Appl Microbiol Biotechnol (1987)26:294--298 Applied Microbiology

Biotechnology © Springer-Verlag 1987

Bacterial corrosion of mild steel under the condition of simultaneous formation of ferrous and sulphide ions

C. O. Obuekwe ~, D. W. S. Westlake 2, and J. A. Plambeck 3

1 Department of Microbiology, University of Benin, Benin City, Nigeria 2 Department of Microbiology, University of Alberta, Edmonton, Alberta, Canada 3 Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada

Summary. Corrosion of mild steel in cultures of a Pseudomonas species under the condition of si- multaneous formation of Fe(II) and S 2- was ini- tially inhibited by inhibiting the anodic reaction, but after long incubation the corrosion process was allowed to continue. When only S 2- was pro- duced, the initial corrosion rate increased for up to 60 h but later declined, probably due to a pro- tective FeS film formed on the metal. Cathodic reactions were affected in a similar fashion as the anode.

Extensive pitting corrosion was observed when the mild steel coupons were immersed in bacterial culture producing Fe(II) and S 2-, but not in the uninoculated control.

Introduction

Coatings formed by the oxidation of iron and steel to ferric compounds have been used for the protection of steel against corrosion (Uhlig 1979; Lumsden and Szklarska-Smialowska 1978), and the removal of such films can cause the corrosion of the metal they protect. Obuekwe et al. (1981a) demonstrated that a Pseudomonas species isolated from a corroding crude oil pipeline caused corro- sion of mild steel by the depolarization of the anode. Such corrosion of mild steel by the bacte- rium arose from the reduction of insoluble ferric film to soluble ferrous compounds (Obuekwe et al. 1981b). The presence of large amounts of sol- uble iron compounds in culture solutions is known to increase the corrosion of mild steel (Booth et al. 1967a), and a high content of soluble iron compounds increases the aggressiveness of

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soils, augmenting the corrosion of iron and steel buried in them (Booth et al. 1967b).

Apart from the ability to transform protective ferric films on steel surfaces to nonprotective sol- uble ferrous compounds, bacteria can cause cor- rosion of iron and steel by generating sulphide, which reacts with the metal to cause corrosion (Von Wolzogen Kuhr 1961; Booth et al. 1968). Sulphide production in a pipeline system has been associated with the corrosion of the pipeline (Obuekwe et al. 1983).

Although it has been shown that the reduction of Fe(III) to Fe(II) or the formation of S 2- by bacteria can cause the corrosion of iron and steel, no work has been done on the corrosion of iron and steel when Fe(II) and S 2- are being produced simultaneously. The aim of this investigation is to determine the bacterial corrosion of mild steel un- der the condition of simultaneous production of Fe(II) and S 2-.

Materials and methods

Bacterial culture and inoculum preparation. The organism em- ployed for this study was Pseudomonas sp. no. 200, which re- duces Fe(III) to Fe(II) and also reduces thiosulphate and sul- phite to sulphide. The organism was originally isolated from a corroded crude oil pipeline (Obuekwe et al. 1981a). The bacte- rium was grown for 18 h in a complex medium of Torriani and Rothman (1961), washed and resuspended in 0.1 M phosphate buffer, pH 7.2, to a final concentration of 1 g wet weight per 80 ml buffer, yielding approximately 3.0 x 108 colony-forming units per millilitre.

Medium for polarization, The medium for polarization and growth of the bacteria in the corrosion cell was an Fe(IIl)-rich Blo medium described previously (Obuekwe et al. 1981a) and modified by the addition of 0.7 g Na2S203 per litre medium.

Potentiostatic polarization. The polarization of the sterile work- ing electrode (AISI 10-18 mild steel coupon; dimensions

C. O. Obuekwe et al.: Bacterial corrosion during Fe(II) and S 2 production 295

50× 12 x 1.0 mm) was undertaken in B10 medium in a two- chambered polarization cell described previously (Obuekwe et al. 1981a). The auxiliary electrode was a plat inum flag of large surface area, while the potentiostat was a Princeton Applied Research (PAR) Potent iosta t /Galvanostat Model 173 coupled to a PAR Model 175 Universal Programmer to provide the de- sired potential. The potential between the working electrode (mild steel coupon) and the reference electrode (saturated Ca- lomel Electrode, Fischer Scientific) was measured using an Electrometer Probe Model 178. The current output was re- corded on a Houston Instrument Omnigraphic Recorder, Model 2000.

Polarization was undertaken after inoculation of polariza- tion medium with 1 ml of the bacteria suspension, de-aeration with deoxygenated N2 and blanketing of the surface of the me- dium with the nitrogen gas. The mild steel working electrode was polarized anodically (positive direction) and cathodically (negative direction) with reference to the open-circuit poten- tial at 25 ° C. The potentiostat rate was 2 mV s -1 and chart speeds were 25 mV c m - 1 (x axis) and 0.5 mV c m - t (y axis).

