Indi an Journal of Chemical Technology Vol. 6 . Jul y 1999. pp. 207-2 18

A review of electrochemical techniques applied to microbiologically influenced corrosion in recent studies

R Sagar Dubey, R S Dubey* & S N Upadhyay

Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India

Received 14 October 1997; accepted 10 June 1999

Mic robiologicall y influenced corrosion (MIC) has been receiving attention from different areas in the last years , as an answer to the demand of various industries. Remedy for most of the MIC problems can be achieved by establishing a close link between mi crobi ology, electrochemistry, medical science and metallurgy. The present paper critically reviews the ·elec· trochemi cal techniques employed in several recent publications on the subject, with the aim of clarifying basic and applied aspects of mi crobio logically influenced corrosion (MIC) and biofouling. During MIC corrosion products migrate from the metal to the so luti on ro llowed by the settelment of biofilm from the bulk solution to the metal surface. These processes can be studi ed by e lectrochemical techniques, such as the measurement of open circuit potential, potentiodynamic polarization , electrochemical impedance techniques, pol arization resistance measurements, electrochemical sensors, and electrochemical noi se analys is.

Corrosion is an irreversible interfacial reaction of a material (metal, ceramic, polymer) with its environ­ment which results in consumption of the material or in dissolution into the material of a component of the environment J. Around 70% of the earth sruface is marine and seawater contains about 3.4% salt and is slightly alkaline (PH 8) in nature. Sulphate reducing bacteria (SRB) thrive frequently in sea water2

.3 and

even in deep ocean sediments4.5

. Their survival in the well oxygenated (225 11 mollL) seawater is due to the presence of the oxygen-protectiv.e enzyme catalase and superoxide dismutase6

. When suitable growth s'lbstrates are available, the high sulphate content of sea water (29 11 moUL) allows rapid SRB ,prolifera­tion .

Microbiologically Influenced Corrosion (MIC) Microbiologically influenced corrosion is the dete­

rioration of metals by corrosion processes that occur directly or indirectly, as a result of metabolic activity of microorganisms? Alternatively microbial corrosion may be defined as the degradation or deterioration of metallic structures due to the activity of various or­ganisms which either produce aggressive metabolites to render the environment corrosive or are able to participate directly in the electrochemical reactions

*Present address: Department of Chemistry. R. J. College, Ghat· kopar. Mumbai.

occurring on the metal surface. The presence of mi­crobial growths or deposits on a metal surface en­courages the formation of different aeration cells between the deposit and the surrounding environment which stimulate the corrosion processesB

•

The biodeterioration of off-shore structures, stor­age facilities, transmission lines associated with oil and gas production, propellers, shafts, heat exchang­ers, ship hulls and ropes etc. causes substantial in­vestment loss and danger to the human life. MIC is not only limited to metals, but concrete, ceramics, polymers and a variety of materials are involved in this process. There are some empirical environmental conditions which can be used to predict the environ­ment of microorganisms in the corrosion process. These are:

(i) Proper environment - suitable pH values ranging between 4 and 9 (some sulphur oxidisers toler­ate even a lower level), temperatures generally below 50-60°C, approximately redox conditions, and an appropriate nutrient for the activity of the

. . mIcroorganIsms.

(ii) Typical morphology and chemistry of the corro­sion products.

(iii) Typical changes in the environment due to mi­crobial activity, for example marked changes in pH in either acid or alkaline medium.

208 INDIAN 1. CHEM. TECHNOL., JULY 1999

Bacterial corrosion is electrochemical in nature like other corrosion reactions, except for the fact that one never knows the exact chemicals involved in the cor­rosion process. Corrosion may be enhanced biologi­cally, through direct enzymatic action of bacteria, or abiot icall y, as a result of reaction with metabolic by­products or changes in local conditions (for example pH) by the bacterial activity. Many workers9

-2o have

reported the process of biofouling, mechanisms, method of detection, case histories, treatment and prevention of microbio logically influenced corrosion in biological processes. Sugars, carbohydrates, succi­nate, fumarate, pyruvate, NH" N02-, H2S, S20/-, Fe2+ , H2 are substrates which may be oxidized in biological proeesses and O2, NO,-, SO/-, Fe'+, H20, CO2 etc are reducible species. Therefore, it can be concluded that most of the reactions involved in bio­logical corrosion processes are electrochemical in nature and e lectrochemical measurement techniques are the most widely applicable techniques In ex­plaining the corrosion phenomenon.

