(1997) a survey of argentinean atmospheric corrosió-copper samples

8/12/2019 (1997) A survey of Argentinean atmospheric corrosi-copper samples

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Corrosion Science, Vol.39, No. 4, 655-679, 1997p.0 1997 ElsevierScienceLtdPrinted in Great Britain. All rights reserved001&938X/97 17.00+ 0.00

PII: s0010_938x( )0015&3

A SURVEY OF ARGENTINEAN ATMOSPHERIC CORROSION:II-COPPER SAMPLESJ. R. VILCHE,* F. E. VARELA,* E. N. CODARO,* B. M. ROSALES,+

G. MORIENA+ and A. FERNANDEZ+* Instituto de Investigaciones Fisicoquimicas Teoricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas,

Universidad National de La Plata, Sucursal4, C.C. 16, (1900), La Plata, Argentina+CITEFA-CONICET, Centro de Investigaciones en Corrosion (CEICOR), Zufriategui 4380, (1603). VillaMartelli, Argentina

Abstract-Copper samples were exposed at six sites with known ambient parameters in Argentina and theatmospheric corrosion was investigated after different outdoor exposition periods. Weight-loss measurements usedto determine corrosion damage were complemented with both DC and AC electrochemical techniques, performedin 0.1 M Na#Od solution employing the exposed face of the test samples, in order to characterize theprotectivenesses of the surface layers generated on copper in the distinct environments. While the ambientaggressiveness could be well evaluated from meteorological and pollution parameters and from weight-loss data,the product protective characteristics estimated through SEM and EDAX observations and electrochemicalmethods yielded valuable information to understand mechanistic aspects concerning the effects of physicalproperties, structure and contaminant content of surface corrosion products. 0 1997 Elsevier Science Ltd. Allrights reservedKeywords: A. copper, B. weight-loss, B. EIS, B. SEM, B. potentiostatic, C. atmospheric corrosion.

INTRODUCTIONSince atmospheric corrosion is a slow process, natural tests for the direct measurement ofcorrosion require very long periods of time. For this reason, new methods must bedeveloped to replace the classical gravimetric procedures in the fight against atmosphericcorrosion. Since atmospheric corrosion has been shown to be an electrochemical typephenomenon, it is possible to use electrochemical techniques for studying it as an alternativeapproach to traditional tests.

Copper is an important functional material used in industrial, commercial and homeenvironments. Found in its native state approximately 6000 years ago, it is considered anoble metal. The corrosion of copper, a common problem, can be aesthetically beautifuland frustratingly complex. The uniform corrosion, passivation and pitting corrosionprocesses of copper have been studied in different solutions since more than three decadesago. The literature on the subject until 1980 is reviewed in references.2 The corrosion ofcopper and its alloys depends to a great extent on the make-up of the electrolyte in contactwith the metal. The reaction mechanism involves copper dissolution at local anode sites andelectrochemical reduction of some species such as oxygen at cathodic areas. A given surfacearea may alternate from being anode and cathode to give uniform corrosion. Pitting in

Manuscript received 28 August 1996.655


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656 J. R. Vilche et al.copper is the result of some spots remaining anodic for relatively long periods. Natural freshwater generally promotes the formation of protective coatings. From detailed examinationby electrochemical and surface analytical methods, this passive layer appears to consist of asimple 0.1~0 or a duplex CuzO/CuO, Cu(OH)z film,3 the entire layer structure undergoingshort and long time range phase transitions4 These layers are important in different regards.They are useful for corrosion protection and they influence electrochemical processes atcopper electrodes. Actual rates of corrosion are usually very low; consequently copper iswidely used in water lines, water tanks and heat exchangers.

The protective properties of the anodic layer produced on copper in alkaline solutionsvary with the nature of anions in the solution. Thus, in aqueous solutions containing eithera carbonate or a phosphate salt, the protective properties of the anodic layer could beimproved. Occasionally, insoluble copper-containing salts as malachite (CuCO,.Cu(OH),)or azurite (XuC03.Cu(OH)z) become a part of the anodic layer. Whether the anodiclayer produced on copper behaves as a protective or a non-protective layer depends not onlyon its chemical composition but also on its compactness and adhesion to the coppersubstrate. The existence of soluble species, aqueous copper carbonate complexes, has beenpostulated,5 as corroborated by the high solubility of the following complex anions:CUCO~(,~, CU(CO&~- and CUCO~(OH)?-.~ In contact with very soft water containinglarge amounts of CO2 and 02, the protective films do not form due to carbonic acid effects.The rate of copper corrosion may become excessive.

