equation 12.
M -- ► Mn+ + ne
and a passivation process involves
M + (x+y) H20 --- — [m(OH)x <£] n'(x+2y)+(x,t-2y)H++ne (21 )
In all cases dissolution of a metal ion requires more energy than the oxide formation; this is due to a large negative free energy of hydration.
Similarly water molecules in the Helmholtz double layer at a metal surface under anodic conditions tend to orientate with oxygen atoms towards the metal,
presenting the possibility of the following r e a c t i o n s ^ ^ ,
M — — M-OH+H+ + 2e ... (22a)
M . .. . 0 < C ^ — » -M-0+2H+ + 2e (22b) Again the formation of a surface monolayer of solid hydroxide or oxide needs a smaller energy due to the large negative free energy for cation association with the
surrounding oxide ions.
The reactions 21 and 22 may prevent the anodic dissolution or the cathodic deposition; this depends on the potential, the composition of the solvent, the specimen geometry and microstructure and the properties of the
solid film. The surface oxide film separates the reacting substances, and further reaction is possible only if there is diffusion of the reacting species through the oxide layer. The diffusion of the ions depends on the proper ties of the oxide film such as; crystal structure, thermo dynamic stability, adhesion to anode surface, thickness, conductivity, and transport processes to and from the boundary^
(11
^
Bockris et alv 7 have suggested the following mechanism for iron dissolution
Fe + O H ~ W — FeOH + e" (23a) r * d . 5. ,
FeOtf FeOH + e“ (23b)
FeOH+ Fe2+ + OH' (23c)
It has been reported that the critical 0H~ concen tration for passivation of mild steel with a given
chloride level and PH in the range 5-11 reached higher values in solutions not containing sulphate^^. Rice
(
12)
et al 7 reported cobalt passivation at PH >6 by the
(
13) formation of a stable complex oxide, Ohatsku and Sato pointed out that cobalt passivates in alkaline solutions but this effect is minimal in acid solutions. Fig. 9 shows a polarization curve as well as cobalt dissolution versus potential and film charge potential relationships in borate buffer solution at PH of 8.4. In addition to the active dissolution region, the passive potential range may be divided into three regions. Passive 1,passive 2, and passive 3. The transpassive region appears beyond the passive regions. These passive regions tie up with the stability regions for a divalent cobalt oxide
CoO, for a spinal type oxide CogO^, and for a trivalent (13}
oxide respectively. Ohatsku and Satov y also applied a cathodic reduction technique to measurements of the passive film formed in different potential regions. The film formed in the passive 1 potential region was
cathodically reduced. Further, no cobaltous ions entered the solution from the film during cathodic
reduction: possibly direct reduction of cobaltous oxide to metallic cobalt took place instead.
The film formed in the potential region, passive 2 and passive 3, dissolves during cathodic reduction,
2+\
producing cobaltous (Co ) ions in the solution as shown in fig. 10, which gives the amount of dissolved cobaltous ions against the cathodic charge passed. The broken line indicates the theoretical reduction curve of CogO^ to cobaltous ions. The film formed in the passive 2 region follows the broken line in its initial stage of reduction, which may be evidence that the outer layer of the film has
(13^
a composition close to CogO^' . The film in the passive
Pes'/tx/
3 region shows an initial induction^where small quantities of cobaltous ions appear in the solution before it follows the same slope as that of the broken line. Ohatsku and Sato also concluded that the outer layer in the passive 3 region is composed of Co20g. The film previously formed
in the passive 2 region is not a single layer but two layers with different optical constants.
Reduction of the passive film which is formed in the passive 3 region, progresses in three stages, as shown in fig. 11. In the first stage the outer C^ O g layer changes to COgO^ with little reduction in the film thickness:
subsequently the COgO^ film is reduced to CoO and finally to Co"*""** Fig. 12 shows the thickness of the film which is estimated by the elipsometrie technique as a
function of potential. The thickness of the outer layer increased almost linearly with the potential^^ • The almost linear relationship between the thickness of this layer and the potential suggests that the outer layer acts as a barrier which gives rise to the greater part of the overpotential•
In aqueous solutions film formation may be effected by electrochemical reduction or oxidation, chemical
dissolution, mechanical disruption, and specially by the presence of active anions (chlorides). These may either encourage the activation of the ’weak1 sites in the film (mechanical flaws, inclusions) or introduce new ones.
Depassivation was usually observed by a fairly rapid fall in potential to a value slightly more noble than that required for passivation (the Flade potential).
Finally the observed relationship between current and potential is a function of several processes including
film formation, film dissolution and metal dissolution. The latter was suggested by Ohatsku and Sato^"^ to be mostly responsible for the magnitude of the current at all potentials. In the active potential region dissolution was hindered by a decrease in the free electrode area, and
in the passive region dissolution depended on the prop erties of the passive film.
2.2 Non Electrolytic Reactions