- Email: [email protected]
An example of microbiologically influenced corrosion
341
Bioelectrochemistry
and Bioenergetics,
16 (1986) 347-355
A section of J. Electroanal. Chem., and constituting Vol. 212 (1986) Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
945 - AN EXAMPLE CORROSION THE BEHAVIOUR
OF MICROBIOLOGICALLY
OF STAINLESS
INFLUENCED
STEELS IN NATURAL
SEAWATER *
VITTORIA SCOTT-O, GIORGIO ALABISO and GIUSEPPE MARCENARO Istituto per la Corrosione Marina dei Metalli, C.N.R.,
Via della Mercantia
4, Geneva (Italy)
(Manuscript received March 19th 1986)
SUMMARY The aim of this paper is to summarize the most significant experimental data collected so far by our Institute on the corrosion behaviour of stainless steels in natural seawater. The purpose of the review is to illustrate the microbiological interference mechanisms with corrosion processes suggested by these experiments. The higher corrosivity of natural, as compared with sterile, seawater seems to be due to a change in oxygen reduction kinetics caused by the presence of microbiological slime on stainless steel surfaces. The observation that the accelerating effects occur only with biologically active slimes suggests the hypothesis of an enxymic catalysis affecting the oxygen reduction rates. The aerobic microbial slime components are confirmed to be the main cause of this effects. Any external action able to remove the slime __ nhvsicallv_ or to inhibit its viabilitv bv toxics or by temperature rises affects the electrochemical phenomenon also.
INTRODUCTION
The corrosive effects of seawater on metals have always been considered greater than the corrosion caused by salt solutions having the same composition. This difference is usually attributed to the presence of biological activity in natural environments [l]. The adhesion of marine organisms on the metal surface (the so-called marine fouZing) has often been mentioned in the literature as a contributing cause of corrosion in seawater, although its actual incidence on the overall corrosion process may vary according to the alloy composition [2].
* Contribution presented at the VIIIth International Symposium on Bioelectrochemistry and Bioenergetits, Bologna, June 24th-29th 1985. 0302-4598/86/$03.50
0 1986 Elsevier Sequoia S.A.
348
Stainless steels are particularly susceptible to biological corrosion, since the very presence on their surface of a microbiological fouling (the so-called slime or primary film) drastically increases the initiation probability and propagation rate of localized attacks (pits, crevices, etc.) [3-61. However, even though the possibility of microbiological interference with metal corrosion in seawater is often mentioned in the literature, little is known about the mechanism through which slime affects the corrosion process. This paper is aimed at summarizing the most significant experimental data collected so far on this subject by our Institute in order to illustrate the microbiological mechanisms interfering with corrosion processes suggested by these data. SUMMARY OF EXPERIMENTAL PROCEDURES
All data illustrated in this paper refer to stainless steels (21 Cr, 3 MO or 20 Cr, 18 Ni, 6 MO) exposed to strongly differentiated environments. The data refer to stainless steels: directly exposed in the sea at about 1 meter depth in natural light and temperature conditions [7]; fitted in dark ducts through which seawater from a thermoelectric power plant condenser was flowing. This condition was not only characterized by a significant temperature rise (lo-15°C beyond the seawater temperature), but also by the presence of antifouling and anticorrosion products [5]; in closed circuits, where natural and artificial seawater was run through stainless steels pipes. The natural seawater was renewed every 7-10 days, whereas the sterile water was kept so by continuous filtering over 0.22 pm meshed Millipore filters [6]. exposed in natural-seawater settling tanks, which were completely renewed every 8 h and thermostated at temperatures of 25, 30, 35 and 40°C [S]. The following measuring techniques were used for virtually all tests: control of the zero-current potentials (also called free corrosion potential UC,,,); reconstruction of the cathodic oxygen reduction kinetics on metal substrata by measuring the current outputs of potentiostatically polarized test specimens in the 0 to -600 mV (uersus s.c.e.) potential range; characterization of the slime nature under the microscope, evaluation of inorganic detritus trapped in the slime (identified by the dry residue weight after treatment at 920°C); quantification of the biomass through biochemical analyses such as: measurement of the chlorophyll a, proteins, carbohydrates and adenosine triphosphate (ATP) content and evaluation of the maximum activity of the respiratory electron transport system (ETS) [9]. SUMMARIZED
DATA
All results of the biochemical analysis, except ATP values, were found to be biomass-equivalent indexes and showed that the microbiological fouling, on atoxic substrata, grows with time following the pattern of Fig. 1. In this graph, both time
349
Time Fig. 1. Schematic
evolution
of slime biomass
versus time.