Atmospheric corrosion of copper has been studied extensively. Reviews by Leidheiserand Rozenfeld lo provide the reader with an historical perspective. The majority of thesestudies focused attention on outdoor urban corrosion. The atmospheric corrosion process ismore complex than high temperature oxidation, where theories exist that explainreasonably well the observed rates. No such theory exists for atmospheric corrosion ofcopper, although attempts have been made to correlate observed outdoor rates tometeorological and pollution parameters.2-4 The corrosion rate of copper in laboratorytests has shown to be sensitive function of relative humidity, sulphur dioxide, nitrogendioxide, hydrogen sulphide, ozone, hydrogen chloride and chlorine concentrations.4-8 Itwas observed that indoor corrosion rates obey log normal statistics over the field populationof each study, although copper corrodes significantly faster outdoors (indoor rates areabout 1% of outdoors values).15 Combined quartz crystal microbalance measurements withion chromatography used to analyse the evolution of the absorbed electrolyte layer duringthe exposure of copper revealed that the adsorption of water on to the metal surfaces fromthe atmosphere reaches a steady state within 30 min at a constant relative humidity, and thatthe water adsorption is not the rate-limiting step in the establishment of the absorbedelectrolyte and the initiation of corrosion.6 The analysis of comparative effects of SOZ, NO2and O3 studied on laboratory and field-exposed copper7,8 suggested that synergistic effectsdetected for sulphur dioxide and ozone can explain the unexpectedly high corrosion rates ofcopper found at rural sites, which are characterized by low sulphur dioxide and nitrogendioxide, but high ozone concentrations. Although sulphate and nitrate ions were thedominating surface species, additional constituents detected in the laboratory, but not in thefield, were sulphite and nitrite. In addition, Cu20 was identified as an important compoundat some sites. Both chloride and ammonia were detected as surface constituents on all fieldsamples. 7x8 The use of multilamellar electrochemical cells, known as electrochemicalatmospheric corrosion monitors, to evaluate the atmospheric corrosion rate of copper hasbeen also reported. I9


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A survey of Argentinean atmospheric corrosion: II 657Kinetics approaches rather than thermodynamics are suitable for studying processes

involved during atmospheric corrosion, in which chemical changes occur on time scalesmuch too short for multiphase equilibria to establish themselves. Among other in situtechniques, infrared reflection absorption spectroscopy (IRAS) and combined X-raydiffraction, infrared spectroscopy, scanning Kelvin probe measurements, and pHmeasurements2 have been applied to the monitoring of corrosion process in real time.Thus, copper samples were exposed in flowing corrosive air and the initial formation of afilm of Cu20 was followed by in situ IRAS; the kinetics of the oxidation was found to obey alogarithmic rate law.20 In experiences of copper corrosion in sulphur dioxide-containing air,the formation of sulphite species on the surface was detected by IRAS. In urbanenvironments, where ammonium and sulphate are the most abundant ions in fine dustparticles, the corrosion mechanism was explained as dissolution of Cu followed byformation of Cu20, oxidation of Cu(1) ions to Cu(I1) ions, and precipitation of antlerite[Cus(SO,)(OH)& brochantite [CU~(SO~)(OH)~]or posnjakite [CU~(SO~)(OH)~.H~O].~Thisprocess is markedly temperature dependent. At 300 K the corrosion products formed arebasic copper sulphates, while at 373 K a higher fraction of Cu20 is formed.21

Complementing previous work on aluminium and zinc samples,22 data obtained fromelectrochemical and microscopy techniques are discussed in this work in order to investigatethe characteristics and properties of corrosion products formed on Cu samples afterexposure in six test stations covering different environmental conditions in Argentina,within the frame of the MICAT (Iberoamerican Map of Atmospheric Corrosion)Project.3,23

