alternative to MOS2 coating on inconel 718 to avoid SCC?

[davefitz]

We are installing flanges on a ahigh temperature steam line, and the flange seal ring normally is supplied with a baked-on MOS2 molybdenmum disulfide coating. The service is steam service to 500-540 C ( 932-1004 F).

We have been advised that MOS2 forms H2S when exposed to steam, and will cause SCC on high strenght alloys such as inconel 718.

Can anyone confirm this is an issue with a baked on MOS2 coating, or can advise an equivalent graphite style coating to be used in lieu of MOS2?

"Nobody expects the Spanish Inquisition! "

 

[EdStainless]

What is the decomp temp for MoS2?
You could look at HBN (hex boron nitride) or WS2 (tungsten disulfide)

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Plymouth Tube

 

[btrueblood]

Trying to understand the why - the INCO 718 flange has the MoS2 coating to prevent galling or diffusion bonding of the flanges when made up? Would nitriding the flanges help? Boron nitride spray may work too, but like graphite spray coatings it's not very permanent - you would apply it just before making up the flange joints.

 

[metengr]

To avoid galling of the seal ring during assembly, I would use graphite versus the MoS₂ baked coating.

 

[moltenmetal]

Sounds like a Grayloc seal ring, and if so the coating is necessary for more than just the initial mate-up. Being a pressure energized seal, you would expect some movement with pressure fluctuations. Can't suggest a coating alternative for you that will be durable under those conditions.

 

[EdStainless]

WS2

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Plymouth Tube

 

[davefitz]

It looks like we may use MolyKote P-37- works well at HP steam turbine inlet casing and valve flange bolts.Graphite would work as well, but the seal ring supplier has not accepted that solution.



"Nobody expects the Spanish Inquisition! "