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Rouging of welds in 316 SS equipment

At my plant we use several 316LSS portable storage vessels. We have found that the welds on the 4 corners of about 12 out of our 30 containers are rouging. The corrosion typically runs the entire length of the weld. Occasionally there will only be a single spot or pit that is rouging. It found it interesting that none of the flat, unwelded, sheet metal surfaces have signs of corrosion. We store dry and water wet non-chloride pharmaceutical salts in these containers. These salts do not attach 316L SS when using test coupon.

2 years ago we sent out several containers for a citric acid passivation. All these containers are now rouging at the welds. Some of these containers have not even been used since they were passivated.

We recently sent some containers to a local electro polisher to try to remove and prevent future rouging. Their process turned the rouged spots from red to black, but the spots were still there. They rubbed the seams with a pickling solution. This removed the black marks, but after a short time the marks returned.

Any thoughts as to cause, or suggestions on how to remove our rouge stains and stop them from returning.

Peter Falk
manufacturing - Coventry, RI
2005

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First of two simultaneous responses --

Hi Peter !

Go and have a look-see in the archives at # 19760 .... I found that some of the later comments to be very interesting.

Have fun !

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).
2005

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Second of two simultaneous responses --

Reading your letter I can't conclude anything else than that the problems you describe are the results of poor welding.
Just passivating or pickling is not enough, you've got to replace the welds, grind hem with grit 320 and electropolish the vessels. Otherwise the stains will return.

K. de Boer
- Derustit, Holland, the Netherlands
2005

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Peter:

Welding generally increases the iron content of the surface in the weld bead and creates a heat affected zone on both sides of the weld. In the heat affected zone, concentration of contaminants can occur and lead to formation of inclusions and pits. In addition, if the atmosphere of the welding activity and quality of the purge gas is not adequate, contaminants may be included into the surface of the weld area and lead to future corrosion.

If this damage or contamination is relatively deep in the surface, greater than 50 angstroms or more, this damage may not be removed by typical passivation treatments. The use of pickling paste and electropolishing are generally excellent methods to remove typical deep weld damage.
Corrosion of the weld area is typical without effective passivation or electropolishing treatment.

In your case, it appears that the damage is in and below the surface and needs to be removed by grinding and possibly welding to remove the spot or pit and inclusions. Corrosion within a pit is very difficult to stop and likely, residue from the cause of the pit (an inclusion of iron, aluminum or other metal) still remains. Physical removal by grinding is usually the only solution.

Welding quality and surface cleanliness, as well as the effectiveness of the passivation treatment are critical factors in the corrosion resistance of stainless steel.

Daryl L. Roll
- Costa Mesa, California, USA
2005

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Excellent article on rouging at
www.corrosion-doctors.org/MatSelect/rouging.htm

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2005

Ask the Manufacturer if they used Argon when they welded the tanks together. Argon is the only true shielding gas for any type of welding. You can use other gases but, they allow dirt and other inert contaminates in the weld. That is what you are seeing. Try this, take a SS plate, put 2 welds on it, one using Argon, one using another gas, send it to your electropolisher. You will see what I mean.

JEFF W LENTZ
- Lebanon, Pennsylvania
2005