Stainless steel ennoblement in freshwater: From exposure tests to mechanisms

Corrosion Science 50 (2008) 2342–2352

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Stainless steel ennoblement in freshwater: From exposure tests to mechanisms Cyril Marconnet a,*, Catherine Dagbert a, Marc Roy b, Damien Féron b a b

Laboratoire de Génie des Procédés et des Matériaux, Ecole Centrale Paris, Grande Voie des Vignes, 92290 Chatenay-Malabry, France Service de Corrosion et du Comportement des Matériaux dans leur Environnement, CEA Saclay, Bât. 458, 91191 Gif-Sur-Yvette, France

a r t i c l e

i n f o

Article history: Received 4 March 2008 Accepted 15 May 2008 Available online 23 May 2008 Keywords: A. Stainless steel B. Potentiostatic B. Polarization B. SEM C. Microbiological corrosion

a b s t r a c t This work confirms the systematic character of the open circuit potential (OCP) ennoblement for stainless steels immersed in natural freshwater, and highlights the mechanism(s) responsible for this evolution. To achieve these results, electrochemical measurements and analysis/observations of the surface were realized in situ during exposure tests in the river Seine on two exposition sites using an original immersion device. Electrochemical results show that on both immersion sites a new oxidizing compound is produced close to the surface and that its reduction, occurring at potentials higher than the initial OCP value, leads systematically to the ennoblement phenomenon. Surface analysis, electrochemical and chemical tests show that the oxidizing compound is not the same on both sites: on one site hydrogen peroxide is produced within the biofilm, while on the second one oxidized manganese is deposited on the surface. Thus the two mechanisms mentioned in the literature can occur on stainless steels immersed in the same water and lead to similar evolutions of the electrochemical behaviour. These two mechanisms are not specific of a type of water and seem to be complementary rather than opposed. They are both based on enzymatic processes. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The surface of a metallic material immersed in natural water is progressively colonized and covered by a consortium of microorganisms (bacteria, algae, fungi) and their exopolymers (lipids, polysaccharides, proteins, nucleic acids. . .), which altogether form a complex biological matrix called biofilm [1–2]. The metabolism of microorganisms enclosed in the biofilm induces changes in the local physico-chemistry at the material/biofilm interface, which may influence the corrosion processes. Previous experiments performed on stainless steel (SS) samples immersed in natural seawater have shown that the open circuit potential (OCP, or free corrosion potential Ecorr) increases during the exposition from between 100 and 50 mV/SCE to between +100 and +400 mV/SCE, and that this shift towards the positive direction is due to the settlement and the development of a biofilm on the surface of the samples [3–10]. This increase is sometimes called ennoblement. It is generally associated with an increase of the cathodic reaction rate [11–12], that is to say either the catalysis of the pre-existent cathodic reaction (O2 reduction in aerated waters), or the appearance of the reduction of a new oxidizer produced by the biofilm. In natural seawater, this new oxidizing compound is very often considered to be hydrogen peroxide H2O2, whose formation could be catalysed by oxidase-type enzymes. * Corresponding author. Tel.: +33 141674105; fax: +33 141674103. E-mail address: [email protected] (C. Marconnet). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.05.007

These enzymes would thus generate an acidic and oxidizing local environment leading to the OCP increase [3–5,8,13–17]. This enzymatic model has been validated in vitro: when glucose-oxidase (GOD) and its substrate D-glucose are added in artificial seawater (international norm ASTM D1141-90), the OCP increases in the same way as in natural seawater [15]. Besides, hydrogen peroxide has been found in natural seawater biofilms at a concentration of about 1 mmol/L [4–5,14]. Immersion of stainless steel samples in waters with lower chloride concentration, either artificial (waste water in treatment plants, sewage water) or natural (rivers, lakes, estuaries), has also been carried out in the literature. Results show that the global trend is an increase of the OCP to values between +250 and +400 mV/SCE approximately [6,18–23]. The ennoblement also seems to occur in fresh waters. Nevertheless, in a few exposition tests, the OCP does not evolve with time [22,24]. The systematic aspect of the ennoblement has not yet been fully demonstrated in natural freshwater. When it occurs, the increase of the OCP is linked to an increase of the kinetics of the cathodic processes [20,25–26] – as in seawater. A model involving the synthesis of a new oxidizer by and within the biofilm has been thoroughly exposed in the literature. In natural freshwater, this new oxidizing compound is very often considered to be manganese oxide MnO2 and hydroxide MnOOH. The reduction of these surface deposits at high potential would lead to the OCP increase [20,26–32]. This model has been validated in vitro: both the electrodeposition of manganese oxides [20,33] and the immersion in a culture medium

