Hi Everyone,

Our client has asked us to use Teflon coated UNISTRUT SUDNUT (see attached). Since these STUDNUTS are standard items and are available only as Hot Dip Galvanized, we have decide to get these STUDNUTS as Hot dip galvanized and then get it coated with Teflon (Xylan).

My doubt is that is it a good practise to coat Teflon over Hot dip galvanized bolts.
Also since during Hot dip galvanizing they will already undercut the nut threads , after Teflon coating we will have to undercut the nut further more.Since the Teflon coating will be done at a different supplier how will he know or control the undercut , is there any standard we can refer to.

Appreciate you help

Unless someone has made a break through it will not work. The only way would to strip the HDG and apply the Teflon. One more undercut and you will have no threads. If quantities are good enough Unistrut may supply uncoated components.
You will have to work with Unistrut, not your local distributor.

What is the purpose for the Xylan coating? Corrosion resistance? Friction reduction? What size fasteners? You have some options, just provide some more info.

@corypad ,These are used inside of desalters where there is high salt content. The client has asked specifically for this coating. Basically this coating is for corrosion resistance. The bolt sizes are M12

For water service I would use either SS or Monel. HDG is just a short term stopgap in the situation. The Xylan will allow corrosion at any wrenching points and on the threads.

According to UNISTRUT technical documents (http://unistrutconstruction.com/pdfs/S04_hardware....), the zinc coating is not hot dipped galvanizing, rather it is electrogalvanizing (or electroplating) that meets ASTM B633 Type III Class SC 1. This is a very thin coating (~ 5 micrometers thick), which will have limited corrosion resistance. Adding a Xylan coating will improve the corrosion resistance greatly, and the total coating thickness of zinc plus Xylan will be just fine for M12 fasteners.

@corypad, thanks mate, i have the same datasheet but i totally missed the material details on the first page.
You reply was very helpful, but my doubt is whether TEFLON will adhere to Zinc coated bolt.

Corrosion of fasteners in desalination plants have been a big issue, even with the variety of stainless steel solutions that have been tried. Does your client have prior experience with Xylan coated fasteners in this environment?

@hokie66, i guess they have done some research. The requirement for Teflon coating is from their standard documents/specifications, which i believe have been complied from proven research and code.

Thanks to all you guys @CoryPad, @unclesyd, @Screwman1 for all the information.

Xylan over zinc plating is very common.

@corypad i found this very nice article regarding xylan coating over zinc

http://www.appliedbolting.com/pdf/Coated_Bolts_and...

http://www.whitfordww.com/pdf/literature/Fastener%...'s%20oil.pdf

You have two different types of corrosion protection. The zinc plating is decorative but gives sacrificial corrosion protection. The teflon is a barrier protection designed to exclude elctrolyte from creating a corrosion cell. In this application no better than a coat of grease or paint.

As far I know Xylan is PEEK not TFE. It came to prominence in the North Sea oil industry. I have had experience of poorly applied Xylan in desaliantion plants where it had no strength and simply was abraided in use and made matters worse.

If there are wet areas and need a barrier protection then painting is after assembly is just as good. If the client wants Teflon this is available in spray packs. If the areas are dry then there is no corrosive cell and not additional protection is required. beware of considering stainless steel as a "solve all". The effect of stainless steel fastenrs in connection with a zinc surface where an aelectrolyte is present could result in corrosion.

With ss fasteners you have the added problem of galling of nut and thread.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/

There are multiple forms of Xylan. Some are PTFE in a PAI binder while others use an epoxy binder. There are spray and dip-spin versions, and solvent-based or water-based.

A modern version that is applied correctly will provide very good corrosion performance.

I'm a little confused and maybe wrong about the HDG question. In my old paper literature they don't specifically mention HDG either yea or nay. They do site Zinc, elctro, is as good candidate for coating. Also the use of conversion coatings, zinc phosphate , is good primer. In the new literature they stated that their primers are better than the conversion coating.