Corrosion underside tank bottom

[Kungkmutt]

Regarding the tank covered with insulation, The service is hydrocabon that service temperature is around 60C to 80C. I found corrosion underside/ leaking through bottom plate during tank shutdown.The corrosion was created from underside because the topside steel surface quite smooth and there was no scale, no significant metal loss (MFL evaluation). The tank is on concrete ring wall with sand cushion wiht leak detection.
Can anyone stimulate the situatuion of underside corrosion occurrence incase the CP is still active?

 

[KnaT]

There can be several reasons depending upon the site conditions and tank design.

A probable cause might be the failure of bottom underneath coating.

 

[Kungkmutt]

Thank you KnaT for kind advice. Is coating of bottom underneath required normally? Do it affect CP?

 

[chicopee]

I have been involved with a bottom leak of an AST made by a food can that had been left by one worker when the tank was erected. It took a couple of decades to notice the leak has it traveled underground and reached the basement of a home downhill from the tank. The impression where the corrosion occured was perfectly circular.