Passivation directed metal oxidation

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

Titanium as a carrier metal Titanium (or a similar metal such as tantalum, etc.) cannot work directly as anode because a semiconducting oxide layer inhibits any electron transport in anodic direction ( valve metal ). But coated with an electrocatalytic layer, for example, of platinum or of metal oxides (see below), it is an interesting carrier metal due to the excellent corrosion stability in aqueous media, caused by the self-healing passivation layer (e.g. stability against chlorine in the large scale industrial application of Dimension Stable Anodes DSA , see below). 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

R ently, the use of oxidized PANI has been r ognized as an inter ting means of protecting oxidizable metals. Debeny (I) was the first to show that a PANI film coated on stainless steel by electrochemical oxidation of aniline acted as a protective primer. This protection has been explained by a redox effect of the doped PANI, which maintains the metallic oxide film in its passive state. These result were confirmed by MacDiarmid et al. (2) and later, Wessling et al. (3) showed that chromating could be replaced by the direct deposition of a PANI dispersion on the metal. 

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