Molybdenium in stainless steel 1

[smgmr2]

Hello there,

I have a question about molybdenium.

Why does molybdenium care for the sea water resistance in 316 (1.4404) according to 304 (1.4301)?

 

[metengr]

Molybdenum alloy addition in 3XX stainless steel increases corrosion pitting resistance.

 

[smgmr2]

Thaks for your reaction.

Is that all molybdeniumdoes in stainless steel for sea water resistance?

 

[metengr]

You can review the information below....

http://www.climaxmolybdenum.com/products/whatismolybdenum/WhatDoesMolybdenumDo.htm

 

[rmw]

Smgmr2,

Does your question indicate that you think 316SS with higher Moly than 304SS makes it a good selection for seawater service?

rmw

 

[smgmr2]

@ RMW

My question is based on the fact that I want to know why it is exactly that molybdenum cares for the sea water resistance in 316ss, compared to 304ss.

WHat does molybdenum do for sea water resistance?

 

[cloa]

Is your question, what does Molybdenum do for pitting corrosion resistance which is hole forming type of corrosion.
If so this is good.

http://www2.steel.sandvik.com/sandv.../HandbookWeb/00CB4E68B7653027C1256761002178E3

Basically molybdenum delays the start of pit formation. Why noone really knows but there are several theories each backed with some evidence including one that molybdenum improves the passive layer.

If your question, what does Molybdenum do for general (layer) corrosion?

 

[smgmr2]

I shall ask my question a little bit different.

How can it be that only a small percentage of molybdenum makes stainless steel sea water proof?

 

[metengr]

smgmr2;
The addition of Mo to stainless steel does not make it sea water proof. The addition of Mo helps to increase resistance against pitting corrosion from chlorides in sea water.

 

[moltenmetal]

I don't think anyone would characterize 316SS as "seawater proof". 316 is certainly more pitting resistant than 304SS, but it is far from "seawater proof".

Molybdenum improves the chloride pitting resistance of austenitic stainless steels. A small amount improves the resistance significantly. A larger amount (6%), with the proper composition of the other alloying components, improves it more still. But an even smaller amount of nitrogen (i.e. in duplex stainless steels) similarly improves the pitting resistance.

http://www.eng-tips.com/faqs.cfm?fid=285

 

[stanweld]

Moly effectively promotes a more adherent/dense passivating surface oxide; it has been postulated that fewer vacancies in the oxide layer are availble for corrosion to initiate. It in no way makes 316 corrosion proof in sea water.

 

[EdStainless]

If you look at the pitting of SS in chloride containing water you can roughly approximate the pitting resistance as
Cr + 3.3xMo + 16xN
So with 316 you remove 2% Cr and add 2% Mo this is about the same as adding 4.5% Cr (18% to 22.5%) but it doesn't require as much of an increase in Ni in order to maintain the austenite structure.

316 is by no means resistant to pitting in seawater. If you keep it very clean and you are at ambient temp ten it will last a while. If you get any surface fouling or rust from iron contact then it will pit easily.

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