An electrochemical study of the corrosion processes of Ni-based glassy metals in aqueous solutions

The electrochemical behavior of amorphous Ni-Co alloys in carbonate/bicarbonate solutions was investigated using electrochemical impedance spectroscopy combined with voltammetry and stationary polarization techniques. The results indicate that the kinetics of the dissolution processes are dependent on the pH and ionic strength of the electrolyte. The composition of the glassy metal is an important factor in determining the kinetic parameters involved in the complex electrochemical behavior of these materials.


In tro duc tion
Ni-Co amor phous al loys pres ent an electrocatalytic activ ity on the OER in al ka line so lu tion which is higher than that of their crys tal line coun ter parts [1][2][3][4][5][6][7][8], and ref er ence therein . The use of these ma te ri als re quires a sat is fac tory re sis tance to gen eral and lo cal ized cor ro sion. It has been found 9 that the cor ro sion be hav ior of dif fer ent polycrystalline Ni-Co alloys is a func tion of al loy com po si tion.
A cor ro sion study of Ni-Co based glassy met als in carbon ate-bicarbonate so lu tions at pH 8.9 showed that the disso lu tion pro cesses are de pend ent on the ionic strength of the elec tro lyte 10 .
The pres ent study was un der taken in or der to eval u ate the in flu ence of the pH and ionic strength of the elec tro lyte, as well as the al loy com po si tion, on the cor ro sion be hav ior of a glassy Ni-based al loy (Vitrovac 0080), and to com pare these re sults with Ni-Co based al loys.

Ex per i men tal
Amor phous al loys (Vakuumschmelze GmbH, Hanau) with the com po si tions Vitrovac 0080 (Ni 78 B14 Si 8), G15 (Ni 58 Co 20 B12 Si 10 ) and G16(Ni 25 Co 50 B10 Si 15 ), were employed as the work ing elec trodes. A con ven tional three-compartment glass cell was used with a large area Pt coun ter elec trode and a cal o mel ref er ence elec trode. Po tentials in the text are re ferred to the nor mal hy dro gen electrode (NHE) scale.
Elec tro chem i cal im ped ance spec tros copy (EIS) combined with steady-state cur rent den sity/po ten tial (j/E) curves, and sin gle (STPS) and re pet i tive (RTPS) tri an gu lar po ten tial sweeps, were per formed un der pu ri fied N 2 gas sat u ra tion in the fol low ing so lu tions: x M KHCO 3

