finishing.com -- The Home Page of the Finishing Industry
no popups, no spam
HomeFAQsBooksHelpWantedsForum letter 51980
Serious Education ... plus the most fun you can have in metal finishing.

Finishes for Bolts on Galvanized Structural Steel

adv. pointer    

April 9, 2009

Q. We are a structural steel fabricator/erector out of Detroit Michigan.
We are in the planning phases of a structural steel project. The entire steel structure will be hot dipped galvanized. We would like to use plated ASTM A325 [link by ed. to spec at TechStreet] bolts, in lieu of our standard bolts which are usually painted after the structure is bolted and assembled.

All the bolts will be exposed to the elements, but not a salty environment. (West Virginia) Which is the best system to use?

Hot dipped would offer best corrosion protection, but will gum up threads, and proper tensioning of bolts is critical in our industry.

Zinc Plated-These should have less problems with tensioning bolts, but more susceptible to corrosion.

CAD Plated-will cadmium be compatible with the rest of the zinc galvanized structure, I understand cadmium will be the best for lubrication of the threads.

Are there any other downfalls I should think about?

Patrick Bell
Steel Erector - Detroit, Michigan

April 28, 2009

A. Mechanical Galvanizing with zinc offers good salt spray (better than hot-dip), good thread profiles, and no hydrogen embrittlement. With a wax, it gives good lubricity. That's why a lot of structural fasteners are done by this process.

tom_rochester
Tom Rochester
Plating Systems & Technologies, Inc.  

Jackson, Michigan, USA



April 30, 2009

A. Zinc plating is OK; the least expensive. Cadmium is very good, but not many platers offer cadmium plating due to the regulations. It is highly toxic. I suggest zinc-nickel alloy + a conversion coating. (Zn-Ni is very corrosion resistant even without a conversion coating.)


Don Baudrand
Consultant - Poulsbo, Washington

(Don is co-author of the
book "Plating on Plastics")


May 4, 2009

A. If the parts are Heat Treated, doing the electrical plating is Risk due to Hydrogen embrittlement. to avoid the risk better do some electrolytic method coating like mechanical zinc plating or zinc-aluminum coating

Kannan Boopathi
- Salem, Tamil Nadu, India


June 19, 2009

A. Choosing ZnNi alloy plate with ~12% Ni and a tri-Cr based seal coat (aka conversion coat) will give good performance and will keep your product "green" versus the choice to go with Ni-Cd or Cd coated parts.

Note that AMS2417 [link by ed. to spec at TechStreet] makes no mention of tri-Cr and the most common Type II coating specifies use of a hex-Cr seal -- again, don't go there if you want a "green" stamp. There are numerous coating suppliers using a tri-Cr finish with excellent results and depending on the specific requirements for your parts, barrel plating may be an option. Barrel plating tends to make for a very inexpensive coating operation, though even if rack plating is required, it should still be most viable in regards to cost.

Best of luck,


Thomas Hanlon, Materials Engineer
aerospace finishing - East Hartford, Connecticut, USA


June 19, 2009

A. Hi Patrick,

This thread has a lot of time gaps in it, so maybe it's too late to consider more options. But...

Hot-dip galvanizing would be good. Bolts are done by cutting the thread diameter undersize so that the hot dip coating brings them up to size - this is an established practice. Nuts can be galvanized as blanks and threaded afterwards, which avoids the possibility of excess zinc not draining from the internal thread. The absence of zinc on the nut threads is of no consequence, as they are in contact with the zinc on the bolt threads.


Bill Reynolds
   consultant metallurgist
Ballarat, Victoria, Australia

We sadly relate the news that Bill passed away on Jan. 29, 2010.


June 22, 2009

A. I would look into a zinc rich dip/spin coating. Specifically you should look into GEOMET, Magni or Dorken. I know that the GEOMET is approved for structural bolts, I am not sure about the others. These finishes offer excellent corrosion protection at a very thin film thickness.

David J. Wolf
Medina, Ohio


August 17, 2009

A. Patrick,

Check out my article in my reply to letter 52457. For your application, I recommend hot dip galvanizing, but from a good spin operation. There are a few good spin lines in the USA, which will provide the required bolts.

Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota


September 25, 2009

A. I would recommend hot dip galvanized coating. ASTM A153 [link by ed. to spec at TechStreet] clearly states that the threads must be clean enough to allow a nut. There are a number of galvanizers around the country qualified to do this job. If hydrogen embrittlement is an issue, have the bolts mechanically cleaned and avoid the acid bath in the galvanizing process.

David Jaye
Houston, Texas



August 3, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. What is the difference between Geomet coating and Hot dip galvanizing? Are Geomet and Zinc flake coating the same? What are the advantages of both? I'm buying some Geomet plus bolts.

Mohammed Afeef
- Kannur, Kerala, India


August 2017

A. Hi Mohammed. Geomet is one brand name of zinc flake coatings. We don't compare specific brands against each other on this site, but you can contact the vendors for their sales info about why they think their zinc-rich dip-spin coatings are better than their competitors'. Hot dip galvanized coatings are generally more corrosion resistant and much thicker; that thickness is probably an advantage on large fasteners and a disadvantage on small threaded fasteners.

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"

ADD a Q or A to THIS thread START a NEW THREADView This Week's HOT TOPICS

Disclaimer: It is not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices


©1995-2017 finishing.com     -    Privacy Policy
How Google uses data when you visit this site.