MadSci Network: Chemistry

Re: What do the different colors of rust mean, chemically.

Date: Sun Mar 3 18:11:17 2002
Posted By: Charlie Crutchfield, Retired
Area of science: Chemistry
ID: 1014495812.Ch

Hello David;

Iron rust can be several different chemicals, all containing different amounts of Iron [Fe], Oxygen [O], the Hydroxyl {OH} ion, and water of hydration [H2O]. There are [so far] eleven such chemicals known, but only five are of interest to us here. Note: I can't show the proper subscripts of numbers in chemical formulas. [Well, maybe my computer can, but I cant.] The formulas here are simple enough so this should be no problem.

There are usually several different "Rust" chemicals formed when iron corrodes. which ones is affected by the amount of Oxygen present, the amount of water,and of course, time.

Some form in layers which may be rather impervious and prevent oxygen and/or water from contacting the iron or other rust and thus stop any more xchemicalreactions. Some bacteriamay do the same, especially those that form "slime" coatings..

The colors of these different rust chemicals often dependon on whether they are dense solids or fine particles. [In general,if a dark colored, transparent solid is ground to fine particles, the color becomes lighter.]

These substances have chemical names, and also mineral names. Look at for more information on their properties.

When iron rusts quickly and is kept wet, the first product is usually Fe3O4, the mineral with this formula is called Magnetite. It is magnetic, colored grey to brownish-black. When finely divided it is black.

Sometimes the Magnetite will change by oxidation to Fe2O3 [Hematite]. This compound is non-magnetic, grey to shades of red, the fine powder is cherry- red to reddish brown.

Hematite changes slowly to gamma-FeO[OH], called Lepidocrocite. Lepidocrocite is not magnetic, it is colored red to red-brown, to yellow-brown. When finely powdered, it is dull orange. The "Iron" stain that is seen under dripping water faucets is Lepidocrocite.

In time, the Lepidocrocite changes [by crystallization] to alpha-FeOOH, called Goethite. It is also non-magnetic and orange to yellow-brown, the fine powder can be orange, yellow,or brown. It is sometimes iridescent.

Dry rusting is of no interest to your situation, but I include it here for general interest. The first stage is gamma-Fe2O4, Maghemite, which is magnetic and brown-yellow both as a solid and as fine powder. The next stages are Lepidocrocite and then Geothite as above.

Some of these compounds are hard to identify or even distinguish without laboratory testing. Also,to make things interesting, several of them can be present at the same time as a mixture in the same material.

The red material formed by the bacteria may be formed of Hematite and Lepidolite, both containing iron in the higher [Ferric] oxidation state. Remember, when you weigh this it contains some bacteria as well as the iron rust.

The sterile yellowish matter may contain some Magnetite or Geothite.

The apparent difference in volume may be due merely to how compacted the products are.

Did you measure the weight change of the pieces of the iron before and after your experiment. Look at the corroded surfaces under a microscope, the "bacterial" specimen may be pitted.

If you can find a badly rusted piece of iron, such as an old nail,look closely at the rust. Scrape some rust off [use a piece of wood or plastic], crush it to powder. he back of a stainless steel spoon on a saucer will do well for this. Apply a magnet; the black brittle material that jumps to the magnet is Magnetite, and the brown-yellow may be Maghemite

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