Chloride Stress Crack Corrosion 2

[samoht]

A Heat Exchanger is used to pre-heat nitrogen with an oxygen content of 0.5%, from 20degrees Celcius to 140degrees Celcius (Design temp: 375degrees C). The gas has been mixed with seawater and then separated leaving the gas saturated with water and some seawater droplets. The pressure is 3 barg.
1. Is Chloride Stress Crack Corrosion likely to occur
in this case.
2. What other types of corrosion could occur.
3. We have evaluated the following materials in this application: 316L, 904L, SMO, Hastelloy C22 and Inconel 625.
If you look at it from the cost side 316L or 904L is preffered, but they are less resistant to CSCC at these temperatures.

 

[EdStainless]

1. Yes
2. Pitting could be an issue. It sounds like you could get some wet/dry issues and the fouling and concentration effects that go along with it.
3. Consider a duplex grade. Much better CSCC resistance htan the austenitics. If you really think that 904L has enough corrosion resistance then 2205 would be a logical choice.
Given the higher strength of 2205 you could make walls thinner also.

To be honest I doubt that eihter 904L or 2205 will have enough pitting resistance to give you long life.
When you looked at the 6%Mo alloys (254SMO or AL-6XN) did you recalculate the wall thickness? They are a lot stronger than 316 or 904, you will use a lot less metal. This would offset some of the cost.

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Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[samoht]

Please note that the Design Temperature of the Heat Exchanger is 375degrees Celcius. Isnt it so that with 2205 (Duplex 22% Cr) embrittlement occur above 300degrees Celcius?

 

[EdStainless]

Sorry, missed the "C".
Doesn't it say that the temp is 20C to 140C?
But I agree with you Samoht, I wouldn't go above 625C with 2205.

It looks like the 6%Mo alloys are your option.

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Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[samoht]

The temperature on cold side inlet is 20C and the cold side outlet is 140C. The temperature on the hot side is close to 300C.
I guess you mean 625degrees Fahrenheit (329.5 Celcius)?

 

[SJones]

Assuming the nitrogen/seawater is on the tube side, what is on the shell side to provide the heat? At 3 bar g, SCC is likely to be a lower risk than pitting. 6Mo alloys will not like seawater above 30 deg C, particularly if the Cl content is enhanced by water evaporation and oxygen is present. You really have to be looking at a high Ni alloy such as 625 or higher.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[samoht]

The Nitrogen gas is heated in catalytic reaction and then used on the shell side of the Heat Exchanger. The maximum content of oxygen at the tube side is 0.5%.
1. I guess the low Oxygen content and the low pressure reduces the effect of SCC?
2. What effect does oxygen content and pressure have on Pitting corrosion?
3. Is it so that Pitting corrosion can intiate SCC?

 

[EdStainless]

Sam, Sorry that I keep messing up the temp units. I really meant to convert it.
1. I have never seen data that suggests CSCC is a function of O2 at these levels in high alloy materials. If youa re talking about very low O2 (ppb) then there are some effects, but the difference between 0.5% and 20% is not significant.

I am not too concerned about CSSC of AL-6XN (that is the specific alloy that I have a lot of data on). It will not crack in boiling 26% NaCl at 100% of yield stress.

Pitting is not the real problem, it is crevice corrosion. If you get any crevices or fouling on the surfaces then you will get crevice corrosion under them.
If the tubing is clean and the pH is neutral to slightly basic then you may be fine. I have crevice corrosion data to 75C ( I double checked this time) that shows no attack.

If you want better corrosion resistance you will need to look at a Ni-Cr-Mo alloy like 22, or 59, or 686. Don't waste your money on 625. It has good strength but the corrosion resistance is not significantly better than AL-6XN.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm