Sulfuric Acid Anodizing Bath Makeup

Q. I'm a beginner in the anodizing field. Please explain to me about the ratio of acid in the anodizing bath. Usually the ratio is from 15 percent to18 percent, but is this ratio calculated by volume or by weight? —H.N.T.

A. Here is the formula for calculating the amount of sulfuric acid to use for the anodizing bath makeup. Please note that this formula is for using 66^o Baume’ (93 percent) sulfuric acid. If you are not using 66^o you will have to adjust the formula accordingly. The bath is always made up by weight of sulfuric acid, not volume. If you make up the bath by volume it will give results that are almost twice the concentration that you want. This is because 93 percent sulfuric acid weighs almost twice as much as water.

Metric calculation

Liters of 66^o Be’ H₂SO₄ per 378 liters of anodizing bath = 100 x % by weight bath desired x 2.37.

Example:

Liters of H₂SO₄ required for a 500 liter 15 percent by weight anodizing bath = 100 x .15 x 2.37 x (500/378) = 47 liters.

English calculation

Gallons of 66^o Be’ H₂SO₄ per 100 gallons of anodizing bath = 100 x % by weight bath desired x 0.625.

Example:

Gallons of H₂SO₄ required for a 500 gallon 15 percent by weight anodizing bath = 100 x .15 x 0.625 x (500/100) = 47 gallons.

The concentration of the anodizing bath should be expressed in grams per liter (g/l).

The amount of dissolved aluminum in the bath should be controlled between five g/l and 15 g/l for “general purpose,” Type II anodizing baths. Some specifications call for less than five g/l of aluminum. It’s best to follow the specification being used. Add aluminum sulfate to a brand new bath in order to get some aluminum in the bath to start with. It’s best to have about two g/l of Al in the bath to start with. This “buffers” the bath and helps prevent burning of the parts.

Aluminum sulfate is usually sold in the hydrated form with 13 molecules of water attached to each molecule of aluminum sulfate. The formula for adding Al₂(SO4)_{3 *} 13H₂O is: 11 grams Al₂(SO4)_{3 *} 13H₂O per liter of bath to raise the aluminum concentration by one g/l.

Metric calculation

To raise the aluminum to two g/l in a brand new 500 liter bath, it would require two g/l x 11g/l x 500 liters = 11,000 grams, or 11 kg of aluminum sulfate.

English calculation

To raise the aluminum to two g/l in a brand new 500 gallon bath, it would require two g/l x 11 g/l x 500 gal x 3.785 liters/gal = 41,635 grams, or about 92 pounds of aluminum sulfate. It helps to remember that 3.785 liters = 1 gallon and 454 grams = 1 pound.