Methanol SCC Steel Vessel

[123harv]

We have a methanol vessel (25yrs, atmospheric, ambient temp service carbon steel no PWHT) which we believe has suffered SCC, (multiple longitudinal cracking in a dished end adjacent to a weld) and we will be taking sections to confirm the dished end metallurgy, the nature of the cracking and make sure we have no impurities in the methanol. However I can't find much in the literature that details methanol SCC, I have found some tentative references to methanol causing SCC in papers concerning ethanol SCC, but I'm struggling to get detail of the actual mechanism involving methanol. Can anyone provide background to the mechanism or any experience of similar failures.
Hope someone can help.

 

[unclesyd]

Can we assume that this is internal?

What is the tank and head material specification?

Could you post some pictures of the cracking?

 

[123harv]

We think it is internal, but the cracks are through wall and they appear to have similar internal and external lengths. We will confirm when we section the item.

The vessel is 25yr old and exact material specefication has been lost in the midsts of time, but no reason to believe that it is anything but plain carbon steel.

We haven't got any pictures to post, cracks are only visible after MPI suggesting they are fine, they are predominatly longitudinal but not dead straight and there is some evidence of fine branching.

I'm really looking for background or experience of the Methanol SCC mechanism.

 

[metengr]

Is there any ammonia in your process stream w/methanol?

 

[metengr]

123harv;

I did a Google search using key words like "stress corrosion cracking of carbon steels in methanol" and came up with several hits that require you to either purchase technical papers or to be a registered user.

I would investigate the following;

1. Check the contents of your process stream - water content, ammonia, etc.

2. If what you report is correct, it does indeed seem like SCC and could be related to lack of thermal treatment to the vessel components during original fabrication.

3. I would remove one or more boat samples containing the cracks and perform a comprehensive metallurgical analysis - chemical composition of the plate or head material, characterization of the cracking, and hardness of the plate material.


I will see if I can check several other sources.

 

[123harv]

No as far as we are aware methanol is normal industial grade used for injection into a propane line to clear ice.
I'm getting a sample checked for impurities but it comes from a reputable supplier.

 

[metengr]

123harv;
Ok. I did some checking with ASM Handbooks, and the only material I could find regarding SCC of carbon steel in methanol was Volume 13, "Material Selection and Corrosion Control".

According to information in Table 3 of this section, the table lists some environment-alloy combinations known to result in SCC. According to this table, for carbon steels in methanol and halides, there is NO SCC.

So, i would suspect that you had a contamination problem in your process stream that was severe enough to expose any weak links (fabrication problem) with the vessel. By the way, H₂S in the process stream can result in SCC.