Transmitted Off Cross Polarized Light Illumination
Limonite is FeO-OH-nH2O, the first form of iron corrosion product evident in a corroding transformer. It is
an iron hydroxide with waters of hydration. The initial form is pale yellow in color. As waters of hydration are lost it
becomes more orange and will eventually become hematite (Fe2O3). Limonite is amorphous
or cryptocrystalline but as it looses more water the nuclei convert to a more evident crystal
structure. There are at least five forms of FeO-OH that have distinctive crystal structures.
They include limonite, lepidocrocite, goethite, and others. These limonite particles show two of the hydration conditions.
The more orange particle has released some water and hydrogen back into the system.
Significance in the Environment:
Free water is accumulating on the surface of sheet iron and is corroding the iron. Growth of crystal sites in the
limonite film indicate that this has been occuring for at least many months. The presence of pale yellow limonite suggests
that the corrosion is still occuring.