Mechanisms of Microbial Corrosion

When a metal is immersed in water, damp soil or other aqueous environment, the initial reaction is the di ssoluti on of the metal as metallic cations which leave behind an excess of electrons. For example in case of iron:

... (I)

These e lec trons are consumed at near by cathodic sites by the bal ancing reaction, which in near neutral so luti on is usuall y the reduction of oxygen to hy­droxyl ions,

. . . (2)

In the absence of oxygen the usual cathodic reaction for corrosion processes is the reduction of hydrogen

... (3)

a reac ti on is rapidly limited to neutral solutions thus stifling the corrosion process.

Many workers reported that for microorganisms to participate in corrosion, the production of oxidising agents through their metabolism is an essential proc­ess. The ro le of microorganisms is to participate di­rect ly in one or both of the e lectrochemical reactions on the metal thereby initiating or exacerbating the reaction. Previous reviews23

.24 classify microbial cor-

rosion on the basis of metabolism of organisms as follows:

(a) Absorption of nutrients (oxygen) by microbial growths adhering to the metal surface,

(b) Liberation of corrosive metabolites or end­products of fermentative growth,

(c) Production of sulphuric acid, and (d) Interference in the cathodic process under oxy­

gen free condition by obligate anaerobes.

Electrochemical techniques include. measurement of redox potentials and potential s of corrosion elec­trodes, measurement of current density between cou­pled electrodes in inoculated and sterile media, po­tentiodynamic polarization, polarization resistance, potentiokinetic curves, electrochemical impedance spectroscopy, electrochemical noise and electro­chemical sensors.

Open Circuit Potential Measurements When a metal specimen is immersed in a corrosive

medium, both oxidation and reduction processes oc­cur on its surface. Typically the specimen oxidizes (corrodes) and the medium (solution) is reduced. The specimen must function as both anode and cathode, and both anodic and cathodic currents occur on the specimen surface at anodic and cathodic areas . .

When a specimen is in contact with an aqueous medium and is not connected to any instrumentation, it assumes a potential , which is termed as the open circuit potential or corrosion potential. The Eoc can be defined as the potential at which the rate of oxidation is exactly equal to the rate of reduction . It is usually measured relative to a reference electrode. A speci­men at Eoc has both anodic and cathodic currents pre­sent on its surface. However, these currents are ex­actly equal in magnitude so there is no net current to be measured. The specimen is at quasi-equilibrium with the environment.

Open circuit potential measurements have been . d b I k 2526· j 'ff . carrIe out y severa wor ers . In (I erent mICro-

bial corrosion systems. Gundersen et al .25 reported the variation of open c ircuit potential with time for 254 SMO, SAF 2507 and SIS 2343 stainless steel immersed in natural seawater with and without chlorination . When OCP shifts in negative direction, it· indicates the increase of corrosion rate whereas a shift of OCP in noble direction is generally responsi­ble for decrease of corrosion of the metal or alloy un­der study. Fig. la-c show Eoc during continuous chlorination with 0.1, 0.2 and 100 ppm res idual chlo-

DUBEY et at.: A REVIEW OF MICROBIOLOGICALLY INFLUENCED CORROSION 209

(0 ) Pt

0 u '" <t

'" ( D)

<t 800 Pt

.;. lAF 2507 >

> '254 SMO E 400

--0 \

SIS 2343

.. 0

0

a. (c) PI

800

"- "-SIS 2343 254 SMO

400

o

o 20 40

Exposure time, doys

Fig. I-Open ci rcuit potenti al for 254 SMO, SAF 2507,SIS 2343, and Pt vs time of immersion in natural seawater during intermit­tent chlorination wi th 0. 1. 0 .2 and 100 ppm residual chlorine respec ti ve ly. 30 min/day.

rine, respec tive ly. The time dependence of Eoc con­tinues to ri se s low ly even afte r 40 days of exposure at 0.1 ppm. The response in potential for various mate­ria ls are di ffe rent at low chlorine concentration. The least noble of the s ta inless steel SIS 2343 has a con­sistently lower ElK than the more nobel alloys. The open c ircuit pote nti a l for SAF 2507 is generally higher than that for 254 SMO in the chl orinated sea­water.

