Passive film chemical composition

A modification of the chemical composition of the passive films. The composition of the passive films formed on nitrogen-bearing austenitic stainless steels has been analyzed by Auger and ESCA (Clayton and Olefjord, 1995, and references therein, Sadough Vanini et al.. 

There is evidence (8)(21 -24)(71 -75) that during eiectrodissoiution, a passivation fiim is formed on the anode. The film is gradually eliminated by diffusion when the current is switched off. The etching and polishing behavior of an anode are perhaps associated with the chemical stability of this passive film whose composition is usually speculative (21 -23)(72)(74). 

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]. 

Passivating films may change their chemical composition after their formation due to reactions with water or carbon dioxide lithium alkylcarbonates react with traces of water to yield lithium carbonate (see Table 8). 

The corrosion resistance of lithium electrodes in contact with aprotic organic solvents is due to a particular protective film forming on the electrode surface when it first comes in contact witfi tfie solvent, preventing further interaction of the metal with the solvent. This film thus leads to a certain passivation of lithium, which, however, has the special feature of being efiective only while no current passes through the external circuit. The passive film does not prevent any of the current flow associated with the basic current-generating electrode reaction. The film contains insoluble lithium compounds (oxide, chloride) and products of solvent degradation. Its detailed chemical composition and physicochemical properties depend on the composition of the electrolyte solution and on the various impurity levels in this solution. 

Jin and Atrens (1987) have elucidated the structure of the passive film formed on stainless steels during immersion in 0.1 M NaCl solution for various immersion times, employing XPS and ion etching techniques. The measured spectra consist of composite peaks produced by electrons of slightly different energy if the element is in several different chemical states. Peak deconvolution (which is a non-trivial problem) has to be conducted, and these authors used a manual procedure based on the actual individual peaks shapes and peak positions as recorded by Wagner et al. (1978). The procedure is illustrated in Figure 2.8 for iron. 

Even if LiPFe is replaced by more thermally stable salts, the thermal stability of passivation films on both the anode and the cathode would still keep the high-temperature limits lower than 90 °C, as do the thermal stability of the separator (<90 °C for polypropylene), the chemical stability of the insulating coatings/sealants used in the cell packaging, and the polymeric binder agents used in both cathode and anode composites. 

In situ methods permit the examination of the surface in its electrolytic environment with application of the electrode potential of choice. Usually they are favored for the study of surface layers. Spectroscopic methods working in the ultra high vacuum (UHV) are a valuable alternative. Their detailed information about the chemical composition of surface films makes them an almost inevitable tool for electrochemical research and corrosion studies. Methods like X-ray Photoelectron Spectroscopy (XPS), UV Photoelectron Spectroscopy (UPS), Auger Electron Spectroscopy (AES) and the Ion Spectroscopies as Ion Scattering Spectroscopy (ISS) and Rutherford Backscattering (RBS) have been applied to metal surfaces to study corrosion and passivity. 

It is now well accepted that the passive film is not a single layer, but rather has a stratified structure. The inner layer plays the role of a barrier layer against corrosion and the outer layer plays the role of an exchange layer. The chemical composition is a... 

Prevention of Localized Corrosion. Available data on the various physical and chemical aspects of passivity, including the composition, thickness, structure, growth, and properties of passive layers should be used in the studies of localized corrosion. A good understanding of the surface reactions involved in the formation and composition of passive films, passivation/repassivation, is necessary for the development of highly... 

Steigerwald et al. reported that the Cu-BTA passivation film was almost 20 nm thick after a 10-min immersion in a solution at pH 2 [22]. Cohen and coworkers also studied the stoichiometry, thickness, and chemical composition of the Cu-BTA using in situ ellipsometry and ex situ X-ray photoelectron spectroscopy [13]. The authors reported that film grown on CU2O and bare Cu under oxidizing conditions are on the order of 5 40 A thick and the chemical composition of this layer is mostly Cu -BTA. Similar to the schematic view portrayed in Fig. 8.3, Walsh et al. suggests that the BTA film is composed of a monolayer that is in direct contact with the copper film and a multilayer built on top of the monolayer [6]. They reveal that in the monolayer, BTA molecular plane is oriented within 15° of the surface normal. In the multilayer, the molecular plane is tilted by about 40° from the plane of copper surface. 

Data on the chemistry and structure of thin oxide layers (passive films) produced by anodic polarization of metallic electrodes are necessary to understand and predict the properties of these films, in particular their corrosion resistance. There are now many available data on the chemical composition of passive films formed on metals and alloys. Surfece chemical analysis techniques have been, and still are, very useful to obtain such data. In sharp contrast, there is a lack of data on the structure of passive films. This is in part due to the difficulty of any structural analysis of very thin films on... 

Passivators are inorganic substances possessing oxidative properties whose reaction products with metals form a passive film on the substrate surface, which shifts the corrosion potential of the substrate to the positive side by a few tens of volts. Like a depolarizer, the passivator generates a current on the anodic areas of the substrate of 1 > i density, where i is the critical density of the passivation current. This means that the chemical composition of the passivating film on a metal substrate is the same whether the substrate is passivated by anodic polarization in an acid or is treated with solutions of chromates (CrO ), nitrates (NO ), molybdates (MoO ), tungstates (WO ), ferrates (FeO ) or pertechnates (TcOj). 

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