Indian Journal of ChemistryVol. 17A, June 1979, pp. 567-569

Corrosion of Copper in Chloroacetic Acid SolutionsE. A. HASSAN, S. M. ABD EL-WAHAAB* & M. M. HEGAZ!

Chemistry Department, Faculty of Science, Al Azhar University, Nasr City, Cairo, Egypt

Received 16 September 1978; accepted 1 January 1979

The corrosion behaviour of copper in solutions of mono-, di- and trichloroacetic acids ofdifferent concentrations has been investigated. The maximum steady state potentials areachieved from positive direction, indicating the destruction of the preformed oxide film on theelectrode surface. An increase in the concentration of acid solution is accompanied by a markedshift of the potential in the positive direction. The maximum steady state potential is found tobe related to the concentration of the acid solution by the equation E = a+b log c, where a andb are constants. The rate of corrosion decreases with increasing solution concentration.The effect of temperature 011 the steady state potentials and the rates of corrosion of copperhave also been investigated in 2M solution of these acids.

THE corrosion and corrosion inhibition of copperand copper-base alloys have been studiedext ensively-?" Shaya! and Hill" proposed that

the rate of corrosion of copper and its alloys inaqueous solutions is partially dependent on theoxygen content as well as the pH of solution.Manning and Dillon+ were also of the opinion thatthe rate of corrosion of c'Jpper and its alloys inacetic acid solutions is neither dependent on tem-perature nor on acid concentration, but is mostlydependent on the degree of aeration of the .solution.

The anodic behaviour of copper in alkaline solu-tions has also been studied by several investi-gators12-1? Cuprous and cupric oxides or cuprichydroxide were reported to be formed on the" elec-trode surface before oxygen evolution12-15. Muller-"and Shams El-Din17 described the formation of ahigher oxide, Cu20a, upon oxidizing copper instrongly alkaline solutions. .

It is now generally accepted that aqueous environ-ment must be sufficiently oxidizing in order tomaintain the oxide film intact; the latter is essentialfor maintaining the metal passive. Such a conditionis usually brought about only in the presen~e ofinhibitive anions, since these prevent or hinderpassivation by breaking down the oxide film18.

In the present work, corrosion behaviour of ?opperis studied in stagnant conditions of mono-, di-, andtrichloroacetic acid solutions. Steady state poten-tials have been studied as a function of both timeand acid concentration. The effect of temperatureis also examined in the range of 30°-60°. Thecorrosion rate of copper metal has been furtherdetermined using the weight-loss procedure.

Materials and MethodsThe copper electrodes used were machined from

electrolytic copper sticks (BDH) in the form ofsmall rods (1·5 em length and 0·5 .cm dim;t.),which were fixed to pyrex glas? tubes WIth arald\te.Electrical contacts were achieved through thickcopper wires soldered to one end of the rods.

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Before being used, the electrodes were abradedsuccessively with 1, 0 and 00 grade emery papersand then degreased with acetone. The electrodevessel used was made of pyrex glass without rubberconnections, and could accommodate 50 ml of thetest solution. The potentials of copper electrodeswere measured in solutions of AR grade mono-, di-and trichloroacetic acids of concentrations varyingbetween 1M and SM. Potentials were measured(versus SCE), as a function of time using a Pyepotentiometer in conjunction with sclamp galvano-meter type MSZ-SOS.

Steady state potentials were considered when thepotential value did not alter by more than 1 mV/5 min. Each experiment was carried out with anewly polished electrode and with a fresh portionof the solution at 24-26°. Similar experiments werecarried out at 30°, 40°, 50° and 60° using 2Macid solution.

Weight loss experiments were carried out usingcopper specimens of the following dim ensions: length5 cm ; width 2 cm; and thickness 2 mm. Thesewere abraded, degreased, washed and immersed in50 ml of the test solution and thermostated at therequired temperature.

The type and extent of attack were determinedat different time periods and at the end of thetest period. The samples were then cleaned bybrushing under running water, dried and weighed.Solvent loss by evaporation was compensated by theaddition of doubly distilled water. The dissolvedcopper ions were determined volumetrically usingSchwarzenbach's method'".

Results and DiscussionPotential-time measuremenis-« Potential-time curves

for copper electrodes in stagnant solutions of mono-,di- and trichloroacetic acids of concentrations1-5M are obtained by studying the variation of theelectrode potential under open circuit conditions.For these solutions, the maximum steady statepotentials of copper electrodes are approached from

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INDIAN J. CHEM., VOL. 17A, JUNE 1979

positive values, indicating the destruction of thepreformed oxide film on the electrode surface20•21.

However, for the three acids, the increase of solutionbll:lk concentration is accompanied by a markedshift of the steady state 'potential towards thenobler values, indicating tendency for inhibition.

The plots of steady state potentials of copperelectrodes versus logarithm of the molar concen-tration C of the different acids used are linearand the data fit relation of the type (1)E = a+b log c ... (1)where a and bare constants depending on thenature of the electrolyte and surface preparali)n18-20•

However, Brashers? and Shams EI-Din21, obtaineda negative s~ope for the linear plots in the case ofsteel and zinc electrodes respectively in neutralsalt. solutions. This behaviour was explained on thebaSIS of the fact that increase in the anion concen-tration was accompanied by a shift of the steadystate potential in the active direction.

