Passive film layer

Often the metal bipolar plate develops a passive film layer, which can increase the contact resistance. This resistance can be measured using the contact electric resistance technique (Kim et al., 2002). In this technique, the two sample surfaces are broughf info contact and then separated repeatedly with a chosen frequency as depicfed in Figure 8.15. The passive... 

The weight gains ( 6 and 7) could be experimental errors caused by some reaction products deposited from the corrosion product removal solution or a passive film/layer formed in the coolants. 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

From polarization curves the protectiveness of a passive film in a certain environment can be estimated from the passive current density in figure C2.8.4 which reflects the layer s resistance to ion transport tlirough the film, and chemical dissolution of the film. It is clear that a variety of factors can influence ion transport tlirough the film, such as the film s chemical composition, stmcture, number of grain boundaries and the extent of flaws and pores. The protectiveness and stability of passive films has, for instance, been based on percolation arguments [67, 681, stmctural arguments [69], ion/defect mobility [56, 57] and charge distribution [70, 71]. 

There are various theories on how passive films are formed however, there are two commonly accepted theories. One theory is called the oxide film theory and states that the passive film is a diffusion-barrier layer of reaction products (i.e., metal oxides or other compounds). The barriers separate the metal from the hostile environment and thereby slow the rate of reaction. Another theory is the adsorption theory of passivity. This states that the film is simply adsorbed gas that forms a barrier to diffusion of metal ions from the substrata. 

The basic mechanism of passivation is easy to understand. When the metal atoms of a fresh metal surface are oxidised (under a suitable driving force) two alternative processes occur. They may enter the solution phase as solvated metal ions, passing across the electrical double layer, or they may remain on the surface to form a new solid phase, the passivating film. The former case is active corrosion, with metal ions passing freely into solution via adsorbed intermediates. In many real corrosion cases, the metal ions, despite dissolving, are in fact not very soluble, or are not transported away from the vicinity of the surface very quickly, and may consequently still... 

Hitzig et al. have produced a simplified model of the aluminium oxide layer(s) to explain impedance data of specimens prepared under different layer formation and sealing conditionsThe model also gives consideration to the formation of active and passive pits in the oxide layer. Shaw et al. have shown that it is possible to electrochemically incorporate molybdenum into the passive film which, as previously noted, improves the pitting resistance. 

Calcium hydroxide leached from incompletely cured concrete causes serious corrosion of lead (see Section 9.3). This is because carbon dioxide reacts with the lime solution to form calcium carbonate, which is practically insoluble. Carbonate ions are therefore not available to form a passive film on the surface of the lead . Typically, thick layers of PbO are formed, which may show seasonal rings of litharge (tetragonal PbO) and massicot (orthorhombic PbO) . 

The corrosion current due to diffusion of metal ions through the passivating film, and dissolution of metal ions at the oxide-solution interface. Clearly, the smaller this current, the more protective is the oxide layer. 

Thus inhibitive anions can retard the dissolution of both the T-FejO, and the magnetite layers of the passivating oxide layer on iron. This has the dual effect of preventing breakdown of an existing oxide film and also of facilitating the formation of a passivating oxide film on an active iron surface, as discussed in the previous section. 

Figure 5 shows the relationship between the passive film thickness of an iron electrode and the electrode potential in an anodic phosphate solution and a neutral borate solution.6,9 A passive film on an iron electrode in acidic solution is made up of an oxide barrier layer that increases its thickness approximately linearly with increasing electrode potential, whereas in a neutral solution, there is a precipitated hydroxide layer with a constant thickness outside the oxide barrier layer. 

Figure 5. Thickness of the anodic passivating film formed on iron at various potentials.6 9 Lbl and Lr, are the thicknesses of the barrier layer and the precipitated layer, respectively. Temperature is 25°C. , in a 150 mol m 3 phosphate buffer solution at pH 1.85 O, in a 300 mol m 3 borate buffer solution at pH 8.2. (From N. Sato, K. Kudo, and T. Noda, Z Phys. Chem. N. F. 98,271,1975, Fig. 5, reproduced with permission and N. Sato, K. Kudo, and R. Nishimura, / Elec-trochem. Soc, 123,1420,1976, Fig. 1. Reproduced with permission of the Electrochemical Society, Inc.)...

In the potential region where nonequilibrium fluctuations are kept stable, subsequent pitting dissolution of the metal is kept to a minimum. In this case, the passive metal apparently can be treated as an ideally polarized electrode. Then, the passive film is thought to repeat more or less stochastically, rupturing and repairing all over the surface. So it can be assumed that the passive film itself (at least at the initial stage of dissolution) behaves just like an adsorption film dynamically formed by adsorbants. This assumption allows us to employ the usual double-layer theory including a diffuse layer and a Helmholtz layer. 

The autocorrelation distance is determined by the total overpotential (0Q) of the double layer, which is measured from the critical pitting potential and the coverage 0 of the passive film. From the experimental results which will be discussed later, the actual function form is determined as... 

Figure 39. Current-time variation in nickel pitting dissolution in NaCl solution.89,91 1, double-layer charging current 2, fluctuation-diffusion current 3, minimum dissolution current 4, pit-growth current (Reprinted from M. Asanuma andR. Aogaki, Nonequilibrium fluctuation theory on pitting dissolution. II. Determination of surface coverage of nickel passive film, J. Chem. Phys. 106, 9938, 1997, Fig. 2. Copyright 1997, American Institute of Physics.)...

The origin of the observed correlation was not established, and the relation was not interpreted as causal. It could be argued that a sustained elevated potential due to as-yet unknown microbial processes altered the passive film characteristics, as is known to occur for metals polarized at anodic potentials. If these conditions thickened the oxide film or decreased the dielectric constant to the point where passive film capacitance was on the order of double-layer capacitance (Cji), the series equivalent oxide would have begun to reflect the contribution from the oxide. In this scenario, decreased C would have appeared as a consequence of sustained elevated potential. 

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