Corrosion-resistance Crystalline phase

Although the role of crystalline phases in the leachability of HT materials is unclear and must be examined from case to case, the identified silicates and oxides are overall more resistant to corrosion than silicate glass and residues of incineration (Scholze 1991). Thus, a clear assessment of the durability of HT materials as a function of crystalline components must take into account the combined effects of their enrichment or depletion in trace metals, their individual leachability, the increase (but sometimes decrease) in overall reactivity due to local heterogeneities and increased Sspec (Jacquet-Francillon et al. 1982 Bickford Jantzen 1984 Jantzen Plodinec 1984 Scholze 1991 Adams 1992 Sproull et al. 1994 Sterpenich 1998). 

Beside the beneficial effect of the addition alloying metallic elements that contribute to the increased corrosion resistance, the amorphous structure itself is also responsible for the very low corrosion. For example, crystalline alloys with the same composition exhibit high rates of dissolution. The chemically homogeneous, single-phase nature of amorphous alloys is believed to account for their corrosion resistance (8, 100, 101). This also allows for the formation of a uniform, protective film on the surface of amorphous alloy electrodes. 

Rules of thumb would imply that a glass with more network-modifiers is less corrosion resistant to corrosion, materials with less glass content are better and that a greater crystallinity of the grain-boundary phase is of advantage. 

Depending on the rate of formation and the rate of crystallization, the product may be amorphous or crystalline. Aging or slow growth of amorphous phases may result in a transition from the amorphous to the crystalline state. This process can occur through slow transformation in the solid state or through dissolution-precipitation processes. This is illustrated in the transition from amorphous to crystalline basic nickel sulfate, the former being less corrosion-resistant than the latter. 

The GDL sheets are then heated to higher temperatures (2500-3300 K) that are required for graphi-tization (i.e., the phase transition from amorphous carbon to crystalline graphite). This transformation is crucial to enhance the electrical, thermal, and mechanical material properties. It also changes the surface characteristics and chemical response to the fuel cell operating environments (e.g., hydrophobicity and corrosion resistance). Differences in the process conditions (temperature, prevailing atmosphere or vacuum, etc.) can result in large differences in material properties. Hence, care must be... 

Deposits with extremely low porosities or an interconnected network of fine crystalline grain boundaries would be expected to show some loss of phase shift at low frequencies (i.e., a less than fully capacitative response) as the real resistance of the flaws becomes detectable, Fig. 19B. The value of RP obtained is inversely proportional to the corrosion rate. 

Parylene C, which has excellent barrier characteristics, did not perform well in corrosion protection. The salt intrusion resistance is poor, probably due to its semicrystalline nature of the bulk phase. The boundary phase between crystalline and amorphous phase seems to be vulnerable for the salt intrusion. 

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