Iron , crystal growth rate

Small amount of impurities sometimes retard dramatically the crystal growth rate. Chromium(III), for example, suppresses the crystal growth of potassium sulfate(i), ammonium dihydrogen phosphate(2) and ammonium sulfate (5), etc. in aqueous solutions. Other metallic ions, iron(III), aluminium(III) are also effective impurities W... 

The particle size and thus surface area of iron oxides and also the porosity depend strongly on the conditions under which crystal growth takes place. For example, high rates of growth at low temperatures may lead to poorly ordered crystals with surface areas of up to several hundred m g , whereas low surface area crystals may grow at higher temperatures and at slow rates of formation. In natural environments, such ex-... 

In the presence of dissolved aluminum ions, at not too high a temperature, the diflusion rate of the iron ions in the magnetite lattice is low. Hence, nucleus formation proceeds rapidly relative to crystal growth. Therefore, small iron crystallites, about 30 nm, form with correspondingly large specific surfaces. The aluminum probably remains partly in the iron crystallite in the form of very small FeAl204 areas statistically distributed over the lattice [262], [263], [294], [296], where an FeAl204 molecule occupies seven a Fe lattice positions [101]. 

Figures 32(a and b) show typical microscopic pictures of FFC on polymer-coated iron, and aluminum. FFC develops in the presence of pores, mechanical defects, unprotected cut edges, or residual salt crystals underneath the organic coating. The corrosion filaments start growing perpendicular from a defect into the polymer-coated area. FFC occurs only at moderate humidity (60-95%) and therefore, not under full immersion conditions. FFC has been found to be triggered by anions such as chloride, bromide, and sulfate. The filament growth rate increases with temperature. Like for cathodic delamination on iron and zinc the corrosion kinetics depend strongly on the surface pretreatment and coating composition.

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