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inoculated with a Leptothrix discophora mangano-oxidizing bacteria strain [30,34] induce an OCP increase of SS samples similar to the one measured in natural freshwater. The enzymatic model of production of hydrogen peroxide within the biofilm is often thought to be specific of seawater, while the biodeposition of manganese oxides is generally associated to river water. The electrochemical behaviour of stainless steel samples immersed in fresh river water is investigated in this paper. In situ OCP measurements are carried out to confirm the ennoblement phenomenon. Electrochemical cathodic polarization curves are performed before and after ennoblement to examine the factors possibly explaining the OCP increase. In the end, chemical tests and surface analysis/observations lead to the identification of the processes causing the OCP evolution. 2. Experimental 2.1. Specimens and surface preparation The aim of this work is to study the electrochemical behaviour of two austenitic stainless steels (AISI 304 L and 316 L) and one superaustenitic stainless steel (254SMO) during their immersion in natural freshwater. The chemical composition and the UNS denomination of these alloys are given in Table 1. Samples used for OCP measurements are 100  25  1 mm plates, in contact with a wired stainless steel hook allowing the transmission of the signal. Before testing, they are immersed one hour in a bath containing 20% of nitric acid at a concentration of 65% and 2% of fluorhydric acid at a concentration of 40%. This surface treatment provokes the dissolution of non-metallic inclusions and an enrichment in chromium within the passive layer. Samples are afterwards rinsed with distilled water and stored in contact with the air until the immersion (storage time superior to 5 days). Samples used for polarization curves are 30  10  1 mm plates, welded with a copper wire to make the electrical contact, covered with a protective paint by cataphoresis and embedded in an epoxy resin (Epofix, Struers), leaving only the work surface in contact with the electrolyte, so as to avoid crevice corrosion. Samples are then mechanically ground with silicon carbide (SiC) emery papers down to 1200 grit and polished with diamond pastes down to 1 lm. They are ultrasonically rinsed in ethanol between each step. 2.2. Electrolyte Two sites have been chosen on the river Seine: Choisy-le-Roi upstream and Flins downstream from Paris, respectively named

‘‘site 1” and ‘‘site 2”. Mean values of the main physico-chemical parameters have been recorded over a 2-year period (see Table 2, where TOC stands for total organic carbon and SM for Suspended Matter in solution). Immersion depth is approximately 1 m. Water temperature is regularly measured. 2.3. Immersion device Specimen holders have been designed and produced to immerse 20 stainless steel samples and to measure continuously their free corrosion potential with a HP Agilent 34970A multiplexer, controlled by the Benchlink Datalogger software. Four reference electrodes are used to measure the potentials (one reference for five samples): three saturated calomel Hg2Cl2 (s)/Hg(l) electrodes (SCE) and one Hg/Hg2SO4 electrode. Two kinds of reference electrodes are used so as to avoid a hypothetical reference problem which could occur during the experiment. All potentials are reported here versus SCE. Epoxy-embedded samples used for dynamic measurements are also introduced in this immersion device. The electrical contact between the hook and the wire is made by a lug welded on the hook. The weld is covered by an insulating resin (Combisub T150, Chryso Resipoly). 2.4. In situ electrochemical measurements OCP measurements are carried out continuously, at a frequency of one point per hour for each channel, except once, for the immersion realised on April 3rd on site 1, during which the values are collected every 10 min. Cathodic polarization curves are performed in a conventional 3electrode electrochemical cell with a Radiometer PGZ 301/402 potentiostat. The counter electrode is a platinized titanium grid and the reference a saturated calomel electrode (SCE). These dynamic experiments are not realised directly in situ. Samples are collected and immediately reimmersed in the Seine water in a beaker. It is checked that the OCP is the same in the open river and in the beaker. Cathodic potential sweeps are then carried out from Ecorr to 2 V/SCE at a scan rate of 5 mV/s. 2.5. Surface observation/analysis (SEM/EDS) To preserve the biofilm each sample is stored in a sterile tube containing Seine water, then dehydrated by several successive ethanol baths, air dried and observed by scanning electron microscopy (SEM). Electron Diffusive Scattering (EDS) analysis is realised by an Oxford Link-ISIS set on a Jeol JSM-T220A SEM. 2.6. Detection of hydrogen peroxide

Table 1 Chemical composition of SS samples (% weight) Common name

UNS

C

Cr

Mo

Ni

Mn

N

304L 316L 254SMO

S30403 S31603 S31254

0.018 0.019 0.01

18 17.3 19.9

0 2.04 6

8.1 11.3 17.8

0 1.04 0.5

1.5 0.041 0.2

Analytical test papers (Merck 1.10081.0001) are used to determine semi-quantitatively the presence of hydrogen peroxide (H2O2) within biofilms developed on stainless steel samples. The active area of the test paper is put in contact either directly with the surface of the metal or with a swab covered by a biofilm sample.