and biomass amounts may of course vary with the temperature, flow, lighting conditions and renewal of the water during the test. The experimental situations realized for the various tests, the slime organisms, the mean values of ATP content and ETS activity measured on slime in the stationary phase and the maximum zero-current potentials reached by uncorroded
TABLE
1
Experimental
data on seawater
Exposure
T (“C)
corrosion
Seawater speed
of stainless Slime organisms
a
(m/s)
10+3
360+20
B, P, detritus 1% of slime wet weight
0.4-0.1
B, P, detritus 7% of slime wet weight
10+2 8+2 10+2 8k2
In seawater at 1 m depth [7]
13.5 f 1.5
wave movement
tanks at sea [S] In settling
29.5 35 40 26.5
stagnant stagnant stagnant 1 stagnant
In seawater coming from a power plant
28 +4 28 +4 33.5 f 2.5
151
33.5k2.52.2
1.2 2.2 1.2
Vcorr (mv)
B,D,P,F,MF
B,D,P,
+ 1 +0.5 f+11.5
ATP content b (ng cme2)
300+30 350+35 2!90*20
0.05 0.05 0.05
[61
ETS activity b (mm3 0s hh’ cm-‘) -
17.7 + 1.5 19.5 + 0.5 26.5 f 3.5
In lab.
steel
F
a B = bacteria, D = diatoms, P = protozoa, F = fungi, MF = macrofouling b Mean values measured on slime in stationary phase.
380+20 320+20 200*20 -35550 2+2 2+2 2*2 2*2 organisms.
280+20 280+20 150 f 50 150+ 50
di I
I
Time 10
20
30
(days) 40
50
60
65
70
Fig. 2. Free corrosion potential values measured as a function of time on 21 Cr, 3 MO steel exposed in natural seawater (0) or sterilized seawater (0) before and after the addition of sodium azide on the 65th day. Standard deviations were calculated on five test specimens for each environment (Ref. 6).
stainless steels are summarized in Table 1. Comparison tests performed in natural and sterilized seawater showed some substantial differences in the behaviour of stainless steels: (1) The initiation of localized corrosion appeared to be more probable in natural than in sterile water. For instance, free exposure tests, lasting for about 70 days, showed. that localized corrosion occurred only on specimens immersed in natural water. No corrosion was found on stainless steels when exposed for the same length of time in sterile water. (2) Zero-current potentials, U,,,,, measured in both environments on passive specimens (i.e. still uncorroded), showed a very different evolution uersur time. Figure 2, concerning a 21 Cr, 3 MO steel, shows that in sterile seawater U,,,, settles at values of about + 100 mV (uersus s.c.e.) within a few days, whereas in natural seawater the potentials increase towards limiting values which may be higher by about 300 mV, evolving in time according to the slime growth modalities (Figs. 2 and 3). This ennobling process has been observed in all natural situations, although the limiting potential values were found to decrease as the seawater temperature rises (Tab. 1). For the onset of local attack on passive metal it is essential that the metal potential reaches a value higher than the characteristic UP values, called the pitting potential. The data illustrated in Fig. 2 explain why stainless steels are easier to corrode in natural seawater than in similar sterile solutions. (3) The oxygen reduction curves registered in the two conditions also proved to be strongly differentiated. In sterile environments, oxygen reduction occurs according to curve 1 of Fig. 4 with a charge transfer followed by a diffusion overvoltage. Indeed the reaction kinetics follows curve 1 only at the beginning of the test in
351
O-
0
I
20
Time (days) I
I
I
40
60
80
Fig. 3. Biomass respiratory ETS activity uerszu time (Ref. 9).
natural environments; successively the cathodic reaction becomes diffusion controlled over the whole 0 to - 600 mV potential range examined here (curves 2, 2’, 2” of Fig. 4) [3,5,10]. It follows that local&d corrosion propagation rates in natural environments can be from 10 to 100 times greater than observed in sterile situations. It is indeed well
Fig. 4. Oxygen reduction kinetics measured on 21 Cr, 3 MO stainless steels exposed in natural seawater after different immersion times: (1) in the first days; (2) on the 11th day; (2’) on the 13th day; (“) on the 17th day.
