SS 316L vs. EtOH-H2O 90-10 with 65ppm H2SO4, low O2, low Chloride @170°C

[Chumpes]

Dear all
i have to select metallurgy for the following service :
- EtOH-H2O (90-10%) full liquid at 30bar, 170°C.
- low chloride content below 2.5ppm
- low oxygen as the system is deaerated at Tamb under ppO2=1.25mbar
- presence of 65ppm H2SO4, which would correspond to pH=2.9 in pure water.

We already disregarded carbon steel because of the acidity at high temperature.
Austenitic SS seems to be a relevant candidate material but i have a doubt if SS 316L could be acceptable @pH=2.9 and 170°C, low chloride and low oxygen. i cannot find data for the resistance of SS 316L at such a low H2SO4 concentration (65ppm) and at such a high temperature (170°C).
Another problem is that, if ever i consider SS 316L is not adequate for the service, then I also have no data to validate the use of one or the other corrosion resistant alloy within the same process service.

Any advice on material selection in such HT EtOH-H2O service service ?

thanks and regards

 

[EdStainless]

All of the H2SO4 will be in the water, so you will have a solution of EtOH + acid.
The oxygen is a non-issue, I am far more concerned about other oxidizing or reducing contaminants in the system.
Oxygen or oxidizing ions in solution will help the performance of SS.
I have no data for low strength acid at temps above atmospheric BP.
Your higher temp and any Cl in the system suggests that 316L or 317L are not good enough.
The next step are alloys such as AL-6XN (not 254SMO, too lean) or 825.
I had some higher temp H2SO4 data, but it is proprietary.

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P.E. Metallurgy, consulting work welcomed

 

[Chumpes]

Thanks Ed for this fast reply.
I was wondering if the very low O2 levels may allow operation with a very few ppm chloride at high temperature (170°C) without acid. No additionnal oxydative contaminants are foreseen. Agreed O2 and oxydants would help SS resisting H2SO4 by promoting the building of a good passive layer.
What do you mean by "lean" saying 254SMO is too lean ?
Once we agree SS 316L or SS 317L may be not adapted to the service, is there any easily available data that can be used to justify AL6XN (23Ni, 20Cr, 6Mo) or 825 will have satisfactory resistance ?
I m starting to think that we cannot go without a bit of testing...

 

[EdStainless]

You will have to find a lab that do this mixture in an autoclave (bomb) for you.
I have seen AL-6XN and 825 used in waste acid recovery systems, but these systems max out at 110-115C.
I have seen these alloys used in other mildly acidic applications hotter, but not H2SO4.
If I was having testing run I would have coupons of AL-6XN, 825, and C22 all tested.
Other options would be 1925hMo and alloy 33.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed