Definition - What does Microcrystalline mean?
A microcrystalline substance is a material that physically presents in a uniform manner that is in a natural geometrically regular form with symmetrically arranged plane faces. Often it is a rock with small crystals that are visible only by microscopic inspection. Certain metals can exist in crystalline form and are more prone to corrosion due to their greater surface area exposure.
Corrosionpedia explains Microcrystalline
A common microcrystalline substance that experiences corrosion is aluminum. The corrosion behavior of microcrystalline pure aluminum, produced by a sputtering technique, has been investigated in both acidic and aqueous solutions. Corrosion resistance of the microcrystalline aluminum coating has been observed to be lower than regular aluminum particles. However, aluminum oxide has a greater resistance against pitting in acidic solutions. Generally, the higher pitting resistance can be attributed to more acidic isoelectric points that an aluminum oxide film achieves.