Biocorrosion of Mild Steel by SRB : Electrochemical Studies

Realizaram-se estudos electroquímicos sobre a biocorrosão de aço carbono na presença e ausência de bactérias redutoras de sulfato (SRB) em meios aquosos de lactato/sulfato e lactato/nitrato. Medidas de potencial em circuito aberto revelaram um deslocamento do potencial no sentido dos potenciais mais nobres, em ambos os meios. Contudo, oscilações regulares de potencial foram observadas somente no meio de lactato/sulfato. As fotografias de SEM revelaram a presença de pites, distribuídas na superfície das amostras em ambos os meios, com pites de pequeno tamanho e de forma mais uniforme no meio de lactato/sulfato, o que está de acordo com os resultados de potencial em circuito aberto. A voltametria cíclica mostrou claramente que as SRBs em meio de lactato/sulfato originam velocidades de dissolução elevadas na região transpassiva, com formação de espécies Fe.


In tro duc tion
The role of mi cro-organisms on the cor ro sion of met als has re ceived in creas ing at ten tion in re cent years [1][2][3][4] .The pres ence and ac tiv i ties of mi cro-organisms can cause se rious prob lems, in in dus try and mil i tary and wher ever met als are pres ent in our so ci ety.
Sul fate-reducing bac te ria, SRB, are an as sem blage of bac te ria that can grow in an aer o bic me dia by the ox i da tion of or ganic nu tri ents, with sul fate be ing re duced to H 2 S.
The pres ence and met a bolic ac tiv ity of sulfate-reducing bac te ria on metal sur faces leads to changes in H 2 S con cen tra tion and pH level in flu enc ing the rates and the type of cor ro sion.
Feio 5 has stud ied the growth of Desulfovibrio desulfuricans ATCC 27774 bac te ria in lac tate/sul fate, as well as in lac tate/ni trate me dia, at 37 ºC, in the pres ence of mild steel.From these stud ies growth curves such as those given in Fig. 1 were ob tained.The curves in Fig. 1 show that there is a pe riod in which the bac te ria pop u la tion increases al most ex po nen tially, and af ter about two days a steady-state of 60 x 10 6 cells/cm 2 is reached.
In the sul fate me dium the met a bolic ac tiv ity of SRB leads to the pro duc tion of S 2-, while in the ni trate me dium the re duc tion pro cess leads to NH 4 + , ac cord ing to the mech a nism pro posed by sev eral au thors 6,7 .
Biofilm for ma tion nec es sar ily plays an im por tant role in the cor ro sion pro cess, but the ex act de scrip tion of the biofilm has not yet been es tab lished.Biocorrosion, like Ar ti cle many other cor ro sion pro cesses, is es sen tially due to electro chem i cal re ac tions; there fore elec tro chem i cal techniques are nec es sar ily good tools to eval u ate, mon i tor, and un der stand biocorrosion both in field and lab o ra tory studies.Ex ten sive elec tro chem i cal stud ies of mild steel samples ex posed to the bac te ria cul tures of Desulfovibrio desulfuricans ATCC 27774, in ei ther the lac tate/sul fate or the lac tate/ ni trate me dium, have been per formed in our lab o ra tory [8][9][10] .
Cy clic voltammograms (CVs), open cir cuit po ten tial curves, and re dox po ten tials of mild steel in lac tate/sul fate and lac tate/ni trate me dia will be re ported in this pa per.
Elec tro chem i cal data is com ple mented with scan ning elec tron mi cro graphs and other tech niques.

Ma te rials and Methods
Studies were per formed with pure cul tures of the strain Desulfovibrio desulfuricans ATCC 27774, grown in a lactate/sul fate or a lac tate/ni trate me dium with the com po sition as given in Ta ble 1.The me dia were pre pared by se quen tial ad di tion of the com pounds to pure wa ter.The pH was ad justed to 7.6 af ter deaeration with pure ni tro gen, and the so lu tions were ster il ized at 120 ºC for 30 min.In ocu lates were made at 10% (v:v) in jec tions of steady-state (1 day) cul tures.
Steel sam ples were made from steel with the nom i nal com po si tion given in Ta ble 2.
All ex per i ments were per formed at 37 ºC (op ti mum growth tem per a ture for the strain un der study).
The open cir cuit po ten tial of each sys tem was mea sured against a com mer cial sat u rated cal o mel elec trode from Radi om e ter ® , model K401.The ref er ence elec trode was placed in a sep a rate com part ment, con nected to the main so lu tion by a KNO3 salt bridge.The work ing elec trode, a 0.2 cm 2 mild steel disk was pre-treated, as de scribed above.
A HP ® 34401A dig i tal multimeter con nected by an inter face RS-232 to an IBM ® PC model 80286, al lowed au tomatic data ac qui si tion at short enough in ter vals to as sume a quasi-continuous mea sure ment.For the mea sure ments in the bac te ria cul tures, in oc u la tion was made at time t = 0.
Potentiodynamic curves were run with a PARC waveform gen er a tor, model 173, cou pled to a PARC potentiostat, model 178 and re corded on an x-y re corder from Philips, model PM 8120.
A con ven tional three-electrode two-compartment glass cell was used.The sec ond ary elec trode was a long he lix of Pt wire and the ref er ence was a com mer cial sat u rated cal omel elec trode (E 0 = 0.242 V vs. SHE).
Be fore each run the work ing elec trode (a 0.2 cm 2 mild steel disk) was pol ished when nec es sary, and then washed and dried.

