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Electrochemical Noise Analysis of Type 316LStainless Steel in a LiBr + Ethylene Glycol +H 2 0 SolutionE. Sarm iento, * J. Uruchurtu, ** J. G. Gonzalez-Rodriguez * " C. Menchaca, ** and O. Sarmiento**

ABSTRACTThe corrosion behavior of Type 316L (UNS S31603) stainlesssteel in a lithium bromide (LiBr) + ethylene glycol (CzHgOJ +H20 solution at different temperatures was evaluated usingelectrochemical noise and electrochemical impeda nce s pectros-copy. The evaluation wa s performed from the fractal dimen -sion of the electrochemical noise time series obtained usingthe so-called Rescale Range Analysis (R/S) proposed byHurst. The fractal dimensions were calculated from the timeseries obtained for the different condition signals. Also, thesurface fractal dimension from the depression angle of theNyquist plot was obtained, and both dimensionswere cor-related. The fractal analysis of electrochemical noise helpsto evaluate the surface condition and electrochemical perfor-mance und er the corrosion conditions tested.KEYW ORDS: aqueous corrosion, fractal a n a l y s i s , Type 316Lstainless steelINTRODUCTIONSta inless s tee ls represent the best candidates forstructural mate r ials under a wid e var iety of conditions.It is not always, however, the most corrosion resistantfor ma ny indu str ia l env i ronm ents , espec ia l ly in thepresence of chlor ides.1"3 For this reason, i t is expectedthat high cor ros ion e f fec ts o f the l i th ium brom ide

Submitted for publication March 25, 2011; in revised form, June10,2011.* Corresponding author. E-mail: ggonzalez@uaem.mx.* UAEM-CIICAp Av. Universidad 1001, 62209-Cuernavac, Mor.,Mxico. Present address: UTEZ, Av. Universidad Tecnolgica 1,62760- Emiliano Zapata, Morelos.** UTEZ, Av. Universidad Tecnolgica 1, 62760-Emiliano Zapata,Morelos.

(L iBr) aqueous so lut ions , used in absorpt ion heattrans formers , 4 are some o f the most prom is ing e le -ments to make an improvement o f the industr ia l wasteheat . L iBr heavy br ines are among the mos t w ide lyused absorbents ; however , they are extremely cor ro-sive.5 8 An a l ternat ive system that reduces some o fthe d isadvantages o f the mixture water/LiBr is to addethylene glycol (C 2H 60 2 ) to this system, 5 because somethermo-phys ica l proper t ies o f the L iBr/water mixture ,such as thermal conduct iv i ty , v iscos i ty , maximumconcentrat ion e tc ., are improved. 5 I t i s cons idered th atstainless steels can be used as possible mater ials inthese industr ia l sys tems, or in combinat ion wi thinhib i tors . However , scarce inform at ion regarding i tscor ros ion behav ior has been publ ished, espec ia llyunder operating temperature conditions.15"7 Electro-chemica l impedance spectroscopy (E IS) i s a techniquewide ly used to eva luate cor ros ion per formance , s incei t prov ides inform at ion abou t the cor ros ion processtaking p lace on the m eta l sur face . Par t icular ly , it g ivesinformat ion about the po lar i zat ion res is tance va lues ,Pip (more precisely, the charge-transfer resistance, R,.),the double- layer capac i tance , C dl , so lut ion res is tance ,R, as wel l as kinetic information.

Somet imes impedance data obta ined at the f r eecor ros ion potent ia l have the shape o f a depressedsemicircle with its center on the real axis. The sim-plest equivalent e lectr ic c ircuit corresponds to a par-a l lel combina t ion o f a capac i tance an d a res is tance . Inthe e lec tr ic c ircui t , the constant ph ase e lem ent (CPE)represents the double- layer e lec trochemica l inter -face, R s is the so lution res istance , a nd th e R,., rep-resents the charge-trans fer res is tance . The complex

ISSN 0011 -9312 (print), 1938-159X (online)C O R R O S IO N V o l . 67 , NO. 10 11/000129/$5.00+$0 .50/0 2011 , NACE International 105004-1

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impedance, Z( jco) , of a depressed semicircle could beexpressed as :

