Heat Tint/Staining of SS 6

[hougy1]

Hello all,

I'm new to this site and a relatiely new engineer. I'm working on a project and I was hoping you could help me understand what I am dealing with. I have a 304 SS part (basically no more than a 2' x 2' piece of sheet metal) that is mounted about an inch above a gas heating element. The max. temperature at any point is roughly 1100 F. As expected, portions of the steel have turned blue. I have two questions:

1. What is the mechanism that causes the discoloration?
2. Is this preventable through surface treatments, a different alloy, etc.?

I've read through several threads on the forum, but most questions on this topic had to do with welds.

Thanks in advance,
Michael

 

[dgallup]

1) Oxidation
2) Insulation

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[hougy1]

Thanks. I don't think insulation is a realistic option. The heating element is an open flame - same thing that heats a commercial gas oven.

 

[mcguire]

No stainless alloy will avoid discoloration by oxidation at that temperature. That's why stoves aren't stainless on the inside.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[Compositepro]

Enamelled steel is often used in these applications.

 

[EdStainless]

The color is actually because of layers of oxide (mixed Cr and Fe) on the surface of the metal. In the early stages (gold, yellow, tan, light blue) the oxide layer is thin enough to be transparent and it actually has color due to diffraction of the light. Then they get thicker they simply scatter the light.
Higher Cr SS grades will discolor but suffer less damage over time from the oxidation.
You need to know if the heating is constant or cyclic before selecting an alloy.
If this is near a flame you also need to be concerned about other products of combustion (SOx and NOx).
All metals will discolor, but some will last much longer than others.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[TVP]

hougy1,

Just for clarification, Compositepro is referring to porcelain enamelled steel. It is a hard ceramic coating, such as that found on older steel bathtubs. It's a good suggestion if discoloration is not acceptable for your application.

 

[moltenmetal]

I've often wondered why the enamel on my gas range's cast iron support grid didn't just have the enamel flaking off of it after repeated thermal cycles, but I guess you can get enamels that will stand up to direct flame exposure followed by rapid cool-down. Mind you, it has flaked off a bit, but it's black already so you don't notice.

Equipment which is enamelled (glass lined) for corrosion resistance has temperature limits much lower than 1100 F and is quite sensitive to thermal shock even within that lower temperature range.