stainless steel bolts on carbon steel valves and equipment

[kevlar49]

I am contemplating the use of stainless steel bolts on carbon steel valves in sweating service in the alky. Operations has been having this problem on control valves, block valves, and other equipment. I don’t believe the issue is external Cl SCC as the services are cold, however, I am concerned with both crevice and galvanic corrosion. What is your opinion? Galvanic corrosion may be minimal due to the large anode to cathode surface area ratio. I could use a Teflon washer, but then I’d be concerned with crevice corrosion; Teflon is used for crevice corrosion tests. Would Teflon-coated carbon steel bolts be better? I have heard that they may not last long if the Teflon coating is scratched off during rough installation.

 

[Metalguy]

Can you use 410 SS bolts?

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[CoryPad]

What is the environment? Temperature range, fluids, other?

Based on the somewhat limited information available, I think PTFE coated bolts would be an excellent suggestion.

There is more than corrosion to consider. If you were considering austenitic stainless steel, then you need to consider the differences in thermal expansion coefficients.

What would rough installation do? Damage the coating on the drive surfaces? That seems to be of much lesser concern.

 

[TVP]

kevlar49,

I am a little confused: is the current setup C steel bolts w/ C steel valves, or is it SS bolts w/ C steel valves? What is the current failure mode that operations is concerned about? Corrosion of the bolts or of the valves? In general, you would use the same material for both, and use coatings for corrosion protection if necessary.

 

[cloa]

This is what kevlar wrote for sweating service in another thread-
colder than ambient conditions allowing moisture to diffuse into the insulation in an alkylation unit.

 

[kevlar49]

Metalguy,

yes 410 is a good idea. thanks!

Corypad,
External environment is moisture condensed from atmosphere because equipment operates at colder than ambient temperature. Hence Sweating service. Internal may contain sulfuric acid/caustic. Concern might be for packing leaks.

Rough handling could damage the coating in general, but possibly mostly on the head when tightening.

TVP,
original is cs bolts/cs valve body. Issue is corrosion of the bolts.

 

[Metalguy]

ASTM A-193 B6 bolts are 410 SS. Suggest you specify the hardness to be on the low side, not more than Rc26 (Rc22 if you don't need the strength).

I think B6M are softer than regular B6, and have more resistance to SCC/HC. Fairly inexpensive bolts and will match the carbon steel for thermal expansion.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[CoryPad]

I think a fluoropolymer coating on a carbon steel fastener could be considered for this application.

 

[TVP]

I agree with the suggestion of a fluoropolymer coating on carbon steel fasteners. This is a very common coating for large bolting. The following NACE publication is a good reference for common fastener coatings:

NACE 02107 Coatings for Protection of Threaded Fasteners Used with Structural Steel, Piping, and Equipment

 

[Guest102023]

I like 410SS as a bolt material. I don't trust polymer coatings on fasteners, having witnessed what can happen to those, especially if these will be regularly disassembled in future.

Plastic washer is an oxymoron - you will not obtain the necessary bolt tension with that in place.