EXPERIMENTAL METHODSamples of Cu (99.97%) were exposed at the following atmospheres: dry (San Juan) andsubtropical (Iguazu) rural; temperate (Camet) and polar (Jubany) marine; and urban-

industrial (La Plata and Villa Martelli). Environmental data monitored in these outdoorstations are presented in Table 1, according to ambient characterization given in ISO/DP9225.22 The corrosion products formed after 1, 2, 3 and 4 y exposure times werecharacterized using DC and AC electrochemical techniques, SEM observations, surfacechemical analysis (EDX) and weight-loss measurements. For the weight-loss measurements,surface corrosion products were removed by pickling of corroded samples as indicated inISO/DP 9226.The electrochemical experiments were performed at 20C in conventional three-compartment double wall glass cells containing 0.1 M Na2S04 solution, which wasprepared from analytical grade (p.a. Merck) reagents and triply distilled water purifiedin a Mill&Q reagent grade system. Potentials were measured (and referred to in thetext), against a SCE reference electrode, properly shielded. A large-area Pt plate wasused as the counter electrode. The experimental setup employed for DC measurementsincluded various Tacussel potentiostats, Servovit 13 potential scanners and EPL 2recorders.Electrochemical impedance spectroscopy (EIS) measurements were performedemploying working electrodes of geometrical area 1.33 cm2 and an activated platinumprobe coupled to the reference electrode through a 10 uF capacitor to reduce phase shifterrors at high frequencies. Detailed descriptions of both hardware arrangement and dataprocessing have been given elsewhere.22*24,25EIS measurements were carried out at the


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658 J. R. Vilche ef al.

Table I. Environmental data of the six test stations in ArgentinaMeteorological Contamination

Station Mean Mean Total TotalPeriod (y) (ambient temp. rel.hum. TOW rain

condition) (C) (%) (h) (mm) SOz(mg mm2 day-) Cl-(mg m - day-)I

234I

234I234I234I234I234

Camet 14.1(marine)

13.914.513.5

Villa 16.7Martelli

(urban-ind.)17.117.015.8

Iguazu 20.6(rural)

20.922.12 .1

San Juan 18.0(rural)

20.018.3IX.3

Jubany 2.0(marine)

m-3.-2.9PO.6

La Plata 17.0(urbanind.)

16.713.617.1

79.2 5974 817 3078.8 6202 805go.3 6448 122678.7 5834 115315.3 5063 1729

407048

71.5 4227 98373.6 4509 142070.1 3478 142277.8 5831 2158

IO9IO

73.8 5530 262414.7 5547 172078.6 5167 202050.6 1002 35

_

49.3 847 Ill50.8 865 9351.8 972 6783.8 2693 * 684.0 2425 *84.5 2588 *84.3 3299 183077.5 5198 1178 6.2

63024

76.8 4955 126.1 8.278.0 553 I 361 6. I17.9 4869 1088 6.5

* Not available

corrosion potential in the frequency range 1 mHz


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A survey of Argentinean atmospheric corrosion: II 659

compared with the data obtained in the corresponding candle (IS0 9224) and outdoorstation.422

EXPERIMENTAL RESULTS AND DISCUSSIONWeight-l oss studi es

Annual corrosion rates of Cu were determined over both 100 x 150 mm areas exposed tothe ambient facing to the sky and to the ground, on the basis of mass losses, density andexposure time. The edges of the samples were considered negligible compared to those areas.Figure 1 shows the corrosion rates, expressed in pm y - for different exposure times. Dataobtained in marine, urban and rural environments follow clearly different trends.

The ambient variables with most influence on the experimental results are time ofwetness (TOW), temperature (2) and pollutant concentration.14 The TOW is determining ofthe difference in annual corrosion rates observed between the two rural stations San Juan(dry) and Iguazu (subtropical), and also between the urban ones Villa Martelli and La Plata.On the other hand, T and pollutant concentration account for the distinct magnitudesobserved between the marine stations Camet and Jubany. The similarity of values obtainedfor the first annual corrosion rates of the rural ambient of Iguazu and the urban site of VillaMartelli can be explained as due to the compensation of effects between pollutantconcentrations and TOW, T and RH (Relative Humidity) values. After the first year theeffect of pollutants is almost negligible.Potenti ostat ic st ep polari zati on dat a

The electrochemical character of atmospheric corrosion suggested the possibility ofusing DC anodic and cathodic steps of 20 mV amplitude to evaluate the protectiveness ofthe corrosion products formed after different exposure times in the six distinctenvironments. Pieces of 1 cm2 cut from the outdoor test samples were immersed in thesupport electrolyte 0.1 M Na2S04 for 1, 24 and 48 h before each potentiostatic pulse and

I . I I . I .Co) Camet[I{ ~~Gllartel:(-1 SanJuan(+I Jubany( 1 La Plata

i

-I0 1 2 3 4 5t I years

Fig. 1. Corrosion rates of copper from weight-loss measurements.


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660 J. R. Vilche et al

then the attained stationary current densities were measured. These data can be associated,in principle, with corrosion rates detected in natural exposures during rain periods whoseduration should be similar to the above mentioned immersion times.

