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Biofilm and corrosion on active-passive alloys in seawater
International Biodeterioration & Biodegradation 29 (1992) 213-229
Biofilm and Corrosion on Active-Passive Alloys in Seawater
A. Mollica Institute for Marine Corrosion of Metals (ICMM) Via Scarsellini, 10, 16149 Genova, Italy
ABSTRACT This paper is a summary of work carried out at the ICMM and deals with the effect of biofilm growth on the marine corrosion of stainless steels and other active-passive alloys. There is evidence regarding the important roleplayed by aerobic bacterial settlement, both on the onset and the propagation of localized corrosion on such alloys and on the enhancement of galvanic corrosion of less noble materials coupled with them. These effects are the consequence of the oxygen reduction depolarization induced by biofilm growth. Hypotheses on the mechanisms by which the presence of the biofilm causes oxygen reduction depolarization, and prevention systems against the microbial induced corrosion, are also discussed.
INTRODUCTION S o m e years ago a g r o u p o f researchers did a b i b l i o g r a p h i c s t u d y to try to a n s w e r s o m e o f the following questions:
- - D o e s sufficient p r o o f exist to affirm that metal materials, o f a n y type, are s u b j e c t e d to ' m i c r o b i a l i n d u c e d corrosion' (MIC)? - - I f so, w h a t type o f materials are involved? - - W h a t types o f m i c r o - o r g a n i s m are involved? 213 International Biodeterioration & Biodegradation 0964-8305/92/$05.00© 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
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- - W i t h what type of mechanisms? - - W h a t types of prevention are needed? etc. After reading the results of this inquiry (Pope et al., 1984), the feeling is that, except for the corrosion of ferrous alloys by sulphate reducing bacteria and some specific cases like the corrosion of a l u m i n i u m in kerosine tanks, a scarce and inconclusive documentation exists for entire classes of alloys of industrial interest. There is more hypothesis than data for stainless steels and nothing for N i - C r alloys, Ti, etc. In the last few years, some literature has appeared which gives different conclusions about MIC. This paper is a summary of recent works done at the I C M M (Institute for Marine Corrosion of Metals) in order to answer the above mentioned questions with regard to MIC in seawater on stainless steels and other active-passive alloys. Most of the results have been obtained through field tests.
C O R R O S I O N STUDIES Effect of the presence of biofilm on the onset of crevice corrosion on AISI 316 SS (stainless steel) in seawater
Once exposed to seawater, the process of biofilm formation on the stainless steel surfaces starts. The absorption of organic substances, the settlement (first reversible and then irreversible), of pioneer bacteria (incubation phase), their growth in colonies, the development of new species (micro-organisms and macro-organisms) etc., are steps in this process. The biofilm, certainly aerobic in the first phases of formation, can become locally anaerobic during its development, in particular where crevices are present (Mollica et al., 1988a). As a consequence, when studying the crevice corrosion, (the most frequent type of localized corrosion on stainless steel in seawater), it must be taken into account that the onset of this type of corrosion starts in the presence of both a mainly aerobic biofilm on the areas freely exposed to seawater and a high concentration of sulphides (200-400 ppm), generated by the sulphate reducing bacterial activity, on the shielded areas. Usually, the minor resistance of stainless steels in a natural environment in respect to sterile seawater is attributed to the sulphate reducing bacterial activity on shielded areas. Recently, we tried to deduce the role of the biofilm and its 'components', aerobic and anaerobic, on the onset of crevice corrosion on AISI 316 stainless steel (SS).
Biofilm and corrosion on active-passive alloys in seawater
215
For this purpose, a simple experiment was organized, described in detail elsewhere (Mollica, 1990). The experiment consisted of the exposure of a series of galvanic couples each formed by a large stainless steel surface coupled, through a resistor, to a second small shielded surface. This assembly is known as a 'remote crevice assembly'. In the case of onset o f crevice corrosion, the two surfaces will play, respectively, the role of cathodic and anodic elements o f the couple, a n d a clear galvanic current will flow through the resistor. By taking timed readings of the galvanic currents, it is possible to determine, for each couple, w h e n the crevice corrosion starts and, therefore, to construct a graph which reports the 'survival' S(t) as a function of the exposure time t where
S(t)
=
[ N t o t couples - -
Ncorroded(t)]/Ntot
couples
We can take advantage of the geometric separation of the shielded and unshielded areas, deciding to couple the two elements after an eventual pre-exposure of each element. It is possible, for example, to pre-expose the cathodic surface to natural seawater until a biofilm has settled on them, or to pre-expose the anodic surface in marine anaerobic m u d rich in sulphide, and so on. In this manner, a set of combinations at the m o m e n t of the coupling can be realised. For each of these combinations, a survival-time curve can be obtained. In Fig. 1 four different shapes of the S(t) curve dependent on the preexposure conditions are shown, as examples. In this way the experiment aimed to evaluate by a stochastic approach the influence exerted on the crevice corrosion onset ofAISI 316 by the presence or absence of aerobic biofilm on the cathodic areas; and the presence or absence of biosulphides on the anodic areas. The result of these tests, carried out on about 400 samples, was that each S(t) experimental curve m a y be considered as a specific case of the general stochastic distribution proposed by Weibull:
S(t)
j" 1 [ exp ( - ( t - O)/a)
t< 0
t>0
in which a depends on the pretreatment of the shielded surface (passivation in air, in HNO3 etc.); /3, as well as by the surface pretreatment, is dependent on the presence or absence ofsulphides on the shielded areas (13 tends to 1 in the presence of sulphides);
216
A. Mollica 10
0.1
o
t (days)
~.
Fig. 1. Percentage of survived AISI 316 SS couples (S(t)) to crevice corrosion (from Mollica, 1990). Each couple was made of a AISI 316 shielded surface (anode) coupled with an unshielded surface (cathode). Curves 1 to 4 show different shapes of the curve S = f(t) as a function of pre-exposure conditions of the anodic and cathodic elements before coupling and of the surface treatment of the anodic element. (1) Cathodic surface pre-exposed to natural seawater. Anodic surface passivated in air and pre-exposed in anaerobic mud. (2) Cathodic surface not pre-exposed in natural seawater. Anodic surface passivated in air and pre-exposed in natural seawater. Anodic surface passivated in HNO3 and not pre-exposed in anaerobic mud. (3) Cathodic surface pre-exposed in anaerobic mud. (4) Both cathodic and anodic surfaces immersed in sterile seawater. Anodic surface passivated in HNO3. 0 represents the i n c u b a t i o n time o f the aerobic b i o f i l m on the c a t h o d i c areas. Thus, the following surprising c o n c l u s i o n s a b o u t the role p l a y e d by the b i o f i l m in regard to the onset o f the crevice corrosion was reached: (a) T h e presence o f sulphides, arising f r o m the activity o f a n a e r o b i c bacteria o n the a n o d i c surfaces, plays o n l y a s e c o n d a r y role. (b) O n the contrary, the presence o f aerobic b i o f i l m o n the c a t h o d i c surfaces plays a d e t e r m i n i n g role: the corrosion starts r a p i d l y (S(t) < 1) o n l y w h e n the b i o f i l m has developed (t > 0). Two questions follows from this: (a) W h y does the growth o f the aerobic b i o f i l m o n the c a t h o d i c areas d e t e r m i n e the p r o b a b i l i t y o f corrosion onset o n the a n o d i c areas? (b) W h e n crevice corrosion has started, h i g h e r corrosion rates in n a t u r a l seawater t h a n in sterile seawater were often observed (Mollica & Trevis, 1976; Lee et al., 1984; Nivens et al., 1986; Holte et al., 1988a). So t h a t it c a n be a s k e d w h e t h e r the aerobic b i o f i l m o n
Biofilm and corrosion on active-passive alloys in seawater
217
the cathodic areas is also responsible for the different propagation rate? Effect of aerobic biofilm on corrosion of the S S s
Some years ago, a p h e n o m e n o n was observed, which was confirmed and studied in more detail later: it was observed that depolarization of the oxygen reduction on SSs, during their exposure to natural seawater (Fig. 2), occurred contemporaneously with the biofilm growth on the SS surfaces. F r o m this p h e n o m e n o n a series of practical consequences of SSs corrosion follows which are described below.