Potentiodynamic Polarization Measurements The deve lopment of potentiostat a ll owed the use of

potenti ostati c and potentiodynamic polarization tech­niques, with Oliver's paper on the pass ivation be­haviour of Fe-Cr a ll oys27 The potentiodynamic tech­nique is ll sed to examine the overa ll corrosion be­haviour of a syste m. In a typical potenti odynamic scan, the potenti a l is s low ly swept over a ve ry wide

potenti a l range (typica ll y -2 to +2 volts rel ative to a re fe rence e lectrode). During the sweep, the meta l

2·4

2·0

Hi

1.2

0.8 .. ~ 0·4 >

w' 0·0

-0·4

-1 .2

-6·0 - 5·0 -"·0 -3·0 -2·0

log ~ A/cm 2

Fig. 2-Polarization curves22 at 25°C of 99.99% AI in :

(a) sterile electro lyte (0.25 ~m diamond po li shi ng paste sur-face fini sh)

(b) sterile ele(; trol yte (600 grid surface finish) (c) anodic in inocu lated electrolyte (d) cathodic in inocul ated electrolyte (e) cathodic in steril e electro lyte

sample may undergo diffe rent electrochemical reac­ti o ns, resulting in anodic and cathodic ce ll currents which may va ry over many orders of magnitude. Fig. 2 shows the polarization curves of 99.99% Al in dif­ferent e lec tro lytes with and without the fungu s lIor­moconis resinae2H

. Curves (a), (b) and (c) show the anodic nature , whereas curve (d) and (e) are cathodic. The proliferation of Hormoconis resinae28 on the

tes ted samples decreases the pitting potential (Epit ) by 2.6V for 99.99% pure a luminum.

Pol arizati on tec hniques have also been applied for the eva luati on of locali zed corrosion phenomena. The

pitting potential Epit and the protection potentia l Eprot

have been used to charac terize the susceptibility of meta ls and a ll oys to pitting corrosion. The difference in El'il and the corrosion pote ntia l ELorr has been used to asses the susceptibility of a metal to pitting in a g iven e nvironment. EI'II is the potential value at which the current density begins to increase drasti cally in the pass ive range in so luti ons containing aggressive

ani o ns (Cl , Br- , e tc.). Ep,ol is the protecti on potential , i.e., the pote nti a l be low whic h no pitting occurs and above whic h pits a lready nucl eated can growN

Fig.J shows the anodic pola rization curves of mi ld stee l in ste ril e Pos tagate C med ium and a SRB culture in the same medium'O . marked decrease of the pi t­ting potential as we ll as a diminution of the pass ive

210 INDIAN 1. CHEM. TECHNOL..1ULY 1999

zone can be noticed due to the synergic action of chloride anions contained in the medium and sulphide anions produced by the metabolic activity of the bac­teria.

Tafel Plots The Tafel technique is used to obtain an accurate

estimate of the corrosion rate of a metal in solution. Cell current is measured during a slow sweep of the potential. The sweep typically is from -250 to +250 mV relative to Eoc. An electrochemical half-reaction uhder kinetic control obeys the Tafel equation :31

.

o (231l3(£-C°)th ) I = Ie .. . (4)

In this equation I is the current resulting from the reaction, 10 is a reaction dependent constant called the exchange current, E is the electrode potential, EO is the equilibrium potential ( constant for a given re­action) , and b is the reaction 's ~ coefficient (constant for a given reaction), ~ has a unit of Volts/decade.

The Tafel equation describes the behaviour of one isolated reaction . In a corrosion system, there are two opposing reactions . The Tafel equation for both the anodic and cathodic reactions in a corrosion system can be· combined to generate the Stern-Geary equa­tion32

.34

.

I = I (. 2303(E- Ec",, )lba ) - (2 .303(E - E )/ b ') Carr \e -e Carr C

... (5)

where I is the measured cell current in ampere, l eorr is tbe corrosion current in ampere, E corr is the corrosion potential in volts, ba is the anodic ~ coefficient in volts/decade, and be is the cathodic ~ coefficient in volts/decade.

Log I versus E plot is generally called a Tafel plot. A Tafel plot is performed to experimentally deter­mine l eorr ' from which the corrosion rate is calculated. l eorr as obtained26 by extrapolating tbe linear portion of Tafel plot to E eorr . as shown in Fig. 4 . The corro­sion rate can be calculated from ICorr by using the fol­lowing equation ,

Corrosion rate R(mpy ) = 0 . 1288/~A/cm 2 ) eq.wt(g )

D{g / em3)

... (6)

where I is the current density and D is the specimen density. The extrapolation of polarization curves to

0 ·0

w a u <.f)

.,; -0·4 >

> 0 ... c ClJ ... 0

- 0·8 a.

. 2 - - Current Density, mA/cm

Fi g. 3--Anodic polari zati on curves of mild steel in steri le Posta­gate C Medium.

-0·3,-----------

- 0·4

-0·5

> -0·6

o C -0·7 .. o Q.