352
0
10
20
30
40
50
60
70
Fig. 5. Corrosion current densities determined against time on AISI 316 (0) and AISI 304 (0) (Ref. 6).
that corrosion rates are always controlled by the slower of the anodic and cathodic processes. In the case of stainless steels, corrosion propagation is certainly cathodically controlled. So the data reported in Fig. 4 clearly show that at the most frequent corrosion propagation potentials, which generally range between - 200 and - 300 mV (versus s.c.e.), the corrosion currents may change from 0.1 to 10 PA/cm2 if there is any microbiological slime sticking to the surface. This observation is confirmed in Fig. 5 in which the corrosion current values observed on AISI 304 and 316 steels as a function of their immersion time in natural seawater are plotted. The current values in this graph were determined by weight losses and by the holding times of the single specimen at the corrosion propagation potentials [5]. It can be seen that after the first 15-20 days, when biomass growth has virtually reached its stationary stage, all specimens show corrosion current values of the same order of magnitude, irrespective of the steel composition. (4) Any external action able to remove the slime physically or to inhibit its vital activity, always affected the electrochemical parameters. Thus, mechanical cleaning ‘of the metal surfaces caused a fast unnobling of the zero-current potentials and the simultaneous return of the oxygen reduction currents to sterile environment values [5]. Similar results can be reached by adding an enzimic inhibitor like sodium azide to the seawater at a concentration of 10e3 M. In this case, the response time may differ according to the electrochemical parameter under examination. Thus, cathodic currents will drop back to sterile environment values in 2-3 min after the toxic addition whereas the unnobling of the corrosion potential may require from 1 to 2 days (Fig. 2). known
DISCUSSION
The higher corrosivity of natural, as compared with sterile seawater seems to be due to a change in oxygen reduction kinetics caused by the presence of microbio-
353
Fig. 6. Maximum zero-current potential values reached by stainless steels 21 Cr, 3 MO and 20 Cr, 12 Ni, 6 MO exposed in a wide range of situations, summarized in Table 1, uerw temperature (Ref. 8).
logical slime on the stainless steel surface. The observation that this electrochemical effect occurs only with biologically active slimes would suggest the hypothesis of an enzymic catalysis affecting the oxygen reduction kinetics. The fast response of cathodic currents to the addition of an enzymic inhibitor might be an argument in favour of this hypothesis. An examination of Fig. 6, in which the maximum corrosion potential values determined in various test conditions are plotted against the test temperatures, makes it possible to draw some further conclusions: The catalytic process follows the same pattern in extremely dissimilar environmental conditions, provided the ambient temperature is not higher than 30°C. This observation leads to ascribing the electrochemical phenomenon to the microbial slime component, since it is the common denominator of all slimes investigated here. If the water temperature is higher than 30°C the ennobling process of the zero-current potential, to which the increased probability of local attacks on stainless steel is due, drops in amplitude at the rate of 30 mV for each degree centigrade until it vanishes, returning to sterile-environment values, when the temperature is raised beyond 40 QC.
0.6
25
30
35
40
Fig. 7. Respiratory ETS activity versus temperature measured oq 20 Cr, 12 Ni, 6 MO in seawater settling tanks (Ref. 8).
An examination of the ETS activity and the ATP content data as a function of temperature shows that rising the temperature to 40°C causes attenuation of the biomass and of its physiological activity (Figs. 7 and 8) but certainly not the disappearance of all viability. This would suggest that, from the corrosion view-
25
30
35
40
45
Fig. 8. Slime ATP content uersuF seawater temperature measured on 20 Cr, 12 Ni, 6 MO exposed in settling tanks (Ref. 8).
355
point, the heating of water to 40°C without sterilization may be sufficient to eliminate the microbiological interference from the corrosion process. REFERENCES 1 F. La Que, Mater. Perform., (April 1982) 13. 2 R.G.J. Edyvean, L.A. Terry and G.B. Picken, Int. Biodeterior. Bull., 21 (1985) 277. 3 A. Mollica and A. Trevis, 4th International Congress on Marine Corrosion and Fouling, Antibes, 14-18 June 1976, Centre de Recherches et d’Etudes OcCanographiques, Boulogne, 1976, pp. 351-365. 4 I.M. Krougman and F.P.I. Jsselin, 5th International Congress on Marine Corrosion and Fouling, Barcelona, 19-23 May 1980, Editorial Garci, Madrid, 1980, pp. 214-237. 5 A. Mollica, A. Trevis, E. Traverso, G. Ventura, V. Scotto, G. AIabiso, G. Marcenaro, U. Montini, G. de Carolis and R. Dellepiane, 6th International Congress on Matine Corrosion and Fouling, Athens, 5-8 September 1984, Skouhkidis ed Lab. Physical Chemistry and Applied Electrochemistry, Athens, 1984, pp. 269-281. 6 V. Scotto, R. Di Cintio and G. Marcenaro, Corros. Sci., 25 (1985) 185. 7 G. Alabiso, G. Marcenaro and V. Scotto, 13th Congress of Societa Italiana Biologia Marina, Cefalh, Palermo, 25-29 May 1981 (Poster). 8 Progetto Finalizzato CNR-Metallurgia, Tema A/l/e/4-B, Sintesi delle attivita di Ricerca, Temi, 21-23 May 1985, Consiglio Nazionale Ricerche, Rome, 1985. 9 G. Alabiso, V. Scotto and G. Marcenaro, Nova Thalassia, 6 (1983-84) Suppl., 451. 10 R. Johnsen and E. Bardal, Corrosion NACE, 41 (1985) 296.
Bioelectrochemistry
and Bioenergetics,
16 (1986) 347-355
A section of J. Electroanal. Chem., and constituting Vol. 212 (1986) Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
945 - AN EXAMPLE CORROSION THE BEHAVIOUR
OF MICROBIOLOGICALLY
OF STAINLESS
INFLUENCED
STEELS IN NATURAL
SEAWATER *
VITTORIA SCOTT-O, GIORGIO ALABISO and GIUSEPPE MARCENARO Istituto per la Corrosione Marina dei Metalli, C.N.R.,
Via della Mercantia
4, Geneva (Italy)
(Manuscript received March 19th 1986)
SUMMARY The aim of this paper is to summarize the most significant experimental data collected so far by our Institute on the corrosion behaviour of stainless steels in natural seawater. The purpose of the review is to illustrate the microbiological interference mechanisms with corrosion processes suggested by these experiments. The higher corrosivity of natural, as compared with sterile, seawater seems to be due to a change in oxygen reduction kinetics caused by the presence of microbiological slime on stainless steel surfaces. The observation that the accelerating effects occur only with biologically active slimes suggests the hypothesis of an enxymic catalysis affecting the oxygen reduction rates. The aerobic microbial slime components are confirmed to be the main cause of this effects. Any external action able to remove the slime __ nhvsicallv_ or to inhibit its viabilitv bv toxics or by temperature rises affects the electrochemical phenomenon also.
INTRODUCTION
The corrosive effects of seawater on metals have always been considered greater than the corrosion caused by salt solutions having the same composition. This difference is usually attributed to the presence of biological activity in natural environments [l]. The adhesion of marine organisms on the metal surface (the so-called marine fouZing) has often been mentioned in the literature as a contributing cause of corrosion in seawater, although its actual incidence on the overall corrosion process may vary according to the alloy composition [2].
* Contribution presented at the VIIIth International Symposium on Bioelectrochemistry and Bioenergetits, Bologna, June 24th-29th 1985. 0302-4598/86/$03.50
0 1986 Elsevier Sequoia S.A.