The effect of both weathering and immersion time, the latter being proportional to theTOW of the test site, can be analysed separately. It was expected that an aggressiveenvironment would produce results in agreement with high corrosion rates while mildambient conditions would be reflected in low current densities, corresponding to the lowmass losses measured. Neither high current densities were obtained for corrosion filmsgrown in aggressive ambient nor low figures were determined for corrosion products formedin mild environments (Table 2). On the contrary, an opposite trend for the corrosion rates inthe distinct ambient was observed after pulses, and that can be probably attributed to theincrease in the pollutant content as a function of time (Table 3). Moreover, the mainconsistent result was the loss in protectiveness, corresponding to an increase of the currentdensity, due to the partial dissolution of the products in the support electrolyte (Table 2)also observed in EIS data for different immersion times.

Table 2. Current densities attained in anodic (A) and cathodic (C) potentiostatic polarizations([4 - &,, = 20 mV) after 1, 24 and 48 h immersion time. Copper samples weathered from 1 to 4 y in the

Argentinean stations

Station Period (y) j PA cme2

Immersion time (h) Immersion time (h)

I 24 48 jpAcme2 1 24 48San Juan 1

234

lguazu 1234

V. Martelli 1234

La Plata I234

A 0.03 0.05 I .66 C 0.02 0.05 0.220.05 0.06 0.09 0.06 0.05 0.140.03 0.04 o ib 0.02 0.03 0.050.99 1.79 6.33 0.11 0.25 0.46

.A 0.19 0.20 0.15 C 0.02 0.04 0.050.14 0.04 0.07 0.08 0.08 0.050.03 0.05 0.05 0.01 0.04 0.040.03 0.03 0.05 0.01 0.01 0.02

A 0.02 0.20 0.14 C 0.03 0.07 0.130.05 0.03 0.14 0.03 0.02 0.140.04 0.09 0.17 0.03 0.07 0.090.43 0.26 0.21 0.12 0.13 0.09

A 0.14 0.17 0.24 C 0.11 0.13 0.190.06 0.06 0.10 0.04 0.06 0.090.27 0.13 0.32 0.06 0.07 0.11

Camet 1 A __2 0.02 0.05 0.06 0.01 0.05 0.553 0.1 I 0.17 0.16 0.13 0.18 0.214 0.01 0.02 0.01 0.05

Jubany 1 A 0.05 0.07 0.08 C 0.07 0.07 0.102 0.10 0.03 0.09 0.16 0.10 0.113 0.17 0.14 0.23 0.20 0.19 0.204 0.10 0.11 0.12 0.06 0.11 0.11


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A survey of Argentinean atmospheric corrosion: II 661Table 3. Pollutant content in the corrosion products of copper weathered from 1 to 3y exposure in the

Argentinean test stationsConcentration

Station(ambient

condition)Cl-(mg me2) SOd2-(mg me2)

1Y 2Y 3Y JY 2Y 3YIguazu (rural)V. Martelh(urban-indust.)Camet (marine)Jubany (marine)

29.8 14.4 52.8 83.9 42.1 101.386.0 53.3 40.8 107.5 130.7 83.2491.6 474.9 464.0 6.8 75.1 102.71031.2 1221.3 70.9 111.7

SEM and EDAX dataSEM analysis on atmospheric exposed samples showed that the attack on Cu is lowerthan that previously found on Zn,22 whereas the thickness of the passive layers seems to be

more uniform. The plan and cross-section observations of corrosion products revealed goodagreement with the corrosion rates estimated from weight-loss measurements. UsingEDAX, the presence of Cl and S proceeding from both the marine and industrialatmospheric pollutants as well as the soil were determined. Distribution mapping can beobtained when their weight-fractions are higher than 2%, and then, interesting correlationwith results of the other techniques can be established.EDAX analysis on corrosion products with several morphology and size showed thatthe flat areas, in which uniform Cu corrosion was observed, contain lower quantity ofenvironmental pollutants (Figs 2-7). The large structures present higher pollutant contentsand are associated with localized attacks, with pitting under the nuclei. The larger the nucleisize is, the wider and deeper the pit becomes, as it can be seen in the cross-sectioncorresponding to the Figs 2-7. The reason for this is that both Cl- and SO2 stimulate thelocalized attack and then constitute partly the insoluble corrosion products on the metal.The depth of the pits, which can be seen in the cross-section SEM micrographs, increaseswith the time of exposure in the test sites with high contents in Cl- and SO2 pollutants (Figs6 and 7(b)).Comparing morphology and EDAX surface analysis of the Cu samples exposed from 1to 4 y, an increase in the density of globular corrosion products is observed, but maintainingthe size of each unit which is determined by the pollutant level. Comparable sizes of thestructures shown in Figs 2-7 resulted from similar total amounts of the different pollutant inthe stations, according to the data shown in the corresponding EDAX. Results for 2,3 and4 y showed an increase in the density of particles with the exposure time. In the cross-sectionmicrographs, few pits of small depth and poor cohesion were observed, mainly in Iguazu(Fig. 3b). A correlation between the size of the grown product nuclei and those of themetallic grains beneath them was also found.As in the case of the Zn samples,22 the pollutants are included in the layers of corrosionproducts on Cu, forming chlorides and basic sulphates, but since the corrosion products onCu are insoluble, the mapping allows determination of their distribution along the thicknessof the corrosion product layers. Figure 7 shows a change in the shape of the surface layer


