Bautista, A. Et Al. Electrochemical Study Corrosion Cooper Plates. 2007

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1. INTRODUCTION

Copper chalcographic plates have been frequentlycoated for years (and they still are nowadays) with metalelectrolytic coatings of higher hardness, in order to enlargethe number of printings. For instance, in the National

Chalcography of Spain, there are 230 copper plates en-graved by Francisco de Goya (the 93% of his chalcograph-ic work), and all of them exhibit electrolytic coatings. A damage assessment was carried out in a previous study,which found that 73% of the plates have some kind of dam-age [1]. Thus, our investigation involves obtaining as muchinformation as possible about the influence of the presenceof metallic coatings on the copper plates for designing theiroptimal conservation and restoration strategies.

The nature of the coatings on the Goya's plates has notbeen extensively studied. However, there is some docu-mented information [1] along with X-ray

diffraction (XRD) analysis that offers interesting informa-tion. X-ray diffractograms shown in Figures 1 and 2 corre-spond to “Tauromaquia No 8” and “Capricho No 10”. As canbe deduced from the diffractrograms, “Tauromaquia No 8”is chromium plated, and there are archival documents [1]that prove that all the “Tauromaquia” series is chromium

plated. “Capricho No 10” is iron plated, and this type of metallic coating was also detected by XRD analysis on platesbelonging to “Desastres de la Guerra” series, “Disparates”series and other Goya's plates. Moreover, XRD has alsobeen used to identify some Ni coatings on Goya's plates[1]

Galvanic coatings were the first conservation issue ap-plied on the plates. The presence of the coatings on the en-graving plates not only changes their surface hardness, butalso affects their corrosion behaviour. Nowadays, any con-servation policy of engraving copper plates should be mod-ified due to the presence of these coatings, and it specificstrategies should be

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Electrochemical Study of the Corrosion Behaviour

of Electroplated Copper PlatesA. Bautista, M. Lage, F. Velasco

Materials Science and Engineering Department. Universidad Carlos III de Madrid

Avda. Universidad no 30Leganés, Madrid 28911, SPAIN

Phone: (34) 91 624 99 14Fax: (34) 91 624 94 30

e-mail: [email protected]

For enlarging the number of printings, copper chalcographic plates were frequently coated for years (evennowadays) using electrolytically applied metal coatings of higher hardness. Galvanic coatings present a corro-sion performance different from that of the copper base, which influences their condition. Our paper has de-

termined the corrosion rates of bare copper plates along with copper plates with electrolytic coatings using ei-ther chromium or iron in neutral and acid solutions. Two different dc electrochemical techniques have beenused: polarization curves and polarization resistance measurements. Moreover, many of these electrolyticallycoated plates have areas of bare copper. The effect of galvanic couplings that can appear on these materials hasbeen quantified electrochemically.

Keywords: copper plates, chromium coatings, iron coatings, corrosion rates, galvanic couples

Figure 1 - X-ray diffractrogram correspondingto “Tauromaquia No 8”

Figure 2 - X-ray diffractrogram correspondingto “Capricho No 10”

A P ° Y P O ¶ O Y § O Y T E L O S 3 0 - 0 1 - 0 8 1 1 : 0 2 ™ Â Ï › ‰ · 1 1 0



Electrochemical Study of the Corrosion Behaviour of Electroplated Copper Plates

designed depending on the nature of the coatings and the

characteristics of the environment they are exposed to. Inthis paper, a simple approach to the corrosion rate of bare

copper, and iron and chromium electrolytic coatings is pre-

sented. The studies were carried out in neutral medium,simulating the corrosive conditions of an uncontrolled mu-

seum environment with fluctuating and high RH, as well as

an acid medium, since air borne contaminants can reducethe pH of water vapour that condensates on the surface of

the objects. Moreover, the influence of galvanic couples,

which can easily occur for engraving plates with electrolyticcoatings, was carefully analyzed in this work.

