isolation of Stainless steel bolts and mild steel plates

[nisha1980]

hi all

Can anyone give me some ideas to desperate these two materials to avoid the risk of bimetallic corrosion .

Application is external steel frame roof all roof sections are mild steel flat bars and near coastal area.

I would suggest nylon washers. similarly can we have the nylon sleeves for bolt hole ?

or Any other ways to isolate this ?

Thanks in Advance

 

[FLCraneBuilder]

I believe you are referring to "galvanic reaction"
I encourage you look up a "galvanic scale" that presents these two materials.
My long ago recollection is that they are fairly close on the scale and hence not very reactive
I also note that this is a roof top installation and hence usually dry (?) Wet environments promote such reactions. dry mitigates..

next, are there other installations in the same area that are experiencing problems?

if all of this leads you back to wanting protection, I suspect the nylon (or perhaps rubber) washer is the way to go

 

[bones206]

You can get washers with sleeves. Sometimes called isolating washers or sleeves/ shouldering washers/ gasket washers/ insulating washers, etc.

 

[Settingsun]

Is this what you're after? This page also gives the various other names you help you search. I'm not sure why they don't crush when the load is applied though.

https://www.supfast.com.au/washers-and-spacers/nylon-shoulder-insulating-washers/

Taking a step back, will your mild steel be ok against corrosion given you've decided you need stainless bolts?

 

[nisha1980]

Thanks everyone for your replies .

 

[kingnero]

If the difference between quantity of materials (eg, stainless bolts vs. mild steel bars) is large enough, there might be no need to do anything. However, in coastal regions, this might be just enough of a bad environment to not follow the above advice.

 

[Tomfh]

Stainless steel Fasteners in regular/galvanised steel plate often perform fine without any sort of protection, but adding a separator is a good idea if there’s any concern whatsoever. Especially if it’s thin plate.

An important point to consider is which item will be corroded. With stainless steel fastener in zinc/iron plate it’s the plate which corrodes, and the (relatively) small fastener only has so much ability to destroy it, particularly if it’s a decent sized plate.

But if it’s the other way around - galvanised fasteners in stainless plate - it’s the fasteners which corrode. The plate will destroy them far more quickly, particular small fasteners. I’ve seen this with galvanised TEK fasteners in aluminium. The fasteners were completely gone within a year.

I’ve also seen cases where a stainless steel nut was used on galvanised bolt. The nut corroded the crests of the bolt threads, and effectively destroyed the threaded connection. The result was similar to a bolt with stripped threads. You don’t need a huge amount of corrosion to wreck a thread.

 

[HouseBoy]

Has anyone used epoxy paint as a separator?
I suppose there would be some scraping off of the paint (but was wondering).

 

[MJCronin]

Exactly the same situation exists for piping systems.....

There is no reason to try to electrically isolate the joint ...

Stainless steel fasteners have been used with carbon steel piping flanges for decades.

There is no SIGNIFICANT electrical potential difference to cause galvanic corrosion in most industrial situations ....

Perform a site-wide search on this matter.

Pay particular attention to the posts in the Corrosion and Piping forums

MJCronin
Sr. Process Engineer

 

[kingnero]

Agree, however what is "coastal area", is this 20 feet or 20 miles from the coast?
I would strongly advise against the use of (austenitic) stainless steel bolts in this situation if 20 ft from salt water.

@ Houseboy, painting is particularly bad, specially when you're coating the mild steel, as a damaged coating "forces" all corrosion to happen in that particular place.

 

[MJCronin]

Nisha,

You have not given us the reason for your selection of stainless fasteners.

Are you selecting these because you do not want corrosion due to coastal effects of salt water ?

If so, that would be a mistake, IMHO ....

Hot-dipped galvanized fasteners will give you adequate service.

How are you protecting the adjoining mild steel bars ????

For ultra-reliable (but expensive) corrosion protection, consider a PTFE coatings system such as XYLAN

Please respect us and complete this thread ..... tell us what bolting system you finally select.

Regards

MJCronin
Sr. Process Engineer

 

[trustthemodel]

Just wanted to chime in and mention AISC design Guide 27 or ASCE 8 may provide some guidance for the specific application you are considering.

From what I recall most recommend insulating washers such as neoprene between stainless steel and carbon steel.

However, as others have mentioned, the materials being connected and their galvanic reactivity may determine whether this is necessary.