Duplex crevice corrosion 1

[SteveyB]

Hi All,

I was wondering if anyone could give some advice on a potential component issue I have which is made from UNS S32760. Basically I have a component that will be part of a subsea manifold consisting of several intricate parts and contains multiple small gaps and sealing faces. The component itself will be connected to a portion of the manifold which has a max working temp of 40 deg C, but is situated next to pipe work that has fluid passing through it at a max temp of 120 deg C. My question is will crevice corrosion be the most likely form corrosion? Are these temperatures of any concern to Super Duplex and should another material be possibly considered?

 

[SteveyB]

Any advice on this would be much appreciated. Thank you!!!

 

[SJones]

It will be if the gaps and sealing faces are exposed to seawater and are not hooked in to CP. If they are exposed to CP, then you will have other issues to deal with.

Steve Jones
Materials & Corrosion Engineer

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

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

 

[SteveyB]

Hi SJones,

Thanks for your response.

As it is a subsea manifold I presume there will be CP present. So I am guessing HISC may be the dominant corrosion mechanism, but as the component it open to the seawater I wouldn't of thought this would be a problem. Also the gaps are so small that I don't think CP would even penetrate that far due to the high current density of Duplex and the increased resistance of the small gaps.

The reason I initially questioned crevice corrosion is due to a statement in the Norsok material standard M001. Stating that "seawater systems with crevices: Max. operating temp. 20°C, max. residual chlorine 1.5 ppm. Max. operating temperature 120°C in saliferous environment." Therefore it is my understanding that most seawater has this amount of chlorine. Hence I am unclear about this statement as to the suitability of Duplex in this application.

Would I be better with a Inconel material, say X750 for example?

 

[SJones]

The component is, or isn't, open to the seawater? Don't worry too much about the NORSOK standard as that is dealing with seawater on the inside of the pipe and the chlorine comes from hypochlorite treatment. The 120 deg C is for atmospheric exposure, not subsea. For external, submerged seawater exposure, here is a quote from EEMUA 194:

"While it is generally agreed that the 22Cr grades require painting and CP in order to avoid localised corrosion, especially at crevices, the 25Cr grades do not strictly require CP if the operating temperatures are low, typically less than 45 deg C. However, it is not usually practicable to electrically isolate individual items......, hence 25Cr duplex components are also usually painted and cathodically protected."

Inconel 625 would remove most of your worries.

Steve Jones
Materials & Corrosion Engineer

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

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

 

[SteveyB]

Yes the component is open to seawater i.e. not contained or shielded.

So for my understanding when the Norsok standard relates to saliferous environments. It is relating to splash zones etc?

I find that most of these standards are open to ones own interpretation and often unclear, hence my misunderstanding.

So as for Inconel 625, this material should be safe uncoated in this environment. Also will it potentially cause any effects with connected duplex?

Thank you again for your advice. This is much appreciated.

 

[SJones]

Welcome to the world of wishy washy materials selection standards. To be fair, it is impossible to globally standardise a process that has so many subjective elements to it. The 'saliferous environment' is generally taken as the atmospheric zone of an offshore facility, or a coastal marine site.

As to any bimetallic issue, that will be dependent upon the nature and geometry of the junction, the access for CP, and the area ratio of the exposed materials. For example, a flanged joint between the two has a reasonably even area ratio and would allow CP access, resulting in a low probability of bimetallic corrosion.

Steve Jones
Materials & Corrosion Engineer

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

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

 

[NickClark]

A well designed CP system will prevent and galvanic and crevice corrosion in any connected materials. For duplex steels follow DNV-RP-F112 Guidelines and you should be OK. This is a bit of a generalisation but if it weren't close to the truth we would have big issues with all installed subsea equipment