2. EXPERIMENTAL TECHNIQUES

Copper plates of 2.9 mm width were examined both as

uncoated and coated with electrolytic layer containing

chromium and iron.Electrochemical measurements were carried out in

neutral media (0.1 M KClO4 solutions) and acid media

(5% H2SO4 solutions). The traditional three electrode con-figuration was used, using a saturated calomel electrode

(SCE) as reference electrode. The used potentiostat was

PAR 263A from EG&G Instruments.Polarization curves were carried out after exposing the

copper plates for 120 minutes in the test solutions. The po-

tential was swept from -175 mV vs. the corrosion potential(Ecorr) to +175 mV vs. Ecorr at a rate of 0.16 mV/s.

Polarization resistance measurements were performed

periodically from -20 mV vs Ecorr to +20 mV vs. Ecorr. The

potential was swept at 0.20 mV/s.

Galvanic intensities were measured using a zero resis-tance ampmeter. Specimens of similar surface exposed to

the testing solutions were used.Prior to the electrochemical studies performed on repli-

cas, XRD measurements of original Goya's plated engrav-

ing plates were carried out from 2ı=5o to 2ı=100o, at asweeping rate of 0.04o/s, using a X’Pert Phillips equipment.

3. RESULTS AND DISCUSSION

The polarization curves of the bare and coated coppersin neutral medium can be seen in Figure 3. Polarization re-

sults in acid medium are shown in Figure 4. The mostmeaningful parameters that can be calculated from bothfigures are summarized in Table 1.

Bare copper plates exhibited low corrosion intensities(icorr) in both studied media, being slightly higher in acid. Inboth media, copper corrosion rate was controlled by thecathodic process (anodic Tafel slope, ‚a, is always minorthan cathodic Tafel slope, ‚c). In acid medium, the Ecorr isalso less noble, and the cathodic process is less impeded inacid media (bc is lower). Obviously, the long-term behav-iour of the copper would depend on the protective abilityof the formed oxides, which is also dependent on the pH of the media [2,3].

It can be checked that iron coatings dramatically de-creases the corrosion resistance if it is compared with thatof original bare copper engraving plates. In neutral media,the corrosion intensity (icorr) of the electrolytic iron is morethan 20 times higher than that of the bare copper plates.However, in the acid media, both iron coated copper plateand bare copper plate increases their corrosion rates, butiron coated copper plate does it at a higher extent thanbare copper. In 5 %

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Figure 3 - Polarization curves correspondingto the studied materials in neutral medium

Figure 4 - Polarization curves correspondingto the studied materials in acid medium

Table 1. Parameters calculated from polarization curves

Medium Material Ecorr icorr ‚a ‚c

(mV) (A/cm2) (mV) (mV)Cu 26 6.0e-7 16 236

Neutral Cr -240 7.1e-8 255 210Fe -490 1.5e-5 260 260Cu -78 1.1e-6 74 140

Acid Cr 11 1.0e-7 46 250Fe -506 1.0e-4 185 120




A. Bautista et al.

H2SO4 solution, electrolytic iron coated copper plate cor-

rodes 100 times faster than bare copper plate. This data ex-

plains the extent and frequency of the damages that iron

coated engraved plates shown all over their surface (see ex-

ample in Figure 5).

XRD carried out on the back side of some iron coated

plates, where relatively large deposits of corrosion prod-

ucts had been detected, are compatible with hematite

(Fe2O3) (Figure 6).

On the other hand, chromium coated copper plates ex-

hibit better corrosion behaviour than bare copper plates in

both media (Table 1). The corrosion rates of the electrolyt-

ic coating in both studied media are more than one order

of magnitude lower than those of the bare copper.

These results about the good corrosion resistance of

chromium corresponds to lack of corrosive attack that are

usually observed in chromium coated engraving plates.

In spite of being a relatively simple method to estimatethe corrosion rate of the metals, polarization curves imply

the imposition of high overpotentials to the studied system,

so they are a destructive test. Polarization resistance (Rp)

measurements are faster, non-destructive, and they can be

applied several times to the same specimen to observe how

the corrosion rate changes with time. In Figure 7, Rp mea-

surements

in the slopes of the lines, that indicate that the Rp of thestudied material decreases with time.

The Rp values can be used to calculate icorr, using theStern-Geary equation [4] and the ba and bc values obtained

from the polarization curves (Table 1). Adequate similaritycan be observed between corrosion rates calculated fromRp measurements and from polarization curves (Figure 8).

