316 ss for use in high pressure saltwater experiment 3

[pauljlem]

I am planning a corrosion experiment that will be conducted at high pressure (2000-8000 psi). As part of the experiment I need to have small diameter tubing, fittings and valves that will contain synthetic seawater at high pressure. Will probably use 1/16" or 1/8" diameter tubing and compatible fittings and valves. 316 stainless fittings and tubing are readily available from suppliers of chromatography equipment, but I am concerned about their use and reliability when carrying seawater. The seawater (synthetic) will have normal oxygen concentrations of about 5 ppm , but will generally be stagnant. Temperature is room temperature or lower. I am trying to minimize the use of dissimilar metals when they come in contact.
Is 316 stainless adequate or not? I have seen cautionary notes about its use in stagnant seawater especially regarding pitting.
What is a better choice of material?

 

[unclesyd]

How long will the exposure be?

 

[pauljlem]

The experiment will run continuously for up to 4 months.

 

[roconnor]

Rule of thumb for stainless of all grades - the more polished the surface, the more resistant to corrosion. As the material is polished, the chromium creates a very hard 'shell' at the surface. Obviously, this won't apply to the inner walls of tubing, but a consideration nonetheless for future projects.
Refer to a galvanic series chart for metals, then check your budget and material availability.
(Platinum->Gold->Titanium->Stainless,etc.)

 

[mshimko]

A few thoughts to add to the discussion:

1. Pitting and crevice corrosion will be the concerns, and I would be more concerned about crevice corrosion than pitting, for two reasons: (a) most likely mechanical joints will be used, which introduce crevices, and (b) crevice corrosion will likely lead to a leak before pitting will.
2. Synthetic seawater will not be as severe as natural seawater. How much better????????? I'll make a WAG of 50% better, but that's just a WAG based on data I recall from many, many years ago.
3. Is the temperature ambient? (obvious reasons).

All in all, given the use of syntheric seawater, and the short time frame, I wouldn't expect a problem.

 

[SJones]

It boils (excuse pun) down to risk: what are the consequences of failure in the system you propose to construct? The probability of corrosion is a given as you already point out with your knowledge of pitting in stagnant seawater. If you get a pinhole leak maybe it isn't a problem but if you get a rupture from SCC that initiates from the pit in a 500 bar system then it could be a very large problem. It's difficult to work your circumstances quantitatively in this forum and, therefore, the answer has to be a "safe" one and go for an alloy such as 6Mo (UNS 31254, UNS N08367, etc) or higher.

Steve Jones
Petroleum Development Oman LLC
Muscat
Sultanate Of Oman

 

[EdStainless]

The problem is that the fittings that you are looking at using will create the worst possible (very tight) crevices. Looking at crevice corrosion data for 316 in ambiant temp seawater, you would expect CC to start within 1 week and pitting deapths of over 1mm withing 3 months are typical.
You should look for an alloy that actually works in seawater.
A 6%Mo stainless is good. You might have an easier time with a Ni-Cr-Mo "C" type alloy (276, 622, 59, or 686). Or you could use Ti.

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Corrosion never sleeps, but it can be managed.

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