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Microbiological corrosion of a Sn base alloy
International Biodeterioration 23 (1987) 159-166
Microbiological Corrosion of a Sn Base Alloy
O. A. B i s c i o n e , M. A. C a r i a * a n d B. M. R o s a l e s * C E I C O R - CITEFA/CONICET, Zufriategui 4380- (1603) Villa Martelli, Argentina (Accepted 2 March 1987)
ABSTRACT Babbitt alloy is generally used as an antifriction coating of bearings in plate mills. Samples of corroded alloy which were analyzed showed a localized attack which penetrated the whole thickness in small areas. The bacterium Pseudomonas maltophilia was isolated from the rolling solution and from the lubricant oil in contact with the coating of the bearings. The damage which occurred in service was electrochemically reproduced. Cultures of the bacterium in a nutrient medium were used as electrolyte in the presence of the different substrates. By SEM and EDAX analysis it was shown that the attack nucleates and propagates selectively in the Sn-Sb matrix, while the Cu rich phase remains unaffected. The oligodynamic action of the Sn 2+ ion on the growth of the bacterium was related to the nutritional requirements of the bacterium and to the selectivity of the attack on the alloy.
INTRODUCTION T h e principal effort devoted to research in microbial corrosion o f metals c o n c e r n s a l u m i n i u m a n d iron alloys. In 1977 (Schiapparelli a n d Rosales, 1977) the first e l e c t r o c h e m i c a l analysis o f the p r o b l e m allowed the d e t e r m i n a t i o n o f the influence o f the biological sludge from within wing fuel tanks o n the corrosion b e h a v i o u r o f a n a l u m i n i u m alloy. It was *Researcher of the Consejo Nacional de Investigaciones Cientificas y Tecnicas. 159 International Biodeterioration 0265-3036/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain.
160
o. A. Biscione, M. A. Carla, B. M. Rosales
found that a mixed culture of the microbes usually grown in jet fuelwater produced a decrease of the rupture potential of the alloy. Later, a m e c h a n i s m was proposed by Schiapparelli and Rosales (1980) and the different morphologies of attack observed were explained as a consequence of anodic and cathodic reactions on specific areas of the metal surface. The electrochemical technique previously applied in laboratory cultures was also employed for water drained off jet fuel tanks of planes in service (Rosales and Schiapparelli, 1980). The results were in good agreement with those obtained from visual inspection of the respective tanks. A similar analysis was applied to the damage detected in the Babbitt antifriction coating of bearings in plate mills.
MATERIALS A N D M E T H O D S
Bacteriological analysis The following components of the rolling solution and the lubricant oil used in the plate mill were analyzed: (1) (2) (3) (4) (5) (6)
River cooling water. Rolling solution used in the process (components 1 + 5 + 6). Morgoil lubricant oil (unused). Morgoil lubricant oil used in the process. A wax. B wax (soluble solution).
The materials were seeded in duplicate plates of tryptone soya agar (TSA, Oxoid), TSA supplemented by 0.5% glycerol-tributyrate (TRI) and Sabouraud-glucose agar. Each system was incubated at 25°C and 37°C.
Electrochemical tests The electrochemical tests were performed in sterile TSA (tryptone 15 g, soya peptone 5 g, NaC1 5 g, agar 15 g per litre, p H 7.3) as support electrolyte. Cultures of the isolated bacterium in the same m e d i u m modified by the addition of the substrates 1, 2, 3, 5 and 6 were used to identify the carbon producing corrosive conditions in the in-service environment. Potentiokinetic polarization curves were obtained using a Tacussel PRT 20-2X potentiostat with a Servovit 13 potential scanner at 10 mV min-~. A conventional Pyrex glass cell with a Pt counter electrode,
Microbiological corrosion of a Sn base alloy
161
through a Luggin capillary, was used. Aqueous solutions deaerated by bubbling of 99.99% N2 were used for the anodic polarization from the open circuit potential up to 100 mV over the respective pitting or rupture potentials. The cathodic curves were obtained in the same solutions saturated with air at a constant rate of stirring. These last were traced using as working electrode the Babbitt alloy and Armco steel, because the cathodic reaction can occur in-service on the Sn alloy coated bearing and on its steel lodging. The morphology of the attack was determined using a metallographic Nikon Metaphot and a Phillips 500 scanning electron microscope (SEM). The chemical composition of the alloy was analysed by energy dispersive analysis of X-ray (EDAX).
