Molybdenum and Corrosion resistance 1

[RedVette]

I understand that the presence of molybdenum in alloys contributes to greater corrosion resistance in that it "increases the lattice strain," thereby necessitating greater energy to dissolve the iron from the surface. However, what I'd like to know is whether the increase in corrosion resistance (upon exposure to solutions not selectively corrosive to Mo) is linear for any given base metal or if it functions along a curve where the enhancement is proportional to how anodic the metal inherently is?

Any clarification is greatly appreciated.

 

[EdStainless]

If you are talking about aqueous corrosion then everything is only electrochemical. (and I am a stainless steel guy also)
The higher stability of the MoOx compared to Fe and Cr mean that it will take higher potential to break down the surface passive film and it will re-passivat easier. Adding Mo to a SS will increase the resistance to pitting.
Mo on its own has limited impact, and some critical level is needed for any significant benefit.
The greatest performance gain will come when and alloys base composition is modified for higher corrosion resistance and Mo is added. Since Mo is a powerful ferrite stabilizer care must be taken in balancing the alloys composition to retain the phase balance desired.

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[RedVette]

Ed,

I understand now that, with the proper balance, Mo improves corrosion resistance by enhancing both the lattice strain as well as the oxide film. However, in conditions where passivity potential is at a minimum, is it the case that the noncorrosive benefit of Mo is obscured in alloys where the base elements are about as cathodic as Mo itself, since the dissolution rate of Mo would not be slow enough to allow for any remarkable retention of the other base elements that their respective rates aren't already allowing?

My attention to Mo was drawn by the rusting of this spoon from table salt that was neglected:

For flowing seawater applications, what would be the minimum requirement? I've read from here that the Mo content in today's 316 stainless is too low and that grades like 2205 and 904L are the reliable alternatives. And by the same token, since Mo acts as a stabilizer, is there an upper alloying limit before the susceptibility of a detrimental effect?

Thank you very much for clarifying.

 

[arunmrao]

I endorse Ed's comments. The spoon can be 304 , which is not good for table salt . Usually in salt manufacturing industries, all parts are made in 316 material.

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[EdStainless]

Todays 316 has lower alloy content than in years gone by, because of better melt control. But it is also a much cleaner steel so if you are dealing with highly polished surfaces modern steel may be superior.
A spoon left sitting in salt and open to air, or an use in seawater splash zones will be exposed to a saturated salt solution, roughly 26%.
316 has only ever worked in this application if is washed regularly, either manually or with rain.
And 904L has no place in seawater service, it is designed for acid service.

What limits the amount of Mo in a SS in phase stability. Either you cant keep it austenitic, or you can't suppress the formation of secondary phases.
When you start looking at high alloy duplex and ferritic grades you find alloys that are very sensitive to heat treatment and especially quench rates. This is because of serious secondary phase issues.

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