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REPÚBLICA BOLIVARIANA DEL VENEZUELAMINISTERIO DEL PODER POPULAR PARA LA EDUCACIÓN
* Conclusion * Bibliography INTRODUCTION Exposure to water, oxygen, andother agents can cause material corrosion, resulting in loss of structural integrity or degradation in surface appearance. Corrosion is the chemical transformation of metal due to chemical reactions. The most common form of corrosion is oxidation, where metal atoms combine with oxygen atoms to form oxides. Iron rust is the most recognizable form of corrosion, and appears when iron oxide formson iron or steel components that are exposed to air or water, however, virtually all metals and alloys are susceptible to corrosion. Technically, corrosion can occur in other types of materials, such as ceramics or polymers, but the process is either rare or different enough that the term “corrosion” is generally not used.DEFECTS IN THE SOLID STATE All solids contain defects, where theideal lattice as described in terms of an infinitely repeating unit cell is broken. Defects can have a large effect on a range of properties of the material, such as the mechanical strength, electrical conductivity, corrosion and chemical reactivity. Defects may be formed due to thermodynamic effects, and these defects are known as intrinsic defects, or due to imperfections instochiometry,which are not due to thermodynamics, and which are known as extrinsic defects. Defects which are localized in space, ie. those which occur at single sites in the crystal, are known as point defects, whereas extended defectsare those which penetrate through the crystal in one or more dimensions.INTRINSIC DEFECTS When a defect is formed, disorder is introduced into the otherwise perfectlattice. This increase in disorder leads to an increase in the entropy, S, of the system. Although the creation of a defect is generally endothermic, ie. it has a positive enthalpy, H, of formation, the overall free energy of formation is given by G = H - TS. As the temperature rises, both H and S increase. For T > 0, there will be a minimum in the free energy of formation of the defects at anon-zero defect concentration, and so the formation of defects is spontaneous..THE SCHOTTKY DEFECT This is a defect where an ion is removed from its lattice site, leaving a vacancy. There are generally equal numbers of vacancies on cation and anion lattice points, so as to preserve charge neutrality, and so the overall stoichiometry is unchanged. The concentration of Schottky defects variesfrom a concentration of 10-12 molL-1 in alkali metal halides, to about 12 molL-1 in some d-metal oxides. This corresponds to one defect for every 1014 formula units in the alkali metals, up to one defect for every 10 formula units in the d-metal oxides, so there is a huge range of defect concentrations. In general, Schottky defects are found when the metal ions are able to have more thanone oxidation state.THE FRENKEL DEFECT This is a defect where an ion is removed from its lattice site, leaving a vacancy, and moved into an interstitial site. Ionic solids may have cation interstitial, as in silver halides, anion interstitials, as in BaF2, PbCl2, and PbBr2, and a mixture of both cation and anion interstitials, as in PbI2. Frenkel defect formation is therefore favoured by theready availability of interstitial sites large enough to hold the displaced ion. Open structures, such as sphalerite and wurtzite, with low coordination numbers are those in which Frenkel defect formation commonly occurs. A less common form of point defect is the atom interchange defect, where a pair of atoms are simply swapped to each other's lattice point. This is common in metal...