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WT %2.545.680.381.600.68

SCLCAcu 89.11

looon

,Fig. 2. MEB-EDX of Cu samples after 1 y (a,b) and 4 y (c,d) in San Juan station. (a,c) In plan, (b,d)cross section.

formed on Cu, turning to be rather flat than globular. This non-typical morphology, whichis clearly observed in Fig. 7a, is similar to that found on Zn in the same Antarctic station,22and can be related to its development under an ice layer, which flattens the structuresmaking them lose the globular shape characteristic of the corrosion products formed on Cuin all the explored continental environments.4 The corresponding cross-sectionmicrographs reveal the localized character of the attack, which originates the globularproducts adjacent to areas with uniform attack. These two regions correspond to thebrighter and darker areas, respectively, in the plan micrographs of Figs 2-6. Both themapping of S and Cl and the EDAX diagrams exhibited their heterogeneous distributions,


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(4c

wr %0.52

99.49____--.---Kmo

f< cr-

Fig. 2. (Continued)

being associated with larger and higher roughness of the globular products. These resultswere found in all of the environments with high pollutant level in Latin America, thoughlower levels in Argentinas stations mask the results.14Exfoliation was still found in rural environments such as those of San Juan (Fig. 2

corresponding to 3 and 4 y) and Iguazu (Fig. 3b, 1 y). It can also be observed in VillaMartelli (Fig. 4b, 3 y), Camet (Fig. 6b, 4 y) and Jubany (Fig. 7b, 1 y).Analysis of pollutantsTable 3 shows the results of the pollutants extracted from the corrosion products layersgrown in the different environments of Argentinas six stations within the Corrosion


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664 J. R. Vilche ef al.

Fig. 3 MEB-EDX of Cu samples after 1 y (a,b) and 3 y (c,d) in Iguazu station. (a,c) In plan, (b,d)cross section.

Mapping Project4322 with the object of comparing the influence of the corrosion productcomposition and the pollutant solubility during the wetting-periods. Thus, pollutantcontents were obtained for the whole thickness of the surface layer, and since the includedcopper micrographs point out their detailed distributions, the calculation of the percentageaccompanied by the corresponding EDAX diagrams allows comparison with the totalvalues obtained through this technique.

The best correlation was obtained when comparing the results of both techniques onsamples of the same series, because the corresponding analysis were made on different pieces


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Cc . tfI 8

(4

WT0.21

99 19_________IW 00

Fig. 3. (Continued)

of the same sample. As the soluble compounds can be lixiviated during rain just before thecollection of the respective series, the comparison with the data of other series can show, ingeneral, quite different values. However, the results for pollutant content in corrosion layersgrown on Cu samples are in agreement for both techniques, because the pollutants areincorporated as basic compounds consisting of insoluble Cl and S containing species, whichare not reached by the rain.Since the Cl and S containing compounds present in the corrosion products formed onCu samples are insoluble at ambient temperature, they can not be cleaned by the rain actionduring the outdoor exposition and can be easily detectable with the EDAX technique.Though, they are lixiviated by hydrolysis during the boiling in distilled water. In this way,the results of both techniques are in agreement only for the case of Cu samples.**


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5 ELkM WC xAL 081SI 0 73S 034 r)FE 1.06CU _____~~o_q10000

-.^-._.___ _._-1._.__ _ _ ---._

171g. 4. MEB-EDX of Cu samples after 2 y (a,b) and 3 y (c,d) years in Villa Martelli station. (a,c) Inplan. (b-d) cross section.