Re sults and Dis cus sion
For the char ac ter iza tion of the elec tro chem i cal be havior of the amor phous al loys, voltammograms were run between the po ten tial re gions of the hy dro gen and ox y gen evo lu tion re ac tions. The cur rent den sity/po ten tial (j/E) profiles cor re spond ing to the first for ward po ten tial scan of G15 al loy in the dif fer ent elec tro lyte so lu tions are presented in Fig. 1. The voltammetric pro file in 0.05 M K 2 CO 3 + 0.75 M KHCO 3 (pH 8.9) shows a first an odic cur rent peak (Peak I) at ca. -0.3 V, as so ci ated with Ni/Ni(II) hydrox ide trans for ma tion 11 , a sec ond an odic con tri bu tion (Peak II) at ca. 0.05 V, re lated to the ox i da tion of Co(II) to Co(III) ox ides or hy drox ides 12 , and a third an odic cur rent peak (Peak III) at ca . 0.7 V, as so ci ated with the re dox couple Ni(II)/Ni(III) 11 . The cur rent val ues of each peak are com plex func tions of the pH, ionic strength, and car bonate/bi car bon ate ra tio. j pI in creases and peak II ap pears less de fined ac cord ing to the ionic strength and pH. The in fluence of bi car bon ate ions be comes ev i dent at the top of peak III. The voltammograms of G16 (not shown here) also exhibit on the first an odic scan all of the cur rent con tri bu tions de scribed above. Fur ther more, lower voltammetric charge den si ties of the peaks as so ci ated with Ni trans for ma tion are ob served for G16 al loy in com par i son with G15 al loy. It is in ter est ing to note, that cur rent peaks II re lated to the ox i dation of Co(II) to Co(III) are the best de fined, and that peak III ap pears at more neg a tive po ten tials. This fact is as so ciated with the in creas ing Co con cen tra tion in the al loy. This ten dency is con firmed with data ob tained un der sim i lar exper i men tal con di tions em ploy ing the non-Co con tain ing al loy, Vitrovac 0080, in which case peak III shifts to 0.9 V. This Co ef fect has also been found by other au thors 6,13 .
The steady state po lar iza tion curves of Vitrovac 0080, G15, and G16 elec trodes in 0.167 M K 2 CO 3 + 2.5 M KHCO 3 (pH 8.9) and 0.5 M K 2 CO 3 + 2.5 M KHCO 3 (pH = 9.5) are shown in Figs. 2a and 2b, where the dots in dicate the po ten tials at which im ped ance spec tra were recorded.
The cur rent con tri bu tion at about 0.6 V can be as so ciated with the re dox cou ple Ni(II)/Ni(III) 11 . It is worth noting that this cur rent peak ap pears at more pos i tive po ten tials for the non Co-containing al loy. This ef fect was also found by Lian et al. 6 , who sug gested that co balt ad ditions to Ni(OH) 2 sta bi lize Ni hy drox ide, im prove the conduc tiv ity and charge ef fi ciency of Ni(OH) 2, and in crease the ox y gen overpotential. Fur ther more, Folquer et al. 13 found a cat a lytic ef fect of the Co(II)/Co(III) re ac tion on the Ni(II)/Ni(III) re ac tion in co-precipitated Ni-Co al loys through hy drox ide ions in serted into the metal hy drox ide lay ers.
At a con stant pH, the high value of the cur rent peak (j p ) in creases with in creas ing Ni con cen tra tion in the al loys and the ionic strength of the elec tro lyte. These ef fects hold for all of the tested al loys. On the other hand, as pH in creases (Fig. 2b) the value of the j p be comes higher for the Co-containing al loy and re mains prac ti cally con stant in the case of Vitrovac 0080. At a con stant con cen tra tion of bi carbon ate ions, lin ear log j p vs. pH re la tion ships are ob tained with a slope of 0.42 ± 0.05 for G15 and 0.4 ± 0.02 for G16. Ac cord ingly, the data re vealed a com plex ef fect of car bonate/bi car bon ate ions on the cor ro sion be hav ior of Ni-Co based glassy met als.
The fre quency re sponse of each in ves ti gated amorphous al loy was found to be re mark ably de pend ent on the ap plied po ten tial, on the al loy com po si tion, and on the electro lyte con cen tra tion. Nyquist plots re corded with Vitrovac 0080 in 0.167 M K 2 CO 3 + 2.5 M KHCO 3 (Fig. 3) ex hibit a ca pac i tive semi cir cle at po ten tial val ues close to the cor rosion po ten tial (Fig. 3a), and at more pos i tive po lar iza tions an in duc tive con tri bu tion ap pears at the in ter me di ate frequen cies, where a new time con stant at low fre quen cies can be ob served (Figs. 3b-e). As the an odic po lar iza tion increases, the re sis tance val ues as so ci ated with the low frequency con tri bu tion be come neg a tive (Fig. 3f). The above-described in flu ence of the elec tro lyte com po si tion on the steady-state mea sure ment is also ob served in the imped ance data.
