Proc Indian Acad. Sci. (Chem. Sci.), Vol. 111, No. 2, April 1999, pp. 377-386 © Indian Academyof Sciences
Electrochemical behaviour of copper-nickel alloy in chloride solution
J MATHIYARASU, N PALANISWAMY and V S MURALIDHARAN* Corrosion Science and Engineering Division, Central Electrochemical Research Institute, Karaikudi 630 006, India e-mail: corr @cscecri.ren.nic.in
MS received 12 October 1998; revised 12 January 1999
Abstract. Cyclic voltammetric measurements were made on pure copper, nickel and 70/30 copper-nickel alloy in sodium sulphate and sodium chloride (0.5 M) solutions. In sodium sulphate solution the passivation of copper and nickel is through oxide formation. The passive film on copper nickel alloy is found to have nickel ions ingress along with copper oxide by volume diffusion. Nickel ions cause hindrance to the reduction of copper oxide to copper. The presence of chloride ions increases the dissolution of nickel while copper is passivated. The formation of CuCI2 and its subsequent hydrolysis to hydroxide prevents the dissolution. Introduction of nickel in copper depassivates the copper and hydrolysis of monovalent copper complex is prevented by the ingress of nickel ions. The formation of NiC12 along with CuC12 is facilitated by chloride ions.
Keywords. Chloride solution; copper-nickel alloys; cyclic voltammetry; dissolution; passivation.
1. Introduction
The subject of dissolution and passivation of copper, nickel and cupronickel alloy in NaCI and Na2SO4 solutions is of both technological and theoretical interest. Copper. nickel alloys (70/30) were found to resist corrosion and erosion in seawater. In chloride solutions, the corrosion and passivation of the copper-nickel alloys were studied J-7. In synthetic seawater, the corrosion proceeds through a surface kinetic process probably due to CuzO and Cu2(OH)3CI formation. In chloride solutions they form soluble CuCI2- complex s. The present study focuses on the electrochemical behaviour of pure copper, pure nickel and 70/30 copper nickel alloy both in 0.5 M sodium sulphate and 0.5 M sodium chloride solutions. In chloride solutions copper, nickel and cupronickel undergo dissolution and passivation. In chloride solutions above 40 wt% nickel and cupronickel alloys undergo pitting corrosion. In order to eliminate the harmful effects of an aggressive anion like chloride, a non-interfering anion like sulphate is chosen for this study. Therefore detailed investigations are needed to understand (a) whether the dissolution is specific for less noble metals, (b) the nature of the passivation of the alloy surface and (c) the role of chloride ions in the passivation process.
*For correspondence
377 378 J Mathiyarasu et al
2. Experimental
Cyclic voltametric studies were carried out with copper (99-99%), nickel (99.99%) and cupronickel (70% Cu + 30% Ni) as the electrode material. They were made into cylindrical rods of cross-section = 0.385 cm 2. The rods were embedded in Teflon gaskets and electrical connections were made by the screw-and-thread arrangement. Specimens were mechanically polished with 4/0 emery paper and solutions oxygenated by bubbling purified oxygen for 1 h before the experiment. A standard all-glass 3-electrode cell was used. A large platinum foil was used as an auxiliary electrode and a saturated calomel electrode as the reference electrode. Triangular potential sweep experiments were carried out using a Potentiostat (EG&G 173) and a universal programmer (EG&G 175) of varying sweep rates. A Rikadenki 201T x-y recorder was used to record E-I curves. Sodium sulphate and sodium chloride solutions of 0.5 M concentration were used in this study. The cyclic voltammograms were obtained from -1000 mV to + 500 mV. The sweep rates were in the range of 5 to 100 mV/s. In order to obtain a clean metal surface the electrode was kept at -1000 mV vs SCE for 5 min after which the specimens were immediately polarised from -1000 mV to the desired values. Experiments were carried out at 30 + I°C and duplicated.
3. Results
From the steady corrosion potentials, excursions were made at 5 mV/s, in both directions up to 200 mV in Na2SO4 and NaC1 solutions. Typical E-log/curves obtained are shown in figure 1. In sulphate solutions the alloy exhibits nobler potentials compared to pure metals. In chloride solutions, copper exhibits least corrosion while the addition of nickel to copper enhances the rate (table 1). In order to understand the passivation and dissolution of copper-nickel alloys, detailed cyclic voltammetric experiments were carried out.
3.I Sodium sulphate solutions
When polarised from - 1200 mV to + 500 mV, copper exhibits a single cathodic peak only in the reverse scan at -60 mV (figure 2). This peak potential is more negative with 70 mV per decade change of sweep rate. Nickel exhibits different behaviour. During the forward scan from -800 mV to + 500 mV, zero current potential (Z(~CP) appears at -380 mV followed by a hump at + 200 inV. ZCCP occurs at - 300 mV during the reverse scan suggesting modification of the surface. ZCCP corresponds to the corrosion potential of the metal and the shift of the
Table 1. Parameters derived from E-log i curves.
