Difference in Anods calculation to protect Carbon vs Stainless steel

[davidet68]

Hi,

I need to calculate the quantity of anodes to install inside a pressure vessel. Project specifications ask to use a current density of 100 mA/sqm for the kind of water flowing inside the vessel without any indication about vessel material. The vessel shell is Carbon steel and it is internally coated therefore we have used a reduction factor of 0.08 for the coated surfaces, but we have some internal plates (welded to the shell) which are not caoted. The material of these plates is AISI 316. Is it correct to use the same figure of current density (100 mA/sqm) both for carbon steel and stainless steel or current density depends also from the material of the wetted surfaces?

Thanking you in advance,

Davide

 

[SJones]

It will simplify your calculations to stick to one current density. Resultant over protection on the stainless steel will not do it any harm.

As an aside, leaving the stainless steel bare whilst in electrical contact with the coated carbon steel is not a good idea with regard to bimetallic corrosion effects should the anodes deplete in service.

Steve Jones
Materials & Corrosion Engineer

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