Process of exhaust discoloration.

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Member for

12 years 8 months

Posts: 2,536

Can someone explain what the process, reason is for an exhaust to change colour from this;
to this;
I'm trying to determine if the discoloration is due to heat or rust, or possibly both,
and why does the front ring stay silver?

Original post

Member for

16 years 5 months

Posts: 705

Exhausts do not normally rust. Rusting is an electrochemical process that requires the presence of moisture. Rust is an amorphous, flaky, porous material consisting of hydrated red and brown iron oxides.

e.g., hydrated iron(III) Fe2O3·nH2O and iron oxide-hydroxide FeO(OH)·Fe(OH)3.

Heat oxidation results in a thin, hard patina of black (magnetite, Fe3O4 ) and brown iron oxides. These provide a measure of protection against rusting and it is called a passivation layer. Skilled blacksmiths can control this oxidation to produce a protective layer of magnetite in a process called 'bluing'.

Member for

7 years 2 months

Posts: 78

Simple reason :- It got hot !

If you look at old British motorbikes they have a wonderful range of colours, depending on the temperature of the exhaust, thickness and type of exhaust pipe material etc.

I think "Patina" is a very apt description of the appearance !

Member for

12 years 8 months

Posts: 2,536

I think that answers that :-)
What metal is the exhaust made from? This process is similar to the bluing you get on a carbon steel wok.

Profile picture for user Anon

Member for

14 years 4 months

Posts: 2,841

The exhaust metal is a type of Nimonic stainless steel and, like nearly all metals, can have a different hue dependant on the temperature it is heated to. Exposure to weather can also add to the final colour it achieves.

The section on the intake edge of the Blenheim engine exhaust is still bright silver due to the fact that it is not a part of the exhaust ring chamber, just an additional spot-welded section on the front edge to aid aerodynamic airflow through the cowl. Because of this it does not get anything like as hot as the main collector ring and, therefore, doesn't discolour.


Profile picture for user dogsbody

Member for

17 years 5 months

Posts: 135

Here is a cross-section drawing of Bristol exhaust collector ring. You can see why the front edge didn't discolour.