Z = R e + R c t / [ l + (jcoCdlR ct)nI CDFor a better quality, CPE, replacing the capacitor , areoften used in the data f i tt ing of depressed semicircles.The C PE is determined by the fo l lowing equ at ion :

Z C P E = Z o ( j c o r n = l / Q ( j c o ) n (2 )

where ZCP E i s the CPE imped ance, Q correspo nds to apropo rtionality factor, j is (-1 ) \ m is the angular fre-quency, and n is the surface irregular ity estimation.8The CPE is considered to be a surface irregular ity ofthe electrode,3 9 causing a depress ion in the Nyquistsemic i rc le d iagram. 3 1 0 Th e time consta nt (x) and th ecapacitance value (C) of the CPE element can be cal-culated by the fol lowing equations:8

Q = t n / R t c (3 )

= [QRf c-n) ]1 /n (4 )where is the capacitance of the double layer associ-ated with the CPE, with a as the depression angle:

a = (1 - n ) X 90 (5)Para met er n is 1 for an ideal capacitor . In real

systems, the ideal capacitive behavior is hardlyobserved because o f sur face roughness , heterogene-it ies, or other ef fects that cause uneve n current distr i-butions over the electrode surface. In the case when n= 1, the term (] | ,) reduces to jraCfj|Rt:1, where C dlis the inter facial double- layer capacitance. This canbe interpreted as an indication of the degree of het-erogeneity of the metal surface. 9 Wh en n values aresl ightly higher than 0.5, i t corresponds to a severeheterogen eity, but whe n n is equal to 1, the me talsur face is complete ly smooth . Th is degree o f heteroge-ne i ty has been assoc iated wi th the f racta l d imensionof the surface.10 A " f racta l " i s an object wi th complexstructure, reveal ing new detai ls at increasing degreesof magni f icat ion .1 1 1 2 For metals, a fracture or surfacei r regu lari ty cou ld be quant i tat ive ly descr ibed throughfracta l geometry , by m eans o f the f racta l d imension .

Takin g in to account the degree o f depress ion o fthe semicircle in the Nyquist impedance plot, i t is pos-sible to determine the fractal dimension of the elec-trode sur face by me ans o f the fo llowing equat ion:9

n ="1 / (D fs -1 ) (6 )wher e D fs is the fractal dimension of the surface. D fscan either take values close to 2, for a surface com-

pletely smooth, or close to 3, for a rough surface. Ithas been dem onstrated that the f racta l d imen sion o fan e lectrode can be determined b y means o f e lec tro-chemical impedance measurements and corre latedwith atomic force microscopy. 6 7

Electrochemical no ise (EN) is a technique u sedsucce ssful ly in dif ferent corrosion conditions . ENdata are easi ly col lected in the form of potentialand/or cu rrent t ime ser ies or ensembles o f su f f ic ientlength . Analys is m ethods for EN data inc lude v isua linspection, and statistical and spectral analysis oft ime records .3 1 8 Prev ious s tudies have suggestedthat EN t ime records contain va luable in format ionabout corrosion and its protection.19 Spectra l analy-sis also have been used to study the per iodicity ofthe structure of EN time records. 6 7 The slope of thespectra l dens i ty funct ion (SDF) at h igher f requenc iestypica l ly has the for m of (1 /f ) . Di f ferent va lues o fthe exponen t have been rep or ted for spec if i c pro-cesses.20 "21

Mandelbrot 2 0 f rac ta l mathemat ics prov ides thetoo ls for the connect ion betw een the s tructure o f theEN t ime record and the S DF (character ized by D f and), and the microscopic behavior (oxidation reactions)respons ib le for cor ros ion. The f racta l d imens ion, D f, isde f ined as :D f = (5 - ) / 2 (7)

Th e power spectru m is a graph o f the ampl i tudespectra l dens i ty aga inst f r equency o f vo l tage an d cur -rent no ise . Tw o types o f behav ior were observed in thespectra obta ined: whi te no ise , wh ich is indepen dent o ff r equency , and a l/ f fu nct ion. W he n loca l i zed at tackis the domin ant m echa nism l ike in the p i t t ing cor ro-sion process, the EN signal t ime ser ies present high-f requency trans ients o f increas ing ampl i tude .6

