Need Alternatives to Galvanizing for Power Line Bolts, Nuts, Lock Washers

A. Hi, Yivgeni, welcome to finishing.com. An important thing to remember is that "500 hours of salt spray" is not a proper qualification for a project of this nature, i.e., selecting between potential coatings. The purpose of salt spray testing is to insure that the coating process is under control and consistent from batch to batch, not to anticipate real world life.

The mechanism of corrosion in a salt spray chamber is so different from the mechanism of corrosion in the real world that it is fair to say that there is no correlation at all. Some coatings like galvanizing derive their good real-world corrosion resistance primarily from the formation of stable corrosion products containing small amounts of compounds that cannot form in short-term salt spray chamber tests. In the real world, galvanizing outperforms (by decades & decades!) coatings that may beat it in a salt spray test. I'm not sure that you should give up on galvanizing (which has proved itself from the very beginning as the ideal coating for pole-lines & towers) before making sure that the thickness consistency issue cannot be solved. Despite salt spray ratings, I doubt that anything else at all will even approach the real-world corrosion resistance -- but you won't know for at least 25 years :-)

Geomet and Dacromet are proprietary zinc-rich dip-spin coatings. There are competitive brands but let's restrict the discussion to generic types of coating rather than claims that one brand is better or worse.

Another generic type of coating which would be thinner than hot-dip is mechanical plating / mechanical galvanizing. And still another along the same idea would be zinc diffusion coatings similar to Sherardizing.

Finally we have electroplated coatings. These are usually much thinner, and plain zinc electroplating probably would not do. But zinc alloy platings like zinc-cobalt, zinc nickel, and tin-zinc might. Of these, tin-zinc offers good lubricity (I believe it would be better than anything else mentioned here). You would need to find a good RoHS-compatible hex-chrome-free conversion coating, but I think this could end up a good solution. It is possible to add thread lubricant to other coatings, but when the service truck has been out in the mud on the fire road for 14 hours you'd like to not have specified an overly dainty repair protocol.

After looking into these, and ideas that may be offered by other readers, please come back with specific questions as they arise. Good luck!

A. Silver plated stainless steel fasteners will provide everything you want and more.

A. The problems encountered with galvanizing perhaps stem from a lack of process specifics.
Get ASTM A394, 'Standard Specification for Steel Transmission Tower Bolts, Zinc-Coated and Bare.'
Bolts, before zinc coating, shall have coarse threads with Class 2A tolerances per ASME B1.1. The zinc coating may be either hot dip galvanizing per ASTM A153 Class C or mechanical plating per ASTM B695 Class 50, except with minimum coating weights as per Paragraph 4.4.6 of ASTM A394.

Suitable nuts for Type 1 bolts are Grade DH per ASTM A563 with zinc coating specified. Nuts are to be coated together with the bolts by the same process. After coating, nuts shall be tapped oversize per Table 5. ASTM A563 allows specifying a dry film lubricant when ordering nuts.

Suitable flat washers are zinc-coated Type 1 hardened steel per ASTM F436. Split ring lock washers are ~obsolete (proper bolt pre-loading is preferred); you may wish to use load indicating washers per ASTM F959, 'Standard Specification for Compressible-Washer-Type Direct Tension Indicators for Use with Structural Fasteners.' These can be ordered with mechanically plated zinc per ASTM B695, Class 50.

Thanks Jeffrey, that sounds like a great idea.

Thanks Ken -- what an extraordinarily great posting. We've missed you lately, friend!
Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Dear sir,

I noticed your post from August and wondered how you are doing with your search. We have just finalized a process that is now being used by Centerpoint Energy in Houston. We coated thousands of fasteners for them over the past weeks as Houston rebuilt.
I would love to tell you more, as this process has been tested for over 5 years.