Electrochemical impedance spectroscopy i EIS dataThe impedance measurements of the copper samples were performed to determine the

transfer function of the metal/corrosion products/electrolyte system. This was made inorder to establish a relationship between the environmental data monitored in the outdoortest stations and the characteristics of the passive layers formed on the exposed samples.Typical impedance responses at the corrosion potential, which lies at ca. - 0.40 f 0.05 V, areshown as Nyquist (Fig. 8) and Bode plots (Fig. 9) for copper samples exposed in the test sitesduring different times. The shape of the corresponding Nyquist diagrams exhibits twoslightly distorted capacitive semicircles and a third contribution at very low frequencies,which is related to slow processes on the corrosion layer such as diffusion. It is important to


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wr %0.360.48

99.16-1oooo

,

Fig. 4. (Continued)

mention that the partial dissolution of the corrosion products in the support electrolyteyields a decrease in the impedance values, as it can be seen in Fig. 8 for different immersiontimes. These results indicate clearly that EIS is a very sensitive technique for characterizingsurface layers. Since the passive films formed on copper are usually described as bi-layeredstructures, the two time constants at higher frequencies can be associated with a compactCut0 inner layer and a porous outer layer, composed by cupric sulphate, nitrate or chloridedepending on the atmospheric pollutants. 17**The experimental data can be discussed taking into account the impedance of the two-layer structure of the surface corrosion products in parallel with the cathodic reactionimpedance, cath and an additional time constant for the mass-transport contribution (Z,),


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ELEM WTSI 2.31

J. R. Vilche et al.

(b)

Fig. 5. MEB-EDX of Cu samples after 1 y (a,b) and 2 y (c,d) in La Plats station. (a,c) In plan, (b,d)cross section.

and in series with the uncompensated ohmic resistance (RJ, according to the followingtransfer function:

-I20~): RQ+ (.OC in + R,;')-'+(jwC,, + R,-:)-]-+(&ahL j (1)

where Ci, and Ri, are the capacitance and resistance of the C&O inner layer, respectively,and C,, and R,, those related to the outer porous layer contribution.

For each set of experimental impedance data the parameters involved in the transferfunction (1) were evaluated using non-linear least-square fit procedures. The results of the


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WI-2.655.913.351.280.7386.07----iCo%-

Fig. 5. Continued)

computer fit to measured impedance spectra are shown in Figs 10-12. The excellentagreement between the experimental results and optimum fit data indicates that theinterface is well represented by the proposed model. The fit parameters obtained for eachsample at its corresponding corrosion potential are summarized in Table 4. The ohmicresistance Rn was close to 77.5 Q cm for the whole set of samples.The parameters Ci,, Ri,, C,, and R,, obtained at the corrosion potential of each sampleare in agreement with the data resulting from the previous techniques, although they werefound to be only weakly exposure time-dependent. Provided there are not dielectricrelaxations in the measurable frequency range, Ci, can be expressed by


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3 759 74

CL 14.81K 0 49FE I65

l UJ 69.51loooo

-.._._I. . ._._- -- _J_- I

Fig. h MEB-EDX of Cu samples after 7 5 (a.h) and 4 y (c,d) in Camet station. (a,c) In plan, (b.d)cross section

C:,, = tto/d (2)where to is the permitivity of the vacuum, 8.85 x lo- I4 F cm-, and t and the dielectricconstant and the thickness of the oxide film, respectively. Thus, d can be evaluated from (2)by assuming a conservative value of t = 20 for the CUE0 and experimental values of Ci,.From the C,, data shown in Table 4, the average value for the calculated thickness results38.6 urn, though the values themselves vary between 0.2 and 300 urn. Values of CuzO


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KEL ZM"AL WI %7.01SI 15.42s 0.69CL 12.72K 0.64Ii FE 2.3361.20_________loO.cm

UT %0.231.8197.96-----__-_100.00

Fig. 6. Continued)

resistivity were reported to be between 10 and 50Qm,26 those corresponding to thecompounds present in the outer layer such as cupric sulphate were as high as 7 x 1O*2 m.27This is in agreement with the fact that the calculated values of Riwere smaller than those ofR,, in almost all the experimental measurements (Table 4).As the structure of the corrosion product layers formed on Cu samples after exposure atthe atmosphere is close to that found on Zn samples in a previous work,** i.e. a compactinner layer and a porous outer layer, the transfer functions used to interpret theexperimental results were similar. It is worth noting, however, that Cu samples developed