Open cir cuit po ten tial mea sure ments
The open cir cuit po ten tial, Eocp, of mild steel in the var ious me dia un der study was fol lowed and stored over 50 h of ex po sure.The cor re spond ing Eocp vs. time curves are given in Fig. 2.
Dur ing the first 20 h of ex po sure in the ab sence of the bac te ria cul ture the Eocp kept an al most con stant value of -600 and -625 mV vs. SCE in the lac tate/ni trate and the lactate/sul fate me dia, re spec tively.There af ter, in the lactate/ni trate me dium, Eocp is slightly dis placed in the neg a tive di rec tion (-600 to -625), while in the lac tate/sulfate me dium a slight dis place ment in the pos i tive di rec tion was ob served (-625 to -600 mV vs. SCE).The pres ence of a SRB cul ture in the lac tate/ni trate me dium leads to a slight dis place ment of the po ten tial in the neg a tive di rec tion during the first 5 h, fol lowed by a rapid in crease in the pos i tive di rec tion from -650 to -400 mV vs. SCE.In the lac tate/sulfate me dium with the SRB the Eocp also var ies from -600 to -400 mV vs. SCE, but at a slower rate, over 20 h of ex posure.There af ter, strong and reg u lar os cil la tions in the Eocp val ues are ob served, the av er age value be ing on the or der of -0.389 ± 0.009 V vs. SCE.On the other hand, in the lac-tate/ni trate me dium with SRB, large (-400 to -300 mV) and long (5 h) os cil la tions are ob served be tween 40 and 50 h of ex po sure.
The re sults of the E ocp mea sure ments showed that in both cul ture me dia (lac tate/ni trate and lac tate/sul fate) the SRB lead to a sig nif i cant dis place ment of the cor ro sion poten tial in the pos i tive di rec tion.This en no ble ment may be due to the for ma tion of a pas sive film, the ki net ics of which are fast in the lac tate/ni trate me dium.The strong os cil lations in the E ocp af ter long ex po sures are also in dic a tive of the on set of lo cal ized cor ro sion, i.e., pit ting due to the lo cal rup ture or dis so lu tion of the pas sive film.Videla 11 has dem on strated that the pas sive film formed un der ac tive SRB biofilm may lead to a pit ting at tack of steel.How ever, a de fin i tive in ter pre ta tion of the re sults can not be ob tained based only on these mea sure ments.Re sults need to be comple mented by ad di tional data, such as re dox po ten tial measure ments and re sis tance po lar iza tion mea sure ments.
The en no ble ment does not di rectly dem on strate an increase in the re sis tance to cor ro sion.It may be due to kinetic ef fects, ther mo dy namic ef fects, or both.On the other hand, the en no ble ment may be re lated not only to the anodic re ac tion, but also to changes in the cath odic prop er ties of the sam ple as a re sult of the mi cro bial ac tiv ity on the surface.

Re dox po ten tial mea sure ments
Ta ble 3 gives the re dox po ten tials ob tained for the var ious me dia un der study.
In both cases, the re dox po ten tials in di cate, in agreement with other data, an in crease in the aggressivity of the me dium due to the pres ence of a bac te rial cul ture.
The dif fer ence ob served be tween the re dox po ten tials of both ster ile me dia may be ex plained by the fact that the lac tate/sul fate me dium has been poi soned which sul fide (see Ta ble 1) added to en sure a very re duc ing en vi ron ment, while in the lac tate/ni trate me dium the bac te rial growth is en sured only by deaeration.
The re dox po ten tials in the pres ence of SRB in both media pres ent about the same value ( -450 mV vs. SCE)