Alternat ive ly , the s tructure o f the EN t ime recordcan be analyzed in the t ime domain and descr ibedby the Hurst exponent H.1 The deve lopment o f f rac-ta l geometry by Mande lbrot 2 0 has pr ov ided m athe -mat ica l too ls for the analys is and cha racter i zat ion o fthe s tructure and sca l ing exponen ts o f f rac ta l t imerecords . An EN t ime record is a "random " f racta l ,where the levels of detai l are s imilar but not identical ,shar ing the sam e s tat is t ica l proper t ies . The f rac ta ld imens ion D f descr ibes the structure of a fractal , e .g. ,the "roughn ess" o f an EN t ime record, and the f rac-ta l geometry prov ides the explanat ion for the va lues o fD f, H, and that are obs erved for some o f the EN timeser ies parameters and no ise spectra .

For instance , the Hurst exponent H, wh ich is for -mal ly re lated to revea ls a long-term t im e dep en-dence in a l ime ser ies and can be eva luated f rom theosc i l la t ions occurr ing in the data . W hen the var iat ionin the t ime record over a speci f ic t ime interval ( thelag t ime) is proportional to the lag t ime raised to thepower H, the t ime ser ies is said to be fractal . Accord-

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ing to Hurst 's rescaled range ana lysis (R/S) based onhis empirical law proposed:21R / S = (T / 2)H (8 )

where R represents the di f ference between the maxi-mum and minimum values of the variable, S, thestandard deviation of the time series; T is the periodof t ime measured; H is the Hurst exponent. Th eparameter H describes both the appearance of thetime series ("roughness") and the characteristicswhe n 0.5 < H < 1 undulating signal or "persistence,"or 0 < H < 0.5 jagged signal "anti-persistence." W henH is equal to 0.5, the process is said to be completelyrandom, that is, statistically independent of eachother. These are associated to the p hysicochemicalprocess, e.g., corrosion.20"23Specif ically, the Mandelbrot fractional B rownianmotion ( f f im ) technique provides the conn ectionbetween the structure of the EN time record and SD F(characterized by D f, H, and ) and the microscopicbehavior (oxidation reactions) responsible for corro-sion. Fractional Brown ian motion is a general izationof a random func tion X(t) , whe re the H exponen t isdifferent to 1/2, being any real number, in the range0 < H < 1. The refore , the fractal dimension D f isdef ined as:

D f = 2 - H = ( 5 - ) / 2 (9)The goal of the present work was the electro-Chemical noise analysis of the corrosion perform anceof type 316L (UNS S31603)111 stainless steel as afunction of temperature, by com paring the resultsobtained by two electrochemical techniques, i.e.,electrochemical noise and EIS measurements. Spe-cial emphasis has been made to determine the mor-phology of the metal surface expressed as the fractaldimension of the surface as obtained by EIS and com -pared with the fractal dimension obtained through theH exponent calculated from noise measurements. Thepurpose w as to explore the possibi l i ty of evaluatingcorrosion resistance characteristics at di f ferent tem-

peratures in a LiBr-ethylene glycol-H aO solution.EXPERIMENTAL PROCEDURES

Material tested was a Type 316L stainless steel ,encapsulated in a commercial polymeric resin.Cyl indrical probes with 5.9 mm in diameter and anexposed area of 0.27 cm 2 to the solution were used.Al l were abraded with 600 SiC emery paper and f inal lyrinsed with distilled water and ethanol (C 3H 60). A LiBr+ ethylene glycol + H 2 0 solution at di f ferent tempera-111 UNS numbers are listed in Metals and AUoys in the Unified Num-

bering System, published by the Society of Automotive Engineers{SAE International) and cosponsored by ASTM International.f Trade name.

