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Cracks on corners of 316 SS phosphoric solution tank

We are a manufacturer of rectangular fabricated stainless steel tanks used in ultrasonic cleaning applications. We have recently seen cracks in our 316L 14 ga tanks at corner bends on a tank where our customer is using a 10% phosphoric acid solution at 175F to remove rust from parts. The tank has ultrasonic transducers bonded to it. We have only seen this happen one other time in a similar application. We need to solve this problem for our customer ASAP and do not want to fabricate a new tank with the same material if that is the problem. Is the acid causing a stress cracking problem? Do we need to go to a Hastelloy or a different stainless or is the weld process the problem (tig with argon)?

JOHN WILLEY
STAINLESS TANK FABRICATION - CHESHIRE, Connecticut, USA
2004

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2004

1st look for "stray" currents. 2nd with a very sensitive volt meter see if you can detect a voltage on the outside of the tank, from the bend to the center of the tank the longest distance away from the corner. All you have to do is bend a piece of stainless steel and stick it in a conductor, especially a hot conductor, and you have a voltage between the bend and the non-bend and either the bend or the non-bend will dissolve in practically any electrolyte. Be sure to do this test at 180 F.

If you detect a voltage then apply a counter-voltage with a small rectifier. Attach the negative to the corner and the positive to the center of the tank. Let us know what you find.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina