Austenitic Stainless Steels - Chloride induced stress corrosion cracking (CSCC) 8

[patchlam]

Dear Engineers,

Good day. Recently, I read that austenitic stainless steels may suffer from external chloride induced stress corrosion cracking (CSCC) when exposed to a combination of tensile stresses, chlorides, water, oxygen, and a temperature threshold. The threshold temperature which the stainless steel is essentially immune to external CSCC is 60 degC for AISI 316/316L.

I understand that this is normally caused by exposure to humid marine atmosphere. However, may I clarify, for a heat exchanger such as waste heat recovery unit to be installed on a offshore platform, is the threshold temperature above applicable for my heat exchanger internal stainless steel material?

Thanks!

 

[EdStainless]

The 'threshold temp' is not a fixed value but depends on the chemical environment and stress levels.
316 rebar has failed in concrete roofs of public swimming pool buildings in Northern Europe for chlor-amine exposure. I doubt that hose temps were over 30C.
The situation is the worst when you have cyclic conditions that allow surface to wet and dry. This concentrates salts on the surface and creates deposits that can drive crevice corrosion and serve as crack initiation sites.
Is there a reason that you are not using a duplex SS? They have some useful resistance to CSCC and are much stronger.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

CSCC can occur in a variety of environments, because the chloride ion is ubiquitous. EdS is correct about temperature; I will add that I would never rely on low temperature as my only defense against SCC for a potentially vulnerable material, and even then I would probably never permit 300 series stainless steels to mix with halide ions.
My assumptions about CSCC in austenitic SS are that almost no Cl- concentration is too low, and that sufficient tensile stress is always present, if not applied then residual.
316SS is perhaps useful as trim on a yacht, but is not very useful in offshore industrial environments, where superduplex SS and superaustenitic SS find wide application.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[weldstan]

Are your tubes to be 316/316L? What are the operating conditions and fluid composition? As Ed and Ironic metallurgist have stated, CSCC has occurred in 316 ss at temps much less than 60C.

 

[patchlam]

Thanks for all replies.

Dear EdStainless and ironic metallurgist, actually this is the first time I encountered this issue after reading a Norwegian specification. The basis for material selection is NORSOK M-001. Before that, we just select the material based on ASME VIII and API 560. The material chosen is ranging from carbon steel, low-alloy to stainless steel (SS 316 most of the time).

Dear weldstan, the preliminary selection for the tube material is carbon steel as it is sufficient to withstand design condition. Only the internal of my heat exchanger is made of SS 316 due to the exposure to hot exhaust gas.

May I clarify further on the humid marine atmosphere? When we say that the material is exposed to marine atmosphere in offshore, does it mean that the material has to be in contact with sea water directly?

And about the temperature limitations for uncoated Austenitic stainless steel, is it only specified by Norwegian design standard? As far I know, we never really encountered this design limitations in other projects. All the stainless steel parts are enclosed within a coated carbon steel casing with insulation layers, maybe can I say that the exposure to marine atmosphere is minimal?

 

[pradipgoswami]

Hi Patchlam,

Many of the above statements are true with respect to CLSCC of SS 316L.
NACE MR0175/ISO 15156-3,Table A.2 — Environmental and materials limits for austenitic stainless steels used for any equipment or components, provide detailed guidelines about selection of 316L for chloride/H2S environments.

Additionally this document may be referred as an excellent resource document for CLSCC :-

http://www.hse.gov.uk/research/rrpdf/rr902.pdf

By all means DSS(2205/31803)may be the ideal or optimum material to consider.
However the above documents should provide useful guidelines for the best engineering rationales.
Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[EdStainless]

patchlam,
I have seen 304/316 piping carrying warm fluids at a plant located near the coast crack from the salt picked up from the salt fog every night. IF the tubing is exposed to chlorides in either the process stream or the cooling water then there is a distinct risk. The presence of oxygen and sulfur are also factors, but are usually secondary to stress, temperature, and pH. The Cl levels hardly matter, 10% isn't a lot worse than 100ppm, either is enough to cause cracking.
If you are cooling exhaust from an IC engine, then the exhaust will have significant Cl in it from the intake air. And if you cool enough to get condensation then you will have a low pH (acidic) environment which is worse.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[patchlam]

Dear pradipgoswami,

Those are really good references, especially for someone like me. Thanks. I will explore the DSS(2205/31803), especially on its cost.