RESULTS A N D DISCUSSION Corrosion of Babbitt alloy in service
The failure observed on the Sn alloy (Sb 8.68; Cu 2.52) consisted of drilling fissures, as shown in SEM micrographs of Fig. 1. The structure of the alloy and the EDAX analysis of the two phases are shown in Fig. 2. Bacterial contamination
After 48 h of incubation, evidence of intense lipolytic activity was observed in TRI agar seeded with river cooling water or rolling solution, and later in TRI plates with the B wax soluble solution. There were no significant differences in the growth observed on media incubated at 25 and 37°C. The isolates were of only one type of mucoid yellow colony. Based on morphological, biochemical and nutritional characteristics and enzymatic activity, the bacterium isolated was identified as Pseudomonas maltophilia. This strain had the following characteristics: Gram negative bacillus, straight, non-sporing, multitrico polar flagellum, non-diffusible yellow pigment, cytocrom-oxidase negative, denitrification negative, no accumulation of poly-beta-hydroxybutirate as reserve cellular material, lipase intensely positive, amylase negative, gelatin hydrolysis positive. As carbon and energy sources the organism utilized maltose, glucose, acetate, malonate, pyruvate, proline, lactate. It did not metabolize xylose, ethanol or tryptophan.
162
O. A. Biscione, M. A. Caria, B. M. Rosales
(a)
(b)
Fig. 1. (a) Top view × 40; (b) Cross section × 40. The viability of the bacterium suspected of causing corrosion was investigated in the materials from which it was isolated. A suspension of P. maltophilia was used to inoculate 25 ml of each substrate a n d i n c u b a t e d at 25 °C for 30 days. Subcultures from all of these m a i n t a i n e d their viability with the original intense lipolytic capacity. Growth was stimulated by addition of SnC12 to the culture m e d i u m , while the c o r r e s p o n d i n g Cu a n d Sb salts did not affect it.
Corrosion under experimental conditions The electrochemical results s h o w n in Fig. 3 e n a b l e d us to assume that Pseudomonas maltophilia produces corrosive metabolites in the presence
o f the rolling solution. These metabolites w o u l d modify the anodic
Microbiological corrosion of a Sn base alloy
163
behaviour of the alloy with respect to that observed in the support electrolyte. The results obtained in the distinct inoculated media were almost independent of the substrate present. This is not only valid for the (a)
(b)
II
I
Sn Sb
Cu
I
Fig. 2. (a) Microstructure of the alloy x 300: (b) EDAX diagram of the dark phase: points matrix: dashes.
164
O. A. Biscione, M. A. Caria, B. M. Rosales Esc e (mY) + 200
a
-2oo I -6ooi -1000
6
5
4
-tog I(A cm -2 )
Fig. 3. Polarization of the alloy in TSA, Anodic polarization of the alloy (a, sterile medium; b, inoculated medium); cathodic polarization in both sterile and inoculated media (c, alloy; d, Armco steel). Esce (mV) = the electrode potential with respect to saturated calomel electrode. - log I (Acm -2) = the negative value of the log of the current density.
electrochemical curves but also for the morphologies of the attack produced under laboratory tests. The selective dissolution observed in service was reproduced by anodic polarization in TSA inoculated with the bacterium. The samples in which' the anodic polarization exceeded the pitting potential of the alloy, both in the sterile and the inoculated electrolyte, presented the pitting shown in Fig. 4. There was a characteristic pattern of attack in the uninoculated solution, while in the inoculated solution selective dissolution was also found. When the anodic polarization stopped before reaching the pitting potential, the tested samples showed selective dissolution in the presence of the inoculated medium (Fig. 4(b)), but remained unattacked in the uninoculated medium. The cathodic curves for the Sn and Fe alloys did not show any effect caused by bacterial proliferation in the tested environments. The intersection of those curves with the respective anodic curves occurred in the passivity range of the Babbitt alloy in uninoculated medium, and in the active dissolution range in the inoculated medium. The cathodic curves for steel produced higher corrosion currents than the obtained for Babbit in inoculated media, both of them being possible in service. These results are in good agreement with the type of failures observed. In all cases the EDAX applied to the corrosion products showed that the Sn-Sb phase of the matrix dissolves selectively leaving the Cu rich second phase unattacked. These results suggest that it would be of interest to evaluate further the influence of Sn 2÷ on the growth of the bacteria. The oligodynamic effect determined was not observed for either of the other components of the alloy, or for Pb. This last metal is the principal component of the usual coating of bearings in comparable plate mills, where there is no attack under the same in-service conditions.