Na2SO4 solution NaCI solution Electrode material E~,rr (mV) l~,,rr (A/cm2) Ecorr (mV) l~,,rr (A/cm 2)
Copper 0 1.2 x 10-3 - 15 4 × 10-4 Nickel - 187 5-0 × 10-6 -377 9 x 10--6 Cu/Ni + 33 IO x 10-7 - 36 5 x 10-4 f e d c b Q -10
-10 90 !00 12 t~ -90 ' If
-5( 50 t~ -5(
W 0 -13~ " t~ trl -9( " 1( 28(
5 ILl -1'31 --3( • ( C3 -gc-
Jt'~ -
-17 - -?C -36
I I
-13C L-21, L.-zt -~111 .t~ ~..80| ~ x, ~ I ... , • i b.,c.e40-7 10-6 1O'S 10 -~* g: a,d,f 1 0 "~ I 0 -3 1 0 -2 10-1 ( A/cm 2 } Fig.1.
Figure 1. Typical E-log/ curves for all materials in sodium chloride and sodium sulphate (0.5 M) solutions at v = 5 mV/s. (a) copper in 0.5 M Na2SO4, (b) nickel in 0.5 M Na2SO4, (c) cupronickel in 0-5 M Na2SO4, (d) copper in --.I 0.5 M NaCl, (e) nickel in 0-5 M NaCl, (f) cupronickel in 0.5 M NaC1. ',D 380 J Mathiyarasu et al
Q 00°v /A0
2 -1"0 -0"5 E(V) vS~E
Figure 2. Typical cyclic voltammogram of copper in 0~5 M sodium sulphate solutions at different sweep rates. E~.~ = -1200 mV; E~.~ = 500 mV; (a) 100, (b) 50, (c) 20, (d) 10, (e) 5 mVls. corrosion potential to nobler values suggests passivation of the surface (figure 3). Beyond 50 mV/s, a distinct anodic peak appears at -520 mV followed by a cathodic peak at - 390 mV in the reverse scan. The appearance of the anodic peak is due to the oxidation of the surface of nickel. Further anodic potential excursion stabilizes and thickens the film. The oxides of nickel formed in the anodic scan are reduced at - 390 mV followed by hydrogen evolution. Figure 4 presents the electrochemical spectrum for copper-nickel alloy. In the forward anodic scan, the ZCCP occurs at - 300 mV. No anodic peak is observed, the reverse scan exhibits a hump at - 300 mV at v > 50 mV/s. The charge flow in the range of - 300 mV to 0 mV corresponds to 150-300 lac/cm2, which in turn corresponds to monolayer oxygen coverage on the alloy surface.
3.2 Sodium chloride solutions
The electrochemical spectrum of copper exhibits an anodic peak at 80 mV, at 5 mV/s sweep rate and is followed by oxygen evolution. Increase of sweep rate shifts the Electrochemical behaviour of Cu-Ni alloy in chloride solution 381
Figure 3. Typical cyclic voltammogram of nickel in 0.5 M sodium sulphate solutions at different sweep rates. Ea.c = -800 mV; E~.a = 500 mV; (a) 100, (b) 50, (c) 20, (d) 10, (e) 5 mV/s.
100mV
E (V} vs SCE
Figure 4. Typical cyclic voltammogram of Cu/Ni in 0-5 M sodium sulphate solutions at different sweep rates. Ea.~ = -500 mV; Ea.= = 500 mV; (a) 100, (b) 50, (c) 20, (d) 10, (e) 5 mVIs. 382 J Mathiyarasu et al i I -0.5 0~ E {V~ ~SSCE
Figure 5. Typical cyclic voltammogram of copper in 0.5 M sodium chloride solutions at different sweep rates. Ea.¢ = -1200 mV; Ea.~= 500 mV; (a) I00, (b) 50, (c) 20, (d) 10, (e) 5 mWs.
peak potential to nobler values. The reverse scans exhibit a cathodic peak at - 240 mV (figure 5). AEp = Ee.,,- El,.c =-160 mV varies with sweep rate suggesting irreversible reduction and oxidation of species on the surface, I t.... varies with v 1t2 suggesting a diffusion-controlled process. If the passivation is of the phase type, the dependence of Ep., and it,., on the square root of sweep rate 9 must be linear. The linear plots of El,., with log sweep rate gave 200 mV/decade change suggesting that the adsorption and oxidation processes are irreversible but diffusion-controlled. Nickel in chloride solutions exhibits different behaviour (figure 6). The anodic current starts increasing beyond - 100 mV and in the reverse scan, ZCCP appears at 200 mV. Sweep rates have no influence on the shape of the spectrum. Figure 7 presents the cyclic voltammogram of alloy. During the forward scan the anodic current starts increasing beyond + 100 mV, followed by oxygen evolution. The reverse scan exhibits a cathodic peak at -230 mV, which varies at 70 mV change per decade change of sweep rate. The repeated cycling at 100 mV/s indicate that charges flowing under the anodic peak decrease, while that under the cathodic peak increase, suggesting enhanced reduction of the oxidised species. Electrochemical behaviour of Cu-Ni alloy in chloride solution 383
-0.5 E(V) vs SCE
Figure 6. Typical cyclic voltammogram of nickel in 0-5 M sodium chloride solutions at different sweep rates. E~.¢ =-800 mV; Ea,a = 500 mV; (a) 100, (b) 50, (c) 20, (d) 10, (e) 5 mV/s.
