Corrosion The process by which metals are

Corrosion The process by which metals are oxidized in the atmosphere. 

Core electron an inner electron in an atom one not in the outermost (valence) principal quantum level. (12.13) Corrosion the process by which metals are oxidized in the atmosphere. (11.6)... 

Ordinary corrosion is the redox process by which metals are oxidized by oxygen, O2, in the presence of moisture. There are other kinds, but this is the most common. The problem of corrosion and its prevention are of both theoretical and practical interest. Corrosion is responsible for the loss of billions of dollars annually in metal products. The mechanism of corrosion has been studied extensively. It is now known that the oxidation of metals occurs most readily at points of strain (where the metals are most active ). Thus, a steel nail, which is mostly iron (Section 22-7), first corrodes at the tip and head (Figure 21-11). A bent nail corrodes most readily at the bend. 

The detailed mechanisms of dealloying have recently been reviewed [19]. The four main mechanisms that researchers have developed to account for the processes by which one metal in an alloy is removed by corrosion, leaving behind a porous metal, are ... 

Wetness of a metal surface The lime of wetness of the metal surface is an exceedingly complex, composite variable. It determines the duration of the electrochemical corrosion process. Firstly it involves a consideration of all the means by which an electrolyte solution can form in contact with the metal surface. Secondly, the conditions under which this solution is stable with respect to the ambient atmosphere must be considered, and finally the rate of evaporation of the solution when atmospheric conditions change to make its existence unstable. Attempts have been made to measure directly the time of wetness , but these have tended to use metals forming non-bulky corrosion products (see Section 20.1). The literature is very sparse on the r61e of insoluble corrosion products in extending the time of wetness, but considerable differences in moisture desorption rates are found for rusted steels of slightly differing alloy content, e.g. mild steel and Cor-Ten. 

The topics covered are as follows. The structure of the interfacial region and its experimental investigation are covered in Chapter 1. The following chapter reviews the mechanisms by which heterogeneous catalysis of solution reactions can take place. The third chapter is concerned with the mechanism and kinetics of crystal growth from solution and the final contribution deals with corrosion processes at the metal-solution interface. 

Petroleum, as recovered from the reservoir, contains metallic constituents but also picks up metallic constituents during recovery, transportation, and storage. Even trace amounts of these metals can be deleterious to refining processes, especially processes in which catalysts are used. Trace components, such as metallic constituents, can also produce adverse effects in refining either (1) by causing corrosion or (2) by affecting the quality of refined products. 

Most metals are thermodynamically unstable with respect to many of their compounds, particularly metal halides and sulphides, in addition to oxides. This is reflected in the extensive processing that is required to extract metals from their various ores. This chapter is concerned with the corrosion reactions by which metals return to their stable oxidized state, by reaction with sulphur, carbon, nitrogen, and the halogens. Often the corrosive gases encountered in practice contain oxygen, as well as one of these components. This problem of mixed-oxidant corrosion will be addressed in the next chapter. 

When one speaks of changing the environment to reduce corrosion, the easiest and the most obvions method is to lower the temperature. Since corrosion processes are chemical reactions, every 18°F decrease in tanperature reduces the reaction rate by half. Thus, if one can lower the tempera-tnre, the rate of corrosivity will be retarded. In addition, atmosphere can be changed by use of gases, in the sense that some metals are corroded in the absence of air and others in the presence of air. Other environmental changes involve agitation, aeration, and velocity, all of which have a decided inflnence on many materials. Behind these, a fairly easily made change in some processes is that of adjnsting the pH, which is a measure of the acidity or basicity of the solution. 

When two dissimilar metals are in electrical contact with one another and both are contacting the same electrolyte, one of the metals will preferentially corrode, a process known as galvanic corrosion (also the principle by which certain types of batteries function). The more active metal will corrode, which is the metal having the more negative open-drcuit (or corrosion) potential, when immersed all by itself in the electrolyte the more noble metal (having the more positive open-circuit potential) will support the reduction reactions. The more active metal is, therefore, the anode and corrodes faster than it would all by itself, whereas the other more noble metal becomes the cathode and corrodes slower than it would alone (or maybe not at all). The electrolyte resistance, important in all corrosion processes, may play a particularly influential role in this type of corrosion process. 

Corrosion is defined as the destructive alteration of metal through interactions with its surroundings—in short, rust. It is a redox phenomenon resulting from the fact that most metals are unstable in relation to their surroundings. Thus, the steel in cars really prefers to revert back to the iron oxide ore that it came from by reacting with oxygen in the air. The corrosion process is accelerated by exposure to water, which may contain corrosive salt placed on road ice, or acid from acid rain. It is only by the application of anticorrosive coatings and careful maintenance that the process is slowed down. 

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