Onset of localized corrosion On immersion in the sea, a SS on which, in time, a biofilm forms; it is found that during the period of the biofilm growth the free corrosion potential of SS in the passive state (curve 'p' in Fig. 2) increases from Eco~r~ to Eco~r2. The e n n o b l e m e n t of the free corrosion potential during the exposure to seawater of SSs in the passive state has actually been observed by ES.C.E.(v) Ecorr2r . . . .
'\ X
0.3
N\ \ \ x \\
0.I
\
-0.I
Ecorrli -O'iI~
8
l -i
;4I'
corr 1
tc o r r 2
I
ILog i (^ • ~ m ~ )
Fig. 2. Oxygen reduction rates measured on SSs exposed to quiet natural seawater. (Mollica, A., & Montini, U., unpublished data, 1973). Curve (l) is obtained within 1 to 2 days of immersion; curve (2) is obtained after 12 days of immersion.
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m a n y authors (Mollica a al., 1976, 1984, 1989; K r o u g h m a n & Ijsseling, 1980; M o r et al., 1980; J o h n s e n & Bardal, 1985, 1986; Scotto et al., 1985, 1990; Desestret, 1986; Dexter et al., 1988, 1990; Gallagher et al., 1988). It was shown that the m a x i m u m level o f the free corrosion potential which can be reached by SSs in natural seawater is about 300 to 400 m V higher t h a n those in sterile seawater. Such possible e n n o b l e m e n t , due to the biofilm growth on cathodic areas, is a powerful factor of localized corrosion initiation for m a n y types of SSs; only very high quality SSs can sustain it without onset of localized corrosion (Desestret, 1986), particularly w h e n defects, like crevices, are present on their surfaces.
Propagation o f localized corrosion Suppose that on the SS some form of localized corrosion has already started (active state). The corrosion rate will be determined by the intersection of the anodic polarization curve a n d the active surface (curve 'a" in Fig. 2), a n d the oxygen reduction curve on the s u r r o u n d i n g cathodic areas. Taking into account that in the sea the corrosion potential of SSs in the active state is in general (Mor et al., 1980) over - 2 0 0 mV vs. the SCE, the scheme in Fig. 2 shows that the presence of biofilm on the cathodic areas produces an increase (from icor~l to icorr2) of the corrosion rate by about two orders of m a g n i t u d e (Lee et al., 1984; Ventura et al., 1989). Galvanic corrosion A scheme similar to that m e n t i o n e d above can be used to show h o w the presence of biofilm on SS surfaces stimulates the corrosion of less noble materials c o u p l e d with them. A n increase of the galvanic current, close to one order of magnitude, was often observed on the SS-iron couples during the biofilm formation on SS surfaces (Mollica et al., 1984, 1990). It is c o n c l u d e d that the presence of aerobic biofilm o n SS surfaces exposed to seawater, depolarizing the oxygen reduction, provokes:
an increase of 300 to 400 m V of the m a x i m u m reachable potential, promoting, as a consequence, a more probable onset of localized corrosion; --an increase, up two orders of magnitude, of the corrosion rate w h e n localized corrosion is in progress; a n d - - a n increase of the galvanic currents between SSs a n d other less noble materials coupled with them.
Biofilm and corrosion on active-passive alloys in seawater
219
Effect of aerobic biofilm on other active-passive alloys The correlation between the biofilm formation and the depolarization of the oxygen reduction was verified not only on stainless steels, independently of their composition, but also on other active-passive alloys like Ti, Ni-Cr, C u - N i and N i - C u ( K r o u g h m a n & Ijsseling, 1980; Schiffrin & De Sanchez, 1985; Dexter & Lin, 1988; Holte et ai., 1988; Mollica et al., 1988b). Figure 3 shows as an example the effect of the biofilm formation on a 70Ni-30Cu and a Ti alloy. In the case of the N i - C u alloy, the biofilm provokes an easy onset and a rapid propagation of localized corrosion; on the Ti alloy the presence of biofilm caused corrosion problems only in the case of galvanic coupling. The conclusion is drawn that the biofilm can therefore produce one or more of the above mentioned effects depending on the kind of activepassive alloy. Composition of biofilm and corrosion The word biofilm is a very general term which includes m a n y 'components': bacteria, algae, cellular exudates, organic and inorganic matter coming from the bulk of the solution and trapped in the mucilage, etc. In order to establish what 'component' of the biofilm correlates with the p h e n o m e n o n of the depolarization of the oxygen reduction several tests have been made. The first test consisted in the exposure to seawater of SSs able to
400-
mV (z.c.e.)
-500
Fig. 3. Anodic dissolution curves of Ti (3) a n d 70Ni30Cu alloy (4) specimens reported with the cathodic polarization curves measured on their surfaces in the absence (1) and presence (2) of biofilm (Mollica et al., 1988b).
220
A. Mollica
m a i n t a i n the passive state and, once ennobled, the free corrosion potential. A substance like s o d i u m azide, able to kill the forms of life present in the biofilm without causing the d e t a c h m e n t of the biofilm itself, was a d d e d in the solution (Scotto et al., 1985). The result was that the elimination of the living part of the biofilm brought the potentials back to the levels m e a s u r e d in sterile water. This result, c o n f i r m e d by other m e a n s (Mollica et al., 1984), indicates that the m o d i f i c a t i o n of the electrochemical parameters is correlated more to the living fraction t h a n to the total mass of the biofilm. With the aim of verifying if the presence of algae inside the biofilm is necessary or not, several SS samples were exposed to the seawater u n d e r different conditions of illumination. D u r i n g the tests the free corrosion potentials were m e a s u r e d a n d the biomass was characterized by m e a n s of c o n t e m p o r a n e o u s m e a s u r e m e n t s of chlorophyll a a n d of the electron transport system (ETS) activity, tied to the respiratory chain present in every cell (Mollica et al., 1984). The increase of potential was always observed, both w h e n the algae were d o m i n a n t (ETS = 3.28 Chla) a n d w h e n these were practically absent (Chla = 0). It was c o n c l u d e d that the m o d i f i c a t i o n of the oxygen reduction kinetics is to be correlated only to a bacterial settlement. Finally, it was shown (Lee et al., 1984; Scotto et al., 1986; Holte et al., 1988a; Mollica et al., 1989) that an increase in the seawater temperature near to 40°C erases the modifications of the oxygen reduction curve i n d u c e d by the biofilm and, therefore, its effects on corrosion. It suggests that the bacteria involved are not of a t h e r m o p h i l i c type. The conclusion is that the depolarization of the oxygen reduction a n d its consequences on the corrosion of active-passive alloys in seawater are c o n n e c t e d to the settlement on their surfaces of a sufficient n u m b e r of aerobic n o n t h e r m o p h i l i c bacteria. By what m e c h a n i s m does the bacterial settlement influence the oxygen reduction kinetics? Hypotheses about the mechanisms through which a bacterial settlement causes the depolarization of the oxygen reduction
There are different hypotheses to explain h o w the bacterial settlement brings about the depolarization of the oxygen reduction: - - T h e presence of the biofilm provokes a decrease of the p H on the surfaces and, therefore, a shift of 60 mV/(pHbulk -- PHsurface) in the noble direction of the visible b r a n c h of the cathodic curve (Dexter et
Biofilm and corrosion on active-passive alloys in seawater
221
al., 1988). Furthermore, the acidification of the surface can produce calcareous deposit causing dissolution and cathodic depolarization (Little et al., 1987). - - T h e exchange density current of the oxygen reduction increases because of the effect of a catalyst generated by the bacterial metabolic activity. On the nature of such a catalyst, there are different opinions (Ford & Mitchell, 1990; Gallagher et al., 1988; Holteetal., 1988; Shiffrin & De Sanchez, 1985; Scottoetal., 1985). To select between the two proposed mechanisms (acidification or catalysis) several tests were done (Mollica et al., 1990b), two of which are briefly described.
