel tank the communities of aerobic and anaerobic organisms as well as the exopolymers and acidic metabolites secreted by them accelerate deterioration of material. Further, aircraft fuel tanks are frequently coated with polymeric materials such as Buna rubber films, epoxy polyurethane coatings and polysulphide sealants. These polymeric coatings are also susceptible to biodegradation 5. In the present study, aluminium alloy coated with commercially available polysulphide based sealants were evaluated for their susceptibility to biocorrosion. Efforts have been made to find out relatively better bioresistant sealant/coating with a view to protect fuel tanks from bioattack. Materials and Methods Contaminated aviation turbine fuel (ATF) samples from fuel tanks of aircrafts were collected from Hindustan Aeronautics Limited (HAL), Kanpur and nine fungi, two bacteria, one yeast and two sulphate reducing bacteria () were isolated. These cultures were brought into pure forms and their identities were confirmed at Institute of Microbial Technology (IM- TECH), Chandigarh. The fungal species isolated were Aureobasidium pullulans, Acremonium strictum, Aspergillus niger, Aspergillus terricola, Cladosporium resinae, Paecilomyces varioti, Curvularia lunata, Libertella heaveae and Aspergillus ganus var bremis. Bacterial isolates were Bacillus firmis, Bacillus megaterium, yeast isolate was Candida tropicalis and isolates were Desulphovibrio desulphuricans and Desulphovibrio vulgaris. Anodized aluminium alloy coupons of 15 1 cm 2 were cut and polished using 600 grit emery papers. After polishing these coupons were sealed on both sides with epoxy resin leaving 1 cm 2 area on one side of the surface. Polysulphide sealants RDL, RDL and, which were found to be suitable in preventing corrosion of aluminium alloy, in preliminary experiments, (taking as test cultures, employing Postgate B as test medium and exposure period 30 days) were coated on 1 cm 2 area with the help of spray. Six sets of each type of coupons were immersed in 500 ml of soda glass jars filled with 300 ml sterilized mineral salt medium containing (in M) CaCl , MgSO (NH 4 ) 2 SO /L and 100 ml of sterile filtered aviation turbine fuel (ATF). Four types of inoculum were prepared employing following test cultures and jars were inoculated separately with each type of inoculum alongwith abiotic control.

2 242 INDIAN J. ENG. MATER. SCI., JUNE 2005 (i) Single fungal culture Aureobasidium pullulans (ii) Sulphate reducing bacteria Desulphovibrio, desulphuricans and Desulphovibrio vulgaris (iii) culture Acremonium strictum, Aspergillus niger, Aspergillus terricola, Cladosporium resinae, Paecilomyces varioti, Curvularia lunata, Libertella heaveae and Aspergillus ganus var bremis and Candida tropicalis (iv) Combination culture All the fungal, yeast and bacterial cultures including Aureobasidium pullulans and. All the jars were kept for 60 days at 28±2 ο C in an incubator alongwith abiotic controls. After the exposure period, electrochemical studies were performed with the help of EG & PARC Galvanostat/Potentiostat model No. 273, at room temperature. The electrochemical techniques helped in accurately determining the corrosion rate of exposed coupons in addition to other techniques employed such as physical and electron microscopic observation of test coupons and weight loss method. For this, Tafel plot measurements were taken employing controlled potential scan starting near E c and extending it either to the anodic or the cathodic directions for about 100 mv. When the resulting potential current curve was plotted against log (current density), it characteristically exhibited a linear region. Tangents in the linear regions were plotted to determine the values of β d (anodic Tafel constant) and β c (cathodic Tafel constant). By polarization resistance technique 6, resistance potential (R p ) was determined and corrosion current as well as corrosion rates were calculated using Stern and Geary Equations 7. Results and Discussion Aluminium and its alloys show a particularly high resistance to environmental factors and form a group of widely used materials for aerospace applications 8. Since electro-chemical methods have been successfully employed to study corrosion of aluminium alloys 9, preliminary screening of sealants was carried out using these techniques for test aluminium alloy coated with various sealants and exposing them to cultures for a period of 30 days. Sulphate reducing bacteria () cultures were taken as test cultures since they are most widely recognized out of many organisms implicated in corrosion of metals and alloys. Polysulphide based sealants used in these experiments were RDL, RDL, RDL 840, RDL 938 and RDL 952. Aluminium alloy (uncoated coupons) exposed to four sets of microbial cultures, show the effect of microbial growth on metal surface (Fig. 1). The coupons exposed to show pitting at edges which can be considered as the evidence for microbiallly influenced corrosion [Fig. 1(2)]. The weight loss of coupons was also determined in presence of and combination culture as 1.21% and 0.03% respectively. Test coupons coated with polysulphide based sealants showed weight loss 0.72% in and % in combination culture. It was observed that, coating provided 60% protection substantially in case of exposure to as compared to uncoated coupon. The effect of microbial growth on test metal surface has been very well visualized with the help of electron microscope (Fig. 2), where pitting and intergrannular corrosion have been observed under areas of high microbial growth. Microscopic observations also helped in ascertaining the efficacy of sealants to protect test coupons from microbial attack. The data on corrosion rates of aluminium alloys coated with sealants under test for preliminary screening (Table 1) indicates that maximum corrosion rate was recorded for coupons coated with sealant 840, whereas, coupons coated with sealant were found least susceptible to corrosion. The corrosion rate of coupons coated with sealant [0.01 mils per year (mpy)] was found nearly six times less than that of coupons coated with sealant 840 (0.06 mpy). The effectiveness of various sealants in terms of corrosion protection was observed as > > 952 > 938 > 840. The sealants, viz., and were found to be the most effective as a result of pre- Fig. 1Photograph of uncoated aluminium alloy (1) abiotic control, and showing corrosion in presence of different microbial cultures (2) (3) Aureobasidium pullulans (4) mixed culture (5) combination culture

