Precipitation of secondary phases

Surface precipitation of secondary phases. Over time, ion exchange and matrix hydrolysis drive to slow supersaturation of the released species. Supersaturation occurs in the thick gel layer and in the bulk solution (Altenhein et al. 1981 Malow 1982 Petit et al. 1990). In addition, ionic species initially present in the lea-chant may be complexed and adsorbed at the solution-gel interface and eventually increase supersaturation (Grambow 1985 Strachan et al. 1985 Whitehead et al. 1993). 

Another difficulty presented by precipitation of secondary phases is that other aqueous species, such as those of Al, can be affected, as stated above. Under certain conditions the activity of Al(OH)4 in mildly alkaline solutions can drop to near zero due to precipitation of zeolite phases. The question is how to represent this change in aluminate activity using a rate equation based on chemical affinity concepts. For example, in equation (1), the activity of the... 

Another class of models relevant to chemical weathering is based on the reaction path approach originally developed by Helgeson et al. (1970). EQ3/EQ6 (Wolery et al. (1990), PHREEQC (Parkhurst and Plummer, 1993), and PATHARC.94 (Gunter et al., 2000) are some codes currently used to describe the progressive reaction of primary silicates and the precipitation of secondary phases as a function of time and mass. These codes are discussed in Nordstrom (see Chapter 5.02). They commonly permit the introduction of user-defined silicate reaction rates. Such models also commonly consider solubility controls on reaction kinetics as defined by... 

Mica particles were cast in various aluminum alloys [87, 88]. In 3.5 wt % NaCl solutions, the presence of mica particles depressed pitting potentials by approximately 20-30 mV, in comparison to the monolithic matrix Eilloys, suggesting that the presence of mica particles may slightly weaken the passive aluminum film. Corrosion behavior was also affected by the precipitation of secondary phases. In some cases, precipitates were preferentially attacked. Pits around and away from mica particles, interfacial corrosion of the mica-matrix interface, and exfoliation of mica particles were also observed. 

Furthermore, both corrosion potential and corrosion current density for the aged alloy are affected by the aging time (heat treatment time). This experimental observations may be attributed to the precipitation of secondary phases, which may form localized galvanic cells on the surface of the 2195 Al — Li ahoy. 

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