Effect of seawater acidification Seawater continuously renewed and acidified at different pH, by suitable additions of HC1, was p u m p e d through a series of high quality SS pipes; on these samples the free corrosion potential was periodically measured and the biofilm was observed. The pH range studied from the natural pH of seawater was 8.1-8.2 down to about pH 5. At the end of the experiment, which lasted 40 days, the presence of biofilm was observed on all exposed samples. In Fig. 4(a), three examples of the evolution of the free corrosion potential at different seawater pH are shown; in Fig 4(b), the free corrosion potentials measured during the exposure on all the tested samples are reported as a function of the seawater pH imposed. The data indicate that the e n n o b l e m e n t of the potential due to the biofilm growth can only be observed at the pH of untreated natural seawater. It is sufficient to eliminate the buffering properties of the seawater to eliminate the effect of the biofilm; also further acidification produces an enoblement of the free corrosion potentials, but in this case only due to the decrease solution pH. Effect of continuous cathodic polarization on the oxygen reduction current In Fig. 2, two cathodic curves showing the effect of biofilm growth are shown. It can be seen that the cathodic curves from an envelope of points, each of them obtained in potentiostatic mode. The potentiostatic curves shown in Fig. 2 are schematically drawn in Fig. 5(a). It was noted that, if we want to obtain rapidly the oxygen reduction curve, tracing it in a potentiodynamic way starting from SS samples in a passive state already settled by bacteria, a completely different cathodic curve, similar to curve 3 in Fig. 5(a), will be obtained.
222 400
A. Mollica
mV (sce)
400
~ii
200
i:"
m
untreated seawater
mV (scc)
,.
v v
•
v
pH = 7 . 5
-200
•
o
•
ee
200
pH = 5 8
Yv
•
0 !
I
bl
o!
o
!
-200 10
days
50
6
(a)
pH
8
(b)
Fig. 4. Effect of the seawater acidification on the free corrosion potentials of SSs in passive state (Mollica et al., 1990b). (a) Three examples of the free corrosion potential trend vs. time in different seawater pH; (b) summary of the free corrosion potentials measured independently of the time in function of the seawater pH.
In addition, if we stop the p o t e n t i o d y n a m i c excursion at a given potential Es, the oxygen reduction c u r r e n t increases with time towards the c o r r e s p o n d i n g value o n the 'potentiostatic' curve 2. A similar route in the E-log(ic) plot is s p o n t a n e o u s l y followed d u r i n g the first stages o f localized corrosion p r o p a g a t i o n (Mollica et al., 1989). We studied the transient p h a s e between curves 3 a n d 2 polarizing, at four different potentials, SS samples a l r e a d y covered by biofilm. T h e result (Fig. 5(b)) was that, i n d e p e n d e n t o f the i m p o s e d potential, the o x y g e n r e d u c t i o n c u r r e n t increases, initially, with a rule o f this type: log(it) = a t + fl
(1)
or, by derivative: d(ic)/dt
= a ic
(2)
a n d the final values are r e a c h e d after several days o f c o n t i n u o u s polarization. E q u a t i o n (2) shows that the oxygen r e d u c t i o n depolarization on surfaces covered by biofilm is a n autocatalysing p h e n o m e n o n , since the increase in oxygen r e d u c t i o n rate is proportional to the i n s t a n t a n e o u s r e d u c t i o n current. E q u a t i o n (2) c o u l d also be r e a d in the following way: the bigger the i n s t a n t a n e o u s p r o d u c t i o n o f O H - o n the electrode
Biofilm and corrosion on active-passive alloys in seawater
mV
223
(sce)
o
i (~.~-,)
Es - - ~ -200
(~
Fig. 5. (a) Schematic description of possible oxygen reduction curves which m a y be obtained on SSs in natural seawater (Mollica et al., 1990b): (1) oxygen reduction curve on SSs not covered by biofilm; (2) oxygen reduction curve obtained by potentiostatic way on SSs covered by biofilm; (3) oxygen reduction curve obtained by potentiodynamic way on SSs covered by biofilm. The arrow shows the increase in time of the oxygen reduction current when the potentiodynamic polarization is stopped at a given potential E s.
i (ia~.¢. -z)
10 1
,,,~
0.1 , ,,,'(" ,lp~"
7 0\ \
j~g~ ic.,~t.~
\ \\
\
mV(sce)~ 1 2 3 4 5 t(days) (b) Oxygen reduction current as a function of the imposed cathodic potential a n d time on SS surfaces covered by biofilm.
224
A. Mollica
surfaces for the reduction of 02 the faster the increase of the cathodic current. The data in this paragraph, and other literature (Holte et al., 1988; Mollica et al., 1990b), suggest that the electrochemical effects induced by the biofilm formation cannot be explained through a mechanism which foresees the acidification of the surface. Taking also into account that, according to previous observations (Nivens et al., 1986), a correlation was shown between the modification of the electrochemical parameters and the increase of the superficial concentrations of bacterial exopolymers (Scotto et al., 1990), the following conclusion can be drawn: The most suitable hypothesis to explain the phenomenon of the oxygen reduction depolarization induced by biofilm growth on activepassive alloys in seawater seems to be at the moment that of a catalyst, produced by settled bacteria probably linked to their exopolymers, whose efficiency increases by raising above 8 the pH on the surface.
Prevention of MIC on active-passive alloys in seawater
Cathodic protection and biocide addition are the most suggested anticorrosive measures in the case of MIC. We studied, in particular, the application of these types of measures, separately or applied together, for the protection of heat exchanger pipes against the corrosive effects of the biofilm growth. Cathodic protection
Stainless steel pipes, through which seawater once flowed, were connected to iron anodes placed in the water box; during the test the galvanic currents and the potentials at fixed points inside the pipes were measured (Fig. 6) (Mollica et al., 1984). The data show that, because of the biofilm growth on the pipe walls, the consumption of the sacrificial anode increases by one order of magnitude. As a consequence, the increased ohmic drop inside the pipe reduces the protected pipe length to less than 0.5 m. The only cathodic protection is, therefore, not sufficient to protect the whole tube nest against the detrimental effect of the biofilm. Biocide addition
Sodium hypochlorite was studied since it is used widely in cooling systems; in particular the effect of NaC10 used in continuous doses.
Biofilm and corrosion on active-passive alloys in seawater
225
Biomass as a function o f residual chlorine concentration. A set of SS pipes was exposed for 30 days to o n c e - t h r o u g h flowing sea water c o n t i n u o u s l y treated with different a m o u n t s of N a C 1 0 and, at the e n d of exposure, the protein content on their internal walls was m e a s u r e d (Mollica et al., 1990a). T h e protein analyses s h o w that a c o n t i n u o u s dose of residual chlorine of >/0.1 p p m prevents the growth of a viable biomass.
Free corrosion potential o f SSs in passive state as a function o f the residual chlorine concentration. O n a series of SS samples in the passive state, exposed in the same way as the above m e n t i o n e d conditions, the free corrosion potential trend was followed (Mollica et al., 1990a). At the e n d of exposure the m i n i m u m free corrosion potential was observed for a concentration of residual chlorine equal to 0.1 p p m , c o r r e s p o n d i n g to that strictly sufficient to prevent the biofilm formation. At higher concentrations, the free corrosion potential rises again rapidly, a s s u m i n g a concentration close to 0. 2 to 0. 3 p p m at values above those a s s u m e d by the same steels in untreated seawater.