Dear EdStainless,

For the heat exchanger (or waste heat recovery unit) that we provide, the heating medium is either hot oil or inhibited water. And even the exhaust gas, Cl is normally not there. The only concern is the sulphur content within the fuel, which we will calculate the acid dew point and ensure that our cooled exhaust gas temperature is always above the dew point.

 

[weldstan]

Numerous stainless steel equipment items developed ClSCC during outdoor storage before being placed into service at the San Onofre nuclear plant. Salt concentration similar to that described by EdStainless was the cause. While ambient temperature rarely exceed 80F, surface temperatures can be much higher in direct sun exposure.

 

[Chumpes]

if the environment is oxygen free (<<50-100ppb, internal corrosion), then i use NACE MR0175 for stainless steel grade selection vs. SCC : stainless steels do have a certain level of resistance when oxygen is absent.
when oxygen is present (atmosphere), resistance of stainless steels decreases and i limit their use below 25 to 50°C, depending on chloride concenration, wet/dry cycles....
painting systems may help vs. external SCC depending on external metal temperature

 

[georgeverghese]

If you've got low chloride inhibited (tempered) water on the tubeside of this WHRU, UNS31803(duplex SS) would be a good, almost zero risk choice. In my previous company, in this same application, one project team decided to go with SS316L at the WHRU and dose hydrazine into the inhibited water "whenever required". This may be risky, if you ask me, since monitoring oxygen at ppb concentrations in this temperered water stream would be unreliable, probably next to impossible with run of the mill dissolved O2 meters.

If you've got hot oil on the tubeside, SS316L would appear to be adequate. But with hot oil, make sure the tubes are seal welded into the WHRU tubesheet to avoid any leaks into the turbine exhaust stream to prevent HO ignition at the highest possible turbine exhaust temp, which would be the setpoint for the turbine exhaust high temp trip. 100% RT of all pressure service welds should be requested for this application in turbine exhaust stream service.

At the process system heat exchangers where heat is transferred from HO to the process medium, the HO minimum operating pressure should be maintained higher than the max operating process side pressure to avoid small leaks of process fluids into HO (from roller expanded tube to tubesheet joints or from pinhole tube ruptures), especially if the process fluid contains chlorides, water, oxygen etc. If you cannot maintain this pressure differential, these process exchangers will need to be constructed out of inherently corrosion resistant materials and tubes should be seal welded.

This concern with cross contamination of the recirculating heat transfer medium also would apply for inhibited water.

A good design practice is to account for all the implications of HX leaks - many design teams assume heat exchangers dont leak at all, except when process safety considerations require it.

 

[patchlam]

Dear weldstan, I totally understand that, and probably I would change all the actuation tubing from SS 316 to duplex SS.

Dear Chumpes, Thanks for your input, guess you had just given me the idea on checking the concentration of oxygen and chloride.

Dear georgeverghese, I totally got your point, some of my customers had directly specified SS321 (probably better than SS316?) in their specification. It seems like I would need to put more attention if the heating medium is water.

SIncerely, thanks for all the comments! Really appreciate that.

 

[GGedge]

Could I just add to this comment from Ed "316 rebar has failed in concrete roofs of public swimming pool buildings in Northern Europe for chlor-amine exposure. I doubt that hose temps were over 30C" for the purposes of raising awareness.

It is correct that 316 and 304 have a history of cracking in pool buildings, note not immersed but in the atmospheres above the pool. Typically temperatures are in the range 20 to 30C. Many failures have occurred to all sorts of fixings and supports, some have caused fatalities. Despite this designers are still specifying these susceptible materials for load bearing components in pool atmospheres when they should not. The European steel design code EN1993 has an updated section (EN1993-1-4) of supplementary rules for stainless steel which limits the use of stainless steels for load bearing components to a very limited number of 6% Moly steels and effectively prohibits anything other stainless alloys.