Steel 1.4404 / 316L vs. 1.4571 / 316Ti - actual difference in corrosion resistance? 1

[MartinLe]

The steel type 1.4571 / (X6CrNiMoTi17-12-2, ASTM: more or less 316Ti) is commonly used in wastewater applications due to it's good corrosion resistance. An alternative is 1.4404 / (X2CrNiMo17-12-2, ASTM: 316L), which is often said to be inferior. Several steel plants published information materials to the following effect (don't have an english source, so I summarize the key points):

[ul]
[li]Heat treatment of carbon steels would lead to the formation of Chrome-Carbides, these are attack angles for corrosion. Ni and Ti are added to form 1.4571 because these elements form carbides and so prevent the formation of Chrome-Carbides.[/li]
[li][/li]
[li]Current (since a few decades ...) manufacturing technology allows lower carbon content in steel production, so that addition of Ni & Ti are no longer neccessary to have corrosion resistance as good or better as 1.4571 in many cases.[/li]
[li][/li]
[li]1.4571 is clearly superior in applications with T > 350°C[/li]
[/ul]

All information about the comparative corrosion resistance of these steels I found so far comes from steel plants and traders who have a vested interest in pushing 1.4404: 1.4571 is commonly used in Germany, Austria and some eastern European countries, apparantly the rest of the world moved on to 1.4404 et. al. This means german steel traders need to warehouse two types of steel.

On the other hand, the (German) code DWA-M 275 advises 1.4571 for applications where the Chloride concentration is above 100mg/l. There's a simplified formula for effective resistance, to sujmmarize the effect of different alloying agents on corroson resistance: resistance = % Cr + 3.3 % Mo + 16 % N, 1.4571 would have a value of 25. So far I have not been able to hunt down a proper source for this formula, or even a proper guidance of how to use it (resistance value at least X in conditions Y or so).

To sum it up, I'm not at all sure if the widespread use of 1.4571 in wastewater applications is a historic idiosyncrasy (as the steelmakers say) or if there are sound technical reasons. But every once in a while, the topic pops up - when you specify 1.4571 ("because that's what we always use") and the contractor delivers or offers 1.4404 ("because that's what we always use").
I'm also lousy at metallurgy, maybe someone here can shed some light?

 

[EdStainless]

316Ti will out perform 316L that is at the high end of the C range when you talk about long term high temp exposure or the corrosion of weld HAZ in the as welded condition.
With modern, clean, very low C 316L there is no reason that there should be any difference in corrosion resistance.
You reference the PREn formula. It has been through many iterations but if you want history go back and read the Redmond and Davidson papers on it.
Most of the issues with pitting in SS go far beyond the base metal chemistry. The truth is that welds in 316 (any version) in the as-welded condition will very susceptible to pitting. The segregation of Mo in the welds is behind this.

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P.E. Metallurgy, Plymouth Tube

 

[MartinLe]

Thanks, this is useful. Though so far I had no luck with finding these Redmond or Davidson papers.

 

[EdStainless]

I'll look in my files.
It would be papers on pitting resistance of stainless steels either by 'Redmond and Davidson' or Redmond and Kovacks'

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P.E. Metallurgy, Plymouth Tube

 

[DBreyer]

Another point you may want to consider is the difference in yield strength.
If you are in Germany you'll most likely have material according to EN 10088-3.
According to this code yield strength of 1.4571 is higher at elevated temperatures than the strength of 1.4404.
So your required thickness might be higher which might make your system more expensive.
Might be worth checking if the pipe/ fitting can actually withstand your design conditions if you receive 1.4404 instead of 1.4571.


Daniel Breyer
Inspection Engineer

http://lnkd.in/dFcQA-w