necessity of painting tank bottom plates

[siamak60]

Hi everyone
As you know corrosion protection of storage tank bottom plates are prohibited by two way:1)applying coal tar epoxy or bitumen on external surfaces of bottom plates and 2)cathodic Protection by impressed current method and in Design Criteria it is assumed that half surface of Bottom Plates are bare. Now one of our contractor believes that this is a wrong decision because due to welding bottom plate to each other,paint in adjacent areas of weld line is burned and therefore(opinion of our contractor) an electrical cell will be generated.
Now my questions:
1)Do you know any history of this type corrosion ?(corrosion occurred due to incomplete painted external surface of bottom plate which create Corrosion cell )
2)How validity of these thesis can be proved or rejected ?

Thanks in advance

 

[gr2vessels]

The cathodic protection is a well known method of preventing galvanic corrosion of the steel tanks bottom. The need arrises from the existence of a closed electrical circuit (galvanic circuit) between the bottom plates and the soil charging/stray currents/tank static charge/ etc. For closed circuit you need conductive moisture (lots!) under the tank. If you insulate properly the soil/ring beam from the tank, you don't have closed circuit, ergo no galvanic corrosion. Nevertheless, at the edge of the tank the rain will tend to ruin your protection plans, generating eddies which are hard to prevent or isolate/insulate. That's where the cathodic protection is usefull.
As far as the welding concerns, all the welds end up continuos joints, hence there is a continuous exposed surface. The paint might burn, but again, if there is no moisture, there is no corrosion!
Cheers,
gr2vessels

 

[SJones]

it is assumed that half surface of Bottom Plates are bare

If complete coating is undertaken, the assumption will be very conservative for designing CP to deal with coating degradation caused both by welding and by paint ageing. Protection of the bottom plates will be monitored by potential measurements and some means of incorporating reference electrodes under the tank should be in the design. It is not uncommon to run a perforated plastic tube under the tank through which an electrode can be pulled. What form of anode is being used in the CP system: remote or close grid type?

Steve Jones
Materials & Corrosion Engineer

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