Corrosion phenomena types

The coexistence of the above-mentioned four factors is a necessary but not sufficient condition for the development of the corrosion phenomenon. There are a number of resistances, in a generalized sense, of kinetic type that hinder the corrosive phenomenon and by means of which it is possible to act in order to prevent corrosion. 

A cut or scratch of the film applied on polarized p>anel allows again the flow of protective electrical current metallic zinc is oxidized and the film is sealed again. A substantial difference with other types of coatings is that the corrosive phenomenon does not occur under the film adjacent to the cut (undercutting). 

The discussion of Section 17.2 treated the corrosion of metallic materials in terms of electrochemical reactions that take place in aqueous solutions. In addition, oxidation of metal alloys is also possible in gaseous atmospheres, normally air, in which an oxide layer or scale forms on the surface of the metal. This phenomenon is frequently termed scaling, tarnishing, or dry corrosion. In this section, we discuss possible mechanisms for this type of corrosion, the types of oxide layers that can form, and the kinetics of oxide formation. 

High-temperature corrosion is induced by accelerated reaction rates inherent in any temperature reaction. One phenomenon that occurs frequently in heavy oil-firing boilers is layers of different types of corrosion on one metal surface. 

Different types of crevices are shown in Fig. 1.49, and reference should also be made to Section 19.2 for other examples. The phenomenon is referred to as crevice corrosion, and is characterised by a geometrical configuration in which the cathode reactant (usually dissolved oxygen) can... 

In a container or a loop in which a molten metal is flowing but where there is not necessarily a temperature gradient, a type of corrosion can occur by transfer of material along an activity gradient ( activity being used here in the thermodynamic sense—see Section 20.1). Again it is convenient to discuss this phenomenon under two headings ... 

Another type of nickel alloy with which problems of intergranular corrosion may be encountered is that based on Ni-Cr-Mo containing about 15% Cr and 15% Mo. In this type of alloy the nature of the grain boundary precipitation responsible for the phenomenon is more complex than in Ni-Cr-Fe alloys, and the precipitates that may form during unfavourable heat treatment are not confined to carbides but include at least one inter-metallic phase in addition. The phenomenon has been extensively studied in recent years . The grain boundary precipitates responsible are molybdenum-rich M C carbide and non-stoichiometric intermetallic ix... 

The principal use of gold is as a very thin coating about 0-05 /xm thick for electrical and electronic applications. Because of the thinness of gold electrodeposits, porosity must be very carefully controlled since seepage of corrosion products from substrate or undercoat exposed at these pores can have serious adverse effects on both appearance and electrical properties of the composite. The porosity can vary with the thickness of the deposit (Fig. 13.1), and with the type of plating bath and with its method of operation (Fig. 13.2), and the phenomenon has been extensively studied by Clarke and many other workers. 

In the preceding sections, various types of fluctuations and instabilities essential to corrosion were examined. As a result, it was shown that a corrosion system involves various kinds of problems of stability and instability. Unlike thermodynamic equilibrium systems, in nonequilibrium systems like corrosion systems, a drastic change in the reaction state should be defined as a bifurcation phenomenon. 

It is well known through our experience that material with conduction electrons suffer from the phenomenon called corrosion i.e., metals turning into metallic oxides in time in air. On the other hand, the materials without conduction electrons do not suffer from corrosion. Technically, the presence of conduction electrons implies the existence of free electrons and conduction band. As pointed out in the mechanical property section these two distinct properties exhibit themselves also in term of plasticity . That is, the existence of free electron band allows plastic deformation whereas in the absence of free electron band the plasticity is nonexistent. It is recalled that the theory we are proposing for metals and alloys requires not only the coexistence of covalent bond and free electron band but also that the ratio of the number of these two type of electrons be maintained at a constant value for a given metal. Within such understanding, we now construct corrosion process in steps ... 

What has been described is what is called stress-corrosion cracking. Some common examples of systems that tend to undergo this type of corrosion are given in Table 12.5. But perhaps one should call it yield-assisted corrosion (an electrochemi-cal-plus-mechanical phenomenon) in contrast to normal field-assisted dissolution (an electrochemical phenomenon). 

Types of Corrosion Phenomena. The major categories of phenomena ( include uniform, localized, and pitting corrosion selective dissolution and corrosion acting together with a mechanical phenomenon. In uniform corrosion, all areas corrode at the same rate. Examples of uniform corrosion include tarnishing and active dissolution of metals in acids. In localized corrosion some areas corrode more readily than others crevice corrosion and filiform... 

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