-0·8

- 0·9

- 1· 0

ANODIC

'" '" \ '" - - - - - - - - /l' Ecorr

CATHODIC

1\ I

: teorr

1

/

-1· 1 .......... ~-----'---'--'--_--'-__ ...l-_--' 2 3

log Current Density, JlA Icm2

Fig. 4--Tafel plot constru cted form data obtained during polari­zat ion of mild steel coupon in B Ii) medi um26

E Corr is well applicable in the case of charge-transfer controlled corrosion reactions which give well de­fined Tafel plots. In the case of processes which are diffu sion controll ed, the extrapolated values depend strongly on the properties of the reacting interface and on the hydrodynamic conditions. Lorenz and Mansfeld 15 reported that extrapolation procedure can not be used for sys tems having non-linear behaviour. However, their non linearity can be overcome by means of pul se polari zation conditions, because under these conditi ons the surface properties of the corrod­. I d . 14 mg e ec tro e remam constant" .

)

DUBEY el al.: A REVIEW OF MICROBIOLOGICALLY INFLUENCED CORROSION 211

Polarization Resistance Method Polarization res istance is defined as the slope

(dE/dJ) at the corrosion potential of a poten­tial/current density curve. This slope has units of re­sistance area and is known as the polarization resis­tance. In I'nathematical form polarization resistance,

Rp, is expressed as :

Rp = ( dE) , E = ECorr d I ai

... (7)

Stern and Gear/ 5.J6 simplified the fundamental equa­

tion for s imple charge transfer controlled reaction, in which the relationship between ICorr and Rp has the form:

... (8)

where ~a and ~ c are the anodic and cathodic Tafel slopes. For a quantitative determination of ICorr ; ~a ,

~ c and Rp have to be determined. Computer programs such as CORFITH

, POLCURRJ8 and POLEFITw have been developed by Mansfeld and co-workers for the determination of prec ise va lue of Icorr. Kasahara and Kajiyama40 used polarization res istance technique for active and inacti ve SRB conditions.

Mansfe ld et al.~J have used the linear polarization technique to determine Rp for mild steel sensors em­bedded in concrete . The concrete samples were ex­posed to sewer environment for about nine months. One sample was periodi cally flu shed with sewage in an attempt to remove the acidic environment pro­duced by the biofilm, the other sample was used as control. A spec ial data logging system was used to coll ec t Rp dat a at 10 min interval s simultaneously for the two corrosion sensors and two pH electrodes which were placed on the concrete surface. Fig. 5 shows the cumulati ve corrosion loss LINT, where INT was obtained by integration of the IlRp-time curves as

t , INT= J-=-dtlR p

t J

... (9)

A quantitative measure of corrosion rate can be obtained from slope of the curves in Fig. 5. The cor­rosion loss remained very low during the first two month and then showed a large increase for both samples . The total corrosion loss as determined from the integrated Rp data was less for the control than for the flushed sample.

105 (0 )

70 PH=2 ·0 E r; 0 ...... III

~' 35 :z W

0 0 60 120 180 240 300

DAYS 150

E 100 r; 0 ...... III

~, 50 ~ W

DAYS

Fig. 5--lntegrated corrosion loss LINT for mild steel sensors embedded in concrete3S

.

Some of the most common errors that are possible in this method is the invalidation of results through oxidation of some other electroactive species besides the corroding metal concerned, a change in corrosion potential , Ecorr during the time taken to perform the measurement and the application of a large applied potential resulting in inadvertent departure from lin­ear current density versus potential behaviour.

Electrochemical Impedance Spectroscopy (EIS)

The term electrochemical impedance spectroscopy was first introduced by Mansfeld42

.43 by replacing the previously used term AC impedance. Since its intro­duction by Epelboin et al.~, EIS has become very popular and has been applied to all types of corrosion phenomenon . Some of the advantage of EIS tech­niques are the use of only very sma ll signals which do not di sturb the electrode properties to be measured, the poss ibility of studying corrosion reaction and measuring corrosion rates in low conductivity media where traditiona l DC methods fail, and the fact that polarization resi stance as well as double layer ca­pacitance data can be obtained in the same measure­ment.

