The effects of anions and pH on the passivity of bright or semi-bright nickels were investigated by potentiostatic measurements of anodic polarization curves in the electrolytes of 1.5N SO
42- or 1.5N ClO
4-, and were also studied from the standpoints of electrochemical thermodynamics.
The results obtained were as follows:
(1) The relation between the critical current density for dissolution of nickel (Ic) and pH of the electrolyte was introduced as follows:
∂log(Ic)/∂pH=-1.
It was cleared that this relation was in good agreement with the values observed.
(2) In case of semi-bright nickel in either SO
42- or ClO
4- electrolyte, it was considered that the passivation occurred at the equilibrium potential of E
C by the following oxidation reaction:
C) Ni+H
2O=NiO+2H
++2e; E
C=-0.14-0.06pH.
Whereas, in case of bright nickel, reactions of the 1st and 2nd passivations appeared in complicated state: these reactions and potentials were proposed as follows.
i) The 1st passivation in ClO
4- electrolyte of low pH region and in SO
42- electrolyte:
D) 3Ni+4H
2O=Ni
3O
4+8H
++8e; E
D=-0.05-0.06pH.
ii) The 1st passivation in ClO
4- electrolyte of neutral region and the 2nd passivation in SO
42- electrolyte of neutral region:
G) Ni
2+2NiO+2H
2O=Ni
3O
4+4H
++2e; E
G≅0.59-0.12pH.
iii) The 2nd passivation in SO
42- electrolyte of low pH region;
J) 2NiO+H
2O=Ni
2O
3+2H
++2e; E
J=0.51-0.06pH.
iv) The 2nd passivation in ClO
4- electrolyte:
L) NiO+H
2O=NiO
2+2H
++2e; E
L=0.98-0.06pH
(3) It was found that ClO
4- had a stronger power for breaking the passive film as compared with SO
42-, and the codeposited S in bright nickel also largely contributed to the breaking of the passive film.
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