Corrosion of PVC in Hypo/Cl2 duty
Corrosion of PVC in Hypo/Cl2 duty
(OP)
We operate a continuous sodium hypo chlorite manufacturing plant. Caustic is circulated over a packed tower which is mounted directly to top of the recirc tank . Both vessels are PVC lined FRP. The PVC liner has suffered extensive corrosion after only 13 mnths operation. Corrosion on this duty is usual but 5 to 8 yr life is the norm.
Corrosion is generally in form of close spaced vertical grooving around the inside of the tank – above the main liquid level (there is no corrosion below liquid), similar but deeper than hydrogen grooving of carbon steel acid tanks. Preferential attack of welds is evident.
We believe the problem is associated with overchlorination, followed by decomposition, of the recirculating hypo/excess caustic solution.
We are trying to identify the corrosion mechanism: is it from heat of decomp and/or corrosive products formed after decomp? Also believe free radicals may be released in the above reactions - do these try to combine with the PVC? Can anyone help please?
Corrosion is generally in form of close spaced vertical grooving around the inside of the tank – above the main liquid level (there is no corrosion below liquid), similar but deeper than hydrogen grooving of carbon steel acid tanks. Preferential attack of welds is evident.
We believe the problem is associated with overchlorination, followed by decomposition, of the recirculating hypo/excess caustic solution.
We are trying to identify the corrosion mechanism: is it from heat of decomp and/or corrosive products formed after decomp? Also believe free radicals may be released in the above reactions - do these try to combine with the PVC? Can anyone help please?





RE: Corrosion of PVC in Hypo/Cl2 duty
Typically PVC has reasonably good performance. There are some better materials available however. If you are simply trying to get the most life out of the existing equipment, then controlling the process is key. When you suggest overchlorination, it sounds like control is not good. However, you suggest that you do have excess caustic. As long as the circulation rate is high and the cooling is sufficient, I'd be surprised at a problem unless there was a problem with the materials (i.e fabrication or intital material quality).
RE: Corrosion of PVC in Hypo/Cl2 duty
www.viatec.com
RE: Corrosion of PVC in Hypo/Cl2 duty
With more information, we can help provide an answer. Normally, PVC is an extremely compatible material at ambient conditions. What is the material of construction of the packing? Has it seen the same degradation of the liner? If not and its a thermoplastic, perhaps you should consider that for the lining material.
Let me know if you need more help or want to handle it off-line.
ghopkins@rl-industries.com