Passivity thick film

Katti VR, Debnath AK, Gadkari SC, Gupta SK, Sahni VC (2002) Passivated thick film catalytic type Hj sensor operating at low temperature. Sens Actuators B 84 219-225... 

The local dissolution rate, passivation rate, film thickness and mechanical properties of the oxide are obviously important factors when crack initiation is generated by localised plastic deformation. Film-induced cleavage may or may not be an important contributor to the growth of the crack but the nature of the passive film is certain to be of some importance. The increased corrosion resistance of the passive films formed on ferritic stainless steels caused by increasing the chromium content in the alloy arises because there is an increased enhancement of chromium in the film and the... 

Generally, such a remarkable restriction of metal dissolution results not only from the formation of a thin surface oxide film but also from the formation of a comparatively thick film such as silver chloride or zinc chloride. In this chapter, however, we use the term passive film only for compact and thin oxide films. 

The passive film is composed of metal oxides which can be semiconductors or insulators. Then, the electron levels in the passive film are characterized by the conduction and valence bands. Here, we need to examine whether the band model can apply to a thin passive oxide film whose thickness is in the range of nanometers. The passive film has a two-dimensional periodic lattice structure on... 

The third aspect to consider is the electrochemical stability of the material used. For the oxygen reduction reaction, the electrode potential is highly anodic and at this potential, most metals dissolve actively in acid media or form passive oxide films that will Inhibit this reaction. The oxide forming metals can form non-conducting or semi-conducting oxide films of variable thickness. In alkaline solutions, the range of metals that can be used is broader and can include non-precious or semi-precious metals (Ni, Ag). 

Active metals such as aluminum, titanium, and high-chromium steels become corrosion resistant under oxidizing conditions because of a very adherent and impervious surface oxide film that, although one molecule thick, develops on the surface of the metal. This film is stable in a neutral medium, but it dissolves in an acid or alkaline environment. In a few cases, such as certain acid concentrations, metals can be kept passive by applying a carefully controlled potential that favors the formation of the passive surface film. The ability to keep the desired potential over the entire structure is very critical in anodic control. If a higher or lower potential is applied, the metal will corrode at a higher rate, possibly higher than if it is not protected at all. 

There are studies in which the fact that active metal electrodes are covered with surface films is not so important, e.g., when these metals are used as counterelectrodes, or when they are studied as practical anodes in batteries. However, even in these cases, the native active metals as received may be covered with two thick films. It is therefore, necessary to remove the initial native film covering the active metal under an inert atmosphere. The passivating films of lithium and calcium can be scraped off with a stainless steel knife. In the case of harder active metals such as magnesium and aluminum, an abrasive cloth or... 

A metal is passive if it substantially resists corrosion in an environment where there is a large thermodynamic driving force for its oxidation (also known as thick film passivity). The Evans diagram for this type of behavior is shown in Fig. 1. 

Figure 1 Schematic Evans diagram for a material that exhibits thick film passivity.

Thick film passivity (i.e., protection of a metal surface by a film of visible thickness) can be due either to oxide formation or salt film precipitation. Salt film... 

While thick film passivity has been documented and understood for many years, the difficulties in studying thin film passivity were daunting. It took many years to determine that indeed a film was responsible for the effect, as these films are so thin that they are invisible to the eye (i.e., transparent to radiation in the visible region). Two main types of theories were developed in order to explain the phenomena observed theories based upon the idea of adsorption reducing the corrosion rate, and theories based upon the formation of a new phase, an oxide of the base metal, on the surface. In all cases, an increased barrier to dissolution results upon the increase in potential. This increased kinetic barrier upon anodic polarization contrasts with the exponentially decreased barrier which develops during anodic polarization of an active material. 

Investigation of the effect of particulate properties during CMP of W showed a significant increase in the polish rate in the presence of ferric nitrate compared to the polish rate in de-ionized water, at all alumina bulk density values (shown in figure 4). Kaufmann et al. , attributed the increase in the polish rate in the presence of ferric nitrate to the "softness" of the passivating oxide film compared to W. Potentiodynamic experiments and open circuit potential measured as a function of time indicate passivation of W surface. However, the hardness values of tungsten films exposed for 5 min to 0.1 M ferric nitrate, even at the lowest load (300p,N), were the same as those of as-deposited W films within experimental error. Since a 10 nm indentation depth was observed at the lowest load, it is possible that the thickness of the oxide film is smaller and its effect does not manifest itself on the hardness measurement. 

The electrochemistry of copper anodes in neutral and alkaline media has been studied in detail [223-226]. This complicated process includes, as a rule, the growth of ultrathin (several nanometers thick) films of CU2O, CuO, and also mixed and non-stoichiometric oxides. Until recently, it was assumed that cations do not affect the dissolution and passivation of copper. However, the first attempts to synthesize HTSCs and (or) their precursors showed that copper oxide films, formed when the potential of a copper electrode is cycled in a Ba(OH)2 solution, incorporate substantial amounts of barium [227]. This result was subsequently confirmed not only for potentiodynamic [228] but also for potentiostatic [229] oxidation modes. It has been suggested that Ba[Cu(OH)4] or Ba[Cu2(OH)6] forms in the supersaturated nearelectrode layer [229]. Similar studies with other alkaline-earth cations at high pH are difficult to conduct due to the poor solubility of the corresponding hydroxides. 

Thermal oxidation is an old and common method of forming a primary passivating film of Si02 on silicon. The metal is heated in dry oxygen, in wet oxygen, or in steam. A silica layer grows inwardly from the surface by a thermal oxidation mechanism. The silicon wafer is heated to 600-1200°C to achieve 1 pm thick films in about an hour. 

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