Intergranular Corrosion for AISI 316 1

[Vinzzzz]

Dear all I have had a conversation with one of my client about AISI 316 and it's resistance to pitting corrosion and intergranular attack.

My situation is the following, I have selected materials for an Emergency Shout Down valve sited near a well-head where the reservoir is constituted by gas, oil and associated water.
About associated water sampling says that Oxygen is not present and the PH is about 6.9-7.1, H2S is not present (no need of NACE requirements) and the molar percentage of CO2 is about 0.02%.

I have suggested the use of AISI 316 for valve trim because as I know as law no oxygen (cathodic process) no corrosion (in this case pitting). Now client says OK but what can you tell me about intergranular corrosion?

I know that a fundamental part in this type of corrosion is played by heat treatment done to make AISI material, agood treatment avoid the possibility of precipitation, at grane boundary, the intermetallic phase of chromium carbide which is the cause of intergranular attack.
Now, the environment in contact with metal is constituted by chlorinated water (120.000 ppm = 120mg/l), oil and gas phase (that represent the principal phase).
I suspect that chlorine ions could break the passivity film of AISI 316 but without oxygen no pitting is expected and intergranular attack could not be present if AISI 316 is treated in order to dissolve in the alloy matrix the intermetallic phase of chromium carbide (normally AISI 316 is made avoiding the intergranular corrosion including also teh use of alligant like Mo or Ti).
My question is am I right or have I, really, evaluate the intergranular attack of AISI 316?

Regards Vincenzo

 

[blacksmith37]

It should Cl stress corrosion crack: I wouldn't put austenitic SS on any well head.

 

[SJones]

Don't kid yourself that lack of oxygen renders everything OK. There's plenty in oilfield water to contribute and assist the high concentration of chloride ions in breaking down the passive film. Temperature is also a key parameter. What temperature do you have?

Intergranular corrosion would be a corrosion threat for incorrectly processed material. It would be checked via an appropriate ASTM A262 test.

Steve Jones
Materials & Corrosion Engineer

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

 

[EdStainless]

At these Cl levels I don't see 316 as an option. Period.
You can't prevent all corrosion and these Cl levels can drive serious crevice corrosion. The risk of SCC is also a real concern.

Consult some of the references (NACE or API) for CRA wellhead construction.

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Plymouth Tube

 

[stanweld]

Will you have liquid water in contact with the valve trim during normal "open" operation? What is the temperature?

 

[SJones]

http://www.hse.gov.uk/research/rrhtm/rr902.htm

Steve Jones
Materials & Corrosion Engineer

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

 

[TomEun]

Traditionally transgranular SCC is commonly developed in concentrated chloride-containing environments while intergranular SCC is developed in sensitized SS. Therefore your major concern should be pitting and SCCinstead of IGC unless the valve is under greatly sensitized condition during welding. However the given process data indicate that the valve may not be subjected to remarkable pitting or SCC if the temperature is 60 deg.C (normal)/ 80 deg.C (upset) and below.

In addition, you may also have to consider the higher levels of the chloride contents in the future if there are. Absolutely a new evaluation for the material is required in this case.

Thomas Eun

 

[Vinzzzz]

dear sirs,

thanks for your responses and evaluations.
About the possible corrosion mechanisms I want to underline the fact that I don't have Oxygen inside the Reservoir as indicated by Client with associated water analisys.

Valve is a GATE type and only trim is in AISI 316 while body is constructed with AISI 4130 (normal carbon steel with 1-1.5 % of Cr).
Obviously inox with high chloride concentration is subjected to depassivation, but without cathodic process how these mechanisms could take place?
Trim material is made without welds and all elements should be one piece and if the heat treatment is well done sensitization shall not take place (AISI 316 materials are constructed principally to avoid this problem trough a dedicated heat treatment).

In these conditions the use of AISI 316 permits a great cost saving instead of use superduplex of Nickel based materials like Hastelloy.

Anyway there is a non sense if Client had selected carbon steel materials for flowlines and pretends that valve trim shall be in superduplex or Hastelloy material. I say this only because I'm in the position to select a valve to be installed after a well while others have projected the entire system with carbon steel material.

Regards
Vincenzo

 

[EdStainless]

While this is commonly done I wish that I had $100 for every broken 316 bolt that I have examined in my career. For other hardware 316 would work, the steel will provide galvanic protection. But I would still avoid it for highly stresses parts.

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Plymouth Tube

 

[SJones]

But the carbon steel would not be subject to pitting and cracking. In selecting a stainless steel in a high chloride environment, the risk of pitting and cracking is increased. A valve stem is still a pressure containing part even if it is 'trim'. Oxygen reduction is not the only cathodic reaction that could occur. Try working through API RP 581, Section 2, Clause 13.

Steve Jones
Materials & Corrosion Engineer

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

 

[stanweld]

I would be most concerned with crevice corrosion (pitting) at the interface between the 316 gate and the 316 seals rendering a leaking emergency shut off. Have seen a number of cases of valves with 316 trim exhibiting this type of corrosion and subsequent leakage.