cathodic protection calculation

[davecu73]

I am calculating the number of zinc anodes required for a sea chest on a vessel. The problem I am having is the current density value.
Looking through old literature states that a value of 10 - 15 mA/m^2 is acceptable for current density for new build ships.
However, I was told to use 30 - 50 mA/m^2 by another source. This would double the number of anodes!!!

Does anyone know the appropriate values for current density on commercial vessels. Or can point me to a source of info. I know this value changes as the protection scheme deteriorates but I need a value to provide the appropriate protection.

cheers

 

[JRMacGregor]

On bare steel you need a mean current density of about 90 mA/m2. It is less in warmer waters.

But most marine CP specs refer to coated (or part coated steel). That is where rates like 5 or 15mA/m2 come from (they are based on the total area).

Of course it not coated steel that needs protected but the uncoated part of it.

If you have 16% coating breakdown, then 90mA/m2 on bare steel is equal to 15mA/m2 on the steel made bare by coating breakdown.

I think this is where the confusion may come from.

 

[mshimko]

Most "criteria" or guidelines do not specifically state the assumed amount of bare paint.

For flowing seawater (which applies to a sea chest), I assume 125ma/ft^2 (significantly higher than JR above). In reality, though, other considrations will factor in, such as expected service time befor changeout, how frequently is the vessel drydocked, etc.

 

[davecu73]

Thanks for the info.

The cathodic protection is to be designed for a period between dry docking of 6 years.
This typically has been 3 - 5 years in the past
There will be a paint scheme in place and the Cathodic system is additional probably to take into account paint lost in service.
The method of calculating has been based on the total area required to be protected.
I'll go back to the anode supplier.