Immersion of mild steel coupon in culture medium. Immersion experiments were carried out in 500-ml corrosion chambers (Erlenmeyer flasks with overflow side arm) connected to 44- litre capacity B~o medium reservoir (Fig. 1). Each corrosion chamber, filled with 400 ml medium, was inoculated with the bacterial culture (1 ml cell suspension) and growth was al- lowed at 25 °C for 24 h before opening the medium feedline. This procedure allowed the culture to establish to prevent wash-out. The dilution rate was 0.012 h - ' . This low rate was chosen to simulate the slow bacterial growth observed under natural conditions.

)2

"--3

~ / / / / / / / ~ / / y / / / / / / / / / / / / y ~ "//,~

Fig. 1. Setup for continuous culture of Pseudomonas sp. no. 200 during the corrosion of mild steel coupons in the culture of the bacterium. 1, Filter-tipped aspirator; 2, Flow breaker; 3, Medium reservoir; 4, Corrosion chamber; 5, Mild steel coupons; 6, Effluent

108

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i i i i i i i I i I i t -0.5 "0 1.0 2_0

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-08

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_1ol I I I i I I I I I I I 0 0.2 0.4

Current Density (mA cm -2)

Fig. 2. Polarization curves of mild steel coupons in cultures of Pseudomonas sp. no. 200 in Blo medium lacking Fe(III) but containing Na2S203. The numbers O, 12, 24, 60 and 108 denote incubation time in hours at 25 ° C. The curves with broken lines also show the polarization of mild steel at the designated peri- ods

Results and discussion

To investigate the corrosion of mild steel (AISI 10-18) in bacterial cultures under conditions of si- multaneous formation of Fe(II) and S 2-, the elec- trochemical (polarization) characteristics of mild steel coupons and the behaviour of the metal un- der immersion in cultures containing $20 2- without Fe(III) were determined.

The polarization characteristics of mild steel coupons in bacterial cultures during S 2- produc- tion alone and during simultaneous Fe(II) and S 2- production are shown in Figs. 2 and 3 respec- tively. When only S 2- was produced by the bacte- ria, there was an initial stimulation of the anodic reaction (Fig. 2) up to 12 h. However, with subse- quent reaction the anode became polarized, prob- ably due to the formation of a protective FeS film in the absence of high Fe(II) level in the culture. A similar observation was made by Booth and Tiller (1960). Working with cultures of sulphate- reducing bacteria, these authors observed that an

296 C.O. Obuekwe et al.: Bacterial corrosion during Fe(II) and S 2 production

initial stimulation of the anodic reaction was al- ways followed by inhibition, regardless of -0.2 whether the organism was hydrogenase-positive or hydrogenase-negative. -0.3

Under the conditions of simultaneous produc- tion of S 2- and Fe(II) by the bacteria, the polari- -0.~ zation characteristics showed that the reaction at the anode was initially stifled for up to 60 h but -0.5 was later followed by an increased anodic disso-

> -0.6 lution (anodic depolarization). This observation is - -0.5 the reverse of what was observed only S 2- was

produced in the medium. A high concentration of Fe(III) in the form of Fe3PO4 inhibited mild steel ~- -0.6 (Obuekwe et al. 1981a), and it was not until Fe(II) -0.7 and S 2- were produced that corrosion resumed. In the cathode, the changes in the corrosion char- -08 acteristics of the mild steel coupons were similar to what was observed in the anode, as shown by -0.9 the polarization characteristics.

High amounts of soluble iron have been re- -to ported to prevent the formation of a protective sulphide film on ferrous metal (Booth et al. 1967a) and will also cause a high corrosion rate of

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Anodic Polarization -05

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~ -0.5

~-" -Q6 Cathodic Polarization

-0.7

-0.8

-0.9 .O! 0

-1.0 ~ ~ ' 12 I I I I I [ i I I I I I I

0 Q2 04 0.6 Current Density ( m A c m - 2 )

Fig. 3. Polarization Curves of mild steel coupons in cultures of Pseudomonas sp. no. 200 in B~o medium containing Fe(III) and Na2S203. The numbers 0, 12, 24, 60 and 110 denote incu- bation time in hours at 25 ° C. The curves with broken lines show the polarization of mild steel at the designated periods

9~ ~ ° 2 4

/ / / / - ' " ~ v / / / / . . - - - ~ f

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, ,

0 0.4 0.8 1.2

i I 011 J i i 0.2r I r

Eurrenf Densify (mA cm 4)

[afhodic Po[arizahon

f 013

Fig. 4. Polarization curves of mild steel coupons in uninocu- lated Blo medium (control) containing Fe(III) and Na2S203. The numbers O, 12, 24, 60 and 98 denote incubation time in hours at 25 ° C. The curves with broken lines also show the po- larization curves of mild steel at the designated periods

-0.3

-0,~

-0.5 7 Anodk Polarization

/ > -0.5 -

, I I r - ~ I .I 4 I r I ,I I I I I = 0 08 1.2 g -0.5 o

-0.6

Eafhodic Potarization -0.7

-0.8 60 ~ 1 ~

-0.9

- 1.0 2/+x~60 t I I I 0 / 8 I I I I r i I i

0. 0.16 024 Eurrent Density (mAcm -2)

