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Getting even finish with chromate conversion

How do I get rid of starburst around holes when Iriditing parts. Also, what causes a rainbow effect on the surface. Running a 2 oz. per gallon immersion tank at 90 deg. at 1.6 pH.Holes are punched in part and the Timesavered with a 150 grit belt.

Douglas Pickerd
metals shop - Blairsville, Pennsylvania, USA
2006

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First of two simultaneous responses --

Work hardening around the holes will always give a different shade. Regarding the iridescence, 1.6 is a very low place to measure pH and you may not be as accurate as you think. That said, however you measure, experiment with lower and higher pH until you get less iridescence and more yellow uniformity on the particular alloy and heat treatment and work hardening situation that you have. Also lower the temperature to 75F unless it is a rare process that your supplier says to heat.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2006

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Second of two simultaneous responses -- 2006

Wow - I usually expect this type of question from some budding finisher in China or India, but Pennsylvania! Isn't it bad enough that the finishing world is losing jobs off-shore at an amazing rate; now what little is left on-shore, some hot shot machinist is going to do it himself! How about I go into the machining business or stamping business? :-)

Seriously though, all kidding aside, (1) check your technical data sheet for the chemical you're using (pH sounds little on low side - would expect closer to 2.0), (2) talk to your chemical supplier about your trouble, (3) all chromates are iridescent which by definition means combination of many colors (i.e. don't expect uniformity), (4) chromating woes especially ones dealing with appearance are as likely to arise in the etch, deox, etc. pretreatments as the chromate solution itself, (5) because you're in PA, if you're selling to a certain large connector manufacturer whose multi-national firm has been in the news a lot over the past few years, they have VERY DIFFICULT (OK call it UNREASONABLE) expectations for UNIFORMITY that may not be doable for a chromate finish! They are the only firm I've ever had a reject for "too much pink" in my chromated cadmium plating.

Milt Stevenson, Jr.
Syracuse, New York