Alloy 20 Corrosion in H2SO4

[rcarr]

We have 3% Sulfuric Acid being injected into our Alloy 20 stock line. Downstream of the injection, the heat affected zones of some of the welds are being eaten away. Could this possibly be from the line not being pickeled and passivated after welding? Is this the right material for the application? This line is between a brown stock (brown paper) chest and the pump suction.

 

[metengr]

What is the temperature of the 3% sulfuric acid in your process?

 

[EdStainless]

What is the concentration and temp in the line after injection?

This alloy is Nb stablized, isn't it? If not then sensitization could be the problem.

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Corrosion never sleeps, but it can be managed.

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

 

[rcarr]

The 3% Sulfuric prior to entering the stock line runs from 70 to 90deg F. The acid is mixing with 130deg F stock.
The material is Columbium Stabalized.

 

[metengr]

Here is the Tech Blue Sheet information on Alloy 20 from A-L for your review.

http://www.alleghenytechnologies.com/Ludlum/Documents/al20.pdf

The columbium stabilized grade is fine, and would reduce susceptibility to sensitization in the weld HAZ.

The only other possibility is what you had mentioned earlier, if the heat tint or surface oxidation was not removed after welding it is possible that this could be the reason for the preferential attack along the base metal HAZ's.

 

[EdStainless]

You need to run some microstructure checks. This material may be unannealed, or at least seriously underannealed. If either is the case that the Nb may not have had an opportunity to stablize the HAZ. The welds would be OK because they were hot longer.

Since you don't generally see sulfuric attacking areas of Cr deleation I doubt that surface oxide has anything to do with this.

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

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