The Effects of Temperature and pH on the Dissolution and Passivation Processes of Copper in Carbonate-Bicarbonate Solutions

Foram feitos estudos eletroquímicos de eletrodos de cobre policristalino em soluções de carbonato-bicarbonato, analisando os efeitos da temperatura e do pH, cobrindo uma extensa faixa de composições eletrolíticas num intervalo de 5-65 °C. As condições hidrodinâmicas do sistema também foram consideradas. Os resultados são explicados com base numa modificação das características do filme decorrente dos efeitos de agitação da solução e das temperaturas elevadas. Nestas situações, forma-se uma camada menos hidratada, reduzindo o processo de dissolução. A força iônica torna-se um fator adicional que afeta o estado geral do sistema.

The aim of this study was to at tain in for ma tion about the potentiodynamic be hav ior of cop per in so lu tions containing car bon ate/bi car bon ate ions, cov ering wide ranges of elec trolyte pH, tem per a ture, and ionic strength, in or der to es tab lish their con tri bu tions to the metal dis so lu tion and passivation pro cesses.

Ma te rials and Methods
The elec trochemical equip ment, ex perimental de tails and elec trode treat ment have been pre vi ously de scribed in de tail 7,12 ."Specpure" cop per ro tat ing disks (John son Matthey Chem i cals., 0.15 cm 2 ap par ent area) em bedded in PTFE hold ers were used as the work ing elec trodes.
The potentials were measured against a saturated calomel electrode (SCE), but in the text the potential are re ferred to the NHE scale, tak ing into ac count the corrected po ten tial val ues of SCE ac cording to a suit able equation 13 .A large-area plat i num plate was em ployed as the coun ter elec trode.
The elec tro lyte so lu tions con sisted of a mix ture of xM KHCO 3 + yM K 2 CO 3 (0.0075 ≤ x ≤ 2.25; 0.0005 ≤ y ≤ 1.5), cov er ing the 8.3-11.6 pH range.They were prepared from an a lyt i cal grade re agents and tri ple-distilled wa ter pre vi ously boiled to re move car bon di ox ide.At each pH, the val ues of x and y were set to cover a wide range of Article ionic strength by keep ing the HCO 3 --CO 3 2-con centration ratio con stant.Elec tro chem i cal mea sure ments were performed at 5, 25, 45, and 65 °C un der pu rified ni tro gen gas satu ra tion us ing sin gle and re pet i tive tri an gu lar po ten tial sweeps, be tween pre set cath odic (E s,c ) and an odic (E s,a ) switch ing po tentials at po tential scan rates (v) in the 0.002 V/s ≤ v ≤ 0.300 V/s range.The work ing elec trode was maintained ei ther at rest or un der ro ta tion at speed , which was var ied in the range of 600 rpm ≤ ≤ 2500 rpm.
After elec tro chem i cal mea surements, some spec imens were ex am ined by scan ning elec tron mi croscopy (SEM).

