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Chromium turns black on some exhaust systems

Sometimes when I plate exhaust system they become black or blue, and yellow and some times they don't change the color. what I know about this, is that the chrome stand without changes in colors up to 200 °C and if the temp goes up there is nothing to do.
I would like to know is there is another variable to control like for example: chromium layer thickness (i think that is possible because when we add more chromium to the piece it seems to delay in change color)
or it can be due to a iron content of the nickel bath?

thanks.

DANIEL HERNANDEZ
PLATING SHOP OWNER - BUCARAMANGA , SANTANDER, COLUMBIA
2007

I presume the chrome finishing has color tone after plating. Solution composition is very critical, such as Cr+6/sulphate ratio, Cr+3 content. Keep solution temperature and current density at optimum condition during operation.

HL Eng
supplier - Singapore
2007

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Please excuse me, I did not post my question well.
the problem is that when the piece is working it becomes gray or blue or black.
I have a mirror finish but when the engine start (it is an exhaust system) to work, the piece changes the color but sometimes it does not change the color.
Why does this happen? How can I avoid this?

again sorry for my English.

thanks

DANIEL HERNANDEZ GUERRA
- Bucaramanga, Columbia
2007

Thank you very much, I note it is working better.

Daniel Hernandez Guerra
- Bucaramanga, Columbia
2007