Aluminum Anodizing: Effect of Sulfuric Acid Concentration on Properties

A. It's probably about the reverse, Garry. Sulfuric acid anodizing is a process where the film is simultaneously growing due to the electricity, and being dissolved due to the sulfuric acid.

AESF's "Illustrated Lecture" says that concentration is not as powerful an influence as temperature, but follows the same trend: higher concentration means more porous, and thus softer, coatings.

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A. All other conditions remaining the same, increasing the sulfuric acid concentration will increase the rate of attack of the anodic coating. Remember the anodizing process is a balanced reaction; the coating is being built up by the current, generating oxygen at the surface which converts Al to Al₂O₃. The sulfuric acid is creating the pores in the anodic coating which allows the current to continue to flow but is also attacking the exterior surface of the Al₂O₃ at the same time. The result is larger pores and a slightly thinner coating for a given anodize time.

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A. Hi Chris. 15% acid is probably very slightly more conductive than 5%, such that you could theoretically get the same current at very slightly lower voltage. However, in aluminum anodizing, the sulfuric acid is not there merely as a convenient conductive electrolyte, but also as an acid which beneficially attacks the anodized layer as it builds.

So, if you are trying to build a good anodized layer, as opposed to producing a report on the effect of changes in acid concentration, I think you should just stick to the accepted concentration, which is about 15-16% by weight. Good luck.

Regards,

A. Hi Thomas. NONE -- never add acid to water! ⇩ ^ ^{never add water to acid!} That's a recipe for having it flash to steam and shower you with concentrated acid :-(

Concentrated sulfuric acid weighs 1.84 grams per milliliter. So your 500 mL (1/2 L) weighs 1.84 x 500 = 920 grams.
Nominally, 96% of this is sulfuric acid, so you have .96 x 920 = 883.2 grams of acid.
If you want 883.2 grams to be 15% of the final weight of something, then that something must weigh 883.2/.15 = 5888 grams.
Since what you already have weighs 920 grams, you need 5888 - 920 = 4968 grams of distilled water = 4968 mL, i.e., about 5 liters. If you don't want to make it up all at once, you can proportion that you should add 1 part of your acid to 10 parts of water.

Don't even think about this without wearing googles and chemical gloves and making sure there are no pets or children in the area. And if it wasn't just a matter of wording, but you really didn't know that you can't add water to concentrated sulfuric acid, you need to read more before attempting this. Good luck.

 Ted Mooney , thank you for such a considered reply. I am guilty of phrasing the question badly it seems. I do of course know that one follows the rule "Always Adds Acid" not the other way around, and I have goggles, gloves and smock at hand. I also have a large bag of bicarbonate of soda, in case of accidental spillage.
Regards Ted.

A. I know I'm late to the party, but it appears you BOTH phrased it wrong...
Mr. Mooney, I've been learning from you anonymously for a couple years now, so I knew when you said "never add acid to water!" that it was the infrequent typo... I just wanted to let you know that I did indeed catch it ...