Re sults and Dis cus sion
The voltammograms of cop per in still 0.75M KHCO 3 + 0.05M K 2 CO 3 so lution at pH 8.9 run at dif fer ent v, at 25 °C, between E s,c =-0.95 V and E s,a =0.55 V (Fig. 1), show three anodic cur rent con tri bu tions, at ~ 0.03 V (Peak 1), 0.18 V (Peak 2), and 0.40V (Re gion 3).Peak 1 is re lated to the Cu 2 O electroformation (first pas si vat ing layer) 1,4 , while Peak 2 was at trib uted to the for ma tion of a Cu(OH) 2 -CuO complex layer through a nu cle ation and growth mech a nism under dif fu sion con trol 1,4,10 .The broad cur rent con tri bution cor re spond ing to Re gion 3 may be re lated to the presence of a pro tec tive com pos ite CuCO 3 -Cu(OH) 2 layer (second pas si vat ing layer) [5][6][7] .Peaks 1 and 2 over lap at v > 0.025 V/s, so Peak 1 can not be clearly dis tin guished.At v < 0.025 V/s, re verse scans show a re ac ti va tion of the electroxidation pro cess in the po ten tial re gion cor re sponding to Peak 2 and Re gion 3, sug gest ing a de crease in the pas sive film sta bil ity.The re verse scan shows three cathodic con tri bu tions, which ex hibit com plex con tours (Peaks 4, 5, and 6), whose rel a tive heights and lo ca tions de pend on tem per a ture, v, E s,a , elec tro lyte com po si tion, and so lution pH.Peak 4 in volves two con tri bu tions, Peak 4' and 4'', which were as sociated with the electroreduction of Cu(II) to the Cu(I) spe cies, in the -0.07 to -0.06 V po ten tial range 9 .Peaks 5' and 5'' have been re lated to the electroreduction of the com posite CuCO 3 -Cu(OH) 2 layer in the -0.28 to -0.43V re gion 6,11 , and Peak 6, in the -0.60 to -0.80V re gion, has been as signed to the electroreduction of the Cu(I) spe cies to Cu(0) 6 (Fig. 1).Peak 1 is better de fined when ei ther tem perature or ionic strength in creases, but its rela tive con tri bu tion di min ishes; also, it ap pears at more negative po tentials, so a lower overpotential is re quired for the Cu 2 O electroformation re ac tion (Fig. 2).As tem per ature rises, the height of Peak 2 in creases, Peak 1 oc curs at more neg a tive po tentials, and the changes cor re spond ing to Region 3 in di cate greater sta bilization of the pas sive layer.In di lute so lu tions, a sig nif i cant tem perature ef fect is not ob served.
A lin ear log j p2 vs. log v re la tion ship is ob tained in still car bon ate/bi car bon ate so lu tions cov er ing the 5-65 °C range, with the cor re spond ing slope be ing ~0.45 0.08, indi cat ing a diffusional pro cess.At con stant v, j p2 in creases as tem perature and ionic strength are raised (Fig. 3).
At 25 °C, in the 8.3-9.5 pH range, at con stant bi car bonate con cen tra tion, j p2 is in de pend ent of pH, whereas a lin ear log j p2 vs. pH is ob served at pH > 9.5 (see Fig. 9 in Ref. 7).These data, to gether with the anal y sis of the cor responding stoichiometric equa tions and the as so ci ated re action orders, show that in the first pH re gion HCO 3 -be haves as the ag gres sive an ion, but in the sec ond pH re gion the lo cal pH value at the re gion in ter face be comes an ad di tional fac tor 7 .Sim i lar re sults are ob tained un der stir ring.Under stir ring, an odic cur rent peaks and the re ac ti vation pro cess dur ing the back ward po ten tial scan in crease with tem per a ture, ionic strength, and (Fig. 4).This ef fect is as so ci ated with the in crement of sol u ble Cu(II) spe cies ob tained from the dis solution of the CuO-Cu(OH) 2 film and ba sic cop per car bon ates.This sit u a tion im proves the dis so lu tion-precipitation mech anism in volved in the forma tion of the pro tec tive sec ond pas si vat ing layer con stituted by Cu (II) com pounds.A sig nif i cant ef fect of hydrodynamics on Peak 1 is not ob served.In gen eral, cathodic peaks de crease un der stir ring, but they in crease with T.
Linear de pend ence of jp2 vs. 1/2 at a con stant v are observed, whose slopes in crease with tem per a ture.Greater slope changes are ob tained with in creasing ionic strength (Fig. 5).
In still so lu tions the ac ti va tion en ergy (Ea ) for the CuO-Cu(OH)2 electroformation is ~15 kJ/mol at pH 8.9 and ionic strength 1, but di min ishes to 6.5 kJ/mol when the ionic strength is 0.1 at the same pH.In gen eral, Ea in creases with in creasing ionic strength, but a clear re la tion be tween Ea and pH does not ex ist.
Under stir ring ( = 1000 rpm), E a at tains higher val ues, thus, at pH 8.9 and ionic strength 1, E a is 25 kJ/mol, suggest ing changes in the pas si vating layer com position, partic u larly in its wa ter con tent, which is re duced.The in fluence of both T and ω on the sur face la yers was analyzed through SEM mi crographs in car bo nate-bicarbonate buf fers at dif ferent pHs and io nic strengths.Considerable dif ferences can be ob served when T, ω, or ionic strength are chan ged.
At 25 °C and ω = 1000 rpm, the cop per sur face is completely co vered with the cor rosion pro ducts, the ap peara When T ri ses to 65 °C the film se ems to be less hydra ted (Figs. 6 and 7).This is an in dication that pro tective cha racteristics of the film are bet ter ob served un der stir ring and at high tem perature through a loss of wa ter mo lecules.Also, these ob servations are in agre ement with the E a va lues obtained.
Solution stir ring and high tem perature chan ge the characteristics of the pas sivating la yer, ma king it more pro tective; this si tuation is ob tained through a de crease of the water con tent, so the film be comes less hydra ted.
The ac tivation energy ob tained for the CuO-Cu(OH) 2 electroformation de pends on io nic strength.This re sult is also ob tained un der stir ring, the refore the elec tro dis so lu tion pro cess is hin dered by films con taining less wa ter.

Fig ure 1 .Fig ure 2 .
Fig ure1.The in flu ence of v on the voltammograms run with STPS in still 0.75 M KHCO3 + 0.05 M K2CO3, pH 8.9 at 25 °C.

Fi gu re 6 .
SEM mi cro graphs of polycrystal li ne Cu spe ci mens af ter po ten ti al hol ding for 10 min in the re gi on of Peak 2, in 0.075 M KHCO3 + 0.15 M K2CO3 so lu ti on pH 10.5, ω = 1000 rpm, at 25 °C.

Fi gu re 7 .
SEM mi cro graphs of polycrystal li ne Cu spe ci mens af ter po ten ti al hol ding for 10 min in the re gi on of Peak 2, in 0.075 M KHCO3 + 0.15 M K2CO3 so lu ti on pH 10.5, ω = 1000 rpm, at 65 °C.