High Pressure Corrosion resistant studs needed

[BexBex1000]

Hi Everyone.

I am currently building stainless steel pumps which are used in high pressure (up to 200 bar) applications and mounted on environments such as Oil platforms in the oceans. At current the main fasteners I am using are high tensile steel such as 12.9 grade. The pump is then painted with maritime paint usually to customer specifications. The problem is that the environment is getting through the paint and attacking the bolts and I fear is weakening these bolts. Can anybody recommend to me any grade of material for the bolts which give me better resistance to the environment but at the same maintain the tensile strength? I have thought of materials such as Inconel 718 but this is very expensive and I was hoping for some other suggestions?

Kind regards
Anthony

 

[EdStainless]

There are a number of high strength marine fasteners used. Yes, they are all expensive.
Alloy like K-500 Monel are traditional, alloys such as 625, 255 and 2507 have also been used successfully. There are also some high strength Cu/Ni alloys that have been used.

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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[CoryPad]

Titanium fasteners are excellent for chloride corrosion resistance, but expensive, though probably not as bad as the nickel alloys.

Stainless steels, even some of the highly alloyed ones, may need cathodic protection, which is why Ni and Ti alloys are used in spite of the higher material costs.

By the way, painted 12.9 is likely a disaster waiting to happen.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TVP]

As the others already mentioned, Ni-based and Ti-based alloys offer the best corrosion resistance but at the highest price. The superduplex alloy SAF 2507 (UNS S32750, EN 1.4410) that EdStainless mentioned is one good option for fasteners exposed to seawater, but I am not sure that it can reach the 12.9 property class requirements. The superaustenitic alloy 254 SMO (UNS S32154, EN 1.4547) is also an excellent choice, and is offered in fasteners with property class 12.9 properties. I suggest contacting Bufab Stainless AB (

http://www.bufab-stainless.se/eng/index.htm)

and asking them about their Bumax 129 grade using 254 SMO. I also concur with CoryPad that using painted 12.9 high-strength steel fasteners is a disaster waiting to happen. I would replace those fasteners as soon as possible, because stress corrosion cracking is going to occur.

 

[EdStainless]

You might also contact Langley alloys.
The duplex grades (2507 and 255) will reach higher strengths than the 6%Mo alloys.

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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[BexBex1000]

Thank you for all the information above, this is of great interest. Would any type of coating such as PTFE make the 12.9 bolts suitable?

 

[EdStainless]

No, coatings would only be suitable in applications where the chances of getting contaminated were slight. In your case you know that you will have have seawater exposure. A coating will always have weak spots.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[BexBex1000]

Thank you all gentlemen, with your help I have decided upon Super Duplex grade UNS32760 which gives me more than enough tensile strength to cover the applications i will encouter normally and also suitable corrosion resistance.

 

[malmosnino]

You may use DACROMET 320 in 3 thin coats and PLUS for top-coat.
DACROMET is a zinc flake coating, when applied, thickness is about 10 (ten) microns.

For more details, see:

http://www.dacral.com

 

[CoryPad]

Property class 12.9 screws coated with Dacromet 320 and PLUS topcoat in a marine environment? I think the only way that would be a good solution is if every step were controlled as closely as possible. That would mean no bulk processing (which induces coating damage), hydrogen embrittlement testing of every lot, routine inspection of the installed fasteners, and likely removal/replacement at specified intervals (e.g. 1 year). The risk is not worth it, in my opinion.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.