Table 2 Mean values of physico-chemical parameters of the Seine water on both sites Mean values Unit

T °C

Conductivity (25 °C) lS/cm

pH

Dissolved O2 mg/L

Cl mg/L

SO24 mg/L

NO3 mg/L

PO34 mg/L

TOC mg/L

SM mg/L

Choisy Site 1 Flins Site 2

14.2 14.2

524 611

7.9 7.8

10 8.1

20 24.4

31.1 40.7

25.4 21.8

0.51 0.68

2.9 3.9

15 19.9

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3.1.2. Influence of seasons The influence of seasons is determined by realising several immersion tests on site 1 at three different dates (January, June and October). OCP evolutions are reported on Fig. 3. Whatever the season of immersion, the OCP increases with time and reaches values comprised between about +100 and +300 mV/SCE. The final value of the free corrosion potential is always in the same range and is not a function of water temperature. The latency times found for the 3 different immersion experiments, performed on January, June and October, are indicated on Table 3. Water temperatures are 8 °C, 22 °C and 14 °C, respectively. The highest latency time (25 days) is found for the lowest temperature (8 °C), whereas the lowest latency time corresponds to the highest water temperature. The OCP increase begins much earlier for high water temperatures (see also Fig. 4). The latency time seems to depend strongly on the temperature of the electrolyte, which is coherent with the fact that temperature is a limiting factor for the growth rate of the biofilm. Other parameters varying with season and temperature should be taken into account. Particularly the total organic carbon (TOC) content of the water increases with temperature and is also a limiting factor for the growth rate of the biofilm, so the incubation time may also depend on TOC values. Low TOC values would limits the biofilm growth, hence its influence on the evolution of the free corrosion potential. Even if it was not studied in this work, it has to be mentioned here that the flow rate may also strongly influence the latency time [35].

3. Results 3.1. Evolution of the open circuit potential (OCP) 3.1.1. Influence of the immersion site The OCP evolution for SS samples immersed in Seine water on site 1 is presented on Fig. 1. Initial values are always comprised between 250 and 50 mV/SCE. In all cases the OCP increases with time until it reaches values ranging between +130 and +320 mV/ SCE, comparable with the literature [6,12,19–23]. There is no obvious difference between the three grades of stainless steel. During the immersion experiments on this site ennoblement is found to be systematic. It can be mentioned that OCP instabilities often occur; they are probably related to very quick losses of electrical contact but may also come from local biofilm removals due to the water agitation (naval circulation on the river, waves). The OCP increase is not immediate. Indeed there is a lapse of time between the immersion and the beginning of the increase. This lapse of time before the increase is called ‘‘latency time” or ‘‘incubation time”, for it must be linked to the time needed by the biofilm to settle and grow on the surface of the samples. The OCP evolution for SS samples immersed in Seine water on site 2 is presented on Fig. 2. Initial values are always comprised between 300 and 50 mV/SCE. In all cases the OCP increases with time until it reaches values between +140 and +400 mV/SCE. During these immersion tests, ennoblement is found to be systematic. As on site 1 there is a latency time between the immersion and the OCP ennoblement.

0.2

316L-1 316L-2 316L-3 316L-4

0.2

0.1

Ecorr (V/SCE)

0.1

0.0

0.0

-0.1

-0.1 -0.2

-0.2 -0.3

-0.3 0

10

20

30

40

50

0

60

10

20

30

40

50

time (days)

time (days)

b- Type 316L SS

a- Type 304L SS 254SMO-1 254SMO-2 254SMO-3 254SMO-4

0.3 0.2

0.1

Ecor r (V/SCE)

Ecorr (V/SCE)

0.3

304L-1 304L-2 304L-3 304L-4

0.3

0.0

-0.1

-0.2

-0.3 -0.4 0

10

20

30

40

50

60

time (days)

c- Type 254SMO SS Fig. 1. OCP evolution for SS samples (4 identical samples of each grade) immersed mid-January in freshwater on site 1 (T = 8 °C).

