What drives SCC in Caustic solutions at 150F? High pH or Chlorides? 5

[sta07750]

We are looking to replace a 50% Caustic line. Our pipe spec references NACE and says to use PWHT Carbon Steel up to about 170F. We shouldn't get higher than 150F but it gets hot in Texas and the caustic is not always flowing in the pipe. I've read conflicting reports that 150F may be too high for carbon steel and 50% caustic. I'd like to move to 304L SS but the chlorides worry me. Our upper limit for NaCl in our caustic is 1.3%. There is no telling what our environmental chloride situation is but the line will be insulated so it shouldn't be a huge problem. The Chlorine Institute calls for 304L for this temp/concentration range but there is no mention of chlorides in there.

Is 1.3% NaCl enough to cause Chloride SCC in a 50% Caustic solution?

Should I be looking at a Duplex instead (2205?)? I'd rather not get into the high nickel alloys if I can avoid it because in reality the fluid should never get hot enough to warrant those.

Thanks.

 

[moltenmetal]

That was an excellent question- it gives the information necessary to help you as well as demonstrating that you have done some research on your own to understand the boundaries of the problem.

Good on you for realizing that there's a LOT of chloride in modern membrane-cell caustic- a lot of people don't realize that. I didn't, until I had a failure which was likely caused by that chloride!

If the caustic is not being used at a continuous process temperature of 170 F, it sounds like you're within the NACE suggested limits. As long as your process need for the caustic can tolerate a little iron in it, and also presuming that the solution isn't continuously aerated, you're probably OK with your PWHT carbon steel with an adequate corrosion allowance based on the literature you've reviewed- I have no practical experience to contradict it.

As to chloride SCC in stainless steels, high pH alone does not suppress it as far as I know, nor is the high pH alone to worry austenitic stainless steels in the absence of chloride as far as I can remember- I'm away from my references to check. I do have a dusty recollection of a risk of non-chloride caustic embrittlement in stainless steels, which is probably why NACE is recommending PWHT.

For chloride SCC in austenitic SS you only need the following conditions: stress (which can be residual stress from forming or welding etc. or continuous stress from internal pressure etc., so you can usually assume it's always there in some part of any piping system!), chloride above some threshold which varies with alloy and temperature (at 170 F, 1.3% as NaCl is PLENTY high enough), something to accept the corrosion-generated electrons (oxygen typically, but not limited to oxygen), water, and time. When considering Cl concentration, any process which may form concentrated localized brines via intermittent exposure and subsequent evaporation of the water, is particularly dangerous- salt spray falling on insulation being the classic example. Since I understand that chloride SCC cracks generally initiate at pits, conditions which enhance pitting will also enhance SCC risk: acidic pH, low velocity, deposits or crevices, poor surface condition (embedded iron, interrupted passive layer) etc.

If the service is high pressure, I'd stay away from 304 or any other austenitic. That's just a gut feel based on the above reasoning, and again I don't have my refs nearby to check. Others should chime in and contradict if I'm off base on that.

 

[sta07750]

I've done quite a bit of background, mostly because we are putting up just over a 1/4 mile worth of pipe for this and it's all in a crowded piperack. The pipe is cheap enough but the charge for fabrication/install/trace/insulation is going to be a few hundred thousand so I want to do it right the first time.

It sounds like the 3XX series stainless it out of the running. The pressure is low enough but chlorides aren't something we really care about as far as the product goes so I wouldn't be surprised if a batch of high salt Caustic was accepted, anything for a discount. We don't mind iron in the product so it looks like PWHT carbon steel might just be our best bet as long as we can control the temperature correctly.

>chloride above some threshold which varies with alloy and temperature (at 170 F, 1.3% as NaCl is PLENTY high enough)

Can you point me to any good resources that deal with this? I've looked at some publishings but most seem to deal with very specific cases and don't make any general statements (i.e. Chlorides over XX PPM will always crack austenitic steels). Are there any good textbooks that deal with general metal corrosion topics? My understanding of corrosion and materials selection is admittedly limited and these things are starting to pop up more frequently.

Thanks!

 

[moltenmetal]

There is no recipe regrettably. There are conditions that are plainly no good or dangerous, but they're pretty obvious. The trouble with chloride SCC is that it's a risk, not a certainty- but when it happens, cracks propagate fast and can be through the wall before you have time to react. There are services where Cl SCC has apparently occurred as low as 20 C, but above 60 C you definitely have enough temperature for the risk to be real. As to the concentration of chloride to be considered a risk, it depends greatly on how much of the other factors are at play: places where water might evaporate and concentrate the chloride are the most worrisome such as tide lines or services which are alternating wet/dry, heat transfer services etc., The surface condition, residual stress and pitting initiating conditions I previously mentioned are also important.

Do a search here- I've populated my own records with numerous excellent references gleaned from Eng-Tips users over the years, but most of them are on paper rather than saved as .pdfs or as a link list. I also have a couple good textbooks, C.P. Dillon's book in particular and one by Jones if I recall correctly. Nickel Development Institute publications are also top-notch. API 571 has a concise and accurate but brief section about chloride SCC.

I'm hoping that somebody with actual years-worth of service experience with hot 50% caustic will chime in because like reduced oxygen concentration, increased pH has a tendency to suppress SCC to some degree. I don't have the direct experience you're looking for- we always used caustic at or near ambient and only heat traced it to keep it from freezing, and under those conditions 316SS (tubing) seemed to have an acceptable service life. Whether welds or HAZ would eventually crack or not is a different matter.

 

[tomwalz]

You may wish to consider the effects of welding in regards to hot caustic corrosion on the selected steel.

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[sta07750]

@tomwalz - Yes we plan to stress relieve all of our welds and bends to help out with that some. There are

 

[EdStainless]

In heat exchangers they often use ferritic stainless for caustic service, that way they don't have to worry about CSSC.
For piping I would look at using a duplex, perhaps even a lean duplex (2101, 2102, 2201).
They have usable CSCC resistance, and better pitting resistance than 316L.
They also have very little reduction in corrosion resistance at welds.

The real problem with 304 is that if this line was ever flushed with water and the pH dropped all hell would break loose (literally).

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P.E. Metallurgy, Plymouth Tube

 

[SJones]

Rebak at Lawrence Livermore has published several reports on SCC of CRAs and offers that 70 deg C is the application temperature limit for 3XX alloys in 50% caustic without any mention of chloride - see NACE Corrosion 2007, Paper 07480 for example. He also suggests that 100 deg C is the threshold temperature for caustic cracking of these alloys.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[EdStainless]

Caustic cracking in 3xx alloys is very species dependent, not just the caustic but also very minor amounts of other ions.
Every failure that I have seen in caustic was NOT caustic cracking, something else happened faster

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P.E. Metallurgy, Plymouth Tube

 

[moltenmetal]

EdStainless: thanks for confirming my suspicions in that regard.