Corrosion product mechanical stability

In dry air the stability of zinc is remarkable. Once the protective layer of zinc oxide formed initially is complete, the attack ceases. Even under under normal urban conditions, such as those in London, zinc sheet 0 -8 mm thick has been found to have an effective life of 40 years or more when used as a roof covering and no repair has been needed except for mechanical damage. The presence of water does, of course, increase the rate of corrosion when water is present the initial corrosion product is zinc hydroxide, which is then converted by the action of carbon dioxide to a basic zinc carbonate, probably of composition similar to ZnCOj 3Zn(OH)2 . In very damp conditions unprotected zinc sometimes forms a loose and more conspicuous form of corrosion product known as wet storage stain or white rust (see p. 4.171). 

Mechanisms regulating deterioration processes in the burial state are still rather unknown and little research has been done on the consequences of long-term corrosion on the stability of the objects. Nevertheless, some studies have been performed on metal artifacts, seeking to characterize the surface layer as well as establish a relationship between the composition of the corrosion products and the environment where they formed. 

Solubility of corrosion product A deposit is usually produced on a metal s surface in neutral waters, which stifles the corrosion reaction at the same time the deposit dissolves in the water in contact with that surface. Initially the rate of corrosion exceeds the rate of dissolution and the deposit increases in thickness. This increased thickness reduces the corrosion rate further until the latter equals the rate of dissolution and a dynamic equilibrium is achieved. For this point to be reached, the deposit has to have some combination of effectiveness at stifling the corrosion with sufficient mechanical stability to maintain the thickness of the l er required. When this dynamic equilibrium is achieved, the rate of contamination is then, under defined flow conditions, controlled 1 the solubility of the corrosion product in the water. The standard sit-and-soak pro-cedme used in this project tacitly assmnes that this is the major controlling mechanism. 

Mechanical stability of corrosion product layer Where the corrosion deposit has insufficient mechanical stability to maintain the thickness required to achieve dynamic equilibrium between the corrosion and dissolution rates, particles of the deposit will break off on an intermittent basis producing peaks of contamination. This type of breakdown would only be expected after prolonged ageing (>70 days for brasses) of the test coupon. Determining the difference between the total metal leached into the test solution and its filtered metal content could give an indication of the presence of this problem. 

In recent years, a large body of work emphasized the use of zeolites for production of fine chemicals (refs.1-4). The interests stand in replacement of liquid acids to lower corrosion of equipment and pollution, and to reach specific selectivities. However, the hopes raised up in a rapid development of processes seems restrained nowadays. Many patents claimed zeolites as catalysts but very few have received industrial applications. Actually, basic research on the stability, the origin of deactivation, the regenerability of the catalysts have to be developed. Moreover, fundamental aspects of the mechanism of this new kind of reactions are lacking, in particular, the possibility of radical mechanisms, which are rather scarce with hydrocarbons, but can likely occur when heteroatoms are involved in the reactant. Those were our objectives in the study of the isomerisation of substituted halobenzenes on zeolites (refs.5-7). Indeed this reaction was claimed to occur readily on zeolites (refs.8-9), but it is supposed that no industrial development has followed. 

Corrosion-inhibitive properties of the compound Na2P03F have been tested by Andrade et al., either by incorporating it in a mortar or as a penetrant[68]. This compound, which is currently available as a proprietary product, is reported to act as an anodic inhibitor, possibly with some cathodic action. The minimum required ratio of phosphate to chloride was suggested as 1 1. The mechanism of action of this admixture is to stabilize the passive layer of iron oxide on the steel and also increase the density of concrete, thus decreasing the permeability... 

The second reason the region of stability is important is that if the metal falls in the region of immunity (the Cu region), it is thermodynamically impossible for the metal to dissolve into solution or form a solid oxidation product. If there is no oxidation of the metal, there is no chemical component to the polish. Without a chemical component, the polish becomes strictly mechanical polishing (MP). An example of a slurry where copper is immune to corrosion is given in Section 4.2.4. 

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