Potential corrosion mechanisms

Reaction 4 has a free energy change, AG, of +13 kcal/mole at 400°C, indicating that CaO should not be severely corroded by Li. This calculation, however, using the free energy of formation for solid Li O and CaO, does not represent the chemical reactions as they occur in flowing Li. Consequently, two additional potential corrosion mechanisms are considered here. Liquid Li is known to dissolve both Ca and O. The solubility of O in Li is 800 wppm... 

Steam traps are installed in condensate, mechanical return systems and are a frequently overlooked item for reducing operating costs. Large industrial process plants typically have many hundreds of steam traps installed to recover low-energy condensate and remove (potentially corrosive) air and carbon dioxide. 

Davis, K. and McDonald, L.K., Potential corrosion and microbiological mechanisms and detection techniques in solution mining and hydrocarbon storage wells, in Underground Injection Science and Technology, Tsang, C.F. and Apps, J.A., Eds., Elsevier, New York, 2007. 

LPG vessels may be directly buried below grade or mounded above grade to reduce exposure to an external fire. Both of these methods require special precautions, careful preparation, and special design features, since they introduce other substantial hazards. These hazards include undetected corrosion, undetected leaks, and the potential for mechanical damage when the vessel is unearthed for inspection. 

There is a special importance in the mechanism of 02 reduction on iron because of its relevance as the counter-cathodic reaction in corrosion mechanisms that involve Fe more often than other metals. Many of the practical costs of Fe corrosion occur in neutral solution, so that the pH range in the study described here (Jovancicevic, 1986) is between 6 and 9. The experimental methods involved the use of ring-disk analysis (see Section 7A. 14) to detect H202, an obvious possible intermediate in the measurement of the log /—potential relation (Fig. 7.101) to give Tafel constants and the reaction order with respect to 02 and pH. 

Fig. 3.1. Schemes of electrode potential settling mechanisms on metal substrates under polymer coatings [1]. (1) substrate, (2) coating, (3) corrosive medium, (4) and (5) coating defects (a pore or local thrrming). See explanations in the text...

With the aid of electrochemical tests [DIN 50918] quantiative statements can be made about the corrosion mechanism. The experimental determination of the current-potential relationship provides the most important information. It can be used to determine the limiting potentials for the occurrence of, for example, pitting corrosion and stress corrosion cracking. 

Differences in detected Volta potentials between pristine and corroded Al-Mg alloy surfaces could be related to the factors influencing thickness and conductivity of the corrosion product layers [219]. Corrosion layers developed in the presence of ion-containing solutions yielded lower Volta potentials and showed higher conductivity. Cathodic delamination of poly aniline-based organic coatings on iron have been studied with SKP [220]. The role of dioxygen reduction and of the poly aniline fraction in the coating were included in a proposed corrosion mechanism. 

The sit-and-soak test procedure used in this study, adapted finm that in BS 7766, tacitly assiunes that the migration of metals in water is governed by the solubility of corrosion products. However, adaptations of the procedure could be possible to cover many of the other potentially controlling mechanisms. 

The unstable behavior of some solder-replacement adhesives has been attributed to galvanic corrosion. Similar to most corrosion mechanisms, condensed or absorbed moisture on the surface and dissimilar metals are required to form a galvanic cell. The silver filler acts as a cathode while the substrate metallization acts as an anode and is oxidized. In the case of tin-lead solder surfaces, the solder, which has a lower electrochemical potential (0.13 V) than silver (0.79 V), becomes the anode at which corrosion and oxidation occur. A smaller potential difference between a copper surface and silver accounts for some improvement in contact resistance over the solder-silver couple. 

This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

Critical Pitting Potential and Evaluation of Pitting Corrosion Mechanism of Pitting Corrosion... 

Metal coupons placed in the sonication reactors were analyzed for potential corrosion and scale formation (e.g., calcium carbonate, magnesium hydroxide, and metal salts). This information will be used in conjunction with the reaction mechanism model. 

The microstructure of sintered Nd-Fe—B magnets is mainly composed of the Nd2Fei4B phase, the Ndi.iFe4B4 phase, and the Nd-rich phase at grain boundaries. The corrosion mechanism of the sintered body has been discussed in several papers. Nakamura et al. (1989) revealed that the Nd-rich phase has a more negative electric potential than the matrix Nd2Fei4B phase, the Ndi.iFe4B4 phase and, if it exists, the Fe-B phase. They also pointed out that the Nd-rich phase is easily dissolved in water. 

Porous photoetching vras also done on sintered pellets of Ti02. The influence of applied potential on corrosion mechanism is illustrated in Fig. 34. When anodization is done at 0.2 V vs SCE, the grains are selectively dissolved, whereas the grain boundaries are not dissolved creating a skeleton structure . By contrast under 1 V bias, the grain boundaries of the pellet are selectively dissolved and a characteristic porous pattern appears on each grain surface [249]. 

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