3 SRIVASTAVA et al.: PROTECTIVE SEALANTS USED IN AIRCRAFT FUEL TANKS 243 Table 1Corrosion rate of aluminium alloy coupons coated with different types of sealants and exposed to culture for 30 days* S. No. Culture/sealants type I corr (A) Corrosion rate (mpy) culture *Comparative effectiveness of sealants 840- B > > 952 > 938 >840 Fig. 2SEM micrographs showing effect of microbial corrosion on uncoated aluminium alloy (a) Abiotic control, and in presence of (b) Aureobasidium pullulans (c) culture liminary screening and were subjected to further studies. In addition, sealant, obtained from HAL, Kanpur, was also assessed for its effectiveness against biocorrosion. In electrochemical analysis, the steady state behaviour of aluminium alloy for a period of 96 h in media containing A. pullulans, culture, mixed and combination cultures are shown in Fig. 3. (a, b, c & d). The potentials after following an initial negative trend showed gradual reduction in values with time. This may be taken as indicative of corrosion initiated as a result of breaking down of the passive layer on aluminum alloy due to secretions of microbial metabolites. With polysulphide-based sealants such as, and, gradual increase in potential values in varying degrees was noted. This indicated effective binding of passive film with test metal surface indicating that sealants were unaffected by metabolic secretions of microbial cultures. In case of A. pullulans (Fig. 3a) and combination cultures (Fig. 3d), sealant was found more efficient as compared to and. Sealant showed gradual increase in E corr values (about 20 mv/s), which stabilized for subsequent 96 h revealing its effectiveness against culture (Fig. 3b). showed better efficacy as a sealant to control biocorrosion in aluminium coupons exposed to mixed culture (Fig. 3c). In further electrochemical experiments where coupons were exposed to test cultures for a period of 60 days, Tafel plots for uncoated coupons (abiotic control) as well as coupons exposed to test cultures, viz., Aureobasidium pullulans,, mixed and combination cultures are shown in Fig. 4 (a, b, c & d) respec-

4 244 INDIAN J. ENG. MATER. SCI., JUNE 2005 Fig. 3Time versus potential plots for aluminium alloy exposed for 96 h to (a) Aureobasidium pullulans (b) culture (c) culture (d) Combination culture [( ) control; ( ) uncoated; ( ) ; ( ) and ( ) 840B] Fig. 4Tafel plots of aluminium alloy coupons in presence of (I) Aureobasidium pullulans (II) sulphate reducing bacteria (III) mixed culture (IV) combination culture (a) abiotic control; (b) coupons (uncoated) exposed to test culture. Coupons exposed to culture and coated with (c) sealant RD ; (d) sealant and (e) Sealant

