Adding Aluminum to a New Anodizing Bath: The Math and Chemistry

Q: When making up a new sulfuric acid anodizing bath (Type II and Type III), is it important to have at least a small amount of dissolved aluminum in the bath to act as a sort of buffer to help prevent burning of the parts?

A: Yes, and aluminum sulfate is the preferred method for providing that aluminum. In North America, it is normally supplied as aluminum sulfate in the hydrated form as Al₂(SO₄)₃ × 14.3H₂O_. In other areas of the world, it is in its non-hydrated form as Al₂(SO₄)₃. The amount that is added to a new bath makes a big difference. Here is the math and the chemistry:

For hydrated aluminum sulfate, start with the atomic weights of each of the elements in Al₂(SO₄)₃ × 14.3H₂O. Don’t confuse the atomic numbers for the atomic weights, also known as the molecular weights–they are not the same. In a chemical compound, the sum of the atomic weights of each of the elements is the compound’s molecular weight.

The atomic weights (rounded off) of each of the elements in hydrated aluminum sulfate are: aluminum 27, sulfur 32, oxygen 16 and hydrogen 1. The math actually is simple; it just looks complicated:

Al₂ + (SO₄)₃ + H₂O × 14.3 = 2(27) + 3((32 + (4x 16)) + (2(1) + 16))14.3 = 54 + 3(32 + 64) + (2(17))14.3 = 54 + 288 + 257 = 599.

The atomic weight (also known as the gram molecular weight) of aluminum sulfate with 14.3 waters attached is 599. Since we want to know the ratio of the aluminum to aluminum sulfate, we divide the atomic weight of the aluminum by the atomic weight of the entire aluminum sulfate molecule:

54 ÷ 599 = 0.0902

So, there is 0.0902 gram-atom of aluminum per molecule of the hydrated aluminum sulfate compound. If we invert the result, 1 ÷ 0.0902 = 11.11. This means there are 11.11 grams of hydrated aluminum sulfate per gram of aluminum. Adding 11.11 grams of the hydrated compound to 1 liter of water will raise the aluminum concentration by 1 gram per liter.

A more practical way of saying this in metric terms: Add 1,111 grams of hydrated aluminum sulfate per 100 liters of sulfuric acid bath to raise the dissolved aluminum by approximately 1 gram per liter (1,111 grams = 1.11 kg). In English terms: To raise the concentration of 100 gallons of sulfuric acid bath by 1 gram per liter using non-hydrated aluminum sulfate: 11.11 × 379 (liters per 100 gallons) = 4,211 grams or 4.2 kg; 4,211 grams ÷ 454 grams per pound = 9.28 pounds of hydrated aluminum sulfate.

For non-hydrated aluminum sulfate, follow the same procedure, but leave off the weight of the water in the molecule: Al₂ + (SO₄)₃; 2(27) + 3((32 + (4x 16)) = 54 + 3(32 + 64) = 54 + 3(96); 54 + 3(96) = 342. This gives the result that the aluminum sulfate molecule (no water attached) has an atomic (or molecular) weight of 342. We divide the aluminum by the aluminum sulfate to find the ratio: 54 ÷ 342 = 0.158. So, 1 ÷ 0.158 = 6.33, or 6.33 grams of Al₂(SO₄)₃ added to a 1-liter bath of brand new sulfuric acid will raise the concentration of dissolved aluminum by 1 gram per liter.

In metric terms: Add 633 grams of non-hydrated aluminum sulfate per 100 liters of sulfuric acid bath to raise the dissolved aluminum by approximately 1 gram per liter. In English terms: To raise the concentration of 100 gallons of sulfuric acid bath by 1 gram per liter using non-hydrated aluminum sulfate: 6.33 × 379 = 2,399 grams or 2.4 kg; 2,400 grams ÷ 454 grams per pound = 5.3 pounds; 379 liters = 100 gallons and 1 pound = 454 grams.

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Q: Usually the ratio of acid in an anodizing bath is from 15 to 18 percent, but is this ratio calculated by volume or by weight?

A: The makeup of an anodizing bath is always by weight of sulfuric acid, not volume. If you make up the bath by volume, it will give you almost twice the concentration that you want. This is because 93 percent sulfuric acid weighs almost twice as much as water (15.3 pounds per gallon).

Following is the formula for calculating the amount of sulfuric acid to use for anodizing bath makeup. Note that this formula uses 66-degree Baume’ (93 percent) sulfuric acid. If you are not using 66-degree, you will have to adjust the formula accordingly. (The numbers in these calculations are rounded off.)

The concentration of the anodizing bath should be expressed in grams per liter (g/l). Note that 1 percent by weight sulfuric acid is approximately 11.1 g/l. The metric calculation for 15 percent (weight) or 165 gallons per liter of 66-degree Baume’ H₂SO₄ per 100 liters of anodizing bath:

• 100 liters of water weighs 100,000 grams
• 0.15 × 100,000 = 15,000 grams
• Sulfuric acid weighs approximately 1,834 grams per liter
• 15,000 ÷ 1,834 = 8.2 liters sulfuric acid + 91.8 liters water

For example, to determine the amount of H₂SO₄ required for a 500-liter, 15 percent by weight (165 g/l) anodizing bath: 5 × 8.2 liters sulfuric acid + 5 × 91.8 liters water = 41 liters sulfuric acid + 460 liters water. The English calculation for 15 percent (weight) or 165 grams per liter: gallons of 66-degree Baume’ H₂SO₄ per 100 gallons of anodizing bath:

• 100 gallons × 8.35 pounds per gallon = 835 pounds
• 0.15 × 835 = 125.25 pounds
• 125.25 pounds ÷ 15.3 pounds per gallon = 8.2 gallons sulfuric acid + 92 gallons of water

For example, to determine the amount of H₂SO₄ required for a 500-gallon, 15 percent by weight anodizing bath: 8.2 gallons/100 gallons of bath × 5 = 41 gallons sulfuric acid + approximately 460 gallons of water.

The amount of dissolved aluminum in the bath should be controlled between 5 g/l and 15 g/l. for “general purpose,” Type II anodizing baths. Some specifications call for less than 5 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 is best to have about 2 g/l of aluminum 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 14.3 molecules of water attached to each molecule of aluminum sulfate. The formula for adding Al₂(SO4)₃ × 14.3H₂O is: 11 grams Al₂(SO4)₃ × 14.3H₂O per liter of bath to raise the aluminum concentration by 1 gram per liter.

The metric calculation to raise the aluminum to 2 grams per liter in a brand-new 500-liter bath requires 2 grams per liter × 11 grams per liter × 500 liters = 11,000 grams or 11 kg of aluminum sulfate. The English calculation to raise the aluminum to 2 grams per liter in a brand-new 500-gallon bath requires 2 grams per liter × 11 grams per liter × 500 gallons × 3.785 liters per gallon = 41,635 grams or about 92 pounds of aluminum sulfate.