60

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0.5

0.3

0.4 0.3

E corr (V/SCE)

Ecorr (V/SCE)

0.2

0.1

0.0

-0.1

0.1 0.0

316L-1 316L-2 316L-3

-0.1

304L-1 304L-2 304L-3

-0.2

0.2

-0.2 -0.3

-0.3 0

5

10

15

0

20

5

10

15

time (days)

time (days)

a- Type 304L SS

b- Type 316L SS

20

0.3 0.2

Ecorr (V/SCE)

0.1 0.0 -0.1

254SMO-1 254SMO-2 254SMO-3

-0.2 -0.3 -0.4 0

5

10

15

20

time (days)

c- Type 254SMO SS Fig. 2. OCP evolution for SS samples (3 identical samples of each grade) immersed mid-September in freshwater on site 2 (T = 14 °C).

0.3

0.4

254SMO-1 254SMO-2 254SMO-3 254SMO-4

0.2 0.2

Ecor r (V/SCE)

Ecorr (V/SCE)

0.1

0.0

-0.1

316L-1 254SMO-1 254SMO-2

-0.2

0.0

-0.2

-0.4

-0.3 0

5

10

15

20

25

0

5

10

15

20

time (days)

time (days)

a- Immersion in June (T = 22˚C)

b- Immersion in October (T = 14˚C)

25

Fig. 3. OCP evolution for SS samples immersed in freshwater on site 1.

Table 3 Latency time as a function of water temperature Immersion month

Temperature (°C)

Latency time (days)

January June October

8 22 14

25 6 7

3.1.3. Influence of the dark/light cycle The influence of the dark/light cycle (night/day alternance) is seen on the OCP evolution measured during an immersion test carried out in April on site 1 (Fig. 4). The frequency of the OCP acquisition is set higher during this experiment (10 min instead of 1 h between each point). The evolution of the free corrosion potential presents oscillations with a period equal to 24 h, which shows that

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3.1.4. Synthesis The results obtained during these experiments in the field confirm the literature: when a SS sample is immersed in freshwater its open circuit potential increases with time, which can be dangerous if it exceeds the pitting potential characteristic of the alloy. The final OCP value is always in the range 100–400 mV/SCE. The latency time before the OCP increase is found here in the range 0–26 days. It depends strongly on the experimental conditions, particularly on water temperature, and may also depend on the TOC content and flow rate, which are limiting factors for the growth of the biofilm. Indeed low water temperature, low TOC content and high flow rate are likely to increase the latency time. It has been shown that the latency time before the OCP increase depends strongly on the temperature. Another parameter seems to have an influence on the ennoblement phenomenon. During one of the experiment, 24-h period oscillations were found on the OCP plot. The maximum amplitude of the oscillations was 70 mV, so these oscillations are barely a modulation of the signal. Still they may be important in the understanding of the process of ennoblement. The day/night cycle has an influence on the free corrosion potential. Consequently OCP is affected either by temperature variations, or, more likely, by light variations (or by both). Even if temperature may play a role, a light effect is surely preponderant. Indeed the alternation light/dark has a powerful impact on the photosynthesis, which needs energy from sunlight to produce O2. Local production of oxygen by photosynthetic algae within the biofilm could create high local O2 concentrations, which would give higher O2 reduction cathodic

304L-1 304L-2 316L-1 316L-2 254SMO-1 254SMO-2

0.3

Ecorr (V/SCE)

0.2

0.1

0.0

-0.1

-0.2 0

2

4

6

8

10

12

14

16

18

time (days)

Fig. 4. OCP evolution for SS samples immersed in freshwater on site 2 on the 3rd of April (T = 15 °C).

the day/night cycle has an influence on the OCP variations. This could be related either to a temperature effect (lower during the night) or to a light effect. The river being a thermostat, the hypothesis of an effect of the light variations on the OCP value is quite probable and is in line with the literature [36–40].