tures (25, 50, 80C) in a concentration mixture of614 g/L and 217 g/L for LiBr and ethylene glycol,respectively, was prepared ,and used.The electrochemical free corrosion potential of theworking electrode, Ecorr, wa s measu red using a sat-urated calomel electrode (SCE) reference electrode,whereas a platinum wire was used as auxi l iary elec-trode. Tripl icate electrochemical measurements wereobtained by using a ful ly computerized potentiostatand the average values were obtained. EIS measure-ments were done with Gamry 300 f EIS equipment inthe frequency interval of 0.005 Hz to 10,000 Hz withan amplitude of 10 mV at the free corrosion poten-tial, 5 min after immersion in the solution. Afterward ,for the di f ferent temperature conditions, the depres-sion angle from the Nyquist plots were obtained usingcommercial software. EN measurements in both cur-rent and potential were recorded using two identicalworking electrodes and a reference electrode (satu-rated calomel electrode [SCE]). The electrochemicalnoise measurements were made recording simultane-ously the potential and current fluctuations at a sam-pling rate of two points per second during a periodof 1,024 seconds. A fully automated zero-resistanceamm eter (ZRA) was used in this case. Remo val of theDC trend from the raw noise data was the f irst step inthe noise analysis whe n needed. To accom plish this,a least-squares f i tt ing method was us ed. Final ly, thenoise resistance, R,,, was calculated as the ratio ofthe potential noise standard deviation over the cur-rent noise standard deviation (R^ = Gv/OJ. Fro m theEN time records, the H exponent was also calculatedto have one quantitative parameter to compa re sig-nals. This exp onent was eva luated using the Hurst(R/S) analysis based on his empir ical law proposed in1965.12RESULTS AND DISCUSSION

Polarization curves for di f ferent temperatures a represented in Figure 1, showing the mos t active poten-tial for the 25C roo m tempera ture conditions. Al lcurves present a passive region, and a pitting poten-tial is present only at 80C for the potential rangeconsidered. The active corrosion rate is greater forhigher temperatures since the curves are displacedto the right, presenting higher current densities. Allcurves show a current limit region in the cathodicpolarization. At 25C a mixed corrosion control exists,while for 50C the passive current density is lowerand it controls the corrosion process. The oppositeoccurs at 80C because it is the lower current densityl imit, hence und er concen tration or di f fusion control .

Nyquist and Bode plots of stainless steel in thebromide solution with di f ferent temperature condi-tions are shown in Figure 2. The Nyq uist plots aresimilar, presenting a depressed charge-transfer resis-tance semicircle with a second low-frequ ency loop or

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Log (i) (A/cm 2)FIGURE 1. Temperature effect in the polarization curve in Type 316Lstainless steel in the LiBr + ethylene glycol + H 20 solution.

semicircle a ssociated to mass-transfer ef fects, con-sistent with polarization cu rves observations. Thecharge-transfer resistance was obtained by extrapo-lation of the charge-transfer semicircle correspond-ing to the diameter of the semicircle. On the otherhand, Bo de diagrams, Figure 2(b) , show that the totalimpedance h as its highest value at room tem pera-ture and decreases by increasing the temperature,fluctuating between 104 Q-cm 2 and 105 Q-cm 2. Elec-trochemical parameters have been calculated fromthe obtained impedance data considering the elec-tric circuit show n in Figu re 3. In this circuit, Rf rep-resents the film resistance, C f is its capacitance, R,.,represents the charge-trans fer resistance, a nd Cdlrepresents the double-layer capacitance. The experi-mental results are presented in Table 1. The corro-sion potential show s a tendency to be more negativeas the temperature increases, and, co rrespondingly,

- Z re (Q-cm 2)(a)

FIGURE 2. (a) Nyquistand(b) Bode diagrams forType 31t271 mVSC E, respectively) in the LiBr + ethylene glycol + H2 \1 N' s '

i; s V80 CV

50 C iiii ;iit V 25 C '

i1

ii1 . 1 . 1

200 400 600 800 1,000 1,200Time (s)

( a )

1E -3 0.01 0.1Frequency (Hz)(b)

F I G U R E 4 . (a) Voltage time sees and (b) FFT plot obtained for Type 316L stainless steel at different solution temperatures.

