Letter 34004

Evaulating the existing electrolite solution [Massachusetts] 

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We have a small anodizing unit that we have used occasionally for many years... How do we know if the sulphuric acid is depleted (does it ever go bad), diluted over the years or is the correct concentration for optimal results. Any suggestions on best working strength?.. We are having a bit of dificulty getting green dye to color well. The dye has worked before and works on some other anodized material (chromic anodized)... any suggestions?..thanks

Bruce J
photo panels - Princeton, MA, USA

If you're against having a high school chemistry student doing a very basic titration with sodium hydroxide of a standardized concentration (i.e. normality), a simple hydrometer which is immersed in the bath and the specific gravity of the electrolyte directly read works great. Most reference books, such as the Metal Finishing Guidebook published annually have conversion tables which coorelate specific gravity and acid concentration.

As for green dye, it typically is a blend of 2 colors - blue and yellow. Our experience has been that "bugs" grow in the dye and consume one or the other color preferentially and that over time, it's impossible to match green anodize one time to the next! Anodically speaking.... good luck!

Milt Stevenson, Jr. Anoplate Corp Syracuse, NY, USA

How to know if depleted? Titrate for sulfuric acid and dissolved aluminum.

Does it ever go bad? Yes

Working strength for dye work? 180 grams per liter,which is 13 fluid ounces per gallon of high quality electronic grade 66 degree Baume sulfuric acid.

For the green dye to be consistent in shade from piece to piece several parameters must always be the same: Acid concentration, dissolved aluminumm, dye concentration, dye pH, dye temperature, anodize voltage different for each alloy.

If the dye works on the chromic acid anodizing and not on the sulfuric acid, then some parameter is off. First guess is acid concentration. Second guess is dissolved aluminum. Third guess is temperature. Call me for the fourth guess.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

After a sulfuric acid solution has been used and loaded up with "dissolved aluminum" and other dragged in contaminants, the hydrometer reads an error. The hydrometer and cook book tables are only viable for a fresh unused reagent grade sulfuric acid aolution.

Regarding algae, most shops avoid such growth, but once it gets started it is almost impossible to get rid of it.

Some proprietary dye sellers offer analytical services and general telephone technical service to their customers.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

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