Stainless steel Issue 4

[ChihebG]

Experts,
As we know, there are several grades of stainless steel (304/04L; 16/316L, Duplex 2205, Superduplex 2207...).
How can I choose the right stainless steel for a specific application?

My problem is: I have a water flow (water & gas) (production unit piping) flow with CO2 content 5% in the gas, Temperature 35°C and total pressure 35 barg. I assessed the corrosion rate and I found that it is very high for carbon steel so I decided to upgrade to austenitic stainless steel (304 or 316).

Do I choose 304 or 316? and why?

How can I convince my boss of the choice made?

Regards,

 

[EdStainless]

Sorry to say this, but if you are asking that question you need to be hiring a specialist.
Use your work to show that CS is not suitable, and then hire a consultant how specializes in this field.

There are many books on the subject. A good general one is "Stainless Steel for Design Engineers" by McGuire from ASM.
There are also specialty books focused on the petrochem industry.

In general, the alloys become more corrosion resistant (resistant to Cl pitting) as the Cr, Mo, and N levels increase.
304L is the basis.
316L is not economically viable, it is being replaced with 20-21% Cr lean duplex grades (2101, 2002)
2205 is a duplex that holds the mid-ground between lower alloys and expensive ones.
Above here are the super-xxxxx grades, they come as austenitic (AL-6XN, 254SMO) which are very expensive, ferritic (Sea-Cure and AL29-4C) which are only available as thin sheet or thin wall tube, and duplex (255, 2507) which are difficult to work with.

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P.E. Metallurgy, Plymouth Tube

 

[ChihebG]

Thank you EdStainless. I am sorry if the question looks evident but I am inexperienced in this field and I hope to learn more. Sometimes when I face new issues I look to the quick solution but I have to take to time lean and to resolve it.



Chiheb Gouta
M.Eng Mechanical

 

[EdStainless]

This is a very difficult subject.
You need to understand corrosion theory and the chemistry behind it, and then you need to know the environment, and then consider the pros and cons of each alloy group, then consider alloys within the selected group.
One of the serious issues is that minor contaminates in the system often control the corrosion.
You need some classes, and you need to hire an expert to lead you through this.

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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

a)What is the range in operating temperatures - is the max normal operating = 35degC?
b)What about max normal external surface temp? Is this pipe in a coastal location or offshore?
c)What pipe dia are we looking at?
d)This is production gas, so youve got dissolved chlorides ? Any other contaminants?. List these even if these are in trace concentrations

 

[ChihebG]

Thack you Edstainless for your helpful advices.

georgeverghese,
a) yes max temperature is 35degC.
b)the pipe is in a coastal location.
c)18".
d)we are dealing with gas free of any contaminant.

Chiheb Gouta
M.Eng Mechanical

 

[EdStainless]

a) So objects left laying out in the sun will never get hotter than 35C there? And the pipeline is always flowing, never shut off?
d)It may be clean from a process point of view, but 10ppm Cl makes a difference in corrosion, and if this water came out of the ground it is loaded with minerals and salts.

From a corrosion perspective the questions are different from the process issues sometimes.

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P.E. Metallurgy, Plymouth Tube

 

[JHWC]

The cost for both 304 and 316 steels is minimal. 316 is more resistant to pitting corrosion.

Based on your initial question, I will choose 316 instead of 304.

However, I do agree with the rest to get more information first before you make the decision. There could be other alternative materials better than 316 after you access the whole situation.

Thanks,
Jeffrey

 

[georgeverghese]

Max solar radiation temp (applicable to external pipeskin temp when there is no production) in the tropics/subtropics is 55-60degC, while in the Middle East, it could be up to 70degC. So where is this facility?
External chloride SCC sets in at approx 55-60degC on austenitic SS.
BTW, is this pipe insulated?

 

[ChihebG]

Thank you for your help. I have to assess the whole situation because I have some missing inputs.

Chiheb Gouta
M.Eng Mechanical

 

[EdStainless]

In practice there is almost no difference in the corrosion (pitting) resistance of good 304L and good 316L.
I have been testing samples for years and find that while there once was a gap, it has almost vanished.
316 is becoming a grade with a high price and no reason to exist, the lean duplex alloys are pushing it out of the mainstream.

If a SS is the right answer for this application go with a lean duplex (2101, 2202, etc).
They are strong so you can use thinner walls (less metal to buy, less to ship, and faster welding when you install), they also have some functional CSCC resistance (I have seen external SCC on 3xx pipe that was indoors and never got above 40C).

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P.E. Metallurgy, Plymouth Tube

 

[AkhilMetallurgy]

Materials selection for corrosion is largely empirical despite large amount of theoretical knowledge. Corrosion behavior is full of discontinuities and sharp changes of reaction rates. Even manufacturers of proprietary alloys some times do not fully know the uses of their alloy.

Better not to depend on materials selection but adopt maintenance rules.

If there is time it is better to make trials in real operating ( and shutdown ) conditions. Variations are as important as design conditions. There usually is not enough time in an actual project. Corrosion exercises have to begin years before the project.