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Fig. 7. MEB-EDX of Cu samples after I y (a,b) and 3 y (c,d) in Jubany station. (a,c) In plan, (b,d)cross section.

corrosion product layers with higher impedance values than those found on Zn. In this way,the impedance contribution corresponding to the oxygen reduction reaction was consideredin parallel with that due to the passive film and characterized by its charge transfer resistancecathnd a diffusional contribution ZWwhich accounts for the mass transport controlled

oxygen electroreduction reaction.** This cathodic reaction occurs in the conditions of theexperimental impedance measurement and it does not necessarily correspond to that duringthe atmosphere exposition. In those cases in which the impedance of the passive layers onthe sample is lower than that of the oxygen reaction, the influence of this cathodic process on


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A surveyof Argentinean tmospheric orrosion: I

wr%2.032.620.9594.40----___-_ Icm.00

Fig. 7. Con t i n ued )

the impedance spectra becomes negligible. Thus, the parameters related to the latter processare hidden and can not be determined by the fitting procedure, as it can be seen in Table 4.The Warburg component, Zw, related to a process under diffusion control through afinite region of length 1corresponding to the thickness of the composite passive layer can bedescribed by2

Zw = Roo(jS)-12tanh(jS) 112 (31where the diffusion resistance R~0 is the limit of Zw(iw) as o+O and the parameter S = 1 w/D. For high values of either w or 12/D tanh(iS) approaches 1 and the Warburg impedance isrepresented by


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674 J. R. Vilche et a

-803G,g -40

00 40 80 120 160

Re/kQcm2Fig. 8. Impedance diagram obtained with Cu samples weathered 1 y at La Plata test station after 1to 72 h immersion times in the electrolyte.

ZW = R~0/1 2w/D)-~ l -j) = cm- 12(1 -j) 4)the corresponding values of the Warburg coefficient (Tare given in Table 4.From the corrosion rate data, an increasingly protective effect of the products formedwith exposure time (Fig. 1) evidently depends on their pollutant content (Table 3) and on theTOW (Table 1). DC potentiostatic steps did not show any monotonous trend with exposuretime. Factors such as the heterogeneous morphology associated to an irregular pollutantdistribution in the corrosion products limits the generalised analysis. For this reason aseparate discussion will be done for each different environment.

(0) 2 years(0) 3 years

T

2 0 2 4log [f/Hz]

Fig. 9. Impedance diagram obtained with Cu samples weathered at Jubany test station afterdifferent exposure times.


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7 5fEN=4 3

80

8 60i i8 40

20

0

)lyeaY w(0) 2 years(v) 3 years(v) 4 years ST

2 0 2 4l og [f/Hz]

Fig. 10. Impedance diagram for Cu samples after different exposition times at Iguazu test station.Full line traces correspond to results fitted by the transfer function (equation (1)).

(0) 1 year(0) 2 years(v) 3 years

LI-2 0 -2 4

l og [f/Hz]Fig. 11. Impedance diagram for Cu samples after different exposition times at San Juan test station.Full line traces correspond to results fitted by the transfer function (equation (I)).


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0 -2 0 -2 -4log [f/Hz]

Fig. 12. Impedance diagram for Cu samples after different exposition times at Villa Martelli teststation. Full line traces correspond to results fitted by the transfer function (equation (1)).

Table 4. Fitting parameters for copper samples exposed at the different test sites

Test stationExposure C,time (y) (nF cm-*)

Ri(kR cm) (nF>-) (kQk2) -ah SD(kn cm*) (kR cm2)San Juan

Camet

IguazuL34

Villa Martelli I34

La Plata 12

Jubany 123

26.2 0.52789.2 0.53040.7 108.7

0.32 1.3152.18 715.90.057 26.650.214 21.231.77 3015.

47.3 909.8112.0 0.662

0.745 4.7370.157 61.52

6.03 20.363.65 36.15

13.3 7.8193.52 87.550.468 35.10

1.46 34.71 13.348.44 61.76 6.3425.58 17.09 14.74

10.3 1890. 142.40.350 1799.0.388 1686. 109.30.289 217.1 212.50.010 4124.0.260 2.2220.094 183.71.99 64.20 33.588.90 1390.

10.4 483.1 84.797.29 176.1 1189.2.34 365.2 147.93.01 17.25 49.792.53 6.079 46.565.16 0.3977 0.1010.545 27.24 41.771.41 230.5 187.9

13.8769.6698.01

247.73730.466.7

45.488897.

