Family: Chromic Acid Cleaning Solutions
Formulation: Chromium Trioxide dissolved in water
Chromic acid solutions are often used for removing corrosion from magnesium or for sealing phosphate coatings applied to iron or steel to improve their corrosion resistance. Other applications in the metal finishing field include its use for bright dipping of brass and acid cleaning of aluminum and magnesium.
Because high-valence chromium is carcinogenic, teratogenic and has been linked with adverse environmental effects, the commercial use of chromic acid is falling from favor in some municipalities. But, even so, it still has its applications.
Chromic acid is a powerful oxidizing agent which lends itself to a variety of uses in the metal finishing field to impart corrosion resistance. Conversion coatings, chromate-containing films of various oxides, are formed on metal surfaces by using sodium dichromate, potassium dichromate or chromic acid.
For Aluminum Cleaning:
Chromic Acid 5% by Weight—Chromium Trioxide Crystals 0.4 oz. for 8 fl. oz.
Put 7.5 fl. oz. of water in a beaker and pour in crystals stirring with a wooden stick or glass rod. This solution can be used on freshly machined aluminum surfaces or to treat mild corrosion. Surfaces must be clean and free of oil or dirt.
For Magnesium Cleaning:
The treatment of magnesium metal differs from the treatment of Aluminum in that in the case of heavy corrosion, two solutions must be used. For light surface corrosion, only one is necessary.
Immerse parts in an alkaline cleaner until parts are chemically clean and free from waterbreaks.
The first solution is a stronger version (20% chromic acid by weight) of the Aluminum treatment formulation. It is mixed in the same manner but more Chromium Trioxide is used in the following proportions:
· Chromium Trioxide 1.6 oz. for 8 fl. oz.
· Put 7.0 oz. of water in beaker and pour in the Chromium Trioxide crystals and mix the same as the aluminum solution.
Surface must be clean and free of oil and dirt. On magnesium, allow the 20% Chromic Acid to stand for 2-5 minutes or until it has stopped working (or bubbling), wash with clear cool water to remove remaining acid and dry with an air hose or wipe with a clean cloth until dry.
The second step in the treatment of magnesium is by the use of another acid solution but more care must be
exercised in mixing the chemicals in this step. A chrome pickle solution (5% by weight) is made from:
· Sodium Dichromate - 1.5 oz.
· Nitric Acid - 1.5 oz.
· Water to make 8 fl. oz. total
Allow the chrome pickle solution to stand on the magnesium from 1-2 minutes then rinse in clear cool water. The chemical reaction on the magnesium will turn the surface of the metal to a light brown or gray depending on the alloy.
These solutions are made up in accordance with MIL-G-5541 and T.O. 1-1-2. These solutions have a shelf life of about 30 days so unless you will be using a lot, there is no sense in mixing more than a half pint at a time.
For Glass Cleaning:
This is a solution of chromium trioxide (CrO3) in concentrated sulfuric acid (H2SO4). A pre-measured mix is available under the name "Chromerge,” which should be treated in the same way as aqua regia or other acidic peroxide solutions.