The role of Molybdenum in 316L steel 4

[gsjs]

Hi Everybody:

The Molybdenum in austenitic stainless steel 316L is supposed to increase the corrosion resistance.

Can some GURUs help me understand the metallurgy of Mo to improving the corrosion resistance.

Thanks in advance.
Regards,
gsjs

 

[metengr]

gsjs;
Have you tried to Google this information? If you want detailed information, see the list of hits below for your reading pleasure;

http://www.google.com/search?hl=en&...istance+of+stainless+steel&btnG=Google+Search

 

[unclesyd]

In addition to the above reference that focuses on the chloride pitting resistance of alloys containing Mo. there are other important uses of Mo containing alloys, say 316 SS in the chemical industry. Mo containing alloys are the workhorse of the fine chemical industry.

Taking our industry where we make organic acids by HNO3 oxidation of alcohols and ketones. We use 304L for the oxidation and purification process as long as the process temperature is below the melting point of any of the organic acids. Once we reach the melting point temperature it is a necessity to switch to 316 SS as the corrosion rate on 304l SS approaches 200 IPY. Any alloy containing over 2.5% Mo. will work with the corrosion rate proportional to the Mo content. At one time we used molten Adipic Acid in a process with the Material of construction being Mo, containing alloys with 316 SS being the lowest grade acceptable. Mo is essential if there are organic acids and heat.
In the same vain there are areas in our distillation process where we use 304L SS up to a point where we have to switch to 316 SS due to excessive corrosion rates to the 304L. This is mainly in the area where we process nitriles.

One of the first materials we look at in new bench scale applications is 316 SS as the base material of construction in a new process unless things, like the presence of HNO3, dictate other wise.

 

[stanweld]

gsgs,
To truly understand why Mo acts the way it does in terms of providing enhanced pitting resistance, you would need an advanced course(s) in thermodynamics of solid surfaces and vacancy theory. Suffice it to say that the empirical evidence provided in published reports substantiate the the effect.

 

[EdStainless]

Mo serves to alter the passivation kinetics of the material. Adding Mo improves pitting and crevice corrosion resistance because it makes it more difficult to break down the surface passive film. To some extent it also improves how the surface of the material repassivates or heals after the film fails and the surface becomes active.
Virtually all of the corrosion that we care about is electrochemical. Understanding ion transport and passivation is at the root of controlling corrosion.

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Plymouth Tube

 

[strider6]

one of the effect of the MO content is to increase the PRE 8Pitting resistance equivalent of the alloys)
i think that the one of the best source regarding the effect of Mo on corrosion resistance is the IMOA website (international molybdenum association), you can find a lot of good paper on this issue

http://www.imoa.info/Default.asp

regards

S.

http://www.corrosionist.com