Passive films definition

The fields marked Fe203 and Fe304 are sometimes labeled passivation on the assumption that iron reacts in these regions to form protective oxide films. This is correct only insofar as passivity is accounted for by a diffusion-barrier oxide layer (Definition 2, Section 6.1). Actually, the Flade potential, above which passivity of iron is observed in media such as sulfuric or nitric acid, parallels line a and b, intersecting 0.6 V at pH = 0. For this reason, the passive film (Definition 1, Section 6.1) may not be any of the equilibrium stoichiometric iron oxides, as is further discussed in Chapter 6. 

There are two commonly expressed points of view regarding the composition and structure of the passive film. The first holds that the passive film (Definition 1 or 2) is always a diffusion-barrier layer of reaction products—for example, metal oxide or other compound that separates metal from its environment and that decreases the reaction rate. This theory is sometimes referred to as the oxide-film theory. 

This definition excludes inhibitor films or other corrosion-protecting layers such as paint and so on, which have the same function as passive films but a different origin. Such films are formed from components in the electrolyte (inhibitors) or are deposited in a technical process (e.g. cathodic deposition of paint). 

In the eighties, there was a long discussion about an appropriate description of passive films by the properties of a semiconductor or chemiconductor [65]. This problem is, in fact, a problem of definition, the appropriate timescale and the... 

In several cases, materials for combined erosive and corrosive conditions have been evaluated on the basis of separate erosion and corrosion studies and data, with the consequence that the synergistic effects are left out of the evaluation. Since one or the other of these effects may be large, the conclusions may be quite wrong. For materials fliat usually are passive due to a dense oxide film, such as stainless steels, Wc is by definition very low. But since sand erosion more or less destroys the passive film, the corrosion rate increases strongly and may reach very high values, i.e. the contribution of Wce may be particularly high for these materials. The other synergy effect, Wec, is most pronounced for ceramic-metallic materials in which the metallic phase has inferior corrosion resistance, e.g. for a cemented carbide with a metallic phase of cobalt (WC-Co). 

The second holds that metals passive by Definition 1 are covered by a chemisorbed film—for example, of oxygen. Such a layer displaces the normally adsorbed H2O molecules and decreases the anodic dissolution rate involving hydration of metal ions. Expressed another way, adsorbed oxygen decreases the exchange current density (increases anodic overvoltage) corresponding to the overall reaction M -1- ze. Even less than a monolayer on the surface is... 

For metals that are passive by Definition 1, based on marked anodic polarization, the films are usually invisible, about 2 to 3 nm thick. Metals and alloys in this category have been the source of extended debate and discussion on the mechanism of passivity over the past 150 years. K the surface is abraded, local high temperatures generated at the surface produce a detectable oxide, but this is not the passive film. 

The current-potential relationship ABCDE, as obtained potentiosta-tically, has allowed a study of the passive phenomena in greater detail and the operational definition of the passive state with greater preciseness. Bonhoeffer, Vetter and many others have made extensive potentiostatic studies of iron which indicate that the metal has a thin film, composed of one or more oxides of iron, on its surface when in the passive state . Similar studies have been made with stainless steel, nickel, chromium and other metals... 

According to Schulze, valve action differs from the related phenomenon of passivity in that in the former case the oxide skin, although very thin, is of definite thickness4 and prevents the passage of the current, whereas in the latter case the oxide film is an electrical conductor of molecular thickness.5... 

By definition, mediated electron transfer of the type discussed in Section 14.4.2(c) cannot occur in blocking films. However for very thin films, e.g., self-assembled monolayers (SAM) of alkane thiols or oxide films, electrons can tunnel through the film and cause faradaic reactions. This phenomenon is important in electronic devices, in passivation of metal surfaces, and in fundamental studies of the distance dependence of the rate of electron transfer. 

There is no question on either viewpoint that a diffusion-barrier film is the basis of passivity of many metals that are passive only by Definition 2. Examples of protective films that isolate the metal from its environment are (a) a visible lead sulfate film on lead immersed in H2SO4 and (b) an iron fluoride film on steel immersed in aqueous HF. 

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