Fig. 5. Polarization curves of mild steel coupons in uninocu- lated B]o medium (control) containing Na2S2Oa but not Fe(III). The numbers 0, 12, 24, 60 and 98 denote incubation time in hours at 25 o C. The curves with broken lines also show the polarization curves of the mild steel at the designated peri- ods

C. O. Obuekwe et al.: Bacterial corrosion during Fe(II) and S 2 production 297

Fig. 6a, b. Pit formation on mild steel coupons immersed for 9 weeks in S 2- and Fe(II)-produc- ing cultures of Pseudomonas sp. no. 200. Magnification x 52.5. a Pits (p) on coupon immersed in cultures during simultaneous Fe(II) and S 2 formation, b Con- trol (uninoculated)

such metal in soils (Booth et al. 1967b). In this investigation, the soluble iron compounds (Fell) were formed in situ by the reduction of Fe(III) and the simultaneous production of S 2- would eventually prevent the formation of protective FeS film on the metal. In the case where only S 2- was produced by the organism, the initial increase in anodic reaction was due to the reaction of S : - produced by the bacterium with the metal. The re- sultant FeS eventually protected the metal, as seen from the polarization curve in Fig. 2.

In the condition of simultaneous production of S 2- and Fe(II), it was possible that the large amount of FeS formed in the medium also con- tributed to increased corrosion by depolarization of the cathode, as seen in Fig. 3, after 60 h incuba- tion. It has been reported that FeS, in the absence of bacteria, can cause corrosion of ferrous metal by increasing the cathodic reaction (depolariza- tion of the cathode), as was similarly observed in Fig. 3 after long incubation.

In the control experiments (Figs. 4, 5) both the anodic and cathodic reactions were inhibited (po- larized) in the absence of bacterial reduction of Fe(III) and $2032- to Fe(II) and S 2- respectively.

Thus, both the production of S 2 and Fe(II) si- multaneously (Fig. 3) and the production of Fe(II) alone by the bacteria (Fig. 2) were responsi- ble for the anodic dissolution (corrosion) of the mild steel coupons, and in the absence of produc- tion of Fe(II) and S 2- no dissolution of the anode was observed, as shown in the controls (Figs. 4, 5).

Mild steel coupons immersed in the bacterial cultures showing simultaneous production of Fe(II) and S 2- showed extensive pitting corrosion (Fig. 6a) and was covered by a loose deposit of FeS. No such pit formation was found on control coupons (Fig. 6b). In real life, the problems of the Pembina pipeline system, from which the bacte- rium was isolated, have been associated with pit- ting corrosion.

In conclusion, it was observed that the corro- sion of mild steel (AISI 10-18) in cultures of Pseudomonas sp. no. 200 was sustained and led to extensive ptting of the coupons when Fe(II) and S 2- were produced simultaneously. In the pres- ence of S 2- alone, the corrosion was inhibited soon. Undoubtedly, the combined effects of des- tabilization of FeS film on the coupon and ca-

298 C.O. Obuekwe et al.: Bacterial corrosion during Fe(II) and S 2- production

thodic depolarization by a large quantity of FeS formed in the culture would contribute effectively to the sustained corrosion of the mild steel under the condition of simultaneous Fe(II) and S 2- pro- duction in the culture.

References

Booth GH, Tiller AK (1960) Polarization studies of mild steel in cultures of sulphate-reducing bacteria. Trans Faraday Soc 56:1689--1696

Booth GH, Elford L, Wakerley DS (1968) Corrosion of mild steel by sulphate-reducing bacteria: an alternative mecha- nism. Br Corrosion J 3:242--245

Booth GH, Cooper AW, Cooper PM (1967a) Rates of micro- bial corrosion in continuous culture. Chem Industr 86:2084--2085

Booth GH, Cooper AW, Cooper PM (1967b) Criteria of soil aggressiveness towards buried metals. II. Assessment of various soils. Br Corrosion J 2:109--115

Lumsden JB, Szklarska-Smialowska Z (1978) The properties of films formed on iron exposed to inhibitive solutions. Cor- rosion 34:169-- 176

Obuekwe O, Westlake DWS, Plambeck JA, Cook FD (1981a) Corrosion of mild steel in cultures of ferric iron reducing bacterium isolated from crude oil. I. Polarization charac- teristics. Corrosion 37:461--467

Obuekwe CO, Westlake DWS, Plambeck JA, Cook FD (1981b) Corrosion of mild steel in cultures of ferric iron reducing bacterium isolated from crude oil. 11. Mecha- nisms of anodic depolarization. Corrosion 37:632--637

Obuekwe CO, Westlake DWS, Cook FD (1983) Corrosion of Pembina crude oil pipeline. The origin and mode of forma- tion of hydrogen sulphide. Eur J Appl Microbiol Biotech- nol 17:173--177

Torriani A, Rothman F (1961) Mutants of Escherichia coli con- stitutive for alkaline phosphatase. J Bacteriol 81:835-- 836

Uhlig HH (1979) Passivity of metals and alloys. Corr Sci 19:777--791

Von Wolzogen Kuhr CAH (1961) Unity of anaerobic and aerobic iron corrosion process in the soil. Corrosion 17:119--125

Received July 9, 1986/Revised January 19, 1987