Applications where 304 is more corrosion resistant than 316

[a7992]

Hello,

I have always assumed that 316SS is an upgrade to 304SS. One of our mechanical engineers informed me that in some cases 304SS is more corrosion resistant than the 316SS. I was a little surprised since they are so similar. Can anyone give me any more information on this? I was reading online that in highly oxidizing environments, 304SS is better. Is there any truth to this?

Also, we are thinking about substituting one 304SS valve with 316SS that will be in 20% phosphoric acid and 5% ammonia...would the 304SS be the preferred metal in this case based on corrosion resistance? We would be making the substitution because we have a 316SS valve onsite and cannot wait for 304SS.

 

[ash9144]

304 is better in high temp applications, usually.

There is no such thing as material is X is always an upgrade to material Y. You need to check your conditions and the exact application to determine this.

 

[EdStainless]

Are you having trouble with 304 in this application?
If so then you need to find out what the impurities in the system are.
The alloys should perform about the same in 20% phosphoric. The addition of ammonia (nitrate or sulfate) should have little impact.
On the other hand if you have any chlorides in the system you may need something better than 316 to get much better life.

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Plymouth Tube

 

[Metalguy]

In a highly oxidizing, very low halogen environment 304 can outperform 316 because it has a little more Cr. The Mo and higher Ni don't help in such environments.

You must be careful about welds, because the Cr will partition to the ferrite phase and can allow attack of the lower Cr austenite phase.

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[MikeMet]

Type 304L is much better than Type 316L or any moly bearing stainless steel in nitric acid solutions containing oxidizing species, such as vanadium pentoxide. Also in some areas of urea production where carbamates are present Type 304L is better than Type 316L