Corrosive wear definition

An alternate method of classification of damage mechanisms in terms of environmental conditions such as stress, temperature, corrosion, wear, radiation is known as the failure wheel. In this representation secondary mechanisms are underlined as opposed to primary damage mechanisms. The boiler tube failure discussed can be represented in terms of failure wheel, as shown in Figure 2.31. The aim in any case is to identify definitively the underlying mechanism of failure to enable one to undertake remedial action such that future failures do not occur. 

FYom the multitude of intricate corrosion processes in the presence of mechanical action (friction, erosion, vibration, cavitation, fretting and so on) it is justified to touch upon corrosion types joined under a single failure mode induced by mechanical stresses. These are the stresses that govern the corrosion wear rate of metals during friction. Such processes are usually called corrosion stress-induced cracking in the case that the mechanical action is effective only in one definite direction, or otherwise termed corrosion fatigue in the case that compressive and tensile stresses alternate within cycles. In spite of the differences between the appearance of these corrosion types, they have much in common, e.g. fundamental mechanisms, the causes, and they overlap to a certain degree [19]. 

Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with its environment. Deterioration by physical causes is not called corrosion, but is described as erosion, galling, or wear. In some instances, chemical attack accompanies physical deterioration, as described by the following terms corrosion-erosion, corrosive wear, or fretting corrosion. Nonmetals are not included in this definition of corrosion. Plastics may swell or crack, wood may split or decay, granite may erode, and Portland cement may leach away, but the term corrosion, in this book, is restricted to chemical attack of metals. 

Erosion is defined as the loss of hard tissue by chemical means not derived from bacteria, i.e. the dissolution of hard tissue by acid where the acid source is not the oral bacteria [5], Erosion may be caused by either intrinsic (e.g. stomach acid) or extrinsic (e.g. dietary) sources. Erosion is often associated with the consumption of acid products, such as fruits or acid beverages, or with medical conditions where reflux of acidic into the oral cavity is present. Interestingly, the term erosion is widely used in other fields where the definition is rather different. For example, in the field of tribology, erosion refers to the loss of material from a surface by solid or liquid impacts [6], In the classical tribological definitions, the mechanism dentists refer to as erosion would be described as corrosion, or tribo-chemical wear. 

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 ... 

Corrosion (of metals) and fracture (of all materials) each amounts to approximately of the GNP in the U.S., and probably in other industrialized nations. Wear, the other degradative process of capital equipment, has not been discussed as yet. However, from the results presented in Ref. 15 and 16, it can be concluded that the application of tribology could save 1.1 to 1.5 of the GNP in the U.S. in 1971. From the definition used (16) this represents costs somewhere between avoidable costs and total costs, the reference "World being essentially the World IV of the NBS fracture study. While the subject merits further study, the results indicate that the costs of wear are roughly comparable to the costs of corrosion and fracture, but perhaps somewhat less. It can thus be concluded that the overall costs of these three degradative processes amount to somewhat more than 10 of the GNP. Costs associated with the other degradative process mentioned earlier are not known to this author, but clearly will raise this total. 

ASTM G 40, Terminology Relating to Wear and Erosion— This contains terms and their definitions relating erosion corrosion and cavitation of materials. 

One definition of fretting is "Wear phenomena occurring between two contacting surfaces having oscillatory relative motion of small amplitude." Note Fretting is a term frequently used to include fretting corrosion. This usage is not recommended. 

Adsorption equilibria for polymers out of concentrated solutions as function of concentration frequently exhibit very pronounced maxima (Fig. 12). These unusual curves can be accounted for if one assumes that the adsorbed species are in aggregation equilibrium in the solution, depending upon the amount of surface area per unit volume of solution. Hence one expects that the adsorption equilibrium out of concentrated polymer solution may not only be approached with "infinite slowness but is also a function of the system characteristics, and the definition of reproducible conditions contains many more variables than one is used to from the more common work with dilute solution. This complexity is particularly awkward when one deals with the important case of competitive adsorption of polymers out of concentrated multicomponent solutions, a common phenomenon in many industrial processes, such as paint adhesion, corrosion prevention, lubrication, especially wear prevention, etc. 

According to the definition, corrosion is not a part of wear, since it is not caused by mechanical action, but by chemical reactions, and can therefore also occur in completely stationary systems. Since corrosion increases wear or even makes it possible. It Is considered as the fifth wear mechanism [3]. The main types of corrosion are summarized in Figure 5.28. Corrosion is largely the dominant damage mechanism in molds and is therefore discussed in fuller detail below. 

According to this concept, the wear process starts with the detachment of particles from the sliding partners. The detached particles remain for a certain time trapped between the two contacting bodies where they form a separating third body that influences contact stress distribution and thus the overall tribological behaviour. The effective material loss associated with wear is given by the definitive ejection of third body particles from the contact. In case of tribocorrosion, the third body approach should be adapted to include the mass flow associated with corrosion processes. 

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