Anodizing Q&A: Calculation for Adding Aluminum to a New Anodizing Bath
What is the calculation for adding aluminum to an anodizing bath?
Q. We can only buy non-hydrated aluminum sulfate [Al2(SO4)3] to add dissolved aluminum to a new sulfuric acid anodizing bath. I have read your article in the Products Finishing archive (short.pfonline.com/scabm) about the amount of aluminum sulfate to add in the hydrated form [Al2(SO4)3 • 13 H2O]. Would I have to add the same amount of non-hydrated aluminum sulfate, or would it be a different formula? How is that calculation done?
A. The amounts of hydrated versus non-hydrated aluminum sulfate to add to the bath would definitely be different. Here is how the calculation goes:
Featured Content
- Start with the atomic weights for each element in Al2(SO4)3
Aluminum= 27
Sulfur= 32
Oxygen= 16
- Al2 + (SO4)3
2(27) + 3[(32 + (4 × 16)] = 54 + 3(32 + 64) = 54 + 3(96) = 342
The result is that the aluminum sulfate molecule (no water attached) has an atomic (or molecular) weight of 342. We want to know the ratio of the aluminum to aluminum sulfate.
- The aluminum in this molecule is 54 and the entire aluminum sulfate molecule is 342.
- Divide the aluminum by the aluminum sulfate to find the ratio:
54 ÷ 342 = 0.158
So 1 ÷ 0.158 = 6.33, or 6.33 g of Al2(SO4)3 added to a one liter bath of brand new sulfuric acid will raise the concentration of dissolved aluminum by 1 g/l.
- 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 g/l.
- In English terms: to raise the concentration of 100 gallons of sulfuric acid bath by one gram per liter using non-hydrated aluminum sulfate:
6.33 × 379 = 2399 g or 2.4 kg
2400 g ÷ 454 g/lb = 5.3 lbs
Remember: 379 liters = 100 gal and 1 lb = 454 g
Originally published in the September 2015 issue.
RELATED CONTENT
-
Cleaning, Pretreatment to Meet Medical Specs ISO 13485 or FDA 21 CFR820
Maximilian Kessler from SurTec explains new practices for industrial parts cleaning, metal pretreatment and decorative electroplating in the medical device industry.
-
Preventing Anodizing Cathodes from Turning Red
While the red color may not be desirable, anodizing expert Drew Nosti says it poses no particular problem to a successful anodizing process.
-
How to Apply the 720 Rule to Current Density Anodizing
What can you tell me about the 720 Rule as it applies to current density anodizing? Plating expert Sjon Westre, Ph.D., from Chemeon, answers this question.