Application of EIS in MIC studies have included monitoring corrosion over time on carbon steel ex­posed to bacteria~5, carbon steel exposed to bacteria

212 INDIAN 1. CHEM. TECHNOL., JULY 1999

and treated with 'biocides46, stainless steel weldments

exposed to bacteria47 and stainless steel samples ex­posed to natural seawater48

. The advantages of using EIS in MIC studies have been reviewed49

• One of the advantages of using EIS is that small amplitude sig­nals , within the linear response range and generally 5 mY rms or less, are applied. Repeated EIS analysis on stainless steel samples with biofilm caused no change in the open-circuit potential after the analysis, pro­viding indirect evidence that EIS do not damage the metal samples or biofilms50

. Effect of EIS on micro­bial biofi.lm cell numbers, viability and activities of sessile bacteri a on stainless steel have been reported by Franklin et ai.51

• They concluded that EIS can be used to study mechanisms of MIC with little or no damage to the numbers of viable bacteria in a biofilm or to the activity of the bacteria.

It is assumed that e lec trochemical reactions that determine the corrosion behaviour can be defined in terms of charge transfer across an interface between a sol id elec trode and an electrolyte. The driving force for the charge transfer reaction is the ' potential drop across the interface, E, usually measured against a reference e lec trode. Thi s requires that the electric current between the electrode and the electrolyte will be a uni que function of the voltage drop across the interface . All the mechanistic information about the rate limiting steps in sllch a system is contained in thi s functi onal relation , i.e., I(£) . If we introduce a small time dependent perturbation in the voltage, M, the resulti ng change in the cllnent can be extended in Taylor series to yield :

I (E+M)= I ~E) + (oJ /OEh,M + ~0 2 1 /~ E )E,I M 2 ... (10)

If the' mplitude ofJhe perturbation is small enough slIch....t+rat the second and higher derivatives can be neglected then the resulting change in the current can be expressed as :

M = (011 OE)E.I M ... ( II)

Yew) = Z-I(w) == (Ol / oE)E! is defined as the ad­mittance of the system and Z(w) as its impedance; w

is the angu lar freq uency of the perturbation. The measurement is re latively simple. In the most

common app licati on, a small AC component which varies with time in a sinusoidal way: M(t) = fiE Sin WI , is superimposed on the DC voltage and the change in the current is measured . The change in the cunent can be expressed as 6/(t) = M SIn (wt+8) , where 8 is

c

RS

RS = Sotution resistance

Ret= Charge transfer imped-once

C = Double layer capacitance

W = War burg impedance

Fig. &---Equivalent ci rcui t fo r a corroding electrode.

the phase difference between the perturbation and the response.

When the result is expressed in this way, imped­ance measurement do not provide direct information about the system. In order to gain useful information one has to solve the full/(£) equation, or to represent the sys tem in electrical engineering terms, or to model the system as a parallel plate capac itor.

One approach that is applied in many practical cases is the network analysis. Th is implies utilizing electrical c ircuit e lements such as capacitors and re­sistors, correlating these elements with some physical process and charge accumu lation modes in the elec­trochemical corrosion system, building the electrical network and composing the response of the network with the measured response. One usuall y finds (hat the mathematics is relatively easy and much of the work is already done and results can be found in any good book on circuit analysis.

To illustrate the application of these principles let us assume that the electrochemical reactions that de­termine the corrosion behaviour are of very simple nature and can be represented by the familiar Ran­dle's equivalent circuit shown in Fig. 6.

Rs represents the series resistance of the electrolyte, ReI is the charge transfer resistance, W is the Warburg impedance, and C is the double layer capacitance, From Fig. 6, it follows that the system impedance is given by-

Z(w) = Rs + [Ill we + ( IIReT + aw-1n - icrw- 1I2

)]

(J 2) where (J is the Warburg impedance. From this com­plicated expression two limiting cases can be consid­ered. First, when charge transfer is important, and when electroactive species are always at their Nem-

)

DUBEY et aL.: A REVIEW OF MICROBIOLOGICALLY INFLUENCED CORROSION 213

stian concentration at the electrode surface, Eq. (9)

simplifies to -

iroeRb R Z(w) - R + cl

- S 1+ ro2e 2 Rb 1 + ro2e 2 Rb ... (13)

If the imaginary component of this impedance, Z ', is plotted versus the real component of the imped­ance, Z, for each excitation frequency, a single semi circle is obtained (Fig. 7), having a diameter of nu­merical value ReI (=l/w*c, where w* is the fre­quency/HZ at the maximum of the semicircle). This representation for evaluating AC impedance data is known as Nyquist plot, or Cole-Cole plot, or complex impedance plane diagram. When charge transfer is small compared with diffusion, the impedance is given by, .