0

3 days (Ecorr = -200 mV/SCE) 3 months (Ecorr = +399 mV/SCE) 100

-2

j (μa.cm )

-2

-j (μA.cm )

-50

-100

10

-150

3 days (Ecorr = -200 mV/SCE) 3 months (Ecorr = +399 mV/SCE)

1

-200

-1.5

-1.0

-0.5

0.0

-1.5

0.5

-1.0

E (V/SCE)

-0.5

0.0

0.5

E (V/SCE)

a- Type 316L SS (linear scale)

b- Type 316L SS (semi-logarithmic scale)

0

30 minutes (Ecorr = -170 mV/SCE) 3 months (E corr = +340 mV/SCE) 100

-2

-j (μA.cm )

-2

j (μA.cm )

-50

-100

-150

10

30 minutes (Ecorr = -170 mV/SCE)

3 months (Ecorr = +340 mV/SCE) 1 -200 -1.5

-1.0

-0.5

0.0

0.5

-1.5

-1.0

-0.5

0.0

E (V/SCE)

E (V/SCE)

c- Type 254SMO SS (linear scale)

d- Type 254SMO SS (semi-logarithmic scale)

0.5

Fig. 5. Cathodic polarisation curves realised in freshwater on site 1 on SS samples immersed in the river Seine, before and after the OCP increase (sweep rate: 5 mV s and semi-logarithmic scales).

1

, linear

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currents and would provoke an increase of the open circuit potential. A few literature cases showed that algae are able to induce ennoblement [41–42]. Moreover algae are often the main microorganisms present within fresh water biofilms. Therefore the OCP oscillations which are observed may be due to local production of oxygen by the photosynthesis realised by algae within the biofilm, this production being more quantitative during light conditions than during dark conditions.

presented on Figs. 5 (site 1) and 6 (site 2). On all the coupons oxygen reduction occurs for potentials lower than 0.4 V/SCE and the production of H2 begins around 1.2 V/SCE. On site 1 the current increases at potentials very close to the OCP, for very low cathodic over voltages. The current density (j) is already of 9.5 lA cm 2 at +300 mV/SCE, whereas this value is only reached at 400 mV/SCE on the curve realised before the ennoblement. Indeed a first local maximum for |j| can be seen between 460 and 380 mV/SCE. This local maximum corresponds to a new reduction, the reduction of an oxidizer which was not present after only 30 min of immersion. This oxidizer is limited by diffusion, which indicates that it is dissolved in the electrolyte close to the surface.

3.2. Cathodic polarization curves Cathodic potential sweeps realised on stainless steel samples immersed in the Seine water, before and after ennoblement, are 0

-50

100

-100

10

-2

-2

j (μa.cm )

-j (μA.cm )

-150

-200

1

0.1

-250

30 minutes (Ecorr = -168 mV/SCE) 9 months (Ecorr = +370 mV/SCE)

-300

-350 -1.5

0.01

30 minutes (Ecorr = -168 mV/SCE) 9 months (E corr = +370 mV/SCE)

1E-3 -1.5

-1.0

-0.5

0.0

-1.0

0.5

-0.5

0.0

0.5

E (V/SCE)

E (V/SCE)

b- Type 304L SS (semi-logarithmic scale)

a- Type 304L SS (linear scale) 0

30 minutes (Ecorr = -161 mV/SCE) 9 months (Ecorr = +214 mV/SCE)

-50 100

10

-150 -2

-j (μA.cm )

-2

j (μa.cm )

-100

-200

-250

30 minutes (Ecorr = -161 mV/SCE) 9 months (Ecorr = +214 mV/SCE)

-300

0.1

-350 -1.0

-0.5

1

0.01

0.0

-1.0

E (V/SCE)

-0.5

0.0

E (V/SCE)

c- Type 316L SS (linear scale)

d- Type 316L SS (semi-logarithmic scale)

0

30 minutes (Ecorr = -155 mV/SCE) 9 months (Ecorr = +355 mV/SCE)

-50 100

10

-150 -2

-j (μA.cm )

-2

j (μa.cm )

-100

-200

1

-250

30 minutes (Ecorr = -155 mV/SCE) 9 months (Ecorr = +355 mV/SCE)

-300

-350

0.1

0.01

-1.0

-0.5

E (V/SCE)

e- Type 254SMO SS (linear scale)

0.0

-1.0

-0.5

0.0

E (V/SCE)

f- Type 254SMO SS (semi-logarithmic scale)

Fig. 6. Cathodic polarisation curves realised in freshwater on site 2 on SS samples immersed in the river Seine, before and after the OCP increase (sweep rate: 5 mV s and semi-logarithmic scales).