Cy clic voltammetry
Cy clic voltammetric ex per i ments were per formed for mild steel im mersed in the var i ous me dia un der study.Typi cal CVs re corded at 20 mV/s are given in Fig. 3.The CVs show clearly no ef fects due to the SRB in the lac tate/ni trate me dium be tween -1.5 and -0.4 V vs. SCE.At the up per poten tial limit of about -0.4 V vs. SCE a de crease in the an odic transpassive cur rent due to SRB is clearly vis i ble.In any case the cath odic side of the CV shows only one peak located around -1.0 V vs. SCE.
Com pletely dif fer ent be hav ior is ob served in the lactate/sul fate me dium.Both the an odic and the cathodic sides of the CVs are in flu enced by the pres ence of SRB.The charge in the voltammetric curve in creases over the en tire po ten tial po lar iza tion re gion (-1.5 to 0 V vs. SCE), the effect be ing par tic u larly strong in the transpassive re gion (at about -0.4 V vs. SCE).Two cath odic peaks are ob served at -0.65 and -1.0 V vs. SCE, while in the ab sence of SRB, only the cath odic peak around -1.0 V vs. SCE was re corded.This is a clear in di ca tion that SRB in the lac tate/sul fate medium lead to the for ma tion of Fe(III) spe cies when ever polar iza tions higher than -0.4 V are ap plied.The H 2 evo lu tion re ac tion is also strongly cat a lyzed by SRB in the lactate/sul fate me dium.Un der the ex per i men tal con di tions of Fig. 3, an hys teresis loop in the re verse scan is ob served for both base cultural me dia (lac tate/ni trate and lac tate/sul fate) in the ab sence of SRB and also in their pres ence, but in this case only in the lac tate/ni trate me dium.The cross over (hys teresis loop) is an in di ca tion of the on set of lo cal ized cor rosion of the base metal (the for ma tion of pits nu cle at ing the base metal af ter or si mul ta neous with the lo cal dis so lu tion of the pas si vat ing film).
The oc cur rence of pit ting in the ab sence of SRB is most prob a bly due to the amount of Cl -in the min eral base me dia (see Ta ble 1).The high transpassive cur rents ob served with the bac te ria cul ture in the lac tate/sul fate me dium, shown in Fig. 3(b), clearly in di cate the high aggressivity of the medium in duced by the mi cro bial ac tiv ity of the SRB cul ture in the lac tate/sul fate me dium, most prob a bly due to changes in pH (pro duc tion of or ganic or min eral ac ids), lead ing to the elec tro chem i cal dis so lu tion of the pas si vating film.
Pitting has been con firmed by SEM mi cro graphs (see Fig. 4).Fig ure 4 shows mi cro graphs of the mild steel samples af ter 21 days of ex po sure in the lac tate/sul fate and the lac tate/ni trate me dia in the pres ence of SRB (af ter biofilm and cor ro sion prod ucts re moval).
The pho to mi cro graphs given clearly show pits dis tributed over the sur face of the steel sam ples ex posed to SRB cul tures in both me dia; how ever the pits of the ex posed sam ples in the lac tate/sul fate me dium are small and more uni form in shape, in agree ment with open cir cuit po ten tial data.

Con cluding Re marks
The pres ence of SRB shifts the cor ro sion po ten tial to more pos i tive val ues, but in tro duces os cil la tions in dic a tive of lo cal ized cor ro sion; the po ten tial os cil la tions are par ticu larly well de fined and reg u lar in sam ples ex posed to SRB grown in the lac tate/sul fate me dium.
With out sul fate the re sults in the ster ile me dium and in the SRB cul ture are sim i lar, while in its pres ence their strong in flu ence is well dem on strated: the re duc tion of SO 4 2-to S 2-or H 2 S is a key pro cess in SRB-induced cor rosion.The es tab lish ment of a mech a nism for the an aer o bic cor ro sion of mild steel in a SRB cul ture in ei ther a lactate/ni trate or lac tate/sul fate me dium re quires com ple mentary stud ies, e.g., cell-counting, bac te ria ac tiv ity, hydrogenase ac tiv ity, de ple tion of ni trate, sul fate and lactate, H 2 S/S 2-and pH eval u a tions, etc.Some of these stud ies are in prog ress, and with them we ex pect to be able to cor relate some of the vari ables and to make con tri bu tions in order to sup port mech a nisms al ready pro posed.
The use of other elec tro chem i cal tech niques, namely, microelectrodes, split-cells and noise anal y sis, may also lead to use ful data, par tic u larly when lo cal ized cor ro sion is in volved.

Fig ure 2 .
Fig ure2.Open cir cuit po ten tial curves of mild steel in the (a) lactate/ni trate and in (b) the lac tate/sul fate me dia, in the ab sence (---) and pres ence ( ____ ) of a SRB cul ture.

Ta ble 3 .Fig ure 3 .
Fig ure 3. Typ i cal CVs of mild steel in (a) lac tate/ni trate and (b) lactate/sul fate me dia, in the ab sence (---) and pres ence ( ____ ) of a SRB cul ture.

Fig ure 4 .
Fig ure 4. Mi cro graphs of mild steel sam ples af ter 21 days of ex po sure to: (a) lac tate/sul fate or (b) lac tate ni trate me dium, af ter re moval of biofilm and cor ro sion prod ucts.

tate/Ni trate me dium
Ta ble 1. Chem i cal com po si tion of the lac tate/ni trate and lac tate/sulphate me dia.

Lac tate/Sul phate me dium
Ta ble 2. Nom i nal chem i cal com po si tion of mild steel (% p/p).