Propagation of localized corrosion as a function o f residual chlorine concentration. Galvanic couples between high quality SS pipes a n d shielded AISI 316 SS samples were exposed, for 75 days, to once-through flowing seawater treated with different a m o u n t s of NaC10, a n d the crevice corrosion p r o p a g a t i o n rate on AISI 316 samples was followed by m e a s u r i n g the galvanic current (Ventura et al., 1988, 1989). T h e m i n i m u m corrosion rate was observed in the presence of a residual chlorine concentration (0-1-0-2 p p m ) close to that strictly sufficient to h i n d e r biofilm formation: the corrosion rate was about two orders of m a g n i t u d e less t h a n that of untreated seawater. Even in the presence o f higher doses o f residual chlorine (1-2 ppm), the p r o p a g a t i o n rate is, however, considerably lower t h a n in untreated seawater. Similar conclusions have been reported in the literature (Wallen, 1990). C o n c e r n i n g the use of c o n t i n u o u s NaCIO additions as the only measure against MIC, we can therefore conclude that the use of N a C 1 0 in c o n t i n u o u s additions, on the o n e h a n d reduces the p r o p a g a t i o n rate of the localized corrosion on the other h a n d it tends to facilitate the onset if the concentration of residual chlorine is over that strictly sufficient to prevent the biofilm formation.
A. Mollica
226
mVs.C.E. 200 /
/
/'J
~
~wilh
slime
/ - ~ lg = 6.2 mA 200 lg = 0.65 m A _.-.-""
~ . w i t h o u t slime
-500 S.C.E. I0
50
1
cm
n Iron "~{SS~ - j
Stainless Steel
Fig. 6. Potential profiles inside a SS pipe cathodically protected by an iron anode in the absence (1) and presence (2) of biofilm on the SS surfaces. (Elaboration from Mollica et al.. (1984.)
Mixed biocide addition-cathodic protection system To examine the effect of a mixed protection system, SS pipes were exposed to once-through flowing natural seawater treated with different amounts of NaC10 and, at the same time, cathodically protected by iron anodes placed in the water box (Mollica et aL, 1990a). Galvanic currents between the SS pipes and iron anodes and potential profiles inside the pipes were followed during the exposure, which lasted 30 days. According to the results plotted in Fig. 6, the data confirm that the elimination of the biofilm through the continuous presence of a residual chlorine concentration close to 0.1 ppm reduces by one order of magnitude the anode consumption, increasing at the same time the protected pipe length, and, in addition, shows that an excess of chlorine (in the range 0-1-0.8 ppm) does not induce further increases of the galvanic currents.
The application of a mixed biocide addition-cathodic protection system appears to be the optimum solution for tube banks combining low anode consumption with high penetration of the protection inside the pipes and not requiring an exact control of the biocide dosage.
Biofilm and corrosion on active-passive alloys in seawater
227
CONCLUSIONS 1. The biofilm growth induces a depolarization of the oxygen reduction on the surface of a series of active-passive alloys, like SSs, Ti, Ni-Cr, Cu-Ni, N i - C u alloys, etc., exposed to seawater. 2. The presence of biofilm on the surfaces, via the oxygen reduction depolarization, causes in turn: - - a greater probability of localized corrosion onset; - - a faster propagation rate of localized corrosion in progress; m an increase of the galvanic currents between the above mentioned alloys a n d other less noble ones coupled with them. 3. The hypothesis of a catalyst produced by non-thermophilic bacteria a n d whose efficiency increases by increasing the p H above 8 seems, at present, the most suitable to explain the p h e n o m e n o n of the oxygen depolarization induced by biofilm growth. 4. If only continuous NaC10 additions are used to prevent MIC on active-passive alloys in seawater, the residual chlorine concentration must be carefully controlled and maintained to a level very close to 0.1 ppm, which is sufficient to hinder the biofilm formation. 5. For MIC prevention on banks of pipes the best solution is a mixed cathodic protection-biocide addition system; such a system permits c o n t e m p o r a n e o u s l y a low c o n s u m p t i o n of the sacrificial anodes, a high penetration of the protection inside the pipes a n d an indifference to an eventual residual chlorine overdose.
REFERENCES Desestret, A. (1986). Corrosion localisre des aciers inoxydables dans l'eau de mer. Mat~riaux et Techniques, 7/8, 317-24. Dexter, S. C. & Gao, G. Y. (1988). Effect of seawater biofilms on corrosion potential and oxygen reduction of stainless steel. Corrosion, 44, 717-23. Dexter, S. C. & Lin, S. H. (1988). Mechanism of corrosion potential ennoblement by marine biofilm. Proc. 7th Int. Congress on Marine Corrosion and Fouling, Valencia, Spain. Organised by Int. Committee for Research of the Prevention of Marine Materials (COIPM), Bruxelles. Dexter, S. C. & Zhang, H. J. (1990). Effect of biofilms on corrosion potential of stainless alloys in estuarine waters, l l t h Int. Corrosion Congress, Italian Association of Metallurgy, Milano, Italy, Vol. 4, pp. 333-40. Ford, T. & Mitchell, R. (1990).Advances in Microbial Ecology, ed. K. C. Marshall. Plenum Press, New York and London, vol. 11. Gallagher, P., Malpas, R. E. & Shone, E. B. (1988). Corrosion of stainless steels
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in natural, transported and artificial seawaters. Br. Corros. J., 23, 22933. Holte, R., Bardal, E. & Gartland, P. O. (1988a). The time dependence of cathodic properties of stainless steels, titanium, platinum and 90/10 CuNi in seawater. Corrosion~88, National Association of Corrosion Engineers Publications, St. Louis, MO, Paper No. 393. Holte, R., Gartland, P. O. & Bardal, E. (1988b) Oxygen reduction on 'passive' metals and alloys in seawater. Effect of biofilm. Proc. 7th Int. Congress on Marine Corrosion and Fouling, Valencia, Spain. Organised by Int. Committee for Research on the Prevention of Marine Materials (COIPM), Bruxelles. Johnsen, R. & Bardal, E. (1985). Cathodic properties of different stainless steels in natural seawater. Corrosion, 41, 296-302. Johnsen, R. & Bardal, E. (1986). The effect of a microbiological slime layer on stainless steel in natural sea water. Corrosion/86, National Association of Corrosion Engineers Publications, Houston, TX, Paper No. 227. Kroughman, J. M. & Ijsseling, F. P. (1980). Crevice corrosion of stainless steels and nickel alloys in seawater. Proc. 5th Int. Congress on Marine Corrosion and Fouling, Barcelona, pp. 214-37. Organised by Int. Committee for Research of the Prevention of Marine Materials (COIPM), Bruxelles. Lee, T. S., Kain, R. M. & Oldfield, J. W. (1984). The effect on environmental variables on crevice corrosion of stainless steels in seawater. Materials Performance, 7, 9-15. Little, B., Wagner, P. & Duquette, D. (1987). Microbiologically induced cathodic depolarization. Corrosion~87, National Association of Corrosion Engineers Publications, San Francisco, CA, Paper No. 370. Mollica, A. (1990). Corrosion d'alliages a transition actif-passif en eau de mer. Mat~riaux et Techniques, 12, 17-24. Mollica, A. & Trevis, A. (1976). Correlation entre la formation de la pellicule primaire et la modification de la cathodique sur des aciers inoxydables exprrimentrs en eau de mer aux vitesses de 0.3 h 5.2 m/s. Proc. 4th Int. Congress on Marine Corrosion and Fouling, Ed. Centre de Recherches et d'Etudes Ocranographique, Boulogne, France, 351-65. Mollica, A., Trevis, A., Traverso, E., Ventura, G., Scotto, V., Alabiso, G., Marcenaro, G., Montini, U. & De Carolis, G. (1984). Interaction between biofouling and oxygen reduction rate on stainless steel in seawater. Proc. 6th Int. Congress on Marine Corrosion and Fouling, Athens, Greece, pp. 269-81. Organised by Int. Committee for Research of the Prevention of Marine Materials (COIPM), Bruxelles. Mollica, A., Trevis, A., Traverso, E., Ventura, G., De Carolis, G. & Dellepiane, R. (1988a). Crevice corrosion resistance of stainless steels in natural seawater in the temperature range of 25 to 40°C. Corrosion, 44, 194-8. Mollica, A., Ventura, G., Traverso, E. & Scotto, V. (1988b). Cathodic behaviour of nickel and titanium in natural seawater. International Biodeterioration, 24, 221-30. Mollica, A., Trevis, A., Traverso, E., Ventura, G., De Carolis, G. & Dellepiane, R. (1989). Cathodic performance of stainless steels in natural seawater as a function of microorganism settlement and temperature. Corrosion, 45, 48-56. Mollica, A., Traverso, E. & Ventura, G. (1990a). Electrochemical monitoring
