Stainless Steel Rusting

[TimBow123]

I was wondering if anyone could describe what is happening in the attached picture. The parts are cast stainless CF8 as per ASTM A351 and then machined. We obtained data from an independent lab that showed that the chemisty and physicals were per the standard. The parts were put through an environmental test with humidity and temperature (80°C). Out of 12 units only 3 showed signs of this corrosion.

Could suggestion be made on how to remedy this issue?

 

[swall]

What is the post machining processing? Are the parts passivated? What type of machining fluid is used?

 

[TimBow123]

There are no post processing steps after machining. I am unaware of what type machining fluids were used?

 

[EdStainless]

It looks like you may have machined into inclusions or unmixed metal in the casting. Are there pits at these locations?
Clean them up and passivate in nitric acid and see it you still get the same response.

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Plymouth Tube

 

[mcgyvr]

free iron ions are everywhere, cutting fluids, dirty hands, shop dust, compressed air used for drying,dirty rags,etc...

 

[BUTCHCWIQAM]

Do all the pieces have the same type of discontinuity or are the other 2 in different areas… can you send pictures of the other 2… if possible?
Thanks

 

[TimBow123]

I stand corrected, there was only 1 out of 12 that had any signs of corrosion. These parts are still at a test house Attached is a picture with the surface corrosion cleaned off.

 

[desertfox]

Hi Timbow123

The last picture looks more like a casting defect to me.

desertfox

 

[stanweld]

Agree with desertfox. Looks like you machined into casting flaws.

 

[PaulvdH]

These are definitely inclusions in the cast material.


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Paul van der Heijden
Composite Engineer
Vekoma Rides Manufacturing BV
Internet:

http://www.vekoma.com

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[arunmrao]

That is a slag or sand defect observed. . Probably this went unnoticed by the foundry, else they could have salvaged it. Actually defect is over a large surface area, and should have been noticed while grinding or after shot blasting.

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
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