Selective Corrosion of 2205 Duplex SS

[Guest102023]

I am investigating some badly corroded 1" HX tubes from a biodiesel production facility. The inputs include fatty acids, methanol, secret sauce, and a fraction of 1% sulfuric acid. Sludge deposits were found, but the corrosion was 360°, with some preferential corrosion at the weld. I have not yet examined the tubes up close, so this is all the information I have.

Is there anything listed (or not listed) here that could cause aggressive corrosion?

 

[EdStainless]

1. Was the tubing tested to ASTM A923 when it was manufactured? (do you know that it was heat treated correctly in the first place)
2. What is the minimum pH in the system?
3. What is the maximum temperature?
4. What is the Cl content?
5. Which fatty acids?
6. What about other metal ions, Fe?

Mixed acids with any chlorides present are not good.

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Plymouth Tube

 

[metengr]

Improper material choice given the stated environment.

 

[Guest102023]

What about duplex in peroxide? I'm told it could be present as a reaction product of the 'secret sauce'.

 

[EdStainless]

peroxide is no issue.

Temp, pH, and chlorides are what you need to know.
My guess is that pH and chlorides are unknown/uncontrolled.


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Plymouth Tube

 

[TVP]

I concur with Ed, pH and Cl- are almost certainly not fully understood. Even a 2% concentration of H₂SO₄ at elevated temperature is rated as unsuitable by Outokumpu, and that does not take into account Cl-.

 

[Guest102023]

I have completed the investigation to my satisfaction, although without positively identifying the corrodent(s). Best guess was H2SO4, which as a reducing acid can be quite aggressive to duplex.

The question now is which way to turn. This industry (biodiesel production from renderings) is still fairly young, are there any established alloys so far? I am thinking we will end up with a superaustenitic of some variety.

Any thoughts/suggestions?