Internal errosion/pitting on downpipes

[Legacy777]

I found something kind of interesting in my downpipe. It appears that there is some errosion on the inside of the pipe. Most likely due to bend downward, high temps and high gas velocities. The pipe is stainless steel, 304 I beleive. There's a noticable area of pitting on the main downpipe tube, and just a little bit of surface rust/pitting on the dump tube.

Not sure if anyone else has run across this.

Pics

[93forestpearl]

Interesting find Josh.

I've noticed a difference in color on the outside of the downpipe in that area in the past and wondered if something like that was happening.

Better grades of stainless steel, like 321 or Inconel would probably alleviate that but be rather expensive.

[Legacy777]

Yeah, better SS would probably help, but the cost would probably out-weigh the advantage.

[beatersubi]

Can stainless be coated?

[gijonas]

I don't get it.What is the issue here that needs to be alleviated?

[Legacy777]

Can stainless be coated?

Yeah, the external piping of my dp is ceramic coated.

[Legacy777]

I don't get it.What is the issue here that needs to be alleviated?

There's not really anything that needs alleviating, it's just more of an informative type thread to see if someone else has noticed anything similar.

[beatersubi]

Can stainless be coated?

Yeah, the external piping of my dp is ceramic coated.

I wonder if coating the inside would prevent this. Its just the heat causing the pitting, right?

[Legacy777]

I think coating the inside would probably help, but I don't know if it would totally eliminate the problem. I think there are a couple causes. Elevated temperatures I'm sure don't help the problem, also any residual stresses in that section of pipe from the bending can make it weaker, but I think one of the primary causes is due to high velocity and temperature gasses hitting the pipe. The process is similar to what a river does to erode pebbles.

My exhaust has a splitter that separates the main turbine exhaust gasses from the wastegate. In doing this, the main exhaust flow has a relatively unimpeded path, and I would guess that the section of downpipe may just be it's first contact which forces the gasses to change direction. If I did not have the splitter in there, the wastegate gassses would mingle with the main turbine gasses and likely not cause such a direct focus of gasses at one single spot.

Anyway....that's kind of my theory...

[beatersubi]

A sound theory it is. I think a ceramic coating could deflect some, or even much of, the heat. Enough to prevent the minor pitting? I know one way to find out.