663.7480.3

16.19114.9314.8343.5

24.2566.46

4.79156.52

217.6


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A survey of Argentinean atmospheric corrosion: II 67-lThe lowest corrosion rates (Fig. 1) were determined in the rural test station of San Juan,

with results from 2 to 4y within the values obtained in the three first annual periods.Comparing Fig. 1 and Table 2 from 1 to 4y with Fig. 2b, the morphology is evidentlyresponsible for the poor barrier effect of the corrosion products at 4 y exposure time. Thismorphology corresponds to the highest current values reached after anodic and cathodicpulses, which also increases with immersion time. It can also be noticed that theprotectiveness, estimated though the corrosion rates, was time-independent (Fig. 1).

Protectiveness in Iguazu followed the same trend, which could be attributed to anincrease in pollutant content masking the effect associated with the thickening of thecorrosion products layer after the third year. Otherwise, these product films would increasetheir protective properties with time of exposure, which correspond to a decrease in thecurrent measured when potentiostatic pulses are applied.

At the Villa Martelli urban ambient, protective effect of the products formed in the firstyear was observed, though no further increase was noticed with time. The morphology incross section (Fig. 4) after 2-3 y does not justify better performance with increasingexposure time although the pollutant levels decreased (Table 3). In the urban test station LaPlata no protective corrosion layers were observed, indicating low adherence, especially inthe micrograph in cross section shown after the second year of exposure. A low increase inthe protective character of the products can be noticed with time, both from mass losses andfrom pulses results.

In the samples weathered in the marine stations, corrosion rate decrease is the mostmarked (Fig. l), in spite of the highest Cl- and SOf- contents increase with time. Thecurrent values when pulses are applied are correlative to the pollutant content in thecorrosion products and at Jubany environment during the respective test period. It has beenstated that chloride ions penetrate protective oxide films through pores, flaws (cracks) orother weak spots.30 It was argued that no matter how compact, a passive layer containsflaws through which the chloride ion easily penetrates. The flaws were stated to be largeenough to permit the passage of large aggressive ions. This viewpoint stresses that thesurface oxide layer plays an essential inert role in the pitting process. The role of halide ionscentres around: (a) competitive adsorption with OH- on the available copper surface thuscreating sites that are more liable for electrochemical dissolution and (b) competition withOH- attached to Cu(I1) in a soluble intermediate stage, thus enhancing film rupture(through dissolution).30 Furthermore, it was shown that the first step of the anodicdissolution of copper in chloride solution is the formation of the complex CuC12-, and thatduring the anodic polarization there is always an equilibrium between two compounds: athin layer of CuCl and a dense liquid layer of dissolved CUCI~-.~ Nevertheless, the degreeof the influence of aggressive anions in breaking down protective films depends on both thenature of the surface (oxide) and the added anion. Then, although cuprous chloride or basiccupric chloride (3CU(OH)2.CUC12) can be formed in the surface film,6 the values of thesolubility products indicate that the formation of oxides is favored and that the basic coppercarbonate is less soluble than chloride compounds.6 The insoluble CuO (or CUE) film ismuch less stable in the presence of I- compared with Cl-. Compared with Cl- and I- ions,the presence of phosphates and to a lesser extent sulphates, partly inactivate the coppersurface and cause a pronounced anodic current decrease of Cu(I1) process.30 Solubility of allcompounds increases considerably with decreasing PH.~


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678 J. R. Vilche er al.CONCLUSIONS

Investigations on outdoor exposed Cu samples show that the protectiveness of corrosionproducts as a function of outdoor exposure time is markedly increased in marineenvironments, moderate in the urban sites and almost negligible in rural environments.While corrosion product morphology is the main factor responsible for the variation in thecorrosion rates found among the test stations, pollutant content explains additional andsingular tendencies, specifically detected in each site. Furthermore, electrochemicalimpedance measurements represent a powerful tool to obtain information on theproperties of the passivated interface. The experimental results are interpreted in terms ofa double passive layer structure model, a compact CuzO inner layer and a porous outerlayer, composed by cupric sulphate, nitrate or chloride. The cathodic process, oxygenreduction reaction, present in the experimental measurement conditions was alsoconsidered as contributing to the whole electrode impedance.Acknowledgements-This research project was financially supported by the Consejo National de InvestigacionesCientificas y T&cnicas, the Comision de Investigaciones Cientificas de la Provincia de Buenos Aires, the CYTEDprogram of Spain in the coordination of the MICAT project, and the Fundacion Antorchas.

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(1997) a survey of argentinean atmospheric corrosió-copper samples

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