Z ) R R - 112.( -1/2 2 -.1. ) (w = s + el + crw - l crw + a c .. . (14)

In this case the impedance spectrum in the complex plane is a straight line of slope 45°. For cases where neither charge transfer nor diffusion is solely impor­tant, there is interaction between the interfacial im­pedance and the Warburg impedance. In general, the shapes obtained in the complex impedance plane de­pend on the relative values of RCt , cr, and e, and typi­cal diagram is shown in Fig. 8. Fig. 9 is the Bode plot for the same data as in Fig. 8. This format permits examination of the absolute impedance, (Z), as cal­culated by

(Z) = (i 2 + i '2) 112 .. . (\5)

as a function of frequency. The Bode plot is useful alternati ve to the Nyquist plot to avoid the longer measurement times associated with low frequency ReI determinations. Thi s is because the log (Z) versus log f plot sometimes a llows a more effective extrapola­tion of data from higher frequencies. The Bode for­mat is also desi rable when data scatter precludes ade­quate fitting of the Nyqui st semicircle. In general, the Bode plot provides a c lear description of the electro­chemical system's frequency-dependent behaviour that does the Nyquist plot where the frequency values are implicit.

Other types of plots (e.g. Randell plots, i versus WZ' etc.) optimize data interpretation for special ex­perimental systems, providing better information than the Nyqui st or Bode Plots. Since today most of the more sophi sticated experimental setup attached to some form of computing facility for data handling, the transfer of data from one form to the next is not

Decreasing frequency -

Rct = 2Mtan ex.

Fig. 7--Estimation of charge transfer impedance by tangential method.

W=2nf

Z" f Decreasing

Z'

c= __ 1_ 2nf*Rct

\

\ , I

Fig. 8--Typical Nyquist plot.

very difficult. The importance of a particular data presentation lies in its ability to suggest to the inves­tigator the general feature of a model that might be applicable for analys is of the system.

Analysis of carbon steel affected by Vibrio natri­gens in batch and continuous flow culture using EIS and direct current techniques has been made by Dowling et ai. 52 Most of the impedance diagrams ex­hibited single depressed capacitive loops with centres below the real axis [Fig. \0 a & b) . A comparison of the corrosive effects of V. natrigens in batch culture with that of a sterile chamber over a three day period showed that the presence of the bacteria considerably increased the corrosion rate. If the assumption that a is good approximation of Rp proves true and the ano­dic Tafel slopes (Pa) is assumed to be constant, then,

I =~ ... (\6) C OlT 2.3A

Thus the presence of V. natrigens in batch culture accelerated the corrosion rate compared with the sterile side over the same period and also the intitial rate before passivation.

Hernandez et at. 53 have reported the EIS study of the corrosion inhibition of steel by Psuedomonas sp.

214 INDIAN 1. CHEM. TECHNOL., JULY 1999

N

E u

'" E c. a .><

, "' N -, 1

1·6

' ·4

' ·2

'·0

0 ·8

06

0.4

0·2

, ·0

0·8

06

04

02

(0 )

• •

• • • ·

I

. a ~a

I Dp

(~ )

• •

•

• •

. . •

• • • • e •

0 · 0~-nD-~~-L~~~-L~~~

0.0 0·2 0 4 0·6 0 ·8 , ·0 ' ·2 ' ·4 ' .6

Z' k ohms . cm2

I

Fig. 9--Typi cal Bode plo t.

2·0,------------ ---.

~ (a ) •• 8"':; ... . . DOY4

1·0 0" 000 :-' "

E rr-.;f 8 0 ~o • • ,

u 80

E 0·0 L @ C.

:, 2'0t; b ) Day 9

~ I I! o'to o

1· 0r- 0 " 0 0

lt0 0 ... ••• ' 0 .~. " '\,

" .. 0

, '. ' 0'b: ~ 0.0- 1 I I

00 ' ·0 2 ·0 3· 0 40

Z') k oh ms .c m2

Fig. IO---Impedance data monocu1tute of V I1Glrigells (closed circle) was compared with a cul ture of the vi brio and two SRB (0) in continuous flow (Days 4) and bath culture :Day 9)

S9 in synthetic scaw;.:ter. Fig. II a & b shows imped­ance spectra ohtai ned after 20 days of immersion in : !uile and inoculated ni ne salt solution (NSS). T he !Jhase angle vers us frequency curve showed a maxi-lum at 45" fo r the steri le soluti on ind icati ng a Wai­

burg type impedance, the cu rves of modules versus frequency il1d icated little corrosion occurred du ing the first 20 days in 11e case of bacterial suspension, since the charge transfe r resistance characterizing the

6 90

(0 ) - Without bact.rio

5 - - - With bactcr io

..-- 75 -- ,/ '\~ 4 -'I \ 60 , \

I ....... \ 3 I \ 45 I \

I \ NE 2 I

30 u I

'" I I .. ~ 0.

