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Different shades on same part

HELLO MY NAME IS KEITH SCOTT & WORK IN SOUTH AFRICA, WE MANUFACTURE MV & LV PANELS & HAVE COME ACROSS A PROBLEM WITH DOOR HINGE PARTS THAT HAVE TO BE YELLOW PASSIVATED TO A THICKNESS OF 8 MICRONS,WE HAVE NO WAY OF CHECKING THE THICKNESS & HAVE NEVER SUSPECTED THAT ANY OF THE PARTS THAT HAVE BEEN MANUFACTURED & PLATED ARE ANYTHING BUT PLATED TO THE CORRECT THICKNESS, BUT HAVE NOW COME ACROSS PARTS THAT ARE DISCOLORED OVER THE SAME FACE, IT LOOKS LIKE A PIECE OF STEEL WHEN HEATED WITH A BLOWTORCH, THE COLOR CHANGES FROM A STRAW COLOR TO A SPRING STEEL BLUE WITH A FEW OTHER COLORS IN BETWEEN.
WE HAVE BEEN TOLD BY THE PLATER THAT EVERYTHING HAS BEEN DONE ACCORDING TO THE SPECS LAID DOWN, BUT DON'T KNOW ENOUGH TO REFUTE HIM, IS THERE A COLOR CHART OR SOMETHING THAT THE INCOMING INSPECTION CAN COMPARE THE PARTS TO.
YOUR COMMENTS WOULD HELP US A GREAT DEAL.

KEITH SCOTT
MECHANICAL DRAUGHTSMAN - JOHANNESBURG, GUATENG, SOUTH AFRICA
February 14, 2008

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The passivation layer is not 8 microns; that's the thickness of the electroplating. But we don't know whether the plating is zinc or a zinc alloy; it makes a big difference in the appearance of the conversion coating (passivation). Conversion coating is not decorative plating and you should expect variations in appearance; the colors come both from pigments, which may be the chemicals' natural color or dye, and the refractive properties of this thin film. You certainly can't specify that the color of an oil sheen on water be uniform, and some of the color you are seeing is that. But give us the complete spec for the plating and we may be able to help. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 14, 2008