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316L CATSUP TANK CORROSION

WE FABRICATED A TANK OUT OF 7 GA 316L SS WITH BUTT JOINT SEAMS AND SEVERAL FITTINGS. AFTER A PERIOD OF ABOUT 2 YEARS THERE ARE CORROSION PIN HOLES IN WELD AREAS AND ADJACENT TO WELDS. THE I/S WAS SANDED FLUSH WITH A 180 GRIT FINISH ALONG THE WELDS. THERE IS A BLACK COLOR TO THE METAL WHERE THE WELDS WERE SANDED.WE DID NOT PASSIVATE THE SURFACE. THE TANK HOLDS CATSUP AT approx. 212 DEG F. THE TANK IS CLEANED WITH A CAUSTIC CLEANER AT approx. 100 DEG F approx. 2X PER WEEK. WHAT SHOULD WE HAVE DONE DIFFERENTLY OR IS THERE A DIFFERENT CLEANING SOLUTION WHICH WOULD PREVENT THE CORROSION?

ANY HELP IS APPRECIATED.

TIM AHNER
FABRICATING - SANDUSKY, Ohio
2005

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You should passivate, or even better electropolish this tank. This will stop your problem unless, somehow, you did not fabricate from 316L. If the stainless is not low carbon, you will have carbide precipitation in the heat affected areas adjacent to the welds, and it will rust/discolor forever.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2005

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First of two simultaneous responses --

It should have been welded with 316 ELC (extra low carbon) rod. Gel passivation would have been appropriate on the welds. There is a possibility that too much heat was used in the welding process. Tomatoes are quite acid, the temp is rather high , so it is not a rare case that you would see pitting in two years. Not much that you can do with it now other than live with it or grind out the welds and reweld and passivate. Brush electropolish is another option.

James Watts
- Navarre, Florida
2005

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Second of two simultaneous responses --

I imagine you have extensive experience working stainless, however, did you check the sanding discs for iron oxide content or zinc? A black color would indicate iron contamination and would be hastened by a low pH environment (i.e. catsup.) Sometimes even a wire wheel used once on carbon steel and then used on stainless will contain enough iron to contaminate the stainless. Just a thought. As far as the tank is concerned now, an acid treatment will probably remove the discoloration. You are probably aware that the pitting is the more serious problem. A passivation process will stop further pitting but the damage may be done. Passivation is cheap insurance against these types of problems. Hope that helps.

Joseph Lockrem
- Indianapolis, IN, USA
2005

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For this type of application passivation is an absolute necessity. All of the above recommendations are good, also. Now that you have the pit corrosion it is a real problem to fix it, but we have done it for other food manufacturers using citric acid products with electricity. It is some work, but it can be done.

With this environment you may always have a problem unless you repeat the treatment periodically.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2005