15 .. " ",' ",-

::? 0 0 0-:>

" c

a ( t» - -- 20 da ys 0

:[ :l: 0- 5 -- 30doys 75 0 ~ c

a.

60

45

30

15

0 0 -1 a 2 3 4

tog Fre que ncy, Hz

Fi g. I I-a- EIS spectra obta ined afte r 20 d ays o f the exposu re with and without Pseudo monas sp. S~ . Curves are given fo r indi­

vidual runs. Standard devi:lli on between repli cates was ±IO%. b­EIS spectra obtai ned afte r differen t ti mes of exposu re in NSS with Pseudomonas sp . S ~. Curves are given for ind ivi du al runs. Stan­

dard deviatio n between rep li eales was ± I 0 %.

reaction kinetics at the metal electro lyte interface was very hi gh and could not be calcu lated at low fre­

quency limit used. Mans fe ld54 states that EIS can also be used for the

detec tion and monitoring of locali zed corros ion of aluminium a lloys and aluminum based metal matrix composi tes. Thus it can be concluded that EIS can be best appl ied in mon itoring and study of MIC.

Electrochemical Sensors Various types of e lec trodes, available for the

measure ment of d issolved O 2 , CO2, pH, redox poten­tial (EH) and H concentration (CH) in steel, are of gal­vanic (potent iometric) or poiarographic (am­perometric) types. Hydrogen embri ttle ment (HE) oc­curs du to the penetration of atomic hydrogen (H atoms) into a metal, which results in de arburization, loss of duc tility and tensile strength cf the metal. Hy­drogen can be absorbed by steel in several way and one of th most important of these is the corrosion influenced by sulphate reducing bacteria (SRB). b the presence of SRB, the common cathodic reaction is the eduction of H20 to hydrogen ato 1S . By using

1

DUBEY er af. : A REVIEW OF MICROBIOLOGICALLY INFLUENCED CORROSION 215

Shitld.d Cu wi~

ru~r QOSkel mask

woll Corrosi..- _dium

EQlJIVALEHT : CIRCUIT

CORROSIVE I ~ (H I MEDIUM

I

ShietcMd compen!I<JllId • thermocoUple •• tcnsion wire

TMnnoc"",* juoclion c:ompcnsat i on moduW:

Chort rvc",d~r (mullirongz)

NiO

I I No + OIC oq I Ni

t¢J Fig. 12--Secti onal view and equivalent ci rcuit of Hay sensors.

some of this hydrogen in the con version of sulphate to sulphide, as given in Eq . (14) , the bacteria are tbought to depolarize the cathodic reaction at the steel surface and the corrosion rate is increased55,56.

SO ~- + 4 H 2 ---7 S 2- + 4 H 20 .. . ( 17)

The presence of su lphide promotes the absorption of hydrogen due to the following cathodic reaction .

... (18)

The measurement of hydrogen concentration at the inner surface of steel (CHi) is performed by am­perometric sensors . Potentiometric sensors enable calculation of the equivalent pressure of H2 gas (PH2et:l ' by application of the Nemst equation. In Devanathan-Stachurski cell,57.59 the concentration of hydrogen in the metal is est imated as60

,

CHi = 1JIDapp ... (19)

where, 1 is hydrogen atom fl ux , J is the thickness of the diaphram and Dapp is the diffusion coeffic ient. Hydrogen atom flux (1) is determined by measuring the current density which IS rela ed to each other by tbe equation .

J = ifF .. . (20)

where i is the current density and F is the Faraday constant. The diffusion coeffc ieni: Dapp may be de­temuned from the current iransient at two shor t times tl and l2 as

Fig. 13--Typical response chart for tests using Hay sensors (test solution= NACE TM 0177) .'7

J II t 2 I I I ( J

I / 2 [ 2 ( l~ I:; '~ exp - 4Dapp ~- 12

... (21 )

The amperometric sensor described by Hay is shown in Fig. 1261.62. The metal diapharm, Fe(H) in the equi va lent c ircuit , was the steel pipe line into which hydrogen was introduced by the corrosive me­dium. React ions at the interface I and II could be written as :

Reacti on I : Hl'e ---7 H ~q + e l-

Reaction Il :

e ~ + H IIlI + II2NiO ---71/ 2H 20 + II2Ni

... (22)

(23)

The nicke l-nickel ox ide e lectrode (cathode) com­prised an e lectron si nk, and the current is measured by the pote ntial d rop (E) across a standard resistor (R). CHi is calculated by Eq ( 16). Dapp for H in steel cou ld be de termined by Eq. ( 18) .