Microbiological corrosion of a Sn base alloy
165
(a)
(b)
Fig. 4. MEB of the alloy after anodic polarization: (a) Sterile medium, up to E = + 200 mVsce x 7200: (b) inoculated medium, up to E = 600 mVsce x 3200. Clearly the Babbitt alloy is susceptible to selective dissolution in the presence of metabolites of the bacterium Pseudomonas maltophilia. The Cu-rich phase remains unattacked while the matrix is dissolved, while the Sn thus extracted stimulates the bacterial growth. A n y biological requirements for Sn could be satisfied by the production of specific metabolites which extracted it from the matrix.
ACKNOWLEDGEMENTS This work was carried out for SOMISA, Sociedad Mixta de Siderurgia Argentina. The authors are grateful to the Comosidn Nacional de Energia At6mica for its cooperation with the use of its SEM a n d EDAX.
166
O. A. Biscione, M. A. Caria, B. M. Rosales
REFERENCES Schiapparelli, E. R. de; Rosales, B. M. (1977). III Latin-American Meeting on Electrochemistry and Corrosion, INIFTA, Argentina, pp. 239-43. Schiapparelli, E. R. de; Rosales, B. M. (1980). V International Congress on Marine Corrosion and Fouling, Marine Biology, 1-5. Rosales, B. M.; Schiapparelli, E. R. de. (1980). Corrosion by microorganisms of jet aircraft integral fuel tanks. Part 2. Corrosion. International Biodeterioration Bulletin 16(2), 31-6.
Microbiological Corrosion of a Sn Base Alloy
O. A. B i s c i o n e , M. A. C a r i a * a n d B. M. R o s a l e s * C E I C O R - CITEFA/CONICET, Zufriategui 4380- (1603) Villa Martelli, Argentina (Accepted 2 March 1987)
ABSTRACT Babbitt alloy is generally used as an antifriction coating of bearings in plate mills. Samples of corroded alloy which were analyzed showed a localized attack which penetrated the whole thickness in small areas. The bacterium Pseudomonas maltophilia was isolated from the rolling solution and from the lubricant oil in contact with the coating of the bearings. The damage which occurred in service was electrochemically reproduced. Cultures of the bacterium in a nutrient medium were used as electrolyte in the presence of the different substrates. By SEM and EDAX analysis it was shown that the attack nucleates and propagates selectively in the Sn-Sb matrix, while the Cu rich phase remains unaffected. The oligodynamic action of the Sn 2+ ion on the growth of the bacterium was related to the nutritional requirements of the bacterium and to the selectivity of the attack on the alloy.
INTRODUCTION T h e principal effort devoted to research in microbial corrosion o f metals c o n c e r n s a l u m i n i u m a n d iron alloys. In 1977 (Schiapparelli a n d Rosales, 1977) the first e l e c t r o c h e m i c a l analysis o f the p r o b l e m allowed the d e t e r m i n a t i o n o f the influence o f the biological sludge from within wing fuel tanks o n the corrosion b e h a v i o u r o f a n a l u m i n i u m alloy. It was *Researcher of the Consejo Nacional de Investigaciones Cientificas y Tecnicas. 159 International Biodeterioration 0265-3036/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain.
