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Sulfuric Acid Anodize: Cl-, F-, Sulfide Limits

(-----) September 1, 2021

Q. Hello, I manage chemistries and solution testing for a finishing company. I am new to this industry (3 years experience with finishing solutions).
I was recently asked by a customer what our "limits for chlorides, sulfides, and fluorides" were for our Sulfuric Acid Anodizing. They did not have a tolerance limit themselves and I am wondering if anyone has a tolerance limit for these components in a solution and why. We have had 1000 ppm programmed into our LIMS system for several years, but have no notes as to why and what problems exceeding these limits could cause.

Our most recent results have given a maximum of 68.7 ppm for chlorides and 1.0 ppm fluorides. Sulfides are newly requested and no results as of yet. We use DI water in our anodizing line.

I do not know what is considered acceptable. Any information is greatly appreciated.

Thank You

Leslie Thompson
- Erie Pennsylvania

September 7, 2021

A. Chlorides and Fluorides:

Sheasby/Pinner: 200 mg/L :to prevent pitting"

Arthur Brace:50 mg/L or you get white spots in the dye.

Boeing: 160 mg/L but they are never dyeing.

Reliable Dye suppliers:50 mg/L

Sulfides are not a common impurity in anodizing.

More impurity data is available in some books.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

September 8, 2021

thumbs up sign Thank you kindly good sir.

Q. Is there an "easy" way to remove chlorides and fluorides from an SAA solution without decanting?

Again, thank you!

Leslie Thompson [returning]
- Erie Pennsylvania

September 2021

A. Hi Leslie. There is probably no practical way, but hopefully Robert will find time to follow up.

In case you read about it elsewhere though, there is an 'easy' way to remove chlorides from nearly any solution: add silver nitrate so that silver chloride precipitates ... but the economics of this are usually out of the question.

Luck & Regards,

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

simultaneous September 8, 2021

A. No economic way. First find out where the halogens came from: tap water, well water, some hard coat additives, other. Then, at the price of quality sulfuric acid, dump and recharge. To other readers who may be doing Type I Chromic anodizing, the chlorides will fizz off in time.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

September 8, 2021

Contaminants of the chemical solutions in a standard anodizing process are below;

Chlorides <25 mg/lt
Iron <50 mg/lt
Copper,nickel,manganese <50 mg/lt
Nitrates <30 mg/lt
Phosphates <200 mg/lt

Frankly, I have not any experience with these contaminants during anodizing production except chloride. Also if we consider dissolved aluminium as a contaminant then it should be max .20 ppm. We use acid purification unit to prevent exceeding 20 ppm. I think, best and easy technique is dump the solution for other contaminants if exceed max values.

Alaattin Tuna
Anodizing Supervisor - TURKEY Sakarya

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