Page 18 - MODERN ASPECTS OF ELECTROCHEMISTRY

5
A Critique of the Additivity Principle for Mixed CouplesA
times larger than the value obtained from the anodic polarization curve
for copper under nitrogen. These results are quite contraryto those
expected from the additivity principle.=
To explain their findings, the authors proposed the following mecha-
nism:
-
+
Cu Cu +e (quasi-equilibrium) (8)
-
+
Cu → Cu 2+ +e (rate-determining) (9)
Cu + ½O +H → Cu +½ H 2 ø 2 (10)
+
2+
+
2
-
O +2H + 2e →H O 2 (11)
+
2
2
For the additivityprinciple to hold, steps (8) and (9), the anodic processes
t¸ing place under nitrogen, would be coupled only with the cathodic
reduction of oxygen in step (11). It is step (10), the attack by the constitu-
+
ents of the oxygen couple on the intermediate Cu , which lies outside the
scope of the principle and which explains the observed findings.=
Andersen et al. predicted that similar results would be expected for
the corrosion of other multivalent metals oxidizing via lower oxidation
states. Theyalso pointed out that their interpretation was consistent with
the kinetics of the corrosion of copper in oxygenated HCl solutions.= Here
+
the final product is Cu and thus there is no vulnerable intermediate.= In
consequence, the rate of copper dissolution from either N 2 -saturated or
12
O 2-saturated HCl solutions was the same at a given potential in conform-
ity with the additivity principle.=
11
More recently, Wiese and Weil reported a detailed study of the
mechanism of electroless copper deposition with formaldehyde from
alkaline ethylenediamine-tetraacetate (EDTA)-containing solutions.= The
partial reactions were expected to be
2- - 4-
CuEDTA +2e → EDTA (12)
- - -
2HCHO + 4OH → 2HCOO +H + 2H 2 O + 2e (13)
2
13
The hydrogen gas was later shown to originate from the formaldehyde,
not the water.= Instead of cumbersome and slow ex situ weighing, Wiese
and Weil measured the plating rate in situ with a quartz microbalance.=
They connected the electrode to a potentiostat to keep it at the plating