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HELP on Anodizing pretreatments
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I need help refering on what is the concentration of the caustic soda and of the nitric acid for the anodizing (sulfuric acid) pretreatments.
What kind of anodize pretreatment procedure can I use that require neither etching nor pickling so I can treat precision-machined aluminum?. Thanks
Francia RodriguezCaracas, Venezuela
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Hi. Someone may respond here with a formulation, and if they do then we'll print it here. But I don't like to offer formulations myself for two reasons: First, almost all of the successful chemistry used today is proprietary, and to print textbook formulations here tends to imply that proprietaries aren't really necessary. Second, there is so much that one should know before trying to formulate solutions that it just seems strange that one could know all those things and have all the necessary reference materials on hand, and yet not have the formulas on hand that are printed in those reference books.
It is not necessary to etch the aluminum, but I don't see how controlled pickling can hurt it or change the dimensions. But speaking of precision machined aluminum, are you sure your customer understands the rather large dimensional changes inherent in anodizing?
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 Ted Mooney, P.E. finishing.com Brick, New Jersey |
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On precision aerospace parts, I have seen them prepared by cleaning in a parts washer to remove all oil and dirt, then dried. Rack them wearing cotton gloves, and go directly (dry) into the chromic acid anodizing tank.
For parts with blind holes, this method reduces the damage sometimes done by entrapment of other processing solutions. Search our letters sections for related letters. Make sure that your procedures allows for the bypass of the normal pretreatment steps, or you will be out of compliance with your quality system.
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Tom PullizziFalls Township, Pennsylvania |
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I totally agree with Ted's comments. It sound like Francia needs to anodize critical parts without much knowledge of the anodizing process. It's like buying the machines that made the parts without having a qualified machinist to operate the machines. Operating an anodizing process is not as easy as taking formulas from a book. That's why the anodizing industry, like the plating industry, doesn't buy chemical from commodity suppliers unless there is someone on hand who knows the exact chemistry of each bath and what to do as the chemistry changes. One needs to consider each bath in an anodizing line as a precision machine that must be kept operating at maximum efficiency.
Francia would be better off to send the parts to a reputable, knowledgable anodizer, or if setting up a new anodizing line is more feasible, work with a reputable, knowledgable vendor or consultant in setting up and controlling the new line. The extra costs of buying chemicals from a supplier of chemicals specifically designed for an anodizing line are minimal if one considers the possible replacement costs of ruined parts. Our Sales people, and I'm sure our competitors' Sales people, are trained to have the mindset that we don't sell chemicals, we sell service. Try asking a distributor of caustic soda what caused a rock-hard white precipitate in the etch bath, and he will simply shrug his shoulders.
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Phil Johnson |
December 18, 2011
generally for pretreatment of aluminium before anodizing the concentration of NaOH should be 55-65 gms per liter and nitric acid should be 70-80 gms per liter.
Aboo Baker- Lagos, Nigeria