Polycrystalline corrosion

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

Let us add here that the fabrication of polycrystalline semiconductive films with enhanced photoresponse and increased resistance to electrochemical corrosion has been attempted by introducing semiconductor particles of colloidal dimensions to bulk deposited films, following the well-developed practice of producing composite metal and alloy deposits with improved thermal, mechanical, or anti-corrosion properties. Eor instance, it has been reported that colloidal cadmium sulfide [105] or mercuric sulfide [106] inclusions significanfly improve photoactivity and corrosion resistance of electrodeposited cadmium selenide. 

The electrochemistry of single-crystal and polycrystalline pyrite electrodes in acidic and alkaline aqueous solutions has been investigated extensively. Emphasis has been laid on the complex anodic oxidation process of pyrite and its products, which appears to proceed via an autocatalytic pathway [160]. A number of investigations and reviews have been published on this subject [161]. Electrochemical corrosion has been observed in the dark on single crystals and, more drastically, on polycrystalline pyrite [162]. Overall, the electrochemical path for the corrosion of n-EeS2 pyrite in water under illumination has been described as a 15 h" reaction ... 

The simple model given above does not take account of the facts that industrial refractories are polycrystalline, usually non-uniform in composition, and operate in temperature gradients, both horizontal and vertical. Changes in the corrosion of multicomponent refractories will also occur when there is a change in the nature of the phase in contact with the corroding liquid for example in Ca0-Mg0-Al203-Cr203 refractories which contain several phases. 

Related to the attack of polycrystalline ceramic materials by aqueous media is the hydrolysis of silicate glasses. The following relationship has been developed to describe the effect of time and temperature on the acid corrosion (10% HCl) of silicate... 

The kinetic nature of mixed potentials is, in most cases, responsible for the lack of reproducibility. In a given solution under the same conditions, the same metal with different surface characteristics may adopt a different corrosion potential and, even at a given polycrystalline electrode, the corrosion potential is an average of different local regions of different properties crystal orientation, defects, and chemical heterogeneities. 

Extensive interface research is crucially essential for developing long-life, cost-effective, multilayer, polycrystalline, thin-film stacks for SECS. Microchemical analysis and other interface measuring techniques must be employed to solve the interfacial stability problems in the stacks. Important topical areas in solar materials interface science include thin films grain, phase, and interfacial boundaries corrosion and oxidation adhesion chemisorption, catalysis, and surface processes abrasion and erosion photon-assisted surface reactions and photoelectrochemistry and interface characterization methods. 

The results of studies of copper surfaces by low-temperature adsorption isotherms may be summarized as follows. True surface areas of metallic specimens as small as 10 sq. cm. can be derived with a precision of 6% from low-temperature adsorption isotherms using vacuum microbalance techniques. This method is of special value in determining the average thickness of corrosion films formed by the reaction of gases or liquids with solids. The effect of progressive oxidation of a rough polycrystalline metal surface is to decrease the surface area to a point where the roughness factor approaches unity. 

In the following sections the electrochemical reactivity of single grains of polycrystalline Ti is explored by using the nl-droplet method. The results from electrochemical measurements and the optical laser techniques from the previous section are combined to yield a band structure model for anodically grown anodic oxide layers. Other applications of this method to study laser induced corrosion, texture dependent photocurrent and corrosion of anodic oxide films are described in Refs. [89,90 and 91]. 

Although this approach is the only example in which catalytic activity and stereochemical control are not separated, no synthetic application can yet be foreseen because the concentration of such chiral sites in practically useful polycrystalline metal catalysts is very low and surface restructuring is likely to occur under reaction conditions. A similar conclusion can be drawn about the importance of other chiral metal structures, such as a screw dislocation or a chiral surface produced by asymmetric corrosion. Interestingly, asymmetric leaching of Ni in the presence of tartaric acid has already been proposed as an explanation for the enantio-differentiation by the tartaric acid-modified Ni catalyst [5]. 

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