Subsurface void formation

At least three possible mechanisms have been proposed to explain subsurface void formation during the oxidation of unstressed... 

Constituents of base salts ITF4 or ThF4 content Loop material Maximum salt temperature, >F Time of operation, hr Depth of subsurface void formation at hottest part of loop, in. 

W. Pine, S. Maddela, M. O Keefe, and W. Fahrenholtz, Formation of subsurface voids in aluminum alloy 2024-T3 during deposition of cerium-based conversion coatings . Surface and Coatings Technology, 204(24), 4095 (2010). 

Alloys containing appreciable quantities of chromium are attacked by molten salts, mainly by the removal of chromium from hot-leg sections through reaction with UFi, if present, and with other oxidizing impurities in the salt. The removal of chromium is accompanied by the formation of subsurface voids in the metal. The depth of void formation depends strongly on the operating temperatures of the sy.stera and on the composition of the ssdt mixture. 

Layadi et al. have shown, using in. situ spectroscopic ellipsometry, that both surface and subsurface processes are involved in the formation of /xc-Si [502, 503]. In addition, it was shown that the crystallites nucleate in the highly porous layer below the film surface [502, 504], as a result of energy released by chemical reactions [505, 506] (chemical annealing). In this process four phases can be distinguished incubation, nucleation, growth, and steady state [507]. In the incubation phase, the void fraction increases gradually while the amorphous fraction decreases. Crystallites start to appear when the void fraction reaches a maximum... 

Subsurface formations can be divided into the overburden (unconsolidated) and bedrock according to its solidarity. The upper subsurface can be further divided into the unsaturated zone and the saturated zone depending on pore structure and moisture saturation. The saturated zone is the zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is at the top of a saturated zone in an unconfined aquifer. The unsaturated zone is the zone between the land surface and the water table, and is also called the zone of aeration or the vadose zone. The pore spaces contain water at less than atmospheric pressure, air, and other gases. This zone is unsaturated except during periods of heavy infiltration. 

---