corrosion rate on buried carbon steel 1

[Nomismum]

I am designing subsea steel structures protected by an indium aluminium CP system. The structures are supported on mudmats which have their undersides and skirts buried in the soil. The area buries has not been painted but has been included in the CP calculation as defined by DNV.

The protection of these areas has now come into question as the values in DNV may only be to cover current drainage and not protect.
I am wondering what the corrosion rates are for buried structures at 1km below the MSL.
I do not want to place zinc anodes on the underside of my mudmats unless there is no other option.

Thanks

 

[dhengr]

Look at some of the weathering steels, they build up a self protecting, self healing, skin of oxides as they weather. They worked pretty well as long as the atmosphere or surroundings were not abrasive which would keep eroding this protective oxide formation away. If the mud isn’t moving (like sandpaper or blasting sand) or active (chemically) once the mat is placed, these might help. Look at ASTM A242, A441, A588, I think those are the right specs. They have atmospheric corrosion resistance 4 to 7-8 times better than regular carbon steels. Carbon steel with copper has some improved corrosion resistance too, but not as much. I would assume the steel producers and suppliers would have some testing and literature on this matter.

 

[GregLocock]

Shipwrecks would give you some idea of corrosion rates.

Cheers

Greg Locock


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[SJones]

Your problem is caused by the terminology used in RP-B401, wherein they mix up 'current drain' and 'design current density'. The document gives the details of designing protection of buried items, so it can be assumed that the 0.020 A/m2 is to achieve the stated protection in sediments.

Steve Jones
Materials & Corrosion Engineer

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

 

[Nomismum]

Thanks everyone for taking the time to respond.

Steve,
I have the same interpretation as you for the dnv, and so my design fulfils the requirements,
however, is it possible that the underside of my mudmats is shielded? (mudmats have holes for installation).
...And if it is shielded, how much corrosion would be experienced? (at 4deg, low oxygen environment)

Thanks.

Alison

 

[SJones]

You will note that there are default values of sediment resistivity to be used in the design which implies that current will flow through the sediment to reach the buried steel. Clearly, this will be the case as the sediment is not a solid insulator.

Steve Jones
Materials & Corrosion Engineer

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