Anode sizing for various materials

[DeepBlues]

Hi. I'm doing a comparison of several metals to determine which should be used for a subsea component. How would I calculate the size & quantity of the anodes to be used on the component? I'm looking for the generic equations, I can fill in the required data.
Thanks!

 

[SJones]

DNV RP B401 which can be downloaded at:

http://exchange.dnv.com/BxWmWeb/Tas...&WCE=PMA_6_812!816&uid=ID20050316196738869241

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[DeepBlues]

That's exactly what I needed, thanks! The DNV credibility will go a long way with my clients as well. From one Steve to another - thanks again.
-Steve

 

[AndrewLindsay]

Steve,

While the type of steel will influence the number of anodes, by far the largest factor will be the amount of exposed steel.

The selection of the coating system to minimise the amount of anodes is going to be the best option in most cases. This is also important when you are looking at complex structures and nodes. On an uncoated system, the ability to protect complex nodes is reduced if there is a large current drain affecting the attenuation of current to the structure.

It is also important to note that in many systems used subsea, you rarely only have the one type of steel, with stainless steel components being electrically continuous to carbon steel. In those situations, the carbon steel protection requirements tend to be the driver.

 

[DeepBlues]

Thanks Andrew. I was really looking more at needing to protect an Aluminum structure if connected to a steel one. The carbon steel structure will be coated.

 

[AndrewLindsay]

Steve,

If you are trying to protect Auminium, then you are going to have to go for something like magnesium as the sacrificial alloy. When you start looking at Mag as the alloy for subsea systems, you are then looking at whole areas of concern, such as hydrogren embrittlement, interference/draining of the CP system to other parts (especially if this is a pipeline component such as a PLEM or protection frame). I would strongly suggest that you try to change out the Aluminium for some other material, if you can't, then you need to isolate the aluminium. Also, I would avoid painting it, because if the aluminium is painted and you get some connection to the steel, the aluminium will pit very rapidly at any areas of coating breakdown.

Regards

Andrew