Passive films chemical

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. 

The protective quality of the passive film is detennined by the ion transfer tlirough the film as well as the stability of the film with respect to dissolution. The dissolution of passive oxide films can occur either chemically or electrochemically. The latter case takes place if an oxidized or reduced component of the passive film is more soluble in the electrolyte than the original component. An example of this is the oxidative dissolution of CrjO ... 

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]. 

In most cases, CVD reactions are activated thermally, but in some cases, notably in exothermic chemical transport reactions, the substrate temperature is held below that of the feed material to obtain deposition. Other means of activation are available (7), eg, deposition at lower substrate temperatures is obtained by electric-discharge plasma activation. In some cases, unique materials are produced by plasma-assisted CVD (PACVD), such as amorphous siHcon from silane where 10—35 mol % hydrogen remains bonded in the soHd deposit. Except for the problem of large amounts of energy consumption in its formation, this material is of interest for thin-film solar cells. Passivating films of Si02 or Si02 Si N deposited by PACVD are of interest in the semiconductor industry (see Semiconductors). 

Precipitating (cathodic) inhibitors are simply chemicals that form insoluble precipitates that can coat and protect the surface precipitated films are not as tenacious as passive films and take longer to repair after a system upset. 

The dissolution of passive films, and hence the corrosion rate, is controlled by a chemical activation step. In contrast to the enhancement of the rate of dissolution by OH ions under film-free conditions, the rate of dissolution of the passive film is increased by increasing the ion concentration, and the rate of corrosion in film-forming conditions such as near-neutral solutions follows the empirical Freundlich adsorption isotherm ... 

The dissolution of passive films is, in the main, controlled by a chemical activation step in contrast to film-free conditions at. Many protective anodic films are oxides and hydroxides whose dissolution depends upon the hydrogen ion concentration, and the rate follows a Freundlich adsorption equation ... 

In the main there exists, for each system of a chemical in contact with those metals and alloys that rely on a passive film, the possibility of an increase in corrosion rate with increasing concentration but reaching a maximum and followed by a decrease in rate. If the concentration when this maximum is reached is low, then the chemical is inhibitive . The effect of temperature on corrosion is dependent on the position of the maximum concentration. For many chemical/metal systems this maximum may be at a temperature... 

Lead is characterised by a series of anodic corrosion products which give a film or coating that effectively insulates the metal mechanically from the electrolyte (e.g. PbS04, PbClj, PbjO, PbCrO<. PbO, PbO, 2PbC03.Pb<0H)z), of which PbS04 and Pb02 are the most important, since they play a part in batteries and anodes. Lead sulphate is important also in atmospheric passivation and chemical industry applications. 

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). 

Passivating agents are chemicals that promote the formation of a passivating film on the surface of a metal or alloy, such that the electrochemical behavior of the metal or alloy then approaches that of an appreciably more noble metal. 

After examining the film breakdown process, we have another question Once broken, how is the film reformed To answer this question, it is necessary to calculate the formation energy for a passive-film nucleus on the film-free surface. The contribution of chemical energy is newly added to the electrocapillary energy. The total energy is thus given by... 

After lapping, the sliders will be cleaned, and then a passivation film of diamond-like carbon (DLC) will be deposited on the surfaces of sliders through chemical vapor deposition (CVD) to protect the pole area from chemical-physical corrosion and electrostatic discharge attack. Corrosion in pole areas will result in loss of read/write functions. A corrosion test was taken to examine the ability of the sliders polished by different slurries as shown in Table 6. It can be seen that the MRR change rate of the sliders polished by UFD slurry is much less than that polished by the slurry T5qre III, that is, the capability of anti-corrosion of the former is much better than that of the latter. 

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. 

Metals develop a natural corrosion-resistant film when exposed to the environment. Examples include the rusting of iron, tarnishing of silver, and the formation of the patina on copper. These passive films help prevent further corrosion. However, films do not provide complete resistance to chemical attack and are destroyed by various corrosive agents. 

The anticorrosive action of the chromate pigments is based both on chemical and electrochemical reactions [5.66], [5.108]—[5.113]. Electrochemical passivation and chemical reaction are illustrated in Figure 70 [5.114], [5.115], Passivation is based on electrochemical processes in the cathodic region. In addition, a protective film is also... 

The formation of passivating films on lithium in contact with liquid or soluble cathodic reagents is a prerequisite for the construction of a practical cell. The film acts in the same way as a separator, preventing further direct chemical reaction of lithium and the cathodic reagent. However, film formation involving the action of S02, SOCl2, etc. on lithium is considerably more complex and may produce much more severe voltage delay characteristics than in the case of insoluble cathodes described above. 

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