Cathodic protection of high strength steel

[tljames]

Hi all,

I am looking to cathodically protect a subsea fabrication which includes a number of Hardox 400 plates (supplier determined, currently being manufactured so too late to look for an alternative) in an otherwise S235/275/355 fabrication. Hardox 400 has a minimum yield strength of 900 MPa (nominal 1000MPa) and a hardness of 40.8 HRC so I am concerned about stress corrosion cracking and hydrogen embrittlement.

The fabrication is to be coated with a 3 layer paint system but there may be some areas that will be hard to get to and may be left uncoated. The fabrication is bolted on to other fabrications and so it cannot be isolated.

So far I have learnt that keeping the CP potential at around -0.8V will prevent overprotection and limit the rate of hydrogen production. I understand that research has been done into low voltage anodes but I was unable to find them available anywhere commercially. I’m looking into whether diode controlled anodes are applicable.

Any advice would be much appreciated. Does anyone know more about the progress of low potential anodes? Ideally I would do some testing to determine the susceptibility to HE, however the fabrication is due to be finished by the end of the month.

Many thanks

 

[SJones]

It will be difficult to undertake a CP design with 'low voltage' anodes since the CP design codes do not address them. The cracking risk will be driven by the stress level and the presence of welds. HRC 35 is the typical limit imposed to address hydrogen embrittlement. DNV-RP-B401 states that the use of diodes or lower potential anodes 'has limited practical experience'.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04