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topic 1130

Aluminum Anodizing: Effect of Sulfuric Acid Concentration on Properties

A discussion started in 1998 but continuing through 2017


Q. I would like to know what happens to the anodized coating as you increase the concentration of sulfuric within a sulfuric acid anodize bath. Such as hardness, and other qualities. Also can you increase the rate of deposit to a certain point as you increase concentration.

Garry Pickett
aerospace - Los Angeles, California

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.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


Surface Treatment & Finishing of Aluminium and Its Alloys
Wernick, Pinner & Sheasby
from Abe Books


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.

Lee C. Branch
Albright & Wilson Americas

Is there a relationship between acid concentration and anodizing voltage?

March 7, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I'm doing some aluminium anodising work and looking at parameters ... mainly looking at time and voltage. However, I have seen articles with 5 wt% sulphuric acid, and I am looking at using 15 wt%. Is there a formula of some sort which would tell me that I need to reduce the voltage, or current density by X amount due to the increase in concentration?

Chris Dewires
Student - Loughborough, England

March 2016

probert book
Aluminum How-To

by Robert Probert
$89 New
The Chromating - Anodizing - Hardcoating Handbook

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.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Anodising solution strength without math

October 29, 2017

Q. I have a half litre of 96% sulphuric acid. I want a 15% solution for anodising. How much distilled water do I need to add to achieve this?

Thomas Sheldon
Hobbyist - Caerphilly, South Wales, United kingdom

October 2017

A. Hi Thomas. NONE -- you never add acid to water! 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.

pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

October 29, 2017

thumbs up sign 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.

Thomas Sheldon [returning]
- Caerphilly, South Wales, United kingdom

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