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of the biofilm growth on active passive alloy tubes of heat exchanger using seawater as cooling medium. Proc. llth Int. Corrosion Congress, Italian Association of Metallurgy, Milano, Italy, Vol. 4, pp. 341-9. Mollica, A., Ventura, G. & Traverso, E. (1990b). On the mechanism of corrosion induced by biofilm growth on the active-passive alloys in seawater. Int. Congress on Microbially Influenced Corrosion, Ed. N. J. Dowling, M. W. Nittleman & J. C. Danka, University of Tenessee, Knoxville, TN. 2.25-2.31. Mor, E. D., Scotto, V. & Mollica, A. (1980). Contribution to the discussion on localized corrosion of stainless steels in natural seawater. Werkstoffe und Korrosion, 31, 281-5. Nivens, D. E., Nichols, P. D., Henson, J. M., Jeesey, G. G. & White, D. C. (1986). Reversible acceleration of the corrosion of AISI 304 stainless steel exposed to seawater induced by growth and secretions of the marine bacterium Vibrio natriegens. Corrosion, 42, 204-10. Pope, D. H., Duquette, D. J., Johannes, A. H. & Weyner, P. C. (1984). Microbiologically influenced corrosion of industrial alloys. Materials Performance, 23, 14-18. Schiffrin, D. J. & De Sanchez, R. S. (1985). The effect of pollutants and bacterial microfouling on the corrosion of copper base alloys in seawater. Corrosion, 41, 31-48. Scotto, V., Di Cintio, R. & Marcenaro, G. (1985). The influence of marine aerobic microbial film on stainless steel corrosion behaviour. Corros. Sci., 25, 185-94. Scotto, V., Alabiso, G. & Marcenaro, G. (1986). An example of microbiologically influenced c o r r o s i o n - - T h e behaviour of stainless steels in natural seawater. Bioelectrochemistry and Bioenergetics, 16, 347-55. Scotto, V., Aiabiso, G., Beggiato, M., Marcenaro, G. & Guezennec, J. (1990). Possible chemical and microbiological factors influencing stainless steel microbially induced corrosion in natural seawater. Proc. 5th Eur. Congress on Biotechnology, eds. C. Christiansen, L. Murnck & J. Urlladsei, Munksgard, Int. Publishers, Copenhagen, pp. 866-71. Ventura, G., Traverso, E. & Mollica, A. (1988). Microfouling control procedures as anticorrosion methods of stainless steels in natural seawater. Microbial Corrosion-l, Ed. C. A. Sequeira & A. K. Tiller. Elsevier Applied Science, London, pp. 235-9. Ventura, G., Traverso, E. & Mollica, A. (1989). Effect of NaCIO biocide additions in natural seawater on stainless steel corrosion resistance, Corrosion, 45, 319-25. Wallen, B. (1990). Some factors affecting stainless steel corrosion in seawater. Avesta Corrosion Management, (ACOM). 4.
Biofilm and Corrosion on Active-Passive Alloys in Seawater
A. Mollica Institute for Marine Corrosion of Metals (ICMM) Via Scarsellini, 10, 16149 Genova, Italy
ABSTRACT This paper is a summary of work carried out at the ICMM and deals with the effect of biofilm growth on the marine corrosion of stainless steels and other active-passive alloys. There is evidence regarding the important roleplayed by aerobic bacterial settlement, both on the onset and the propagation of localized corrosion on such alloys and on the enhancement of galvanic corrosion of less noble materials coupled with them. These effects are the consequence of the oxygen reduction depolarization induced by biofilm growth. Hypotheses on the mechanisms by which the presence of the biofilm causes oxygen reduction depolarization, and prevention systems against the microbial induced corrosion, are also discussed.
INTRODUCTION S o m e years ago a g r o u p o f researchers did a b i b l i o g r a p h i c s t u d y to try to a n s w e r s o m e o f the following questions:
- - D o e s sufficient p r o o f exist to affirm that metal materials, o f a n y type, are s u b j e c t e d to ' m i c r o b i a l i n d u c e d corrosion' (MIC)? - - I f so, w h a t type o f materials are involved? - - W h a t types o f m i c r o - o r g a n i s m are involved? 213 International Biodeterioration & Biodegradation 0964-8305/92/$05.00© 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
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- - W i t h what type of mechanisms? - - W h a t types of prevention are needed? etc. After reading the results of this inquiry (Pope et al., 1984), the feeling is that, except for the corrosion of ferrous alloys by sulphate reducing bacteria and some specific cases like the corrosion of a l u m i n i u m in kerosine tanks, a scarce and inconclusive documentation exists for entire classes of alloys of industrial interest. There is more hypothesis than data for stainless steels and nothing for N i - C r alloys, Ti, etc. In the last few years, some literature has appeared which gives different conclusions about MIC. This paper is a summary of recent works done at the I C M M (Institute for Marine Corrosion of Metals) in order to answer the above mentioned questions with regard to MIC in seawater on stainless steels and other active-passive alloys. Most of the results have been obtained through field tests.
C O R R O S I O N STUDIES Effect of the presence of biofilm on the onset of crevice corrosion on AISI 316 SS (stainless steel) in seawater
Once exposed to seawater, the process of biofilm formation on the stainless steel surfaces starts. The absorption of organic substances, the settlement (first reversible and then irreversible), of pioneer bacteria (incubation phase), their growth in colonies, the development of new species (micro-organisms and macro-organisms) etc., are steps in this process. The biofilm, certainly aerobic in the first phases of formation, can become locally anaerobic during its development, in particular where crevices are present (Mollica et al., 1988a). As a consequence, when studying the crevice corrosion, (the most frequent type of localized corrosion on stainless steel in seawater), it must be taken into account that the onset of this type of corrosion starts in the presence of both a mainly aerobic biofilm on the areas freely exposed to seawater and a high concentration of sulphides (200-400 ppm), generated by the sulphate reducing bacterial activity, on the shielded areas. Usually, the minor resistance of stainless steels in a natural environment in respect to sterile seawater is attributed to the sulphate reducing bacterial activity on shielded areas. Recently, we tried to deduce the role of the biofilm and its 'components', aerobic and anaerobic, on the onset of crevice corrosion on AISI 316 stainless steel (SS).
