Stainless Steel corrosion on marine environment 2

[purpletoes]

Hello has anyone have any experience on a stainless steel that is showing some corrosion pits after exposure in the ocean. The metal is not fully exposed to the water, moisture will probably still seep in.

Would you happen to have a recommendation on how this can be prevented in the future. Will it matter if the metal is coated with something first? Thanks

 

[TGS4]

What grade of Stainless steel are you referring to? There are many grades that are susceptible to pitting in chloride environments, and some that are not.

Sounds like someone made the wrong choice of grade.

 

[unclesyd]

Salt water will pit SS the majority of the time. No coating is 100% effective.

Comeback with a little better description of your problem.
What type stainless?
Exactly what type exposure?
For how long?

 

[purpletoes]

It's a 316 SS for submerged in seawater. Some corrosion are evident after 4-6 months of exposure. Thanks

 

[grimmy65]

Check into passivation. If your 316 has too much surface iron then corrision will occur. You could also switch to a 317LMN which has better corrosive resistance.

 

[DAVIDSTECKER]

You may need to use zinc anodes if there is any electrolysis working on your part.

 

[AllHandlesTaken]

Austenitic SS (incl. 316) is definitely not optimum

Have a look at this document.

Guidelines for Selection of Stainless Steels for Marine Environments, Natural Waters and Brines

 

[RobLN]

From a purely practical/experience point of view I can say that SS316 is good but not perfect for marine applications.

I have installed 316 handrails in a marine environment and there is slight surface corrosion. This can easily be removed with some special cleaning fluid and it is never really deep corrosion.

If your application is submerged you may need a higher quality steel than 'noraml 316' as previously indicated by grimmy65.

I would guess that if you specify a swimming pool grade stainless you won't go far wrong.

there is a load of good information here:

http://www.bssa.org.uk/topics.php?article=77

Solid Edge; I-Deas 7 to 12; NX4 & currently NX5.

 

[Niwot]

Maybe a different arena, but with flow meters submersed in sea water, the following materials are spec'd regularly:

1) Monel
2) Hastelloy C
3) Duplex

For offshore applications, even the mounting hardware located outside of the pipeline is usually spec'd monel 400 or hastelloy C because most grades of SS will pit/corrode over time. We see alot of Duplex spec'd in the North Sea environment. Have not run across a coating that will protect SS, although many oil and petrochemical companies (Shell, Exxon Mobil, etc.) provide detailed painting specs to prevent corrossion of external components (not immersed, but exposed to the marine atmosphere). Bottom line: Sea water and marine environments will attack SS over time.

My Compass Corrosion Guide gives 316SS an 'A' rating on seawater to 250F (flowing or not), but also states that it "pits on drying". I've always questioned this rating because most refineries, power plants, and petrochemical companies spec Monel for Sea Water applications.

 

[stanier]

Problem may be that you have stagnant conditions and the CrO2 layer is not reoxidised once removed.

316 is generally not much good for sea water but it is highly temperature dependent.

Desalination plants generally use super duplex UNS 32750 or 32760. On top of that the welding needs to be pickled and passivated. refer ASTM A 380.

Refer

http://www.nidi.org

or Outokumpu/Sandvik/Avesta websites for more information.