1

, linear

C. Marconnet et al. / Corrosion Science 50 (2008) 2342–2352 0

-20

-40 -2

Thus, during the immersion, a new reduction appears at potentials between the OCP and 0.5 V/SCE. These polarisation curves can explain the ennoblement: a new oxidizer, probably produced by and within the biofilm, is reduced at potentials higher than the initial OCP value, hence the increase of the free corrosion potential. Results are identical for both type 316 L and 254SMO stainless steels and can be extended to type 304 L SS. The evolution of the cathodic processes throughout the immersion does not vary with the chemical composition of the stainless steel. On site 2, the current density also increases at lower cathodic over voltages for samples immersed during several months than for samples immersed only for a few minutes. For example, a current density of 10 lA cm 2 is reached before ennoblement only at -370 mV/SCE but as soon as 15 mV/SCE after ennoblement (304 L). But there is a fundamental difference with the polarization curves obtained on site 1: here there is no new diffusion plateau appearing during the exposition. As a consequence, the increase of the absolute value of the current density is due either to an increase of the kinetics of oxygen reduction, or to the synthesis of a new oxidizer in contact with the material. This new oxidizer would not be the same as the one responsible for the ennoblement on site 1, which is dissolved in the electrolyte.

j (μA.cm )

2348

-60

-80

-100

-120 -1.5

immersed during 3 months (Ecorr = 395 mV/SCE) after the introduction of catalase -1.0

-0.5

0.0

0.5

E (V/SCE) Fig. 8. Cathodic polarization curves realised on type 316 L SS immersed during 3 months in the river Seine on site 1, before and after the introduction of a high catalase activity (>5000 l/L/L).

pletely when catalase is added. Consequently, the oxidizer produced by the biofilm during the immersion is hydrogen peroxide, whose reduction at high potentials induces the ennoblement.

3.3. Mechanism of the ennoblement on site 1 3.4. Mechanism of the ennoblement on site 2 3.3.1. Presence of hydrogen peroxide (site 1) A picture of an analytical test paper put in contact with a sample of biofilm developed during 3 months on site 1 is presented on Fig. 7. The reactive area is blue, indicating the presence of a certain amount of H2O2 in the biofilm. This experiment was reproducible: no matter what kind of stainless steel, hydrogen peroxide was detected in all 3 months old biofilms grown on site 1. Besides analytical test papers show that there is no H2O2 in the water. Hydrogen peroxide is formed within the biofilm during the immersion in the river Seine on site 1. The semi-quantitative method used here gives a concentration of about 10 mg H2O2/L inside the biofilm. 3.3.2. Effect of the introduction of catalase on the cathodic current The chemical test presented above has shown the presence of hydrogen peroxide in the biofilm. H2O2 may be the oxidizing compound causing the increase of the cathodic current density at high potential (the absolute value of the current being maximal between 460 and 380 mV/SCE) and thus the OCP increase. Fig. 8 reproduces a cathodic polarization curve obtained on a type 316 L SS sample immersed during 3 months in the river Seine on site 1. The new reduction occurs between 500 mV/SCE and Ecorr. Another cathodic polarization curve is drawn after the introduction in the electrolyte of a high catalase activity (above 5000 l/L), which should decompose quickly hydrogen peroxide. It is here shown that the reduction of the new oxidizer disappears com-

Fig. 7. Picture of an analytical test paper put in contact with a sample of biofilm developed on a type 304 L SS immersed during 3 month on site 1.

SEM observations and EDS analysis are used to investigate the hypothetical presence of a surface deposit on the samples immersed in the river Seine on both sites. They supply precious hints for the identification of the mechanism causing the ennoblement on site 2. 3.4.1. SEM observations Observations of the surfaces of SS samples immersed during 9 months in the river Seine on site 2 are shown on Fig. 9. Surfaces are covered by a complex matrix composed of imbricate biomacromolecules and microorganisms. A few bacteria are present, but the microorganisms within the biofilm are mainly diatoms, which are very numerous. Diatoms were found to be already adhered on the surface after one day of immersion. The connective tissue can be rather scarce between diatoms; stainless steel grains are revealed underneath. SS surfaces present the same visual aspect when immersed in site 1 freshwater. 3.4.2. EDS measurements Local EDS spectra (Fig. 10) are acquired on the locations 1 and 2 on the 316 L surface shown on Fig. 9d. The spectrum noted 1 is performed in the grains area between diatoms; the spectrum of the stainless steel is logically found: presence of Fe, Cr, Ni. The two peaks Ka (5,414 keV) and Kb (5,947 keV) characteristic of chromium are observed, and the area of the Ka peak is significantly superior to the area of the Kb peak. Chromium is accurately identified. The spectrum noted 2 is performed on the white substance in the upper left corner of the picture 7-d. The peak at 5.947 keV has an area largely superior to the area of the peak at 5.414 keV: the ratio of the peaks intensities is reversed. This can be explained only by the fact that the peak at 5.947 keV contains another contribution, which is not present on spectrum 1. As the Ka1 transition peak of manganese has an energy of 5.899 keV and is consequently very close to the Kb peak of Cr, the whole peak could be a superposition of the overlapping peaks of Ka1 of Mn and Kb of Cr. It seems that some manganese has been deposited on the surface of the sample during the 9 months of immersion. The cartography presented on Fig. 11 shows the results of EDS measurements on the whole 316 L surface of the Fig. 9d. Silicium

