316 L seal crevice corrosion? 1

[Jackfeng]

It is well known that in stagnant seawater 316 may be unsatisfactory for piping. However, if I have a 316 L seal with Inconel inlay on the ring groove, will the 316 L seal has the same risk of the crevice corrosion?

 

[MetalMickey]

Depends on the conditions you have in there, is it allwas stagnant seawater?
This kind of arrangement is common, but in this case you may be building up problems for yourself, with even a slight galvanic effects making things worst. If you've got the cash suggest Inconel seal rings and sleep safe at night!

 

[Jackfeng]

Thank you very much for your reply.
The equipment is 25 meters under seawater inside a steel basket. So I would suspect it is stagnant seawater all the time. The body consists two pieces connected using flange. The concern is the seal which is 316L. The mating pieces are low alloy CS with TSA coating. What is your suggestion?

 

[MetalMickey]

Suggest replacing the 316L with an Inconel seal ring to match the ring groove.

 

[Jackfeng]

Thank you very much for your reply.

I have to have sound the justification for the change. Crevice corrosion of the 316L in the stagnant seawater? Do you know any historical data or regulatary info about it? Any website?

 

[MetalMickey]

A good site to try is

http://www.nidi.org

 

[tomosekine]

Recently, 316L with 0.3 to 1.0% nitrogen has become commercialized. These materials show good seawater pitting resistance.
Example: 316L with 0.5% nitrogen showed no pitting, but has about double 0.2%YS and TS.
With 1.0% nitrogen, no Ni was required to get the metallurgical structure of completely austenite. This no nickel austenitic stainless steel was formed to a very small pipe to be used as catheter for the patients with nickel allergy. For these materials several melting and refining processes were developed under Japanese government funding.

Consultant
T. Sekine
Tokyo, Japan
tomosekine@jcom.home.ne.jp

 

[rolledalloy]

In stagnant seawater, 316L is not suggested due to crevice corrosion and pitting concerns. It has been used in aerated high velocity seawater, as long as its always flowing. Typically if a stainless is to be used a 6% molybdenum stainless, such as AL-6XN or better is suggested.

You mention Inconel, however, there are several varieties of inconel. Hopefully, yours is Inconel 625 which has 9% molybdenum and has good resistance to seawater. If it is Inconel 600 (No Moly), or other low molybdenum containing Inconel grades then it will be no better in resisting crevice corrosion than 316L stainless.

 

[MEKANIKEREN]

For crevice corrosion in seawater, the use of AISI 316 in the gasket may act as a &quot;kickstarter for starting crevice corrosion even at the higher alloyed flange (since the environment inside the crevice keep souring from the creation of HCL from corrosion of AISI 316). AISI 316 is thus not recommended for this service. We also recommend to follow MetalMickey's advice to use a gasket of similar material as a seawater resistant flange, just keep in mind to order the ring with a significant lower hardness than the flange. 6Mo or Alloy 625 for low temperature applications < 15 deg.C souds fine to me.

Regards,
Mekanikeren