Corrosion general mechanisms

Investigations of the effects on corrosion of mechanical working of steels indicate some influence on pitting but little on general corrosion. 

In general, the requirements for plate and plate materials listed in Table 5.1 can be classified as electrical properties, corrosion resistance, mechanical properties, gas permeability or soundness, weight, and cost. Although the output power of a stack is determined by many factors, one simple interpretation of the 2010 DoE cost target is US 2 per plate, which has often been quoted as a convenient estimation [8]. In addition to the cost requirements, the other requirements are all linked either directly or indirectly to the functions of the plate mentioned earlier. 

Corrosion can be controlled by Isolation of the metal from the corrosive environment by suppression of the anodic dissolution of metal and by suppression of the corresponding cathodic reaction. Isolation of corrosion prone metals from corrosive environments is probably the most general mechanism of the corrosion protection afforded by paint films, sealers, and similar polymer-based materials. Effective isolation requires that polymeric materials have good barrier properties and remain adherent in the presence of water and the products of metallic corrosion. Barrier properties and adhesion aspects of corrosion control are discussed in detail in subsequent sections. 

Permeable refractories are thus generally more susceptible to corrosion due to their porosity and the consequent high surface area of bond phase exposed to the corrodent. There are very few corrodents which would significantly attack SiC by dissolution in an aqueous medium. If SiC is attacked, the general mechanism is one of oxidation of the SiC to Si02. The Si02 is usually then removed as a reaction product exposing fresh SiC surfaces to corrosion. 

Because of the broad variation in composition and response to thermal treatment of the nickel-base alloys, it is not possible to generalize mechanisms responsible for developing susceptibility to intergranular corrosion. Therefore, the following discussion of the behavior of a Ni-Mo-Cr alloy is used to illustrate the complexity of an interrelationship between alloy composition, heat treatment, corrosion environment and corrosion rate. The alloy has the nominal composition in weight percent of 14.5 to 16.5 Cr, 15 to 17 Mo, 3 to 4.5 W, and 4 to 7 Fe with maximum limits on carbon and silicon. The alloys for which the corrosion data are shown in Fig. 7.59 contained 0.045 to 0.06 wt% carbon and 0.53 to 0.80 wt% silicon and were initially quenched from 1225 15 °C (2235 25 °F), which produced a dispersion of M6C type carbides (M = Mo, W, Si) in austenite (Ref 94). These carbides were not involved in the subsequent corrosion behavior or heat treatments. Heat... 

For nongalvanized steel, cosmetic corrosion generally involves a cathodic delamination mechanism the surface under the paint becomes cathodic and the surface exposed in the hole becomes anodic. To slow down or prevent atmospheric corrosion, it is therefore important that the siuface treatment be a good cathodic inhibitor in the finished product. The phosphate layer increases corrosion resistance by limiting the available free surface for the cathodic reaction. In general, the activity of the free surface is further reduced by passivating posttreatments or by the deposition of amorphous phosphate films between the crystals. 

In addition to the general mechanisms of erosion corrosion described in this section, there are also some special mechanisms occurring under certain conditions of materials and environments (see next section). 

More thorough analyses and mechanism studies show that there often is a considerable synergy effect of erosion and corrosion. Generally, the total material loss rate Wj for such material deterioration can be expressed by... 

Dissolution of alloys follows different principles. Dissolution of a freshly prepared surface can be described by the dissolution mechanism of a pure metal but now with different rate constants for the different alloy components (section General mechanisms). Because of the different corrosion rates a depletion of one component soon occurs. An intermediate region of different composition can develop (section Stationary dissolution conditions). In some cases the matrix of the nobler component is retained and a sponge-like structure develops. Again, this all concerns an oxide-free surface. The complications connected with oxide layer formation will be discussed in Section 10.2. 

However, the general mechanism for the chemical activation is not so well understood as for the physical activation. Other disadvantages of chemical activation process are the need of an important washing step because of the incorporation of impurities coming from the activating agent, which may affect the chemical properties of the activated carbon and the corrosiveness of the chemical activation process [56]. 

The local electrochemical techniques that are used for coatings research are mainly used in the more fundamental studies of corrosion mechanisms. They can be divided into techniques in immersion (SVET end SRET) and in atmospheric conditions (Kelvin). All techniques are based on the observation that corrosion mechanisms underneath coatings are generally mechanisms in which anodic and cathodic processes take place on separated sites with a slightly different local corrosion potential. The techniques are used to visualize these differences. SRET and SVET can also be used on polarized samples to detect pinholes. 

The general mechanism of fretting corrosion is shown in Figs 4.76 and 4.77. 

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