C. Marconnet et al. / Corrosion Science 50 (2008) 2342–2352

2349

Fig. 9. SEM observation of SS surfaces exposed during 9 months in the Seine water on site 2.

is detected on the diatoms, as it is the main component of their skeleton. Chromium and iron are detected on the areas which are not covered by diatoms or macromolecules. Manganese is detected particularly in the area 2. The counts which are attributed to Mn in the cartography may actually be linked to the presence of Cr, because of the overlapping peaks previously mentioned. However certain zones present only Mn counts and no Cr or Fe counts; this confirms the interpretation of spectrum 2: Mn is present on the surface of the sample. Some manganese has been effectively deposited during the immersion. Similar results were found on the two other grades of stainless steel.

Identical EDS measurements have been performed on SS samples immersed on site 1. No inversion of the intensities of 5.414 and 5.947 keV peaks is seen. There is no evidence for the presence of manganese on the SS surfaces exposed on site 1. 3.4.3. Synthesis EDS analysis shows the presence of a deposit containing manganese on the surface of the samples immersed in the river Seine on site 2. The mechanism explaining the OCP increase by the deposition of oxidized manganese by the biofilm is very likely occurring on site 2. On the contrary, EDS analysis shows that there is no manganese on samples immersed on site 1.

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Fig. 10. Local EDS spectra realised on the points 1 and 2 on the 316 L surface shown on Fig. 7d.

Fig. 11. EDS cartography of the surface of the 316 L sample shown on Fig. 7d.

4. Discussion These field results confirm the literature: when a SS sample is immersed in fresh water, its surface is colonized by a complex biofilm whose settlement induces an increase of the OCP (ennoblement), which can be dangerous if it exceeds the pitting potential

characteristic of the alloy. The final OCP value is always in the range 100–400 mV/SCE. The latency time before the OCP increase (or incubation time) was here in the range 0–26 days. It depends strongly on the experimental conditions, particularly on water temperature. Other limiting factors for the growth of the biofilm, like TOC content and flow rate, may also influence the latency time.

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Low water temperature, low TOC content and high flow rate are likely to increase the latency time. The day/night cycle also has a slight influence on the evolution of the free corrosion potential. Cathodic potential sweeps presented on Figs. 5 and 6 show that oxygen reduction may be catalysed during the immersion, as it seems to appear quantitatively at a higher potential after a certain time of immersion. Still the experiment with the addition of catalase, shown on Fig. 8, clearly indicates that oxygen reduction is not responsible for the OCP ennoblement. Therefore the increase of the OCP is due to the reduction of a new oxidizer produced by the biofilm. As the OCP is in the passive region of the SS electrochemical behaviour, even a tiny additional reduction current would cause a large increase of the free corrosion potential. Two main hypotheses are currently developed in the literature [43]: (i) Some bacteria produce hydrogen peroxide H2O2 within the biofilm thanks to oxidase-type enzymes. The powerful oxidizer H2O2 is reduced on the surface, which increases the cathodic current [3–5,8,13–17]. H2O2 is found inside natural marine biofilm [4,14], its concentration can reach 10 mg/L [5]. (ii) Mangano-Oxidizing MicroOrganisms (MOMO) and in particular Mangano-Oxidizing Bacteria (MOB) use bulk Mn2+ to deposit manganese oxide MnO2 on the surface through MnOOH as a reaction intermediate. MnO2 acts as a new oxidizing agent electrically linked to the metal and reduced at the cathode [20,26–33]. In the experimental conditions of site 1 mechanism (i) occurs. During the exposition a new reduction appears at potentials higher than the initial OCP value, the new oxidizer being dissolved in the electrolyte. H2O2 is detected in all the mature biofilms developed on the 3 tested SS grades. The decomposition of hydrogen peroxide by the introduction of catalase provokes the disappearance of the new reduction wave. It is here clearly shown that the ennoblement on site 1 results from the reduction of hydrogen peroxide produced within the biofilm. In the experimental conditions of site 2 the hypothesis (ii) seems to be more relevant. Indeed no obvious sign of H2O2 presence is noticed. There is no reduction wave around 0.5 V/SCE on the cathodic potential sweep plots. On the contrary Mn is found by EDS on the SS surfaces immersed during 9 months. The Ka1 transition peak of Mn at 5.899 keV has an area significantly superior to the area of the same peak obtained on SS surfaces immersed during only 1 day, so the manganese which is detected after 9 months of immersion does not come from the metal itself: it has been deposited by the biofilm. The Mn oxide deposit is very thin, scarce, located on very small areas. As it is in direct contact with the electrode, there is no limitation by mass transport and Mn oxide reduction does not present a diffusion plateau. In the natural experimental conditions established on site 2 the OCP of SS samples immersed in fresh water increases because of the deposition of Mn oxides on the surfaces, surely realised by MOMO. Thus two important results can be highlighted in this work. First of all there is not a unique mechanism causing the OCP increase in freshwater. Here both H2O2 production within the biofilm and biodeposition of manganese oxides are found on two different sites in the same river. These two different mechanisms generate the same global evolution of the electrochemical behaviour of the SS samples, that is to say an increase of the OCP due to the reduction of the new oxidizer at potentials higher than the initial OCP value. These two mechanisms are not antagonist but could rather be complementary. They are both based on the production of a new oxidizing agent by the microorganisms, and this produc-