Moreover, the amount of metal transformed into oxideduring the tests can be calculated from the icorr values usingthe Faraday’s law [5]. Using the results from Rp measure-ments, weight loss for the studied plates, in Figures 9 and10 were estimated. The higher corrosion rates suffered byiron coated copper plates indicated that the weight lossescorresponding to this coating should be represented in dif-ferent vertical axis than those corresponding to bare cop-per or chromium coated copper plates.

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Figure 5 - Appearance of “Disparate no 4”, that is ironcoated, and where extensive corrosion damage can be easi-ly observed.

Figure 6 - X-ray diffractrogram of oxide powders detachedfrom the back side of the Goya's plate “Felipe IV” wherepeaks compatible with Fe2O3 (v) can be identified.

corresponding to bare copper exposed for different timesin H2SO4 solutions are shown. A decrease can be observed

Figure 7 - Temporal evolution of the Rp of bare copper

in acid medium.

Figure 8 - Relationship of icorr values obtained using differ-

ent electrochemical techniques for the studied materials

after similar exposure times.

Figures 9 and 10 indicates that initially, all the studied

plates tend to corrode faster, but after the formation of a

certain amount of oxides on their surface, the corrosion at-

tack slows down. This phenomenon is evident when a large

amount of oxides is formed (see curve corresponding to

iron coating in acid, in Figure 10) or when the formed ox-




Electrochemical Study of the Corrosion Behaviour of Electroplated Copper Plates

ides are highly protective, as chromia (see curve corre-sponding to chromium coated copper plate in Figure 9).

However, the electrolytic metal coatings do not usuallycover the entire surface of the copper engraving plates.

Quite often, coated plates have evidence of areas withoutcoating. These areas can have different origins:ñ Suction pads placed at the back of the plate to hold the

plate in the electrolytic bath [6] (Figure 11).ñ Clamps placed on edges for holding in the electrolytic

bath the plates when they are engraved on both sides(Figure 12).

Figure 11 - Bare copper region caused by a suction padon the back side of “Capricho No 36”.

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Figure 9 - Weight loss suffered by the metals due to expo-sure in neutral medium. The results are calculated from Rp

measurements.

Figure 10 - Weight losses suffered by the metals due to ex-posure to acid medium. The results are calculated from Rp

measurements.

ñ Detachments of the coating due to scarce careful appli-

cation or manipulation of the plates (Figure 13).ñ Loss of the coating due to corrosive attack (Figure 5).

The strength of the galvanic couples that can appear oniron and chromium coated engraving plates was measured

during this research in neutral and acid solutions. Curves

as that shown in Figure 14 are obtained. During the mea-

surements the bare copper and the coated copper coupons

are connected in such a way that a positive the galvanic cur-

rent (igalv) indicates that the metallic coating favours the

corrosion of the bare copper, and a negative igalv means that

the copper reduces its corrosion rate due to the presence of

the coating and it speed the attack on the coating.

Figure 12 - Bare copper region caused by clamps on theone of the edges of “Tauromaquia No 8”.

Figure 13 - Improper metal coating due to greasy contami-nation from finger prints prior to the electrolytic processon “Capricho No5”.

The bare copper, when is short-circuited to electroplat-ed chromium in H2SO4 solutions, tend to be polarized bythe electrolytic coatings. Initially the Ecorr of isolated Cu is

more noble than the Ecorr of isolated Cr (Figure 14). Thatmeans that when both metals are short-circuited, copperprotects itself and favours the attack on the chromium.However, this only occurs during the first few minutes.

While the Ecorr of the Cu hardly changes during exposure,the Cr coated plate in 5% H2SO4 passivates in a very effec-tive way. A very isolating oxide layer grows on this surface,as can be checked by the marked increase on its E corr.Hence, after a short time, the chromium becomes morecorrosion resistant than copper in acid, and the sign of thegalvanic current changes. After the few minutes of stabi-

lization, the Ecorr of the galvanic couple is 30 mV




A. Bautista et al.

Figure 14 - Galvanic current measured between bare Cuand chromium plated Cu in acid medium. Time evolutionof the corrosion potentials of both isolated materials inthat medium are also included.

ply such kind of conservative technique in pieces with artis-tic value. It will be interesting to carry out studies that ana-lyze the possibility of removing the coating without affect-ing the copper base. Besides, the original artefact of the

artist could be appreciated without the metallic coating.