160
o. A. Biscione, M. A. Carla, B. M. Rosales
found that a mixed culture of the microbes usually grown in jet fuelwater produced a decrease of the rupture potential of the alloy. Later, a m e c h a n i s m was proposed by Schiapparelli and Rosales (1980) and the different morphologies of attack observed were explained as a consequence of anodic and cathodic reactions on specific areas of the metal surface. The electrochemical technique previously applied in laboratory cultures was also employed for water drained off jet fuel tanks of planes in service (Rosales and Schiapparelli, 1980). The results were in good agreement with those obtained from visual inspection of the respective tanks. A similar analysis was applied to the damage detected in the Babbitt antifriction coating of bearings in plate mills.
MATERIALS A N D M E T H O D S
Bacteriological analysis The following components of the rolling solution and the lubricant oil used in the plate mill were analyzed: (1) (2) (3) (4) (5) (6)
River cooling water. Rolling solution used in the process (components 1 + 5 + 6). Morgoil lubricant oil (unused). Morgoil lubricant oil used in the process. A wax. B wax (soluble solution).
The materials were seeded in duplicate plates of tryptone soya agar (TSA, Oxoid), TSA supplemented by 0.5% glycerol-tributyrate (TRI) and Sabouraud-glucose agar. Each system was incubated at 25°C and 37°C.
Electrochemical tests The electrochemical tests were performed in sterile TSA (tryptone 15 g, soya peptone 5 g, NaC1 5 g, agar 15 g per litre, p H 7.3) as support electrolyte. Cultures of the isolated bacterium in the same m e d i u m modified by the addition of the substrates 1, 2, 3, 5 and 6 were used to identify the carbon producing corrosive conditions in the in-service environment. Potentiokinetic polarization curves were obtained using a Tacussel PRT 20-2X potentiostat with a Servovit 13 potential scanner at 10 mV min-~. A conventional Pyrex glass cell with a Pt counter electrode,
Microbiological corrosion of a Sn base alloy
161
through a Luggin capillary, was used. Aqueous solutions deaerated by bubbling of 99.99% N2 were used for the anodic polarization from the open circuit potential up to 100 mV over the respective pitting or rupture potentials. The cathodic curves were obtained in the same solutions saturated with air at a constant rate of stirring. These last were traced using as working electrode the Babbitt alloy and Armco steel, because the cathodic reaction can occur in-service on the Sn alloy coated bearing and on its steel lodging. The morphology of the attack was determined using a metallographic Nikon Metaphot and a Phillips 500 scanning electron microscope (SEM). The chemical composition of the alloy was analysed by energy dispersive analysis of X-ray (EDAX).
RESULTS A N D DISCUSSION Corrosion of Babbitt alloy in service
The failure observed on the Sn alloy (Sb 8.68; Cu 2.52) consisted of drilling fissures, as shown in SEM micrographs of Fig. 1. The structure of the alloy and the EDAX analysis of the two phases are shown in Fig. 2. Bacterial contamination
After 48 h of incubation, evidence of intense lipolytic activity was observed in TRI agar seeded with river cooling water or rolling solution, and later in TRI plates with the B wax soluble solution. There were no significant differences in the growth observed on media incubated at 25 and 37°C. The isolates were of only one type of mucoid yellow colony. Based on morphological, biochemical and nutritional characteristics and enzymatic activity, the bacterium isolated was identified as Pseudomonas maltophilia. This strain had the following characteristics: Gram negative bacillus, straight, non-sporing, multitrico polar flagellum, non-diffusible yellow pigment, cytocrom-oxidase negative, denitrification negative, no accumulation of poly-beta-hydroxybutirate as reserve cellular material, lipase intensely positive, amylase negative, gelatin hydrolysis positive. As carbon and energy sources the organism utilized maltose, glucose, acetate, malonate, pyruvate, proline, lactate. It did not metabolize xylose, ethanol or tryptophan.
162
O. A. Biscione, M. A. Caria, B. M. Rosales
(a)
(b)
Fig. 1. (a) Top view × 40; (b) Cross section × 40. The viability of the bacterium suspected of causing corrosion was investigated in the materials from which it was isolated. A suspension of P. maltophilia was used to inoculate 25 ml of each substrate a n d i n c u b a t e d at 25 °C for 30 days. Subcultures from all of these m a i n t a i n e d their viability with the original intense lipolytic capacity. Growth was stimulated by addition of SnC12 to the culture m e d i u m , while the c o r r e s p o n d i n g Cu a n d Sb salts did not affect it.
