Surface deterioration of mild steel in a Muffler

[Wicsteve]

We've recently seen flaking and deterioration on the surface of unplated mechanical tubing used inside a muffler. Upstream of the tube is a catalyst element so temperatures are quite high. High temperature alone does not seem to be all of the cause since this is a rather recent problem. About the time that the flaking problem started it had been noted that there was also a problem with carbon build-up at the exhaust port of particular test engines. We're cureous if there is any relationship here between the excessive carbon, temperature, and exposure of the mild steel. For example: is surface hardness affected?

 

[AEF]

Potentially a whole raft of problems. Excess carbon indicates a rich mixture with high amounts of unburned hydrocarbons or CO. Any sulfur in your fuel? Water content downstream of the cat converter? Any one of these combinations along with high temperatures can cause scaling on carbon steel. Surface hardness will only be affected if the tube was heat treated and your running hot enough to temper it, but normally mufflers dont run that hot. Carburization causing increased surface hardness shouldn't be a concern unless you running really rich with high CO and very high temperature (muffler would be glowing red). SAE should have something on this, if not check out ASM.

 

[Wicsteve]

Unlike automotive muffler, most small engine mufflers run at very high temperatures especially when the muffler is directly attached to the engine port like this one is. The area does get red hot, especially down stream of the catalyst. The two cycle oil for certain contains sulfur and water vapor would also be in the gas stream. The steel tube was not unusual by any nature and had not been heat treated in any manner. The surface of the tube after endurance testing is generally a dull grey with streaks of light reddish brown (but not heavy rust). The surface of the tube is flaking off in large pieces and surface cracks are appearing over the entire surface.

 

[rustbuster1]

Why not have someone do an X-ray Diffraction Analysis of the flakes compared to the desired state? (presumably you have one of each available) This is not expensive and the results will focus your investigation.

 

[AEF]

I agree with rustbuster1. Analyze your scale. Caution on some X-ray diffraction analysis though, some are only qualitative and not quantitative. You may want to do some metallography on the tube also.