5 SRIVASTAVA et al.: PROTECTIVE SEALANTS USED IN AIRCRAFT FUEL TANKS 245 Table 2Corrosion rate and percentage (%) protection of aluminium alloy coated with sealants and exposed to test microbial cultures for sixty days S. No. Sealants Culture βa mv/dec βc mv/dec R p Ω I corr A Corr. rate (mmpy) % Protection A. pull. A. pull A. pull A. pull E E E E E E E E E E E E E E E E tively. Table 2 summarizes the data on corrosion rate for aluminium alloys exposed to test cultures. Highest corrosion rate (2.63 mpy) was found in coupons exposed to A. pullulans. This is not surprising considering the fact that A. pullulans, colonized extensively showing numbers as high as /cm 2. The corrosion rate of aluminium alloy in the presence of this culture was found approximately twice that of either or mixed culture. A. pullulans had a greater effect on corrosion rate individually as compared to its being part of the consortium (viz. mixed or combination culture) suggesting competition from other microorganisms resulting in reduction of its ability to attack test aluminium alloy. Table 2 also reveals minimum value of 0.01 mpy for coupons coated with, exposed to Aureobasidium pullulans and maximum value of 0.99 mpy in coupons coated with exposed to combination culture. Thus, combination culture was observed to cause nine times more damage to aluminium alloy but since such values of corrosion (0.99 mpy) may still be considered negligible, it is visualized that more or less all the sealants were found very effective in protecting aluminium alloys against corrosion induced by test cultures. The highest corrosion rate in the present study was recorded in coupons exposed to A. pullulans. All the sealants under test were found to be fairly effective Table 3Efficacy of sealants against biocorrosion of aluminium alloy induced by fuel resident microbial test cultures S.No. Cultures RDL-840 B RDL- PR Aureobasidium pullulans culture culture Combination culture (98%) (95%) (58%) (58) (92%) (99%) (78%) (68%) (99%) (86%) (82%) (54%) *Corrosion rate in mmpy Values in parentheses indicate % protection offered by sealants against biocorrosion against biocorrosion induced by although the percentage of protection offered was less as compared to that observed against A. pullulans (Table 3). In coupons exposed to mixed or combination culture, the percentage protection was found to be less than that of individual cultures, viz., and A. pullulans (Table 3). This may be attributed to the fact that growth of different species of microorganisms within biofilm facilitates development of a consortium accelerating corrosion of alloys. Corrosion rate of alumin-

6 246 INDIAN J. ENG. MATER. SCI., JUNE 2005 Fig. 5Bar graphs showing corrosion rate of aluminium alloy uncoated and coated with different type of sealants (, 840 and ) exposed to (a) Aureobasidium pullulans (UNCAP), (b) (UNCSR), (c) (UNCMX) and (d) Combination (UNCCM) culture and abiotic control (UNCCT coupon exposed to sterile medium) ium coupons exposed to test cultures has also been depicted by bar graphs in Fig. 5 (a, b, c & d). For mixed culture, the corrosion rate observed was slightly higher than that observed for coupons exposed to. Sealant was found to be the most effective sealant against mixed culture (82% protection) followed by sealant (78%). Sealant offered least protection (58%). Least percentage protection in the present study was also observed for coated coupons exposed to combination culture, the percentage protection values were found to be 68%, 58% and 54% for coupons coated with sealants, and respectively. Sealant has also provided 92% and 99% protection for coupons exposed to A. pullulans and respectively. The results thus indicate the effectiveness of sealant RDL as a broad-spectrum sealant for single cultures as well as cultures introduced collectively in aircraft fuel tanks. Conclusions Aureobasidium pullulans, a fuel residing fungal species was observed to affect the performance of sealants used in aircraft fuel tanks. In the present study, dealing with the corrosion protection of aircraft fuel tank material, it has been observed that polysulphide based sealant RDL can be used as a sealant (broad spectrum) since it has been found effective against the test culture, Aureobasidium pullulans individually as well as collectively in combination with other microorganisms. Acknowledgement The authors thank Aeronautical Development Agency (ADA), Bangalore, for awarding the project and Regional Center for Military Airworthiness (RCMA), C/o HAL, Kanpur, for their help in carrying out this work. Thanks are also due to M/s Choksey Chemicals Pvt. Ltd., Mumbai, for supplying the sealants.

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