Q: My company sanitizes its food processing equipment nightly. Two years ago, we installed a 10,000 gallon stainless-steel storage tank for our hot water, and now there appear to be hairline cracks in the tank. The bottom half of the vessel is leaking. The warranty expired, and the vendor says there are no material or workmanship defects. Do you have any idea what the problem might be and how it can be resolved?
A: It sounds like stress-corrosion cracking is occurring. This can happen when two conditions coexist: the material is in tensile stress, and a corrosive environment exists. In your case the water is in a tank, therefore the stainless steel is definitely in tensile stress. The combination of hot water (140° F or hotter) and chlorine (3.0 ppm or greater) has created the corrosive environment I mentioned.
In fairness to your vendor, if you did not specify the material for this application and just asked for a stainless-steel tank, the tank most likely would have been constructed from series 300 stainless steel, which is susceptible to deterioration. If you gave them the application and asked them to give you the best solution, I think they may have some ownership, if not legally, certainly from the standpoint of getting future business from you.
The bad news is that once the damage is done, the only immediate option is to continue to patch leaks in the short term. You have a couple of midterm options. You can modify your process by using the tank as a storage vessel for ambient-temperature water and put a heat exchanger downstream, prior to sanitation's use. You also can use the vessel to hold non chlorinated water (hot or cold) and chlorinate after storage. The latter option is not very expensive. With either approach, you minimize the cracking, but both of these solutions are mitigation strategies and not long-term solutions.
The long-term solution is to remove the vessel and install one that is designed for this application. You will need to be very specific about your chlorine levels and temperatures, but an austenitic steel alloy like AL-6XN or 254-SMO would be more compatible with this application.
I wish I had a better answer for you, but it is what it is.
This article originally appeared in the June 2013 issue of Food Processing Magazine.