348
Stainless steels are particularly susceptible to biological corrosion, since the very presence on their surface of a microbiological fouling (the so-called slime or primary film) drastically increases the initiation probability and propagation rate of localized attacks (pits, crevices, etc.) [3-61. However, even though the possibility of microbiological interference with metal corrosion in seawater is often mentioned in the literature, little is known about the mechanism through which slime affects the corrosion process. This paper is aimed at summarizing the most significant experimental data collected so far on this subject by our Institute in order to illustrate the microbiological mechanisms interfering with corrosion processes suggested by these data. SUMMARY OF EXPERIMENTAL PROCEDURES
All data illustrated in this paper refer to stainless steels (21 Cr, 3 MO or 20 Cr, 18 Ni, 6 MO) exposed to strongly differentiated environments. The data refer to stainless steels: directly exposed in the sea at about 1 meter depth in natural light and temperature conditions [7]; fitted in dark ducts through which seawater from a thermoelectric power plant condenser was flowing. This condition was not only characterized by a significant temperature rise (lo-15°C beyond the seawater temperature), but also by the presence of antifouling and anticorrosion products [5]; in closed circuits, where natural and artificial seawater was run through stainless steels pipes. The natural seawater was renewed every 7-10 days, whereas the sterile water was kept so by continuous filtering over 0.22 pm meshed Millipore filters [6]. exposed in natural-seawater settling tanks, which were completely renewed every 8 h and thermostated at temperatures of 25, 30, 35 and 40°C [S]. The following measuring techniques were used for virtually all tests: control of the zero-current potentials (also called free corrosion potential UC,,,); reconstruction of the cathodic oxygen reduction kinetics on metal substrata by measuring the current outputs of potentiostatically polarized test specimens in the 0 to -600 mV (uersus s.c.e.) potential range; characterization of the slime nature under the microscope, evaluation of inorganic detritus trapped in the slime (identified by the dry residue weight after treatment at 920°C); quantification of the biomass through biochemical analyses such as: measurement of the chlorophyll a, proteins, carbohydrates and adenosine triphosphate (ATP) content and evaluation of the maximum activity of the respiratory electron transport system (ETS) [9]. SUMMARIZED
DATA
All results of the biochemical analysis, except ATP values, were found to be biomass-equivalent indexes and showed that the microbiological fouling, on atoxic substrata, grows with time following the pattern of Fig. 1. In this graph, both time
349
Time Fig. 1. Schematic
evolution
of slime biomass
versus time.
and biomass amounts may of course vary with the temperature, flow, lighting conditions and renewal of the water during the test. The experimental situations realized for the various tests, the slime organisms, the mean values of ATP content and ETS activity measured on slime in the stationary phase and the maximum zero-current potentials reached by uncorroded
TABLE
1
Experimental
data on seawater
Exposure
T (“C)
corrosion
Seawater speed
of stainless Slime organisms
a
(m/s)
10+3
360+20
B, P, detritus 1% of slime wet weight
0.4-0.1
B, P, detritus 7% of slime wet weight
10+2 8+2 10+2 8k2
In seawater at 1 m depth [7]
13.5 f 1.5
wave movement
tanks at sea [S] In settling
29.5 35 40 26.5
stagnant stagnant stagnant 1 stagnant
In seawater coming from a power plant
28 +4 28 +4 33.5 f 2.5
151
33.5k2.52.2
1.2 2.2 1.2
Vcorr (mv)
B,D,P,F,MF
B,D,P,
+ 1 +0.5 f+11.5
ATP content b (ng cme2)
300+30 350+35 2!90*20
0.05 0.05 0.05
[61
ETS activity b (mm3 0s hh’ cm-‘) -
17.7 + 1.5 19.5 + 0.5 26.5 f 3.5
In lab.
steel
F
a B = bacteria, D = diatoms, P = protozoa, F = fungi, MF = macrofouling b Mean values measured on slime in stationary phase.
380+20 320+20 200*20 -35550 2+2 2+2 2*2 2*2 organisms.