Corrosion under experimental conditions The electrochemical results s h o w n in Fig. 3 e n a b l e d us to assume that Pseudomonas maltophilia produces corrosive metabolites in the presence
o f the rolling solution. These metabolites w o u l d modify the anodic
Microbiological corrosion of a Sn base alloy
163
behaviour of the alloy with respect to that observed in the support electrolyte. The results obtained in the distinct inoculated media were almost independent of the substrate present. This is not only valid for the (a)
(b)
II
I
Sn Sb
Cu
I
Fig. 2. (a) Microstructure of the alloy x 300: (b) EDAX diagram of the dark phase: points matrix: dashes.
164
O. A. Biscione, M. A. Caria, B. M. Rosales Esc e (mY) + 200
a
-2oo I -6ooi -1000
6
5
4
-tog I(A cm -2 )
Fig. 3. Polarization of the alloy in TSA, Anodic polarization of the alloy (a, sterile medium; b, inoculated medium); cathodic polarization in both sterile and inoculated media (c, alloy; d, Armco steel). Esce (mV) = the electrode potential with respect to saturated calomel electrode. - log I (Acm -2) = the negative value of the log of the current density.
electrochemical curves but also for the morphologies of the attack produced under laboratory tests. The selective dissolution observed in service was reproduced by anodic polarization in TSA inoculated with the bacterium. The samples in which' the anodic polarization exceeded the pitting potential of the alloy, both in the sterile and the inoculated electrolyte, presented the pitting shown in Fig. 4. There was a characteristic pattern of attack in the uninoculated solution, while in the inoculated solution selective dissolution was also found. When the anodic polarization stopped before reaching the pitting potential, the tested samples showed selective dissolution in the presence of the inoculated medium (Fig. 4(b)), but remained unattacked in the uninoculated medium. The cathodic curves for the Sn and Fe alloys did not show any effect caused by bacterial proliferation in the tested environments. The intersection of those curves with the respective anodic curves occurred in the passivity range of the Babbitt alloy in uninoculated medium, and in the active dissolution range in the inoculated medium. The cathodic curves for steel produced higher corrosion currents than the obtained for Babbit in inoculated media, both of them being possible in service. These results are in good agreement with the type of failures observed. In all cases the EDAX applied to the corrosion products showed that the Sn-Sb phase of the matrix dissolves selectively leaving the Cu rich second phase unattacked. These results suggest that it would be of interest to evaluate further the influence of Sn 2÷ on the growth of the bacteria. The oligodynamic effect determined was not observed for either of the other components of the alloy, or for Pb. This last metal is the principal component of the usual coating of bearings in comparable plate mills, where there is no attack under the same in-service conditions.
Microbiological corrosion of a Sn base alloy
165
(a)
(b)
Fig. 4. MEB of the alloy after anodic polarization: (a) Sterile medium, up to E = + 200 mVsce x 7200: (b) inoculated medium, up to E = 600 mVsce x 3200. Clearly the Babbitt alloy is susceptible to selective dissolution in the presence of metabolites of the bacterium Pseudomonas maltophilia. The Cu-rich phase remains unattacked while the matrix is dissolved, while the Sn thus extracted stimulates the bacterial growth. A n y biological requirements for Sn could be satisfied by the production of specific metabolites which extracted it from the matrix.
ACKNOWLEDGEMENTS This work was carried out for SOMISA, Sociedad Mixta de Siderurgia Argentina. The authors are grateful to the Comosidn Nacional de Energia At6mica for its cooperation with the use of its SEM a n d EDAX.
166
O. A. Biscione, M. A. Caria, B. M. Rosales
REFERENCES Schiapparelli, E. R. de; Rosales, B. M. (1977). III Latin-American Meeting on Electrochemistry and Corrosion, INIFTA, Argentina, pp. 239-43. Schiapparelli, E. R. de; Rosales, B. M. (1980). V International Congress on Marine Corrosion and Fouling, Marine Biology, 1-5. Rosales, B. M.; Schiapparelli, E. R. de. (1980). Corrosion by microorganisms of jet aircraft integral fuel tanks. Part 2. Corrosion. International Biodeterioration Bulletin 16(2), 31-6.