b 100 mV
i i 5 , 05
\
Figure 7. Typical cyclic voltammogram of Cu/Ni alloy in 0-5 M sodium chloride solutions at different sweep rates. E~,c = -500 mV; Ea,a = 500 mV; (a) 100, (b) 50, (c) 20, (d) 10, (e) 5 mV/s. 384 J Mathiyarasu et aI
4. Discusslon 4.1 Dissolution of copper
In the passive region, the copper ions leave the lattice to form Cu20, which was confirmed by X-ray analysis earlier ~0. The initial passivation is due to
2Cu + H20 ---) Cu20 + 2H ÷ + 2e-, and the subsequent oxidation of CuzO to CuO occurs as
Cu20 + H20 ---) 2CuO + 2H+ + 2e-.
The observed anodic Tafel slope of 160 mV/decade suggests a diffusion and activation controlled dissolution in chloride solutions. The dissolution of copper occurs as
Cu + 2C1- CuCI2- + e-, and the rate of dissolution, if equated to the rate of diffusion from the electrode to the bulk, is given by
ia _ ( FDM+/b)KC M exp(1 - fl)FA~o I RT ( g exp(-flFA~o l RT) + FDM+ / 8 ' where A~o is the absolute interfacial potential, /~,/( are the chemical rate constants for the dissolution and deposition reactions respectively, DM* is the diffusion coefficient for the M ÷ species in the electrolyte and 8 is the diffusion layer thickness. If /~ exp (--flFA~olRT) >> FDM÷I6,
ia = (FDM÷IO)KCM exp FAqgIRT. If (FDM÷I6) >> /(exp(-flA~p/RT),
ia =/~ CM exp (1--fl)FA~olRT.
The observed Tafel slope of 160 mV/decade at 30°C may suggest this. The appearance of the cathodic peak in chloride solutions is due to
CuClz- + e- ~ Cu + 2C1-.
4.2 Passivation of nickel
In figure 3, the initial portion of the anodic curve is assigned to a steady electrodissolution leading to passivation. Before the appearance of the anodic peak at -520 mV, initial surface oxidation can take place. The participation of water and OH- ions may lead to Electrochemical behaviour of Cu-Ni alloy in chloride solution 385
Ni + H20 Ni (H20)ad,,
Ni (H20)ads + OH- NiOH. H2Oads + e-. The reaction
Ni(OH. H20)ads + OH- HNiO2- + H + + H20 + e-, can occur only at high pH. The appearance of peak at - 520 mV is due to Ni(OH)2 or NiO and further polarisation thickens the film. The appearance of the cathodic peak at - 390 mV suggests the reduction of oxides to metal. In chloride solutions, dissolution and passivation leads to the formation of NiCI2 as
Ni (H20)ads + 2C1- --~ NiC12 + 2e- + H20. 4.3 Dissolution of binary Cu:Ni alloy
In the case of alloy dissolution one would expect ~1 (1) both copper and nickel to leave the metal lattice followed by the deposition of nickel, (or) (2) only copper to dissolve and enter the solution, while the nickel atom aggregates by surface diffusion, or (3) only copper to ionise and enter the solution and atoms of both metals to move in the solid phase by volume diffusion. Extensive studies made on seawater m3 revealed that the surface is covered with a black CuO deposit and the presence of nickel in the defect structure of Cu20 indicates a small amount of nickel penetration into the chloro complex. In the present study an observed anodic Tafel slope > 200 mV suggests dissolution through a surface-covered film of chloro complex. The reduction of this complex to copper is controlled both by C1- to and CuCI2 ions away from the surface. In sulphate solutions, the appearance of a cathodic peak at -300 mV suggests the hindered reduction of copper oxide by the ingress of nickel ions in it, and in Cu:Ni alloys in chloride solutions, the nickel enrichment of the surface seems to be a slow process aided by the high rate of copper entry in the corrosion product.
5. Conclusions
In sodium sulphate solutions the passivation of copper and nickel is due to oxide formation. The passive film on copper-nickel alloy has nickel ions ingress along with copper oxide by volume diffusion. Nickel ions cause hindrance to the reduction of copper oxide to copper. The presence of chloride ions increases the dissolution of nickel while copper is passivated. The formation of CuCI2 and its subsequent hydrolysis to hydroxide prevents dissolution. Introduction of nickel in copper depassivates the copper and the hydrolysis of monovalent copper complex is prevented by the ingress of nickel ions. The formation of NiCI2 along with CuCI2 is facilitated by chloride ions.
Acknowledgments
One of the authors (JM) expresses his sincere thanks to the Council of Scientific & Industrial Research, New Delhi, for a fellowship. 386 J Mathiyarasu et al
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