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CP Calculation- Coating Breakdown Factors

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JT100

Mechanical
Feb 19, 2007
71
I am currently designing some anodes for cathodic protection of flexible pipe. I am trying to calculate the mean current requirement and I have the current density and the surface area however I am struggling to find the coating breakdown factors for the end fittings. They are coated in a Nickel Kanigen coating system. I havent come across this before (havent done cp for flexibles before), and I cannot find a value, or even a mention of this system, in any of the standards I use- DNV RP F103, RP 106, B401, ISO 15589 or in Norsok M503. Can anyone direct me to some more information regarding this coating system, or even if it is similar to a coating system that is considered in the standards- FBE for example? Thanks.
 
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It's a form of electroless nickel plating which means, for anode consumption calculations, you would have to treat it as 100% 'broken down' although the potential difference value could be altered if the free corrosion potential of the system in the seawater at location were known. Was it the end user's intention that it be considered a direct substitution for CRA weld overlay?

Steve Jones
Materials & Corrosion Engineer
 
thanks steve, yeah the end fittings are duplex and they plan on coating them with this nickel kanigel for hisc etc reasons. so can i treat it as having no current reducing properties?
 
ok but how would i treat it as 100% 'broken down'? do you know whether it can be taken as an equivalent coating in the standards, i have previously taken the 'a' and 'b' constants that are referenced in the DNV standards and then calculated a cbf using:

cbf = a + b*t

where t is the design life.
thanks,
jt
 
It's a metallic coating. It doesn't have the insulating capacity of an organic coating - so, it's like zero coating, i.e. 100% broken down. Unless you have some detailed electrochemical data for the coating, I don't think you have any option but to adopt the design criteria for 100% exposed surface area of duplex stainless steel as per, say, ISO 15589-2.

Steve Jones
Materials & Corrosion Engineer
 
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