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Problem getting consistent purple dye after clear chromate

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I AM EXPERIENCING A TECH DIFFICULTY WITH USING A PURPLE DYE ON NUTS THAT ARE ZINC PLATED WITH A CLEAR CROMATE. WE ARE PLATING THE PARTS THEN DIPPING IN CLEAR AND THEN DIPPING IN DYE. WE ARE HAVING TROUBLE KEEPING THE DYE A CONSISTENT COLOR AND ARE GETTING A VARIETY OF COLORS. WE HAVE CHECKED THE PH IN THE DYE AND IT RANGES FROM 3.5 TO 4.5. WE ADDED WHITE VINAGEAR TO DILUTE THE PH LEVEL AND HAVE FOUND SOME SUCCESS. WE ARE UNDER THE GUN AND NEED A SOLUTION FAST. CUSTOMER IS KNOCKING ON OUR DOOR. IS THERE A SPECIFIC FORMULA WHEN MIXING THE DYE OR SOMETHING ELSE THAT SHOULD BE ADDED. PLEASE HELP

Joseph J Kolacsky
PLATING - CANTON, Ohio

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Acid zinc deposits iron in random locations as a function of current density, then where the iron is rich the dye color will vary. Also the wetters in acid zinc are tenacious and interfere with the uniform conversion of the clear chromate, so insert a 1/4 %/vol HCl dip between the zinc and the chromate. Any dye will have a different shade at 3.5 pH then is has at 4.5 pH.

Vinegar contains organics other than acetic acid and these impurities interfere with the uniform adsorption of dye. Go get some reagent grade glacial acetic acid.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como