Oth er wise, the im ped ance re sponse ob tained in the case of the Co-containing al loys (G15 and G16) in car bon ate/bicar bon ate so lu tions pres ents the same gen eral fea tures as the Vitrovac 0080 one, but in volves higher in duc tance, which in turn hin ders the ap pear ance of a well-defined induc tive be hav ior at in ter me di ate fre quen cies and a non-complete ca pac i tive semi cir cle at low fre quen cies (see, for in stance, Figs. 4 and 5). The higher im ped ance val -ues ob tained em ploy ing these al loys in com par i son with Vitrovac 0080 are con sis tent with the lower cur rent den sities un der steady-state con di tions for the al loy with a higher Co con tent.
The whole set of ex per i men tal im ped ance spec tra can be dis cussed ac cord ing to the fol low ing to tal trans fer function: stants due to sur face unhomogeneities in the amor phous al loys, R ct rep re sents the charge trans fer re sis tance de fined as Z( ω = ∞), L and R ic are as so ci ated with the in duc tive con tri bu tion which can be in ter preted by as sum ing a strong po ten tial de pend ence of the ac tive site con cen tra tion causing sur face re lax ation phe nom ena, and R ad and C ad can be re lated, in prin ci ple, to a Faradaic pseudo-capacitive contri bu tion of an ad sorbed re ac tion in ter me di ate at the low est fre quen cies. The good agree ment be tween the ex per i men tal and sim u lated data ac cord ing to trans fer func tion (1) anal y sis us ing non-linear least-square fit rou tines is dem on strated in Figs. 6 and 7 in 0.167 M K 2 CO 3 + 2.5 M KHCO 3 (pH = 8.9) and 0.5 M K 2 CO 3 + 2.5 M KHCO 3 (pH = 9.5) so lu tions, respec tively.
The val ues of C dl and R ct were de ter mined from the opti mum fit ting ac cord ing to Eq. 1. In the po ten tial range -0.4 V≤ E ≤ 0 V, it was pos si ble to es ti mate mean val ues by C dl = 30 ± 10 µF cm -2 and α = 0.95 ± 0.05, while the value of R ct de creases lin early. At pos i tive ap plied po ten tials the R ct val ues are nearly con stant and lower ca pac i tance val ues, C = 9 ± 2 µF cm -2 , are ob tained. These low C val ues are typ ical of a metal cov ered with a pas sive film 14 , and may be con sid ered the pas sive film and dou ble layer ca pac i tance con nected in se ries. The po ten tial in de pend ence of the capac i tance in di cates ei ther in su lat ing or semi con duct ing prop er ties of the pas sive films. At E ± 0 V, R ct = 0.434 b/j ss for a charge trans fer con trolled re ac tion, where j ss de notes the steady-state cur rent den sity and b = ∂E/ ∂logIj ss I is the   Tafel slope of the an odic re ac tion, b ≈ 120 mV was found, cor re spond ing to a charge trans fer co ef fi cient of 0.5.
The anal y sis of the im ped ance pa ram e ters as so ci ated with the time con stant con tri bu tion at the low est fre quencies is dif fi cult in the case of al loys G15 and G16 due to an in com plete ca pac i tive pro file, al though it was pos si ble to es ti mate a value of C ad ≈ 860 ± 60 µF/cm 2 . The val ues of resis tance and ca pac i tance as so ci ated with the sec ond ca pac itive loop for Vitrovac 0080 were close to 3.5 ± 0.2 k Ω cm 2 and 4 ± 0.2 mF/cm 2 , re spec tively.
It is pos si ble to con clude, in prin ci ple, that the fit param e ters re lated to the in duc tive loop ob served at in ter medi ate fre quen cies fail to obey a di rect func tion of the Ni/Co con cen tra tion ra tio in the al loy ma trix. This fact is re affirmed by the ex per i men tal re sults ob tained with Vitrovac 0080, which pres ent a clear in duc tive con tri bu tion at in terme di ate fre quen cies.
The im ped ance re sults clearly show that the dis so lu tion pro cesses are tak ing place through a com plex re ac tion mech a nism, and it is nec es sary to de ter mine the com po sition of the sur face films formed on amor phous al loys un der dif fer ent po lar iza tion con di tions. Con se quently, XPS studies are in prog ress.

Ac knowl edg ments
This re search pro ject was fi nan cially sup ported by the Consejo Nacional de Investigaciones Científicas y Técnicas, the Comision de Investigaciones de la Provincia de Bue nos Ai res, and the Fundación Antorchas. Part of the equip ment used in the pres ent work was pro vided by DAAD and the Al ex an der von Humboldt-Stiftung.