Biofilm and corrosion on active-passive alloys in seawater
215
For this purpose, a simple experiment was organized, described in detail elsewhere (Mollica, 1990). The experiment consisted of the exposure of a series of galvanic couples each formed by a large stainless steel surface coupled, through a resistor, to a second small shielded surface. This assembly is known as a 'remote crevice assembly'. In the case of onset o f crevice corrosion, the two surfaces will play, respectively, the role of cathodic and anodic elements o f the couple, a n d a clear galvanic current will flow through the resistor. By taking timed readings of the galvanic currents, it is possible to determine, for each couple, w h e n the crevice corrosion starts and, therefore, to construct a graph which reports the 'survival' S(t) as a function of the exposure time t where
S(t)
=
[ N t o t couples - -
Ncorroded(t)]/Ntot
couples
We can take advantage of the geometric separation of the shielded and unshielded areas, deciding to couple the two elements after an eventual pre-exposure of each element. It is possible, for example, to pre-expose the cathodic surface to natural seawater until a biofilm has settled on them, or to pre-expose the anodic surface in marine anaerobic m u d rich in sulphide, and so on. In this manner, a set of combinations at the m o m e n t of the coupling can be realised. For each of these combinations, a survival-time curve can be obtained. In Fig. 1 four different shapes of the S(t) curve dependent on the preexposure conditions are shown, as examples. In this way the experiment aimed to evaluate by a stochastic approach the influence exerted on the crevice corrosion onset ofAISI 316 by the presence or absence of aerobic biofilm on the cathodic areas; and the presence or absence of biosulphides on the anodic areas. The result of these tests, carried out on about 400 samples, was that each S(t) experimental curve m a y be considered as a specific case of the general stochastic distribution proposed by Weibull:
S(t)
j" 1 [ exp ( - ( t - O)/a)
t< 0
t>0
in which a depends on the pretreatment of the shielded surface (passivation in air, in HNO3 etc.); /3, as well as by the surface pretreatment, is dependent on the presence or absence ofsulphides on the shielded areas (13 tends to 1 in the presence of sulphides);
216
A. Mollica 10
0.1
o
t (days)
~.
Fig. 1. Percentage of survived AISI 316 SS couples (S(t)) to crevice corrosion (from Mollica, 1990). Each couple was made of a AISI 316 shielded surface (anode) coupled with an unshielded surface (cathode). Curves 1 to 4 show different shapes of the curve S = f(t) as a function of pre-exposure conditions of the anodic and cathodic elements before coupling and of the surface treatment of the anodic element. (1) Cathodic surface pre-exposed to natural seawater. Anodic surface passivated in air and pre-exposed in anaerobic mud. (2) Cathodic surface not pre-exposed in natural seawater. Anodic surface passivated in air and pre-exposed in natural seawater. Anodic surface passivated in HNO3 and not pre-exposed in anaerobic mud. (3) Cathodic surface pre-exposed in anaerobic mud. (4) Both cathodic and anodic surfaces immersed in sterile seawater. Anodic surface passivated in HNO3. 0 represents the i n c u b a t i o n time o f the aerobic b i o f i l m on the c a t h o d i c areas. Thus, the following surprising c o n c l u s i o n s a b o u t the role p l a y e d by the b i o f i l m in regard to the onset o f the crevice corrosion was reached: (a) T h e presence o f sulphides, arising f r o m the activity o f a n a e r o b i c bacteria o n the a n o d i c surfaces, plays o n l y a s e c o n d a r y role. (b) O n the contrary, the presence o f aerobic b i o f i l m o n the c a t h o d i c surfaces plays a d e t e r m i n i n g role: the corrosion starts r a p i d l y (S(t) < 1) o n l y w h e n the b i o f i l m has developed (t > 0). Two questions follows from this: (a) W h y does the growth o f the aerobic b i o f i l m o n the c a t h o d i c areas d e t e r m i n e the p r o b a b i l i t y o f corrosion onset o n the a n o d i c areas? (b) W h e n crevice corrosion has started, h i g h e r corrosion rates in n a t u r a l seawater t h a n in sterile seawater were often observed (Mollica & Trevis, 1976; Lee et al., 1984; Nivens et al., 1986; Holte et al., 1988a). So t h a t it c a n be a s k e d w h e t h e r the aerobic b i o f i l m o n
Biofilm and corrosion on active-passive alloys in seawater
217
the cathodic areas is also responsible for the different propagation rate? Effect of aerobic biofilm on corrosion of the S S s
Some years ago, a p h e n o m e n o n was observed, which was confirmed and studied in more detail later: it was observed that depolarization of the oxygen reduction on SSs, during their exposure to natural seawater (Fig. 2), occurred contemporaneously with the biofilm growth on the SS surfaces. F r o m this p h e n o m e n o n a series of practical consequences of SSs corrosion follows which are described below.
Onset of localized corrosion On immersion in the sea, a SS on which, in time, a biofilm forms; it is found that during the period of the biofilm growth the free corrosion potential of SS in the passive state (curve 'p' in Fig. 2) increases from Eco~r~ to Eco~r2. The e n n o b l e m e n t of the free corrosion potential during the exposure to seawater of SSs in the passive state has actually been observed by ES.C.E.(v) Ecorr2r . . . .
'\ X
0.3
N\ \ \ x \\
0.I
\
-0.I
Ecorrli -O'iI~
8
l -i
;4I'
corr 1
tc o r r 2
I
ILog i (^ • ~ m ~ )
Fig. 2. Oxygen reduction rates measured on SSs exposed to quiet natural seawater. (Mollica, A., & Montini, U., unpublished data, 1973). Curve (l) is obtained within 1 to 2 days of immersion; curve (2) is obtained after 12 days of immersion.
218
A. Mollica
m a n y authors (Mollica a al., 1976, 1984, 1989; K r o u g h m a n & Ijsseling, 1980; M o r et al., 1980; J o h n s e n & Bardal, 1985, 1986; Scotto et al., 1985, 1990; Desestret, 1986; Dexter et al., 1988, 1990; Gallagher et al., 1988). It was shown that the m a x i m u m level o f the free corrosion potential which can be reached by SSs in natural seawater is about 300 to 400 m V higher t h a n those in sterile seawater. Such possible e n n o b l e m e n t , due to the biofilm growth on cathodic areas, is a powerful factor of localized corrosion initiation for m a n y types of SSs; only very high quality SSs can sustain it without onset of localized corrosion (Desestret, 1986), particularly w h e n defects, like crevices, are present on their surfaces.
Propagation o f localized corrosion Suppose that on the SS some form of localized corrosion has already started (active state). The corrosion rate will be determined by the intersection of the anodic polarization curve a n d the active surface (curve 'a" in Fig. 2), a n d the oxygen reduction curve on the s u r r o u n d i n g cathodic areas. Taking into account that in the sea the corrosion potential of SSs in the active state is in general (Mor et al., 1980) over - 2 0 0 mV vs. the SCE, the scheme in Fig. 2 shows that the presence of biofilm on the cathodic areas produces an increase (from icor~l to icorr2) of the corrosion rate by about two orders of m a g n i t u d e (Lee et al., 1984; Ventura et al., 1989). Galvanic corrosion A scheme similar to that m e n t i o n e d above can be used to show h o w the presence of biofilm on SS surfaces stimulates the corrosion of less noble materials c o u p l e d with them. A n increase of the galvanic current, close to one order of magnitude, was often observed on the SS-iron couples during the biofilm formation on SS surfaces (Mollica et al., 1984, 1990). It is c o n c l u d e d that the presence of aerobic biofilm o n SS surfaces exposed to seawater, depolarizing the oxygen reduction, provokes:
an increase of 300 to 400 m V of the m a x i m u m reachable potential, promoting, as a consequence, a more probable onset of localized corrosion; --an increase, up two orders of magnitude, of the corrosion rate w h e n localized corrosion is in progress; a n d - - a n increase of the galvanic currents between SSs a n d other less noble materials coupled with them.
