Chp 12 - Chapter 12 Corrosion and surface engineering 12.1...

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12.1 The engineering importance of surfaces The general truth of the engineering maxim that 'most problems are surface problems' is immediately appar- ent when one considers the nature of metallic corrosion and wear, the fatigue-cracking of metals and the effect of catalysts on chemical reactions. For instance, with regard to corrosion, metal surfaces commonly oxi- dize in air at ambient temperatures to form a very thin oxide film (tarnish). This 'dry' corrosion is lim- ited, destroys little of the metallic substrate and is not normally a serious problem. However, at elevated tem- peratures, nearly all metals and alloys react with their environment at an appreciable rate to form a thick non- protective oxide layer (scale). Molten phases may form in the scale layer, being particularly dangerous because they allow rapid two-way diffusion of reacting species between the gas phase and the metallic substrate. 'Wet' or aqueous corrosion, in which electrochemical attack proceeds in the presence of water, can also destroy metallic surfaces and is responsible for a wide variety of difficult problems throughout all branches of indus- try. The principles and some examples of 'dry' and 'wet' corrosion will be discussed in Section 12.2. Conventionally, the surface properties of steels are improved by machining to produce a smooth sur- face texture (superfinishing), mechanically working (shot-peening), introducing small atoms of carbon and/or nitrogen by thermochemical means (carbur- izing, nitriding, carbo-nitriding), applying protective coatings (galvanizing, electroplating), chemically con- verting (anodizing), etc. Many of these traditional methods employ a liquid phase (melt, electrolyte). In contrast, many of the latest generation of advanced methods for either coating or modifying material sur- faces use vapours or high-energy beams of atoms/ions as the active media. Their successful application on a commercial scale has revealed the merits of developing a new philosophy of surface design and engineering. In Section 12.3, we examine some typical modern meth- ods for improving surface behaviour. 12.2 Metallic corrosion 12.2.1 Oxidation at high temperatures 12.2.1.1 Thermodynamics of oxidation The tendency for a metal to oxidize, like any other spontaneous reaction, is indicated by the free energy change AG accompanying the formation of the oxide. Most metals readily oxidize because AG is negative for oxide formation. The free energy released by the combination of a fixed amount (1 mol) of the oxidizing agent with the metal is given by AG 0 and is usually termed the standard free energy of the reaction. AG 0 is, of course, related to A//°, the standard heat of reaction and A5° the standard change in entropy, by the Gibbs equation. The variation of the standard free energy change with absolute temperature for a number of metal oxides is shown in Figure 12.1. The noble metals which are easily reduced occur at the top of the diagram and the more reactive metals at the bottom.
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This note was uploaded on 12/05/2011 for the course MSE 4100 taught by Professor Hennig during the Fall '11 term at Cornell University (Engineering School).

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Chp 12 - Chapter 12 Corrosion and surface engineering 12.1...

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