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Figure 15 - igalv measured between bare Cu and the studiedelectrolytic coatings. Currents that favour the corrosion of

the copper are marked in black. Currents that favour thecorrosion of the coating are marked in grey.

vs SCE, very close to the Ecorr of isolated Cr. When the stabi-lization is reached, the corrosion of the copper is accelerated

by the presence of electrolytic chromium. It is true that theigalv is lower than the icorr of isolated copper in acid (Table 1),but data in Figure 14 has been obtained exposing identicalsurfaces of bare copper and electrolytic chromium to the so-lution. In practice, the surface of bare copper on coatedplates is small in comparison with that electrolytic coating(Figures 5 and 11-13), so it is probably that if this kind of gal-vanic coupling appears on engraving plates, it was more dan-gerous for the copper than in the designed laboratory test.

The ability of chromium to accelerate the attack on cop-per plates has also been detected in neutral media (Figure15). As expected, the strength of the galvanic couple is minorthan in acid solutions, due to the minor aggressivity of themedium. Copper oxides have been identified by XRD in the

boundary between chromium and copper caused by the suc-tion pads (as those shown in Figure 11). A small peak com-patible with Cu2O is found in measurements performed inthat region (Figure 16). In Figure 15, it can also be seen thatcopper forms intense galvanic couples with iron coating.These galvanic couples favour iron corrosion. For these ma-terials, the measured igalv are higher than the icorr of the elec-trolytic iron in the studied media (Table 1). In practice, theeffect of the galvanic couple could be reduced by the area ra-tio. In this case, the anodic area on engraving plates (iron) ismuch higher than cathodic area (copper). In summary, bothchromium and iron coatings can have adverse effects forthe conservation of copper plates. It is not advisable to ap-

Figure 16 - X-ray diffractrogram of Goya's plate “Tauro-maquia 8” where peak compatible with Cu2O can be iden-

tified. Diffractogram taken on a bare copper region of achromium coated plate.

4. CONCLUSIONS

Galvanic coatings increase the hardness of the surfaceof engraving plates, but our study has shown disadvantagesfrom the corrosion point of view:ñ Iron coatings exhibit a low corrosion resistance and cor-

rode easily, increasing the difficulty of the conservationof the engraving plates.

ñ Chromium coatings increase the corrosion resistance of the copper, but tend to form galvanic couples with it, soit can favour the corrosion of copper in those regions

where the copper of the engraving plate is exposed tothe environment.

REFERENCES

[1] Lage, M.: "Goya, ciencia y tecnologí a para la conser-vación de sus matrices de grabado calcográfico", InLage, M., Mota, J.M. (eds.): "Ciencia y Tecnologí a parala conservación de matrices de grabado calcográfico".Fundación BBVA, Bilbao, pp. 143-166 (2005).

[2] Mora, N., Cano, E., Mora, E.M., Bastidas, J.M.: "Influ-ence of pH and oxygen on copper corrosion in simulateduterine fluid", Biomaterials, Vol. 23, pp. 667-671 (2002).

[3] Palit, A., Pehkonen, S.O.: "Copper corrosion in distrib-

ution systems: a evaluation of homogeneous Cu2O filmand natural corrosion scale as corrosion inhibitors",Corros. Sci., Vol. 42, pp. 1801-1822 (2000).

[4] Stern, M., Geary, E.D.: "Electrochemical polarization I.A theoretical analysis of the shape of the polarizationcurves". J. Electrochem. Soc. Vol. 104, pp. 56-63 (1957).

[5] Otero, E.: "Corrosión y degradación de materiales".Editorial Sí ntesis S.A., Madrid, p.60 (1997).

[6] Bautista, A., Velasco, F.: "La electrodeposición en lasplanchas calcográficas", In Lage, M., Mota, J.M. (eds.):"Ciencia y Tecnologí a para la conservación de matricesde grabado calcográfico". Fundación BBVA, Bilbao,pp. 37-58 (2005).


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Bautista, A. Et Al. Electrochemical Study Corrosion Cooper Plates. 2007