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Multicoloured Metal?

J

Jahee

Platinum Member
On some motorbike exhausts and high performance cars, i've noticed sometimes the pipework goes multicoloured... i understand that its the heat, and probably something to do with oxidising the metal, but what makes it turn those colours?

There's some pictures of what i mean here if you scroll down some... Thanks
 
Heat treating metal causes a rainbowish coloration to appear. I would say that it is the repetitive heating/cooling of the exhaust system that does it.
 
My educated guess: Impurities rising to the surface.

We don't use pure iron or pure titanium for exhausts. We use an alloy consisting of the main metal and smaller percentages of "helper metals". Its probably these helpers that are rising to the surface.

 
Originally posted by: Minjin
My educated guess: Impurities rising to the surface.

We don't use pure iron or pure titanium for exhausts. We use an alloy consisting of the main metal and smaller percentages of "helper metals". Its probably these helpers that are rising to the surface.
Click to expand...

Impurities, or perhaps something like what you get with a diffraction grating, or maybe like whatever causes oil on water to look colorful.
 
Good question!

I suspect it's caused by tempering the steel either intentionally (for aesthetic reasons I presume, perhaps by welding) as in the OP's link, or unintentionally caused by hot exhaust gasses. Basic steel is carbon and iron and forms all sorts of compounds based on the arrangement of the carbon and iron and other metals in the steel. I poked around the web and Britannica but the actual chemistry is rather complex, as are the many varieties and grades of steel.

Older style razor blades were often blued steel, and one can blue a drill bit pretty easily. These are heat treatments though other processes can blue steel. Quenching, which is probably a type of tempering but I'm not sure, can turn steel blue. Gunsmiths used bluing - chemical or heat - to resist oxidation (see wikipedia).

Hope you can get better answers!
 
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