General corrosion definition

Chapter 1 includes definitions of different corrosion mechanisms that are classified as general corrosion and localized corrosion. A full description and detailed scientific approach of each corrosion mechanism under the above classification is not included since books on this topic are available in the literature. 

Using this definition, one finds both corrosion and deposit problems in the preboiler system that can manifest themselves as general corrosion, pitting, or erosion-corrosion. The deposit problem can result from either deposition of suspended sohds that should have been removed earlier in the clarifier unit, or else it may be caused by formation of adherent calcium, magnesium, or iron scales. 

Corrosion is generally taken to be the waste of a metal by the action of corrosive agents. However, a wider definition is the degradation of a material through contact with its environment. Thus, corrosion can include non-metallic materials such as concrete and plastics and mechanisms such as cracking in addition to wastage (i.e. loss of material). This chapter is primarily concerned with metallic corrosion, through a variety of mechanisms. 

Soil has been defined in many ways, often depending upon the particular interests of the person proposing the definition. In discussion of the soil as an environmental factor in corrosion, no strict definitions or limitations will be applied rather, the complex interaction of all earthen materials will come within the scope of the discussion. It is obvious only a general approach to the topic can be given, and no attempt will be made to give full and detailed information on any single facet of the topic. 

In general, it is fair to state that one of the major difficulties in interpreting, and consequently in establishing definitive tests of, corrosion phenomena in fused metal or salt environments is the large influence of very small, and therefore not easily controlled, variations in solubility, impurity concentration, temperature gradient, etc. . For example, the solubility of iron in liquid mercury is of the order of 5 x 10 at 649°C, and static tests show iron and steel to be practically unaltered by exposure to mercury. Nevertheless, in mercury boiler service, severe operating difficulties were encountered owing to the mass transfer of iron from the hot to the cold portions of the unit. Another minute variation was found substantially to alleviate the problem the presence of 10 ppm of titanium in the mercury reduced the rate of attack to an inappreciable value at 650°C as little as 1 ppm of titanium was similarly effective at 454°C . 

Chemical reactivity has many different names, such as reactive materials, runaway reaction hazards, instability, thermal sensitivity, and incompatibility. Flammability, toxicity, and corrosion are also forms of reactivity. Since these topics are addressed elsewhere, our focus here will be on those reactions that fall outside the normal definitions of flammable or toxic and that generally occur far more rapidly than corrosion. 

A general definition of the term acute toxicity is The adverse effects occurring within a given time, following a single exposure to a substance. The term usually excludes local irritant or corrosive effects arising from a single application of a substance to the skin or eye (Section 4.5) (EC 2003). 

A general definition of a corrosive substance is A substance, which may destroy living tissues with which it comes into contact. 

There are many types of corrosion, as would be expected from its general definition. It has been traditional (4) to divide the study of corrosion into two areas the study of low temperature corrosion by aqueous or other solutions, controlled by electrochemical processes (wet) and the study of gaseous corrosion at high temperatures, controlled by thermodynamics and diffusion processes (dry). In addition to the obvious differences, the two areas have many phenomena in common. 

Initially, packed beds were also used. They, however, were no success, and at present monoliths are applied exclusively. This should not be misunderstood. Monolith means literally a single stone. However, metal-based analogues are also included in the definition of monolith. In fact, for catalytic converters in cars, in addition to ceramics, metal-based monoliths have been and still are used. A major advantage of metal was the thin wall thickness that could be achieved. Later, industry succeeded in manufacturing ceramic structures of comparable wall thickness. In view of their higher resistance against corrosion, ceramic monoliths are now more generally applied than metal ones. 

This chapter provides general definitions of the defects present after CMP with specific examples and images whenever available. The likely causes for the defects are explained in light of formation mechanism whenever possible. In addition to the defects visible to optical and electron microscopes, such as scratches, corrosion [8,9], and particles, this chapter also deals with residues and nuisances that are not visible yet in some cases more important, such as metal contamination [10] on the wafer surface. For example, a trace of radioactive element left from a CMP [11] process would be detrimental to the transistor structures. At the same time, it is one of the least visible contaminations under conventional defect detection methods. 

The broadest definition of corrosion is the degradation of a material s properties or mass over time because of the effect of the environment. We can think of this in simpler terms by recognizing this process as the tendency for a material to return to its most thermodynamically stable state. For most metallic materials, this means the formation of oxides or sulfides, or other basic metallic compounds generally considered to be ores. For polymeric materials, the end result could be a variety of simple organic compounds. Only in vacuums or under inert atmospheres can corrosion processes be expected to halt entirely. In most cases, these processes are slow enough to afford useful and practical equipment life. 

These considerations bring into sharp focus the issue of the baseline from which costs are calculated. What is generally intuitively meant when costs associated with a given degradative process are discussed is that they are costs above and beyond those that would be incurred in a world in which that process did not exist. This intuitive definition of costs associated with corrosion and wear seems to have been used in all studies prior to the NBS corrosion study (X>l2), where it was specifically defined. In that study, three worlds were defined as follows ... 

Particularly under the broad definition of corrosion as the deterioration of materials by reaction with the environment, the number of mechanisms whereby deterioration occurs is large. In general, a mechanism of corrosion is the actual atomic, molecular, or ionic transport process that takes place at the interface of a material. These processes usually involve more than one definable step, and the major interest is directed toward the slowest step that essentially controls the rate of the overall... 

In accordance with these definitions, stress-corrosion cracking has been a familiar problem in the petroleum and chemical industries for decades. Consequently, measures seem to be rather well established and generally known for preventing stress-corrosion cracking or for keeping it in check. Where conditions are such that this type of hydrogen attack can be expected, appropriate supplemental requirements should be included in the specification. A vessel built only to code requirements could be rendered unserviceable in a matter of hours by stress-corrosion cracking. 

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