Oxide films continued porous

I would like to comment on the application of the Pilling-Bedworth rule. In most cases of oxidation, it is the metal ion that migrates in the oxide film. If this is true, then the metal ion will go through the oxide layer and form new oxide on the outer surface. Since this new oxide is not constrained, there seems to me to be no reason why the difference in the volume of the metal oxide to the volume of the metal should be used to predict the continuity of the oxide film. For example, for sodium, one would predict, by this rule, a porous oxide when actually a dense, protective oxide is found. For tungsten, one would predict a compact oxide while the opposite is found, ft seems to me that the continuity of the oxide layer is determined more by the possible transformations or reactions of the film which forms initially. 

It was mentioned above that a chromium anode continues to dissolve in the passive state forming chromate ions in this case the invisible oxide film may be suflSciently porous to allow ions to penetrate it alternatively, the oxide film may become oxidized to C1O3 which dissolves to form chromate, but is immediately regenerated by oxidation of the anode. 

It may be concluded that the use of a continuous, porous gold film as reference electrode in the given solid oxide electrochemical cell is an appropriate choice. It was shown to be catalytically inert and to have a reasonably stable potential. The estimated error of the latter is less than 20 mV under typical conditions of electro-chemical promotion experiments. The potential distribution in the solid electrolyte was shown to be highly sensitive to the exact position of the electrodes. Nevertheless, estimation of the catalyst potential remains reliable because the ohmic drop correction is almost negligible, e.g., at 375°C it is in the order of 1 mV pA . ... 

If the Pilling-Bedworth ratio is less than 1 the oxide cannot cover the metal completely and the oxide film has an open or porous structure. Oxidation takes place continuously, and the oxidation kinetics tend to be linear. This type of behaviour is found for the alkali and alkaline earth metals. In the rare cases where the PiUing-Bedworth ratio is equal to 1, a closed layer can form which is stress-firee. When the Pilling-Bedworth ratio is greater than 1, a closed layer forms with a certain amount of internal compressive stress present. 

If the oxide film or scale cracks or is porous, that is, if the corrosive gas can continue to penetrate readily and react with the base metal in a catastrophic manner, no protection will be afforded and attack will proceed at a rate determined essentially by the availability of the corrosive gas. In this case, the rate will not sensibly change with time, and, as is apparent from Fig. 15.12, the weight change or depth of penetration from oxidation is a straight line or linear function of time and may be expressed as... 

These differences depend mainly on the nature of the surface layer. If its conduction is electronic, an anodic reaction such as oxygen evolution can proceed freely at its surface, leaving the very thin oxide film unchanged. This is one extreme case the other is that the film is porous to ions, in which case the attack of the metal will continue at a slow rate and the oxide film may grow to visible proportions. 

The essential protective film on the 2inc surface is that of basic 2inc carbonate, which forms in air in the presence of carbon dioxide and moisture (Fig. 1). If wet conditions predominate the normally formed 2inc oxide and 2inc hydroxide, called white mst, do not transform into a dense protective layer of adhesive basic 2inc carbonate. Rather the continuous growth of porous loosely adherent white mst consumes the 2inc then the steel msts. 

Mechanical Passivity.—In certain instances the dissolution of an anode is prevented by a visible film, e.g., lead dioxide on a lead anode in dilute sulfuric acid this phenomenon has been called mechanical passivity, but it is probably not fundamentally different from the forms of passivity already discussed. The film is usually not completely impervious, but merely has the effect of decreasing the exposed surface of the electrode to a considerable extent the effective c.d. is thus increased until another process in which the metal is involved can occur. At a lead anode in sulfuric acid, for example, the lead first dissolves to form plumbous ions which unite with the sulfate ions in the solution to form a porous layer of insoluble lead sulfate. The effective c.d. is increased so much that the potential rises until another process, viz., the formation of plumbic ions, occurs. If the acid is sufficiently concentrated these ions pass into solution, but in more dilute acid media lead dioxide is precipitated and tends partially to close up the pores the layer of dioxide is somewhat porous and so it increases in thickness until it becomes visible. Such an oxide is not completely protective and attack of the anode continues to some extent it is, however, a good conductor and so hydroxyl ions are discharged at its outer surface, and oxygen is evolved, in spite of its thickness. 

Si present in the substrate also creates another problem (Fig. 6). As can be seen in the SEM cross section, a continuous Si02 layer forms beneath Cr203 layers when the metallic 446 interconnect coated with 0.2 im porous LaCrOs thin film is annealed at 900°C for 100 hours. Formation of such insulating layer is also detrimental for the electrical performance of the SOFC. Though the cross section analysis of structure of oxide layers did not... 

Anodic porous alumina is conventionally grown on aluminum foils, as indicated in Fig. 2. Similar self-assembled growth is achieved on Si by depositing an A1 thin film on the front side of a silicon wafer and forming an ohmic contact on the back side that is used as anode. The electrochemical solutions currently used are oxalic or sulfuric acid aqueous solutions. Details for the fabrication of thin alumina templates on Si with adjustable pore size and density are given elsewhere [8]. Electrochemical oxidation of A1 starts from the A1 surface and continues down to the Al/Si interface, following an anodization current density/time curve as shown in Fig. 3. 

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