Biofilm and corrosion on active-passive alloys in seawater
219
Effect of aerobic biofilm on other active-passive alloys The correlation between the biofilm formation and the depolarization of the oxygen reduction was verified not only on stainless steels, independently of their composition, but also on other active-passive alloys like Ti, Ni-Cr, C u - N i and N i - C u ( K r o u g h m a n & Ijsseling, 1980; Schiffrin & De Sanchez, 1985; Dexter & Lin, 1988; Holte et ai., 1988; Mollica et al., 1988b). Figure 3 shows as an example the effect of the biofilm formation on a 70Ni-30Cu and a Ti alloy. In the case of the N i - C u alloy, the biofilm provokes an easy onset and a rapid propagation of localized corrosion; on the Ti alloy the presence of biofilm caused corrosion problems only in the case of galvanic coupling. The conclusion is drawn that the biofilm can therefore produce one or more of the above mentioned effects depending on the kind of activepassive alloy. Composition of biofilm and corrosion The word biofilm is a very general term which includes m a n y 'components': bacteria, algae, cellular exudates, organic and inorganic matter coming from the bulk of the solution and trapped in the mucilage, etc. In order to establish what 'component' of the biofilm correlates with the p h e n o m e n o n of the depolarization of the oxygen reduction several tests have been made. The first test consisted in the exposure to seawater of SSs able to
400-
mV (z.c.e.)
-500
Fig. 3. Anodic dissolution curves of Ti (3) a n d 70Ni30Cu alloy (4) specimens reported with the cathodic polarization curves measured on their surfaces in the absence (1) and presence (2) of biofilm (Mollica et al., 1988b).
220
A. Mollica
m a i n t a i n the passive state and, once ennobled, the free corrosion potential. A substance like s o d i u m azide, able to kill the forms of life present in the biofilm without causing the d e t a c h m e n t of the biofilm itself, was a d d e d in the solution (Scotto et al., 1985). The result was that the elimination of the living part of the biofilm brought the potentials back to the levels m e a s u r e d in sterile water. This result, c o n f i r m e d by other m e a n s (Mollica et al., 1984), indicates that the m o d i f i c a t i o n of the electrochemical parameters is correlated more to the living fraction t h a n to the total mass of the biofilm. With the aim of verifying if the presence of algae inside the biofilm is necessary or not, several SS samples were exposed to the seawater u n d e r different conditions of illumination. D u r i n g the tests the free corrosion potentials were m e a s u r e d a n d the biomass was characterized by m e a n s of c o n t e m p o r a n e o u s m e a s u r e m e n t s of chlorophyll a a n d of the electron transport system (ETS) activity, tied to the respiratory chain present in every cell (Mollica et al., 1984). The increase of potential was always observed, both w h e n the algae were d o m i n a n t (ETS = 3.28 Chla) a n d w h e n these were practically absent (Chla = 0). It was c o n c l u d e d that the m o d i f i c a t i o n of the oxygen reduction kinetics is to be correlated only to a bacterial settlement. Finally, it was shown (Lee et al., 1984; Scotto et al., 1986; Holte et al., 1988a; Mollica et al., 1989) that an increase in the seawater temperature near to 40°C erases the modifications of the oxygen reduction curve i n d u c e d by the biofilm and, therefore, its effects on corrosion. It suggests that the bacteria involved are not of a t h e r m o p h i l i c type. The conclusion is that the depolarization of the oxygen reduction a n d its consequences on the corrosion of active-passive alloys in seawater are c o n n e c t e d to the settlement on their surfaces of a sufficient n u m b e r of aerobic n o n t h e r m o p h i l i c bacteria. By what m e c h a n i s m does the bacterial settlement influence the oxygen reduction kinetics? Hypotheses about the mechanisms through which a bacterial settlement causes the depolarization of the oxygen reduction
There are different hypotheses to explain h o w the bacterial settlement brings about the depolarization of the oxygen reduction: - - T h e presence of the biofilm provokes a decrease of the p H on the surfaces and, therefore, a shift of 60 mV/(pHbulk -- PHsurface) in the noble direction of the visible b r a n c h of the cathodic curve (Dexter et
Biofilm and corrosion on active-passive alloys in seawater
221
al., 1988). Furthermore, the acidification of the surface can produce calcareous deposit causing dissolution and cathodic depolarization (Little et al., 1987). - - T h e exchange density current of the oxygen reduction increases because of the effect of a catalyst generated by the bacterial metabolic activity. On the nature of such a catalyst, there are different opinions (Ford & Mitchell, 1990; Gallagher et al., 1988; Holteetal., 1988; Shiffrin & De Sanchez, 1985; Scottoetal., 1985). To select between the two proposed mechanisms (acidification or catalysis) several tests were done (Mollica et al., 1990b), two of which are briefly described.
Effect of seawater acidification Seawater continuously renewed and acidified at different pH, by suitable additions of HC1, was p u m p e d through a series of high quality SS pipes; on these samples the free corrosion potential was periodically measured and the biofilm was observed. The pH range studied from the natural pH of seawater was 8.1-8.2 down to about pH 5. At the end of the experiment, which lasted 40 days, the presence of biofilm was observed on all exposed samples. In Fig. 4(a), three examples of the evolution of the free corrosion potential at different seawater pH are shown; in Fig 4(b), the free corrosion potentials measured during the exposure on all the tested samples are reported as a function of the seawater pH imposed. The data indicate that the e n n o b l e m e n t of the potential due to the biofilm growth can only be observed at the pH of untreated natural seawater. It is sufficient to eliminate the buffering properties of the seawater to eliminate the effect of the biofilm; also further acidification produces an enoblement of the free corrosion potentials, but in this case only due to the decrease solution pH. Effect of continuous cathodic polarization on the oxygen reduction current In Fig. 2, two cathodic curves showing the effect of biofilm growth are shown. It can be seen that the cathodic curves from an envelope of points, each of them obtained in potentiostatic mode. The potentiostatic curves shown in Fig. 2 are schematically drawn in Fig. 5(a). It was noted that, if we want to obtain rapidly the oxygen reduction curve, tracing it in a potentiodynamic way starting from SS samples in a passive state already settled by bacteria, a completely different cathodic curve, similar to curve 3 in Fig. 5(a), will be obtained.
222 400
A. Mollica
mV (sce)
400
~ii
200
i:"
m
untreated seawater
mV (scc)
,.
v v
•
v
pH = 7 . 5
-200
•
o
•
ee
200
pH = 5 8
Yv
•
0 !
I
bl
o!
o
!
-200 10
days
50
6
(a)
pH
8
(b)
Fig. 4. Effect of the seawater acidification on the free corrosion potentials of SSs in passive state (Mollica et al., 1990b). (a) Three examples of the free corrosion potential trend vs. time in different seawater pH; (b) summary of the free corrosion potentials measured independently of the time in function of the seawater pH.
In addition, if we stop the p o t e n t i o d y n a m i c excursion at a given potential Es, the oxygen reduction c u r r e n t increases with time towards the c o r r e s p o n d i n g value o n the 'potentiostatic' curve 2. A similar route in the E-log(ic) plot is s p o n t a n e o u s l y followed d u r i n g the first stages o f localized corrosion p r o p a g a t i o n (Mollica et al., 1989). We studied the transient p h a s e between curves 3 a n d 2 polarizing, at four different potentials, SS samples a l r e a d y covered by biofilm. T h e result (Fig. 5(b)) was that, i n d e p e n d e n t o f the i m p o s e d potential, the o x y g e n r e d u c t i o n c u r r e n t increases, initially, with a rule o f this type: log(it) = a t + fl
(1)
or, by derivative: d(ic)/dt
= a ic
(2)
a n d the final values are r e a c h e d after several days o f c o n t i n u o u s polarization. E q u a t i o n (2) shows that the oxygen r e d u c t i o n depolarization on surfaces covered by biofilm is a n autocatalysing p h e n o m e n o n , since the increase in oxygen r e d u c t i o n rate is proportional to the i n s t a n t a n e o u s r e d u c t i o n current. E q u a t i o n (2) c o u l d also be r e a d in the following way: the bigger the i n s t a n t a n e o u s p r o d u c t i o n o f O H - o n the electrode
Biofilm and corrosion on active-passive alloys in seawater
mV
223
(sce)
o
i (~.~-,)
Es - - ~ -200
(~
Fig. 5. (a) Schematic description of possible oxygen reduction curves which m a y be obtained on SSs in natural seawater (Mollica et al., 1990b): (1) oxygen reduction curve on SSs not covered by biofilm; (2) oxygen reduction curve obtained by potentiostatic way on SSs covered by biofilm; (3) oxygen reduction curve obtained by potentiodynamic way on SSs covered by biofilm. The arrow shows the increase in time of the oxygen reduction current when the potentiodynamic polarization is stopped at a given potential E s.
i (ia~.¢. -z)
10 1
,,,~
0.1 , ,,,'(" ,lp~"
7 0\ \
j~g~ ic.,~t.~
\ \\
\
mV(sce)~ 1 2 3 4 5 t(days) (b) Oxygen reduction current as a function of the imposed cathodic potential a n d time on SS surfaces covered by biofilm.
