Carbon Steel Bolts for All Alloys 1

[nickypaliwal]

Is carbon steel nuts and bolts with PTFE coat suitable for all alloys suchas SS316, DSS (22% Cr), SDSS (25% Cr), Alloy 825 and Alloy 625?

Our client accepts CS nuts and bolts even without coating for piping flanges and asks for CRA nuts and bolts used for valves.

Also SS316 for non-wetted nuts and bolts are accepted for DSS/SDSS valves. I believe it should be fine as they may not face any issue.

Please comment.

 

[CoryPad]

Carbon steel fasteners, even with PTFE coating, have problems when used with more passive materials like stainless steels and nickel alloys. Galvanic corrosion is an obvious problem, as is general corrosion. Both of these are made so much worse due to the unfavorable anode:cathode area ratio. Differences in thermal expansion coefficients is another tricky problem when elevated temperatures are involved, which leads to higher stresses.

The use of type 316 stainless fasteners with duplex stainless steel is not as problematic if they non-wetted as you state.

 

[nickypaliwal]

Thanks for response.

Since I am working for Oil and Gas Clients and thermal expansion may not be a problem as the operating temperature is generally low (below 70 degC).

Also regarding galvanic corrosion,most of the facilities are in the dessert, do we still expect galvanic corrosion as there may not be moist conditions.

 

[CoryPad]

I cannot answer the question regarding your environment. Why were duplex steels chosen? Presumably for corrosion resistance. If that same corrosive environment can contact the fasteners, then galvanic corrosion can occur.

 

[SJones]

The client's piping classes for duplex stainless steel offer the option of either bare A193-B7 or with it electroplated and PTFE coated. Clearly, the latter option is the expected choice for a C5-M marine environment. The former option has been in the piping classes since day 1 and reflects the client's perception that they have acceptable experience with uncoated low alloy steel fasteners when the exposure environment allows the same. It may be worth questioning the client as to whether this perception is actually holding up in practice, particularly in some desert locations that can intermittently be exposed to maritime influences under certain weather conditions. The client's materials selection specification does state that "CRA bolting should be considered where the piping is CRA, where long life is needed, or where access for inspection is difficult or impossible." However, given the current price of oil, it is likely that the client's considerations lay elsewhere.

It should also be noted that the client is not alone. Process Industry Practices, Piping Class PIP PN03SD1B01, 2008, for 316L piping also shows bare B7s for fasteners.



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.

 

[MJCronin]

The OP actually asks a very complex question about a wide variety of material combinations and environments.

I don't think that his particular question can be answered in a single thread.

There have been many, many questions about nut and bolt selection and applications in these fine engineering forums.

CoryPad states:

Carbon steel fasteners, even with PTFE coating, have problems when used with more passive materials like stainless steels and nickel alloys

Several PTFE (and other)Bolt coating systems are available for a variety of offshore and industrial duties.

http://www.metcoat.com/xylan-coatings.htm

http://www.metcoat.com/assets/files/fk12200hrsaltfogtest.pdf

I have specified and used bolting with XYLAN coatings for decades..... I beg to disagree with that statement



MJCronin
Sr. Process Engineer

 

[nickypaliwal]

Also , Can we use bar material, forged material for bolts ? Is it not compulsory to follow bolt specification ?

 

[CoryPad]

PTFE-style coatings like Xylan and Fluorokote are excellent coatings, and used widely for offshore and other applications. But any bare spots/cracks/etc. in the coating can lead to localized corrosion. If that bare spot is touching a passive metal like nickel, then the corrosion will be even worse. Of course the performance depends on the exact materials, and part geometry, and the environment. But, it is not a leap to say that problems can exist with this approach. Norsok M-001 Materials Selection is one standard applicable to this type of situation, and it provides good guidance for galvanic corrosion.

 

[XL83NL]

The one issue we find with PTFE/Xylan coated bolts is that the coating will wear out if the bolts are torques few times. When this happens, weak spots may initiate corrosion, when they are in contact with more passive materials. You could therefore argue that PTFE coated need to be replaced with the gasket whenever a BFJ is opened/disassembled.