280+20 280+20 150 f 50 150+ 50
di I
I
Time 10
20
30
(days) 40
50
60
65
70
Fig. 2. Free corrosion potential values measured as a function of time on 21 Cr, 3 MO steel exposed in natural seawater (0) or sterilized seawater (0) before and after the addition of sodium azide on the 65th day. Standard deviations were calculated on five test specimens for each environment (Ref. 6).
stainless steels are summarized in Table 1. Comparison tests performed in natural and sterilized seawater showed some substantial differences in the behaviour of stainless steels: (1) The initiation of localized corrosion appeared to be more probable in natural than in sterile water. For instance, free exposure tests, lasting for about 70 days, showed. that localized corrosion occurred only on specimens immersed in natural water. No corrosion was found on stainless steels when exposed for the same length of time in sterile water. (2) Zero-current potentials, U,,,,, measured in both environments on passive specimens (i.e. still uncorroded), showed a very different evolution uersur time. Figure 2, concerning a 21 Cr, 3 MO steel, shows that in sterile seawater U,,,, settles at values of about + 100 mV (uersus s.c.e.) within a few days, whereas in natural seawater the potentials increase towards limiting values which may be higher by about 300 mV, evolving in time according to the slime growth modalities (Figs. 2 and 3). This ennobling process has been observed in all natural situations, although the limiting potential values were found to decrease as the seawater temperature rises (Tab. 1). For the onset of local attack on passive metal it is essential that the metal potential reaches a value higher than the characteristic UP values, called the pitting potential. The data illustrated in Fig. 2 explain why stainless steels are easier to corrode in natural seawater than in similar sterile solutions. (3) The oxygen reduction curves registered in the two conditions also proved to be strongly differentiated. In sterile environments, oxygen reduction occurs according to curve 1 of Fig. 4 with a charge transfer followed by a diffusion overvoltage. Indeed the reaction kinetics follows curve 1 only at the beginning of the test in
351
O-
0
I
20
Time (days) I
I
I
40
60
80
Fig. 3. Biomass respiratory ETS activity uerszu time (Ref. 9).
natural environments; successively the cathodic reaction becomes diffusion controlled over the whole 0 to - 600 mV potential range examined here (curves 2, 2’, 2” of Fig. 4) [3,5,10]. It follows that local&d corrosion propagation rates in natural environments can be from 10 to 100 times greater than observed in sterile situations. It is indeed well
Fig. 4. Oxygen reduction kinetics measured on 21 Cr, 3 MO stainless steels exposed in natural seawater after different immersion times: (1) in the first days; (2) on the 11th day; (2’) on the 13th day; (“) on the 17th day.
352
0
10
20
30
40
50
60
70
Fig. 5. Corrosion current densities determined against time on AISI 316 (0) and AISI 304 (0) (Ref. 6).
that corrosion rates are always controlled by the slower of the anodic and cathodic processes. In the case of stainless steels, corrosion propagation is certainly cathodically controlled. So the data reported in Fig. 4 clearly show that at the most frequent corrosion propagation potentials, which generally range between - 200 and - 300 mV (versus s.c.e.), the corrosion currents may change from 0.1 to 10 PA/cm2 if there is any microbiological slime sticking to the surface. This observation is confirmed in Fig. 5 in which the corrosion current values observed on AISI 304 and 316 steels as a function of their immersion time in natural seawater are plotted. The current values in this graph were determined by weight losses and by the holding times of the single specimen at the corrosion propagation potentials [5]. It can be seen that after the first 15-20 days, when biomass growth has virtually reached its stationary stage, all specimens show corrosion current values of the same order of magnitude, irrespective of the steel composition. (4) Any external action able to remove the slime physically or to inhibit its vital activity, always affected the electrochemical parameters. Thus, mechanical cleaning ‘of the metal surfaces caused a fast unnobling of the zero-current potentials and the simultaneous return of the oxygen reduction currents to sterile environment values [5]. Similar results can be reached by adding an enzimic inhibitor like sodium azide to the seawater at a concentration of 10e3 M. In this case, the response time may differ according to the electrochemical parameter under examination. Thus, cathodic currents will drop back to sterile environment values in 2-3 min after the toxic addition whereas the unnobling of the corrosion potential may require from 1 to 2 days (Fig. 2). known
DISCUSSION
The higher corrosivity of natural, as compared with sterile seawater seems to be due to a change in oxygen reduction kinetics caused by the presence of microbio-
353
Fig. 6. Maximum zero-current potential values reached by stainless steels 21 Cr, 3 MO and 20 Cr, 12 Ni, 6 MO exposed in a wide range of situations, summarized in Table 1, uerw temperature (Ref. 8).
logical slime on the stainless steel surface. The observation that this electrochemical effect occurs only with biologically active slimes would suggest the hypothesis of an enzymic catalysis affecting the oxygen reduction kinetics. The fast response of cathodic currents to the addition of an enzymic inhibitor might be an argument in favour of this hypothesis. An examination of Fig. 6, in which the maximum corrosion potential values determined in various test conditions are plotted against the test temperatures, makes it possible to draw some further conclusions: The catalytic process follows the same pattern in extremely dissimilar environmental conditions, provided the ambient temperature is not higher than 30°C. This observation leads to ascribing the electrochemical phenomenon to the microbial slime component, since it is the common denominator of all slimes investigated here. If the water temperature is higher than 30°C the ennobling process of the zero-current potential, to which the increased probability of local attacks on stainless steel is due, drops in amplitude at the rate of 30 mV for each degree centigrade until it vanishes, returning to sterile-environment values, when the temperature is raised beyond 40 QC.
0.6
25
30
35
40
Fig. 7. Respiratory ETS activity versus temperature measured oq 20 Cr, 12 Ni, 6 MO in seawater settling tanks (Ref. 8).
An examination of the ETS activity and the ATP content data as a function of temperature shows that rising the temperature to 40°C causes attenuation of the biomass and of its physiological activity (Figs. 7 and 8) but certainly not the disappearance of all viability. This would suggest that, from the corrosion view-
25
30
35
40
45
Fig. 8. Slime ATP content uersuF seawater temperature measured on 20 Cr, 12 Ni, 6 MO exposed in settling tanks (Ref. 8).
355
point, the heating of water to 40°C without sterilization may be sufficient to eliminate the microbiological interference from the corrosion process. REFERENCES 1 F. La Que, Mater. Perform., (April 1982) 13. 2 R.G.J. Edyvean, L.A. Terry and G.B. Picken, Int. Biodeterior. Bull., 21 (1985) 277. 3 A. Mollica and A. Trevis, 4th International Congress on Marine Corrosion and Fouling, Antibes, 14-18 June 1976, Centre de Recherches et d’Etudes OcCanographiques, Boulogne, 1976, pp. 351-365. 4 I.M. Krougman and F.P.I. Jsselin, 5th International Congress on Marine Corrosion and Fouling, Barcelona, 19-23 May 1980, Editorial Garci, Madrid, 1980, pp. 214-237. 5 A. Mollica, A. Trevis, E. Traverso, G. Ventura, V. Scotto, G. AIabiso, G. Marcenaro, U. Montini, G. de Carolis and R. Dellepiane, 6th International Congress on Matine Corrosion and Fouling, Athens, 5-8 September 1984, Skouhkidis ed Lab. Physical Chemistry and Applied Electrochemistry, Athens, 1984, pp. 269-281. 6 V. Scotto, R. Di Cintio and G. Marcenaro, Corros. Sci., 25 (1985) 185. 7 G. Alabiso, G. Marcenaro and V. Scotto, 13th Congress of Societa Italiana Biologia Marina, Cefalh, Palermo, 25-29 May 1981 (Poster). 8 Progetto Finalizzato CNR-Metallurgia, Tema A/l/e/4-B, Sintesi delle attivita di Ricerca, Temi, 21-23 May 1985, Consiglio Nazionale Ricerche, Rome, 1985. 9 G. Alabiso, V. Scotto and G. Marcenaro, Nova Thalassia, 6 (1983-84) Suppl., 451. 10 R. Johnsen and E. Bardal, Corrosion NACE, 41 (1985) 296.