Evaportation tank, Chem reactions?

[aamoroso]

We are trying to evaporate our wastewater from a Pre-Paint Rinse Line, which is used mostly for galvanized sheet metal parts. We have constructed our tank from Stainless Steel (316 SS). We have put a Stainless Steel Fire Tube through the tank (it serpentines through the tank). Everything functions properly and works extremely well for concentrating the Zinc into crystals. Here is the problem: After running the tanks for a while (sometimes 3 months, sometimes up to 8 or 9 months) the tanks or the fire tube will “rot” through. Examining the materials at the “rot” we can tell that the stainless parts are being broken down, sometimes rapidly. There does seem to be a faster breakdown of the stainless parts in areas of concentrated heat. I am looking for some thoughts or opinions on this. Should we use different material for the equipment, should try forced circulation of the waste in the tank, perhaps a third factor besides Stainless, Heat and Zinc are the cause. I am a Mechanical Engineer (not Chemical) and I apology for my choices in terminology if they seem crude.