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WHERE DO "WHITE SPOTS" ON ANODIZED ALUMINUM COME FROM?
Most Salts of Zinc from 7000's alloys and Calcium from tap water are WHITE
White salt |
Source, calcium in the tap water reacting with: |
Calcium Oxalate |
Anodizing Additive, proprietary |
Calcium Citrate |
Anodizing Additive, proprietary |
Calcium Acetate |
Dye |
Calcium Fluoride |
Fluoride Type Seal |
Calcium Glyconate |
Anodizing Additive, proprietary |
Calcium Metasilicate |
Down the line from Silicated Cleaner |
White Salt |
Source. Zinc from the 7000 series alloys reacting in: |
Zinc Acetate |
Dye |
Zinc Carbonate |
Starts at the cleaner |
Zinc Fluoride |
Fluoride Seal |
Zinc Nitrate |
Some deox and any nitric dip |
Zinc Oxalate |
Some proprietary anodizing additives |
Zinc sulphate |
Anodizing 7000 series alloys |
Zinc Silicate |
Down the line from silicated cleaners |
Calcium, More. Even if you made up sensitive solutions with DI, many shops rinse in tap water. As tap water drags into the "DI" solutions the calcium begins to build up and react as indicated above. Again, almost all calcium salts are WHITE.
Calcium, More. Finally, on Calcium, the element has sort of "reverse solubility" versus most compounds from other elements in that it tends to precipitate at higher (rather than lower) temperatures, therefore at the anodizing temperature of 72 °F or lower, and at the dye temperature of 140 °F, the Calcium stays in solution and may not show up. But when you hit the 180 °F or the 205 °F of the seal, the Calcium falls out as "White Spots"
Zinc, More. The zinc in the 7000 series alloys forms soluble salts in just about every process tank in the line. When these dissolved salts hit the water rinse the lower solubility causes the salt to precipitate as a white powder.
Zinc, More. If you process any 7000 alloy, you will leave zinc behind in the alkaline etch and this will be picked up by other alloys and carried down the line. Again, Zinc Hydroxide from the etch remains soluble in that tank, but when it hits the cold water it falls out as a white powder, then resolubilizes in the deox, then falls out in the rinse, then some more zinc is pulled out by electrolytic action in the anodizing and is partially occluded in the pores, then on down the line it leaches out of the pores as "white spots".
Dissolved Aluminum, above 12 gm/L, in the anodizing causes white spots, again by occluding in the pores and later leaching out.
Dirty rinse water (also with Calcium, Zinc, Aluminum, Silicates, etc.) causes white spots sulphate drag-in into the dye, especially with 7000 alloys, will cause white spots
In the deox, the use of plain nitric acid usually gives a clean look to the naked eye, but nitric acid does not remove, from the surface, all the alloying ingredients like copper, magnesium, manganese, and other metals. That is why proprietary mixed acid deoxidizers are recommended to help prevent white spots.
Please do not call us and say "the dye is causing white spots". First look up stream, are you cleaning in a silicated cleaner, have you loaded the alkaline etch with zinc, is the deox removing all the non-aluminum "crap" from the surface, have you dragged zinc or calcium into the anodizing tank, look at the work RIGHT AFTER ANODIZING, are you getting the sulfuric acid out of the pores before going into the dye?
Sulfuric Acid is very difficult to rinse, and even more difficult when trapped down inside the 450 pores per square inch. Sulfuric Acid in Georgia Tech Chemistry 101, 1949, text book, "sometimes called 'oil of vitriol'" tells it like it is. Sulfuric Acid IS OILY and it is trapped in the pores. Call me to discuss how to get it out.
There must be dozens of other related reasons for white spots. I will add, revise, and reissue this page when I hear of new white spots, so let me hear from you.