Electrochemical potentials between grain-boundary

Fukuda and Shimotomai (1994) measured the electrochemical potentials between grain-boimdary phases and the Nd-Fe-B and Nd-(Fe,Co,Ni)-B main phases, and concluded that the potential difference between the Nd-rich grain-boundary phase and the main phase is about 7 times larger in the Nd-Fe-B-based magnet than in the Nd-(Fe,Co,Ni)-B magnet. 

Intercrystalline corrosion is caused by a difference in the electrochemical potentials between the bulk of the grain and the grain boundaries where intermetallic phases precipitate. The grain (also called the matrix) comprises a solid solution and dispersed intermetallic compounds. At room temperature, the solubility of iron, nickel, or magnesium in aluminium is so low that the solid solution has a potential very close to that of unalloyed aluminium 1050A. However, when the solid solution is supersaturated or enriched at ambient temperature, the potential depends on the concentration of the alloying element (Figure B.1.8). 

Figure 25. Grain boundary capacitance of a Fe-doped StTiOj polycrystal (rriFe = 6.5 x 10,9cm"3), normalized to the electrode surface and measured at various oxygen partial pressures as a function of reciprocal temperature.100 Typical space charge potentials vary between 300 and 800 mV. (Reprinted from I. Denk, J. Claus and J. Maier, Electrochemical Investigations of SrTiOj Boundaries. J. Electrochem. Soc. 144, 3526-3536. (Copyright 1997 with permission from The Electrochemical Society, Inc.)...

Fundamentally, corrosion is an electrochemical process. That is, it is accompanied and accelerated by the passage of very small electric currents between the corroding metal and any other metal with which it is in electrical contact or between different areas on the surface of the corroding metal. For these currents to flow, a potential difference must exist, either between the two pieces of metal or between the different parts of the same piece, and moisture or other electrolyte must be present on the surface to act as a conductor for the current. Potential differences sufficient to cause current flow can arise from very small local variations in the chemical constitution of the surface phase differences across a grain boundary (e.g., between ferrite and pearlite or cementite) are quite sufficient to constitute an electrolytic cell. 

More grain boundaries are formed because of the disintegration of the nickel particles, and poor electrical contacts between the particles should therefore yield an increased electrical resistance that results in higher losses. However, the most significant effect is that the potential difference in the electrode increases, the three-phase zone can be extended less, and the electrochemical performance therefore decreases. 

Also, indirect insight into the chemistry of the grain boundaries can be obtained by referring to early work by Westbrook (1967), in which electrochemical potentials and the grain-boundary corrosion properties are affected. These results could be explained by direct observation of the chemical compositions at the grainboundary plane. Thereby, the controlling element would be identiHed and also the correlation between these chemical properties and the chemistry at the grain boundary would be understood. 

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