It is well known-s that the corrosion of c)pperdepends on several factors, among which are: thecomplex ion formation, the solubility of reactionproducts and the oxygen content of solution. It isprob~ble that the immersion of copper electrodes insolut~ons of chloro-acids is accompanied by dis-solution of th: metal to yield cuprous ions. Thechloroacetate ions meet the incoming cuprous ions~ear the electrode surface and react to give themsolu~le copper salts which appear as a green layeradhering to the metal surface.

.Th~s, the ~nno~ling of copper electrode potentialWIth mcrea~e m acid concentration may be expla'nedon the baSIS of the probable formation of insolublecorrosion products which adhere to the metalsurface, although the formation of complex ion isnot excluded. .

A~ inspection of the potential-log molar c mcen-tration curves shows that the steady state potentialof the copper electrode becomes more noble as theconcentration of the chloro acids increases. Ac-cordin~ly, the value of b in Eq. (1) is constantlyapproximated at +0·075 V for the three acids.This is in accordance with the previously reportedb valuesw, It is also observed that the value of theconstant a decreases in the order: di- >mono- >tri-chloroacet ic acid.

Weight loss experiments - The results of immersiontests in 1M-5M mono-, di- and trichloroacetic acidso.luti~ns show that the weight loss increases linearlywith time for 80-160 hr (the duration of experiment).The corros~on rates which have been computedfrom the linear plots of weight loss versus timeare collected in Table 1.

The corrosion rate is found to be related to themolar concentration of the acid by Eq. (2)dw([i (me/cm2/10 hr) = KC-n ... (2)

In Eq. (2) C is the molar c::mcentration and Kand n are constants depending on the surfaceprep~ration' of the metal and the type of thesolution 20.

The plo~ of log (corrosion rate) versus log (molarconcentration) of the acid solutions are linear with

568

TABLE 1- VARIATION OF CORROSION RATE WITH TH:E:CONCENTRATION OF CHLOROACETIC ACID SOLUTIONS

[Acid]Corrosion rate (mg/cm2/10 hr)

------M Monochloro Dichloro Trichloro

1'0 0'15 0'875 6·002·0 0'08 0·500 5·003'0 ·0'07 0·400 4'504·0 0·05 0'300 4·005·0 0'03 0·230 2·50

negative slopes, indicating that the rate of corrosionof c )pper in these solutions decreases with theincrease in acid concentration. The corrosion rateis greater in trichloroacatic acid solutions as corn-pared to the other two acid solutions. This beha-viour is attributed to the increase in basicity ofthese acids on going from mono-, di- to trichloro-acetic acid solutionsw.

The results obtained indicate that increase inacid conc mtration is accompa iied by a decreasein c orrosion rate as well as a positive shift in thesteady state potential values. Thus, it may besuggested that the corrosion process is d~termi.nelby depolarization of the cathodic controlling re-action, i. c. the corrosion process is cathodi.callyc )ntrolled22-24.

The corrosion rates, however, increase in theorder: m ono-«; di-< trichloroacetic acid solutions.This is c msist ent with their order of reacivities'" .The change in steady state potentials with acidc)nc~ntration does not obey the same order asobtained from steady state p)tential versus log'molar c mc mtration curves. In the case of dichloro- .acetic acid solutions, the steady s'ate potentialvalues of c)pp:3r electrode are found to be morenobler than those in tri- and monochloroacetic acidsolutions. This could be attributed to the typeand nature of the film formed on the metal surface.A plausible explanation is the greater possibilityof the highly acidic «-hydrojen of the dichloro-acidto participate in a c omplex salt formation. Thissalt is believed to be less soluble than the othersalts. It is evident that this behaviour is completelyruled out in the case of the trichloro-acid, sinceit has no «-hydrog'en atom. In monochloro acid,it is. believed that the tWJ IX-hydrogens are notsufficle!ltly acidic to participate in such complexformation.

Effect of tevnperature - The effect of temperatureon the behaviour of c)pp~r electrode in 2M solutions?f acids under investigation, has been investigatedm the temperature range of 30°-60°. It is observedthat the increase in temperature increases theco~rosion rate and is accompanied by a mar~edshift of the steady state potential to the negatIVedirection. The plots of the steady state potentialsand the logarithm of corrosion rate respectivelyas a function of temperature are linear in theseacid solutions. The energies of activation of thethree chloro-acids in prom )ting the corrosion ofc')pper, calculated from the slopes of the linearplots of log (corrosion rate) versus lIT, are 7·59

n,HASSAN et al.: CORROSION OF COPPER IN CHLOROACETlC ACIDS

kcal/mol for monochloroacid, 4·58 kcal/mol fordichloroacid and 2·53 kcal/mol for trichloroaceticacid solutions which c)nfirm the more aggressiveaction of trichloroacetic acid. A similar behaviourwas noted with iron in chromate, benzoate andnitrite solutions25-27 and with zinc in chromatesolutions" in the presence of aggressive anions.

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