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tion can be catalysed by enzymes (oxidases for H2O2, some laccases or Mn-peroxidase for manganese oxide). They may even occur simultaneously: a coupling between these mechanisms is possible. Indeed Mn-peroxidase needs the presence of H2O2 to catalyse the oxidation of Mn(II) in Mn(III). The other important result is the relevancy of the enzymatic model of production of hydrogen peroxide in some cases of ennoblement in freshwater. This model is classically found in seawater but is often thought not to occur in natural waters with a lower chloride concentration. It is here shown that natural freshwater biofilms can also produce H2O2 and thus induce an increase of the OCP of SS samples. 5. Conclusions (1) For all stainless steel samples immersed in Seine River natural freshwater the open circuit potential (OCP) increases after a certain latency time until it reaches a value in the range 100–400 mV/SCE. The latency time before the ennoblement depends strongly on the water temperature, and may also depend on the total organic carbon content and on the flow rate. It can reach about one month with a very low water temperature, in our experiments. (2) The day/night cycle has an influence on the OCP value. The assumption was made that algae may play a role in the ennoblement phenomenon. Indeed algae and in particular diatoms are generally very numerous within biofilms grown on SS in this particular freshwater. Through their metabolism (particularly through photosynthesis), algae may be able to modify the local physico-chemistry close to the surface and to contribute to the increase of the OCP (by their O2 production for instance). (3) On both exposition sites the ennoblement is due to the synthesis of a new oxidizing compound whose reduction at potentials higher than the initial OCP value induces the OCP increase. (4) The new oxidizer is not the same on both sites. The ennoblement is attributed on one site (site 1, Choisy-le-Roi, upstream from Paris) to the local production of hydrogen peroxide by the biofilm, whereas it is attributed on the other site (site 2, Flins, downstream from Paris) to the biodeposition of oxidized manganese on the surface of the SS samples. There is not a unique mechanism leading to the OCP elevation. The two ‘‘classical” mechanisms are not opposed but may be juxtaposed/superposed. They lead to the same evolution of the open circuit potential and are both based on an enzymatic catalysis producing either H2O2 or MnO2. (5) The enzymatic mechanism of production of hydrogen peroxide within the biofilm does not happen only in seawater, since it is here found in natural river water. Acknowledgements We would like to thank Renault factories in Choisy-le-Roi and Flins, especially Mr. Serge Richard, the people working on the Poses-Amfreville dam, and all the staff which have made the experiments possible in situ. We also would like to thank the European program EA Biofilms for its financial support of part of the exposure experiments. References [1] J.W. Costerton, G.G. Geesey, P.A. Jones, Bacterial biofilms in relation to internal corrosion monitoring and biocide strategies, Mat. Perf. 27 (1988) 49–53. [2] J. Wingender, T.R. Neu, H.-C. Flemming, Microbial extracellular pymeric substances: characterisation, structure and function, Springer, Berlin, 1999.

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