In pract ical app li cation, the surface of the mild steel is c leaned with emery pape r and degreased wi th appropriate so lvent. The sensor is mounted on the steel . urface and sea led wi th sui table epoxy res in . Now the sample is exposed to the corrosive envi ron­ment and measure ment is performed. Fig. 13 shows the response chart from Hay sensor6J. There is maxi­mum potential drop across the standard resistor which is fo ll owed by a ' Iow decline. CHi determined from the Hay sensor was reported in the above experi­ment}' arou nd 4 .62 x 10'c to 7.17 x 10. 2 mL (STP) g tee I.

Electrochemical Noise Analysis

More recent ly , s ign ificant deve;opments describing the use of e lect rochemical no ise techniques have been repo rted fr:om whi ch insigh ts into electrode processes and hence corrosi on phenomena can be obta ined by monitori ng the "pontaneou, fluctuations of poten tial

216 INDIAN J. CHEM. TECHNOL., JULY 1999

TIME RECORD : NP26 + 0.053Or-( 0-)--,.--------=-='.:-:-\-, --~-~ -.t .. -~-. --,

+0·0576 .r'; '. : I.

~ o

.;

: ~

~ -0 ·0530 I- :; -..J -..J

~ -0· 1737

~ -0·2943 I 38·96 39·03

I

39·10 TIME ,h

39·17 39·24

1000 POTENTIAL DISTRIBUTION (+ve & '"'IIt!)

(b) 1000

> -- Magnitude - - - - Population

E 100 t- 100z w' 0

i= 0 ct ::l ~ I -..J l- I v

.t' ::l z 10 b-

el. \!) I !I' 10 0 ct I~ el.

~ .1 1

I

il/\U , I

1 ,

1 0·001 0·01 0·1 1 10 100

MILLIVOLTS

Fig. 14--Tirne record after 216 days for covered reinforcements in media containing active cultures of SRB . ~3

or current that occur at electrode/electrolyte inter­faces . The minute variations in potential can be measured using a sensitive voltmeter, coupled to a microprocessor, and analyzed using Fast Fourier Transforms (FFT) or Minimum Entropy Model (MEM) analysi s routines. For example, the current noise measurements have been made under poten­'tiostatic control in pitting and passivity studies64

.65

.

Voltage and current noise measurements have also been analyzed from corroding electrode under open circuit conditions66

.68

.

Moosavi, et al.69 reported electrochemical noise analysis on the corrosion of reinforced concrete ex­posed to sulphate reducing bacteria in marine envi­ronment. Fig. 14 shows a time-potential record and a potential distribution chart showing the population and the magnitude of potential fluctuation. The po­tential fluctuation f the specimens were measured at Ec orr . The time record after 218 days for the covered rebar reveals that the magnitude of noise fluctuations depends on the total impedance of the system. A cor­roding metal undergoing uniform corrosion with fairly high corrosion rate might be less noisy than a passive metal which shows occasional bursts of noise

due to localized breakdown of film fo llowed by rapid repassivation.

Finally it can be concluded that electrochemical noise techniques are very useful in evaluating the nature of the corrosion process and in monitoring mi­crobial corrosion but presently this technique is under the initial stage of its development.

Conclusion Since MIC is a complex process in the corrosion

phenomenon, it may be concluded that electrochemi­cal techniques are very useful in the study of micro­bial corrosion. Electrochemical tenchiques coupled with chemical and biochemical techniques can solve most of the problems in the diagnosis, surveillance and monitoring of bacterial corrosion processes . Most of the microbial corrosion and microbial metabolisms which are associated with MIC may be studied with electrochemical techniques . Mechanistic investiga­tions and microstructure of biofilm as well as the cor­rosion behavious of various specific-systems in the course of MIC can be best elucidated by applying a combination of electrochemical techniques and sur­face analytical techniques such as electron spectros­copy for chemical analysis (ESCA), Auger electron spectroscopy, strain electrometry, ellipsometery, transmission and scanning electron microscopy (TEM and SEM), energy dispersive X-ray analysis (EDAX), low energy electron diffraction (LEED), reflection electron diffraction (RED) and ion microprobe mass spectrometry. Thus electrochemical techniques are equally applicable in basic research and fie ld appli­

cations and also in es tab lishing a link between elec­trochemistry and microbiology.

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( 1989 ) 19. 2 Hard y J A. J Appl Bacteriol, 51 ( 1981) 505 . 3 Postgate J R. Th e Sulphate Reducing Bacteria, 2nd ed!l,

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