HT sulfidation of Mo containing stainless steels 1

[Chumpes]

Dear experts,

High tempeerature sulfidation is the high temperature (200°C+) generalized corrosion of steel in presence of sulfur containing species.

I have a note in our material selection philosophies that says :

" Stainless steels without Mo are preferable in high temperature sulfiding environments "

I have in mind that chromium scales and content are responsible for resistance of austenitic stainless steels to HT sulfidation, and i have no clue on the influence of molybdenum on the corrosion resistance of Mo containing stainless steels.

does anyone have any feedback of an inferior resistance of Mo containing stainless steels in high temperature sulfiding environements ?

thanks

 

[EdStainless]

In general Mo is not helpful in sulfidation because of the volatility of MoS.
This can result in defects in the surface scale which will lead to accelerated attack.
Higher Cr (>25%), and Ni or Co based alloys are what are seen in severe sulfidation situations.

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P.E. Metallurgy, Plymouth Tube

 

[Chumpes]

Thanks
Now i remember that I 've already seen this MoS sublimation thing... now looking for MoS sublimation temperature =)
I which i could find pictures of cross sections with compared morphologies of SS 304 and 316 corroded in a HT sulfiding environment...
regards

 

[EdStainless]

I have boiled Mo oxide out of high moly SS annealing too hot in open air.
It can get ugly fast.
In fact the reaction path may be Mo - MoS - volatile Mo oxides
The temp is about 400C where the oxidation reaction becomes noticeable, by 550C it is very rapid.

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P.E. Metallurgy, Plymouth Tube

 

[moltenmetal]

The volatility of Mo species is the reason that 304 and its children are superior high temperature alloys to 316 and its children. 304H is our go-to high temperature material, until it runs out of stress values or unless we're worried about sensitization and IGC under shutdown conditions.