Corrosion of bolts on pump casing

[midris]

Hello all,

We have 4140 bolts on a SDSS pump casing on an offshore installation. A concern was raised regarding the possibility of galvanic corrosion occurring between the bolts and casing due to the high humidity and marine environment.

My understanding is that galvanic corrosion initiates in the presence of an electrolyte (i.e. very high humidity will not initiate galvanic coupling). Perhaps atmospheric corrosion of the bolts is the actual concern.

Moreover, a proposition was made to galvanize the bolts. My understanding is that hot dip galvanizing 300 grade stainless steels is not a good idea as it will peel off. How about galvanizing 4140?

Appreciate your thoughts,

midris

 

[cloa]

I think galavanised parts are to be avoided due to the chance of Liquid Metal Embrittlement. Galvanise 300 series and you'll destroy their mechanical properties. And you have a high risk of Hydrogen Embrittlement of steels with strength of 140 psi. Why don't they just paint the bolts e.g. High Build Expoxy or even better use Duplex Bolts which have lots of strength and great fatigue properties.

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[SJones]

It will be atmospheric corrosion of the bolts, but it will be exacerbated by the galvanic interaction with the large amount of SDSS. High humidity could result in water condensation which will be enhanced by the presence of deposited salt.

According to API 610, the use of 4140 is allowed 'unless otherwise specified' in spite of showing SDSS as the first choice. Looks like there was no otherwise. 4140 is used for B7 bolts and that is routinely galvanised.



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]

Galvanizing carbon steel bolts can be a good choice, but not if it is in contact with stainless steel. Either use a stainless steel alloy for the bolts or use a non-metallic coating like those provided by Xylan or Magni.

 

[midris]

Thank you all for your feedback.

Steve: thanks for shedding light on the issue of galvanic corrosion in this application. Would you therefore suggest coating the galvanized 4140 bolts as a preventive measure against galvanic corrosion?

cloa and CoryPad: I have forgot to mention that coating the bolts with Xylan or Takecoat as with the carbon steel bolts has been considered. Thanks for your suggestions.

 

[SJones]

No I wouldn't suggest coating the bolts alone because that will make things worse by focusing corrosion at coating damage. If you are going to coat - coat the whole lot, SDSS and all, after assembly.

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.

 

[andygo11]

Interesting topic. I just am not clear about one thing. It was said galvanized parts should be avoid. But the zinc-related liquid-metal embrittlment usually happen at high temperature service or welding. If the bolt is galvanized and used for the pump with offshore installation, is there actually any corrosion issue because of the zinc embrittlement?

 

[CoryPad]

You are correct that elevated temperature is required for zinc-related liquid metal assisted cracking. The problem with zinc coatings in contact with stainless steel is that the zinc will be consumed rapidly (just like cathodic protection anodes used in offshore applications). So the zinc coating does not protect the carbon steel material for long, and the corrosion process may generate hydrogen that can lead to hydrogen assisted cracking or stress corrosion cracking of the carbon steel bolt if it has susceptibility.