6% Moly Superaustenitics - Performance Ranking in aqueous chloride at high temperature 1

[Guest102023]

I am looking at some 16" diameter Alloy 20 piping that is experiencing fairly rapid corrosion in a pH3 environment @ 300°F, with unavoidable deposits settling on the bottom. There is pitting in the vapour space and more general (at least smoother) corrosion on the bottom side under the deposit. I am looking at the 6% Mo stainlesses, as well as Hastelloy C-276. How do these alloys rank in these conditions?

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

For a general idea you can look at PREN values, and you will see that 20 is well below what you would get with AL-6XN or similar grades.

At 300F and pH=3 you will need something better than a 6%Mo alloy.
Look at C22, 59, 686 and other similar grades.

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[Guest102023]

Thanks Ed,

Do you have any references that compare the half dozen or so most prominent 6%Mo superaustenitics? I know they are all tweaked in different ways for particular characteristics. Much of the data is published by the manufacturers, so they don't include competitor's products.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

There have been some NACE papers presented that include data for multiple alloys.
When you calculate PREN make sure that you use 16 for N, and not a higher value.

The slight differences in alloys don't matter, AL-6XN, 1925hMo, and 25-6Mo as so close that they might as well be the same.

Your temp and pH, (you don't mention Cl levels) are well beyond what I would ask any 6% Mo alloy to handle.
If there are welds in your system I would skip C276 also, the newer generation C alloys are much better in terms of stability.

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[Guest102023]

Sorry I didn't mention that the pH is all due to chloride ions, presumably from hydrolysis of methyl chloride. Lots of steam is used to keep the product moving, so high temperature is unavoidable. The pipe spools with long weld seams as well as circs.

So the differences between the 6%Mo alloys are more about manouevering around each other's patents than any significant technical differences?

I found some reports that showed some 6%Mo alloys superior to C276 in some respects, which puzzles me. What C alloys are newer than C-276?

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

The 6%Mo alloys do have advantages in some environments, but not usually at low pH.
The difference in chem can give slight advantages in specific environments, but not something that I would rely on.
Since your current unit is alloy 20 was there fear of H2SO4 attack? This is where 20 (and other Cu containing grades) excell.

So what is driving it low? HCl?
How much Cl, 100ppm, 1,000, 100,000?

The new C type alloys such as C22, 686, and 59.
These do have very different corrosion resistance and you will need to know the process chemistry in great detail.

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[Guest102023]

Thanks Ed.

Waiting for lab results on Cl- concentration. We think it is HCl, produced by hydrolysis of MeCl.

I know Alloy 20 is specific to H2SO4, but there is actually none present. Alloy 20 has some tragic history that I was not involved in (but no doubt the guilty parties were well rewarded).

Thanks for the alloy suggestions. Sounds like I should start discussing this with the respective alloy manufacturer's metallurgists. Thanks for the alloy suggestions.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Guest102023]

Process water analysis: chloride 900 ppm; sulphate 17 ppm; other anions negligible
Metals scan found high levels of Fe, Ni, Cr contaminant consistent with (stainless) steel corrosion.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

The sulfate actually helps a bit.
If this was not quite so hot I would go for a 6%Mo, but at this temp and pH it is a huge risk.


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