Hot KOH and SS 316 or SAF 2205: SSC? 1

[Harg]

Any experience on stress corrosion cracking in SS 316 used for hot (80 C) potassium hydroxide (KOH) with presence of oxygen? We have had some cases of severe problems, while other systems with (seemingly) identical service are doing fine. Our chosen replacement material in problem areas is SAF 2205 - any comments?

 

[EdStainless]

Any duplex should work, even the leaner alloys.
316 is very touchy in hot caustic service. Your experience isn't uncommon.

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Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[metengr]

According to the information that I have seen for 304 SS and 316 SS, indicates these alloys are not recommended for use with KOH concentrations greater than 20%, at or below 212 deg F. The reason for the performance variations that you reported with the 316 SS could possibly be fluxuations in concentration and temperature of the KOH solution in service?

What is your KOH concentration? Depending on concentration, you could be forced to upgrade even further to a Nickel base alloy - Nickel 200, Inconel 625 for handling concentrated caustic solutions.

 

[EdStainless]

If you can't make the system work using a duplex the next choice would be a ferritic stainless. They are a lot less expensive than the Ni alloys and actually work better. Yes, E-Brite is still made. Even alloys like 430 are not bad.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

Your thread implies that your 316 SS suffered stress corrosion cracking (SCC) from KOH. While there are limited data for KOH, these limited data available indicate that corrosion related problems associated with KOH are nearly identical with those of NaOH. The literature for NaOH shows that 300 series stainless steels do not suffer SCC below 120C. If you have experienced SCC, the cause may be residual halogens, e.g. chlorides or chlorates, in the caustic.

In any case, duplex stainless steels should be an improvement. If for some reason you decide to use a ferritic stainless, I would suggest that you use a 26Cr grade, such as E-Brite. E-brite is still produced, but it may be difficult to get in all of the sizes and product forms you require. I do not think you will get adequate resistance from the conventional 12Cr or 17Cr grades.

Additional reference material can be found in books by Paul Dillon and published by the Materials Technology Institute (MTI):

"Materials Selection for the Chemical Process Industries"
"Corrosion control in the Chemical Process Industries"

Also Volume 13 of the "Metals Handbook," published by the American Society for Materials - International has usedful information.

If you need more specific information, I suggest you contact the Nickel Institute in Toronto (

http://www.nickelinstitute.org