224
A. Mollica
surfaces for the reduction of 02 the faster the increase of the cathodic current. The data in this paragraph, and other literature (Holte et al., 1988; Mollica et al., 1990b), suggest that the electrochemical effects induced by the biofilm formation cannot be explained through a mechanism which foresees the acidification of the surface. Taking also into account that, according to previous observations (Nivens et al., 1986), a correlation was shown between the modification of the electrochemical parameters and the increase of the superficial concentrations of bacterial exopolymers (Scotto et al., 1990), the following conclusion can be drawn: The most suitable hypothesis to explain the phenomenon of the oxygen reduction depolarization induced by biofilm growth on activepassive alloys in seawater seems to be at the moment that of a catalyst, produced by settled bacteria probably linked to their exopolymers, whose efficiency increases by raising above 8 the pH on the surface.
Prevention of MIC on active-passive alloys in seawater
Cathodic protection and biocide addition are the most suggested anticorrosive measures in the case of MIC. We studied, in particular, the application of these types of measures, separately or applied together, for the protection of heat exchanger pipes against the corrosive effects of the biofilm growth. Cathodic protection
Stainless steel pipes, through which seawater once flowed, were connected to iron anodes placed in the water box; during the test the galvanic currents and the potentials at fixed points inside the pipes were measured (Fig. 6) (Mollica et al., 1984). The data show that, because of the biofilm growth on the pipe walls, the consumption of the sacrificial anode increases by one order of magnitude. As a consequence, the increased ohmic drop inside the pipe reduces the protected pipe length to less than 0.5 m. The only cathodic protection is, therefore, not sufficient to protect the whole tube nest against the detrimental effect of the biofilm. Biocide addition
Sodium hypochlorite was studied since it is used widely in cooling systems; in particular the effect of NaC10 used in continuous doses.
Biofilm and corrosion on active-passive alloys in seawater
225
Biomass as a function o f residual chlorine concentration. A set of SS pipes was exposed for 30 days to o n c e - t h r o u g h flowing sea water c o n t i n u o u s l y treated with different a m o u n t s of N a C 1 0 and, at the e n d of exposure, the protein content on their internal walls was m e a s u r e d (Mollica et al., 1990a). T h e protein analyses s h o w that a c o n t i n u o u s dose of residual chlorine of >/0.1 p p m prevents the growth of a viable biomass.
Free corrosion potential o f SSs in passive state as a function o f the residual chlorine concentration. O n a series of SS samples in the passive state, exposed in the same way as the above m e n t i o n e d conditions, the free corrosion potential trend was followed (Mollica et al., 1990a). At the e n d of exposure the m i n i m u m free corrosion potential was observed for a concentration of residual chlorine equal to 0.1 p p m , c o r r e s p o n d i n g to that strictly sufficient to prevent the biofilm formation. At higher concentrations, the free corrosion potential rises again rapidly, a s s u m i n g a concentration close to 0. 2 to 0. 3 p p m at values above those a s s u m e d by the same steels in untreated seawater.
Propagation of localized corrosion as a function o f residual chlorine concentration. Galvanic couples between high quality SS pipes a n d shielded AISI 316 SS samples were exposed, for 75 days, to once-through flowing seawater treated with different a m o u n t s of NaC10, a n d the crevice corrosion p r o p a g a t i o n rate on AISI 316 samples was followed by m e a s u r i n g the galvanic current (Ventura et al., 1988, 1989). T h e m i n i m u m corrosion rate was observed in the presence of a residual chlorine concentration (0-1-0-2 p p m ) close to that strictly sufficient to h i n d e r biofilm formation: the corrosion rate was about two orders of m a g n i t u d e less t h a n that of untreated seawater. Even in the presence o f higher doses o f residual chlorine (1-2 ppm), the p r o p a g a t i o n rate is, however, considerably lower t h a n in untreated seawater. Similar conclusions have been reported in the literature (Wallen, 1990). C o n c e r n i n g the use of c o n t i n u o u s NaCIO additions as the only measure against MIC, we can therefore conclude that the use of N a C 1 0 in c o n t i n u o u s additions, on the o n e h a n d reduces the p r o p a g a t i o n rate of the localized corrosion on the other h a n d it tends to facilitate the onset if the concentration of residual chlorine is over that strictly sufficient to prevent the biofilm formation.
A. Mollica
226
mVs.C.E. 200 /
/
/'J
~
~wilh
slime
/ - ~ lg = 6.2 mA 200 lg = 0.65 m A _.-.-""
~ . w i t h o u t slime
-500 S.C.E. I0
50
1
cm
n Iron "~{SS~ - j
Stainless Steel
Fig. 6. Potential profiles inside a SS pipe cathodically protected by an iron anode in the absence (1) and presence (2) of biofilm on the SS surfaces. (Elaboration from Mollica et al.. (1984.)
Mixed biocide addition-cathodic protection system To examine the effect of a mixed protection system, SS pipes were exposed to once-through flowing natural seawater treated with different amounts of NaC10 and, at the same time, cathodically protected by iron anodes placed in the water box (Mollica et aL, 1990a). Galvanic currents between the SS pipes and iron anodes and potential profiles inside the pipes were followed during the exposure, which lasted 30 days. According to the results plotted in Fig. 6, the data confirm that the elimination of the biofilm through the continuous presence of a residual chlorine concentration close to 0.1 ppm reduces by one order of magnitude the anode consumption, increasing at the same time the protected pipe length, and, in addition, shows that an excess of chlorine (in the range 0-1-0.8 ppm) does not induce further increases of the galvanic currents.
The application of a mixed biocide addition-cathodic protection system appears to be the optimum solution for tube banks combining low anode consumption with high penetration of the protection inside the pipes and not requiring an exact control of the biocide dosage.
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CONCLUSIONS 1. The biofilm growth induces a depolarization of the oxygen reduction on the surface of a series of active-passive alloys, like SSs, Ti, Ni-Cr, Cu-Ni, N i - C u alloys, etc., exposed to seawater. 2. The presence of biofilm on the surfaces, via the oxygen reduction depolarization, causes in turn: - - a greater probability of localized corrosion onset; - - a faster propagation rate of localized corrosion in progress; m an increase of the galvanic currents between the above mentioned alloys a n d other less noble ones coupled with them. 3. The hypothesis of a catalyst produced by non-thermophilic bacteria a n d whose efficiency increases by increasing the p H above 8 seems, at present, the most suitable to explain the p h e n o m e n o n of the oxygen depolarization induced by biofilm growth. 4. If only continuous NaC10 additions are used to prevent MIC on active-passive alloys in seawater, the residual chlorine concentration must be carefully controlled and maintained to a level very close to 0.1 ppm, which is sufficient to hinder the biofilm formation. 5. For MIC prevention on banks of pipes the best solution is a mixed cathodic protection-biocide addition system; such a system permits c o n t e m p o r a n e o u s l y a low c o n s u m p t i o n of the sacrificial anodes, a high penetration of the protection inside the pipes a n d an indifference to an eventual residual chlorine overdose.
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