DNV-RP-B401, XYLAN coating breakdown factor

[KMSymington]

Hi

I'm working on the design of a subsea product which is protected from corrosion by Cathodic Protection and paint/coatings.

I'm currently working through the DNV-RP-B401 calculation. Some of the component is coated with Xylan 1070, 20-50 microns thick. Am I right in thinking that the mean coating breakdown factor for this coating would be 1 i.e. equivalent to bare metal?

Regards

KMS

 

[SJones]

How did you arrive at that thinking since the Xylan could be considered to be at least a Category I coating?

Steve Jones
Corrosion Management Consultant

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

All answers are personal opinions only and are in no way connected with any employer.

 

[KMSymington]

Hi Steve

Sorry-I missed out all the assumptions...

Design life=30 years

Depth=less than 30m

Assume that Xylan is a category 1 coating,so a and b=0.1

Mean coating breakdown factor=a+b*design life/2, which equals 1.6 but max is 1.

I think this means that the Xylan is not really considered to help as far as the calculation is concerned.

Is there any data out there for Xylan that shows that is better than a category 1 coating?

KMS

 

[SJones]

The manufacturer is most likely to have the data. 20-50 microns is pretty thin - that would probably not even make a bolt coating and probably wouldn't offer too much in terms of long term corrosion protection. 1 is looking a reasonable bet.

Steve Jones
Corrosion Management Consultant

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

All answers are personal opinions only and are in no way connected with any employer.

 

[CoryPad]

Not sure if this is a fastener application or not, hopefully the following helps.

The Xylan coatings applied to fasteners are in the range of 20 to 50 micrometers, even for the thicker spray applied versions used in offshore environments. Long-term corrosion resistance of iron/steel components usually requires a sacrificial layer (e.g. zinc rich) plus an organic topcoat like Xylan. For subsea and cathodic protection, that changes things a bit. NORSOK M-001 says:

For submerged applications, where dissolution of a thick zinc layer may cause loss of bolt pretension, phosphating shall be used. For sub-sea installations the use of poly-tetra-fluoro-ethylene (PTFE) based coatings can be used provided electrical continuity is verified by measurements. Cadmium plating shall not be used.

 

[KMSymington]

Thanks for the sanity check guys-that's the answer I was expecting.