Does SCC in MoS2 coated A193 B7 studs stop above 212 degF?

[Bambie]

If condensation is boiled off above 212 degF when bolted joints in steam service are placed in service, is propagation of existing cracks in A193 B7 studs arrested?

Assume there is enough gross stud cross section left to ensure elastic response to stud preload and transient thermal loads as the flange warms up.

Is there a crack propagation threshold stress for this type of cracking?

 

[cap1a79]

Have you checked API 579, especially related to crack growth?

Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[SJones]

From the post title it is assumed that cracks are being ascribed to sulphide stress cracking (SSC) from the decomposition of the MoS2. With the fasteners already cracked, the cracks could propagate by other mechanisms such as fatigue even if the likelihood of further SSC will have been reduced whilst at the elevated temperature.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

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

 

[EdStainless]

The problem with environmental cracking is that the agent that accelerated the cracking isn't consumed by it.
So it is still there.
That combined with how much lower the force is to propagate a crack once it exists basically doom you.

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P.E. Metallurgy, Plymouth Tube

 

[Bambie]

cap1a79,

API 579 doesn't provide specific FFS advice for SCC in studs, however EPRI NP-5769 (1988) provided the Nuclear Industry with very detailed advice.
The scan attached indicates that continuous impingement by Boric acid contaminated steam at 316 degC produced an average crack propagation rate of .16" per year between 65 degC to 175 degC in preloaded studs, which must be the very worst case.

My scenario doesn't include impingement or even the presence of steam.

SJones,

I suspect the only source of significant fatigue for a stud with high preload stress (50-80% yield) would be the thermal transient associated with placing the process in and out of service.

EdStainless,

EPRI cautions against trying to remove MoS2 lubricant (see scan). Do you know if there has been any development in this area since 1988?

 

[Bambie]

Here's the attachment.

 

[cap1a79]

The graph 8-2 showing corrosion rate is for "metal loss" under specific environment while SSC("Sulfide" Stress Cracking) due to molybdenum disulfide is "cracking" mechanism which is different from metal loss. Hence, you cannot apply 8-2 to crack propagation rate.

Referring to the last sentence of the attached, the reason why MoS2 shall be removed is due to that sulfur which is decomposed from MoS2 can be a potential cracking problem. (In other reference, it is known as "promoter of SSC".)

Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[EdStainless]

But don't you have to get the MoS hot enough to decompose in order for it to drive SSC?

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P.E. Metallurgy, Plymouth Tube

 

[SJones]

The belief was that H2S was generated by a reaction with moisture leading to specifying certain lubricant formulations that were expected to keep water at bay.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

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

 

[Bambie]

Gentlemen, to quote the EPRI source:

Very little information is available on the conditions that leads to MoS2 decomposition except that the presence of moisture influences its chemical behavior. In the presence of water or in an oxidizing environment, MoS2 reacts to form an oxide layer and H2S. Following chemisorption of water, H2S04 can evolve in ad adsorbed form. MoS2 which had been exposed to damp air contained sulfuric acid, molybdenum trioxide and both chemisorbed and physically adsorbed water. Sulfuric acid contamination promotes physical adsorption of water, whereas Mo03 promotes chemisorption of water.

Does this explanation imply that in the absence of moisture, mechanical and thermal fatiguing mechanisms, SSC or SCC propagation may arrest or be mitigated?

 

[Chumpes]

hello
in my opinion, no moisture = no wet H2S-SCC (SSC, HIC, ...), continuously keeping a margin (+15°C?) above water dew point will prevent SCC, but can this really be achieved ?
why do you think corrosion will stop above 100°C (212°F) if there is still moisture present ?
regards

 

[Bambie]

Chumpes,

The stud area where cracks have been observed is in the flange hole annulus, not the threads engaged in the nuts.
I realize there is always some water vapour present in the air next to the stud, however, if the stud surface is at 300 degF and atmospheric pressure, what corrosive chemical reaction can proceed there?

 

[EdStainless]

If the system never sees any cycling, never shuts down or starts up, and the flange never leaks, then yes.
In the real world there will be time when it moist and <200F.
We used to use a spray dry film MoS, no binder, no oil or grease, and it was applied to the thread section only.

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P.E. Metallurgy, Plymouth Tube