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WHY DO SOME 
PLASTIC TANKS CRACK?
WHY DO SOME 
PLASTIC TANKS CRACK?
Ever wonder why a welded polypropylene tank severely cracks horizontally across the welds when used with chrome, some chromates, and high concentrations of caustic.

Although corrosion tables show compatibility of polypropylene, that listing is for the sheet alone. Once the tank is heat welded, the molecular structure of the polypropylene at the weld changes and severe cracking across the welds is the result.
The tank cannot be repaired. You will need to insert a new compatible liner into your cracked tank and use the outer cracked tank as support. The liner can be rigid PVC which usually has a temperature limitation of 140F, flexible high temp premium PVC which can withstand 180F-190F depending on the type of PVC, or even CPVC for higher temps.

I have also seen these chemistries cause similar problems in dragout and/or rinse tanks following the chemistry. Even traveling puddles on the floor can attack the welds of tanks near by. 

Molded polypropylene and polyethylene tanks are compatible depending on the operating temperatures because they are not welded. They are roto-molded as one piece. However, they will need to be supported depending on their size and operating temperature because they usually have thin walls. However, sizes are limited to the mold sizes available at the manufacturer and where that FOB point may be. 

Remember not to weld anything to this kind of tank when used with the chemistries noted above because then the welds will crack.
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