0.0005

0.0000Ei g - 0 . 0 0 0 5

-0.0010

- 0 . 0 0 1 5

5 o : cy/ ! Vf

-80C

25C

200 400 600 800 1 ,000 1 ,200Time (s)

(a)1E -3 0 .01 0 .1

Frequency (Hz)(b)

F I G U R E 5 . (a) Voltage time series and (b) FFT plot obtained for Type 316L stainless steel at different solution tempe ratures.

long-term memory ef fect.13 '23 For H = 0.5, the phe-nomenon is Brownian motion (random). Potentialnoise Hurst exponen t results (HE) show persistentconditions. The cu rrent noise Hurst exponent (HI)results also show persistency for changing tempe r-atures, and anti-persistent conditions do not exist.This could be because of the corrosion process takingplace in those sites with su bsequent f i lm formation,which reduces the corrosion rate giving the persistentcondition even at higher temperatures. An extensionto the R/S analysis, related to corrosion and coat-ing performance, was proposed, depen ding on 2H val-ues.22 '23

For the experimental conditions, statistical elec-trochemical parameters have been calculated fromthe obtained noise data and presented in Table 2. Th enoise resistance sh ows the highest values at roomtemperature. According to these results, the bestresult corresponds to this condition similar to the low-

est Rjj value (167 Q-cm 2) . As opposed to that result,the high est R,,t value obtained w as at 50C. This di f-ference could be caused from the sensitivity of electro-chemical noise for localized corrosion, as opposed tothe EIS.The current noise variabi li ty coeff icient or D OLwas calculated as fol lows:

DO L = Oj / im ean (10)where o is the current standard deviation divided byimean, the current mean value. The greater the D OLvalue, the more local ized attack affected the metalsurface.6 7 The D OL revealed more local ized attackfor 50C and 80C. As the temperature increased, theDO L value increased, suggesting a direct relation.Figure 9 shows a comparison between the valuesof fractal dimension calculated from the depressionangle of impedan ce plots and the fractal dimension

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1 E- 3 0 .01Frequency (Hz)FIGURE 6. Change in the impedance spectra with temperature forType 316L stainless steel in the LiBr + ethylene glycol + H 20 solution.

200 400 600Time (s) 800 1,000FIGURE 7. Change in the noise resistance value, R, withtemperature for Type 316L stainless steel in the LiBr + ethyleneglycol + solution.

obtained through the H exponent, determined fromelectrochemical noise potential and current t imerecords as a function of temperature. The beh avior ofthese fractal dimen sions could be observed and asso-ciated to the morp hology (roughness) of the metal su r-face and the electrochemical noise t ime series. Thefractal dimension, D fs, tends to decrease as a func-tion of temperature, wh ile the potential noise fractaldimension, D f(E) , remained almost constant, increas-ing slightly. The current noise fractal dimension,Df(I ), decreased at 50C, rema ining almost constantat 80C. These appear to be sensitive to the ch angesin the experimental conditions. The Df(I) correlatesbetter with the fractal dimension obtained throughthe impedance depression angle, since the current isdirectly related to the corrosion proces s and the evo-lution of the metal surface. Possibly, the Df(E) couldbe associated with the mass -transport process, since

this is related to the free corrosion potential oscilla-tions, as suggested.21 '23

Corrosion is a complex phenomenon, as evi-denced fro m the electrochemical noise osci llations,especial ly du ring local ized corrosion wh ere transi-t ions between stochastic (probabi l istic) and determ in-istic (probabilities not involved) processes can occur,as explained.25 Sometim es, local ized attack consistsof two stages: nucleation consisting of probabilisticbehavior and propa gation, which is a deterministicprocess. These are revealed by the presence of per-sistence (clear trends in behavior, same mecha nism)or anti-persistence conditions (change in the previ-ous behavior, change in mechanism).21 The conditionsappear to be related to corrosion and f i lm forma tionfol lowed by stochastic breakdow n-repair events andpit propagation.

1.5 2.0 2.5Log (r)(a)

1.5 2.0 2.5Log (r)(b)FIGURE 8 . Rescaled range analysis for the electrochemical (a) potential and (b) current noise time series

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F I G U R E 9 . Fractal surface a nd time series as a function of thetemperature.

Corrosion begins when aggressive ions attack alocal site on the surface of the metal substrate. 18 Withfurther ion attack, corrosion spreads gradu ally andoxidizes adjacent sites on the metal surface.19"20 Onthe basis of this model for passive metal surface, it isexpected that H > 0.5 and the electrochemical n oisetime records would becom e persistent. Dif fus ion ofspecies present random conditions (H = 0.5), and gen-eral corrosion would be expected to reduce the persis-tency of the electrochemical noise (H