The question sometimes arises of whether the sulfuric acid anodizing bath is made up of volume or by the weight of sulfuric acid.
The bath is always made up by the weight of sulfuric acid, not by volume. If the bath is made up by volume, it will give results that are almost twice the concentration that is wanted. This is because 93% sulfuric acid — the most commonly used concentration — weighs almost twice as much as water: 15.3 lb/gal for the sulfuric vs. 8.35 lb/gal for water.
Larry ChesterfieldBelow is the formula for calculating the amount of sulfuric acid to use for the anodizing bath makeup. This formula is for using 66o Baume’ (93%) sulfuric acid. If you are not using 93%, you will have to adjust the formula according to the weight of the sulfuric acid being used. The weight can usually be found on the container label. The numbers in these formulas are rounded off.
The concentration of the anodizing bath should be expressed in grams per liter (g/l). Note that 1% by weight sulfuric acid is approximately 11 g/l. The metric calculation to make up a 15% by weight, 165 g/l, 100-liter bath is:
- 100 liters of water weighs 100,000 grams (1 liter of water weighs 1000 grams)
- 15 x 100,000 = 15,000 grams of 93% sulfuric acid needed
- 93% sulfuric acid weighs approximately 1834 g/l (15.3 lb/gal ÷ 785 liters/gal = 4.04 lb/liter. 4.04 lb/l x 454 g/lb = 1834 g/l ).
- 15,000 grams of sulfuric acid ÷ 1834 g/l = 8.2 liters sulfuric acid
So 100 liters of 15% (weight) sulfuric acid is made up using approximately 8 liters of 93% sulfuric acid and 92 liters of water. Always add acid to water; never add water to concentrated sulfuric acid!
The English calculation for making up 100 gallons of 15% (wgt), or 165 g/l, solution of sulfuric acid is:
- 100 gallons of water x 8.35 lb per gallon = 835 lbs
- 15 x 835 = 125.25 pounds of 93% sulfuric acid needed
- 125 lb sulfuric acid ÷ 3 lb/gal = 8.2 gallons of sulfuric acid
So 8.2 gallons of 93% sulfuric acid added to 92 gallons of water will make 100 gallons of 15% (wgt) sulfuric acid.
When making up a completely new anodizing bath, be sure to add enough aluminum sulfate to have approximately 2 g/l of aluminum in the bath as a “buffer” to help protect parts from burning in the new anodizing bath. See a previous article in this series for that calculation.
Larry Chesterfield is President and CEO of Anodizing Technologies, Inc. He has experience in a job shop performing anodizing, painting, and fabrication and now offers equipment systems design and build, anodizing technical expertise, and operations/process troubleshooting. Visit www.anodizingtechnologies.com or email him at larry@anodizingtechnologies.com
