Oxide films, on the metal surface

Sometimes the formation of oxide films on the metal surface binders efficient ECM, and leads to poor surface finish. Eor example, the ECM of titanium is rendered difficult in chloride and nitrate electrolytes because the oxide film formed is so passive. Even when higher (eg, ca 50 V) voltage is apphed, to break the oxide film, its dismption is so nonuniform that deep grain boundary attack of the metal surface occurs. 

Passivating (anodic) inhibitors form a protective oxide film on the metal surface they are the best inhibitors because they can be used in economical concentrations and their protective films are tenacious and tend to be rapidly repaired if damaged. 

He concluded that for aluminium and titanium certain etching or anodization pretreatment processes produce oxide films on the metal surfaces, which because of their porosity and microscopic roughness, mechanically interlock with the polymer forming much stronger bonds than if the surface were smooth . 

Chromate ions, when used as inhibitors in aqueous solutions, passivate by maintaining a coherent oxide film on the metal surface. Passivation is maintained even in a boiling concentrated chromic acid solution, in which many of the oxides in bulk form are soluble. The passivity breaks down rapidly, however, once the chromate is removed. 

Figure 18. Dependence of activation barrier A f for the nucleation of a thin oxide film on the metal surface as a function of electrode potential. Ey is the equilibrium potential of anodic oxide formation.7 The solid line represents the value of A against and the dotted line corresponds to the critical potential for the film formation. AE = 0.2 V, Cd= -1Fm-2, am = 0.411 m 2, a -0.01 J m-2,...

To increase equipment reliability and plant efficiency, corrosion inhibitors are used in boiler and cooling water programs to control fouling and deposition on critical heat-transfer surfaces. In cooling systems, corrosion inhibition is commonly achieved through the use of passivators, which encourage the formation of a protective metal oxide film on the metal surface ( 1). ... 

During the handling of microgram-sized samples of berkelium metal, it was observed that the rate of oxidation in air at room temperature is not extremely rapid, possibly because of the formation of a protective oxide film on the metal surface (135). Berkelium is a chemically reactive metal, and berkelium hydride (123), some chalco-genides (123, 136, 137) and pnictides (138, 139) have been prepared directly from the reaction of Bk metal with the appropriate nonmetal-lic element. 

As the name implies, the affected material disintegrates into fine metal and metal oxide particles mixed with carbon. Depending on the defects in a protective oxide film on the metal surface and the ability of the material to sustain this film, an induction period may be observed until metal dusting manifests itself as pitting or general attack. A possible mechanism was proposed by Grabke [1250] and Hochmann [1251]. 

When some metals or alloys are immersed in an oxidizing solution, corrosion does not occm, although it is favored thermodynamically. For example, iron will dissolve in dilute, but not in concentrated nitric acid. Corrosion is prevented because of the formation of a protective oxide film on the metal surface. This loss in chemical reactivity of a metal is known as passivation. Metals which readily undergo passivation in damp air include Fe, Al, Cr, Ni, Ti, and Pt. 

There exist other explanations for the generation of TSC in M1-P-M2 systems. For instance, the authors of [58] attempted to prove that voltage in the open circuit of the system with a layer of polyvinyl alcohol depended on the difference of work functions of the electrodes. This explanation can hardly be accepted at least because there is always an oxide film on the metal surface that experiences interfacial transformations at the polymer-metal interface. It is evident (Table 4.5) that the work function difference can be correlated neither with the current value in the M1-P-M2 systems nor with its direction. 

Other microorganisms promote corrosion of iron and its alloys through dissimilatory iron reduction reactions that lead to the dissolution of protective iron oxide/hy dr oxide films on the metal surface. Passive layers are either lost or replaced by less stable films that allow further corrosion. Obuekwe and coworkers [60] evaluated corrosion of mild steel under conditions of simultaneous production of ferrous and sulfide ions by an iron-reducing bacterium. They reported extensive pitting when both processes were active. When only sulfide was produced, initial corrosion... 

For example, with nickel-abased alloys, the formation of a nickel oxide film seems to be pre- requisite for obtaining a polished surface a finish of this quality, of 0.2 ym Ra, has been claimed for a Nimonic (nickel alloy) machined in saturated sodium chloride solution. Surface finishes as fine as 0.1 ym Ra have been reported when nick el- chromium steels have been machined in sodium chlorate solution. Again, the formation of an oxide film on the metal surface has been considered to be the key to these conditions of polishing. 

Mechanical damage to the pipe can result in localized corrosion where the surface is scratched or dented, due to the formation of differential stress electrochemical corrosion cells. Corrosion can also occur if local damage occurs to the protective oxide film on the metal surface, resulting in an electrochemical corrosion cell between the clean metal and corroded metal. In the case of active-passive alloys, such as stainless steel, local damage to the protective oxide layer can result in an electrochemical corrosion cell. 

Antimony is a unique metal in that it can provide a direct relationship between pH and its measured potential due to the formation of an oxide film on the metal surface. The potential difference or voltage developed between antimony and a copper/copper sulfate reference electrode is typically between 0.1 to 0.7 V for a variation of pH between 1 and 11. 

Protective film formation. The good corrosion resistance in seawater offered by copper-nickel alloys results from the formation of a protective oxide film on the metal surface. The film forms naturally and quickly, changing the alloy s initial exposure to seawater. In clean seawater, the film is predominantly cuprous oxide, with the protective value enhanced by the presence of nickel and iron. Cuprous hydroxy-chloride and cupric oxide are often also present. ... 

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