Guidelines for galvanize bolts

[yahoo123]

If the nuts and bolts are galvanized, should the threads on the nut or the threads on the bolt be tapped?

I remember reading that the female threads on the nuts should be tapped after hot-dipping such that all the zinc coating is removed from the thread. If so, would it still be necessary to tap the coating on the male threads of the bolts?

By the way, a Simpsons rep told me zinc coating and galvanizing is two different things. Can somebody tell me the difference? Which would you use for bolts and nuts?

Thanks,

 

[hokie66]

I understand that the threads of the nuts (or the bolts, not sure which) are made to allow for the zinc, before the galvanizing. So no need to tap the nuts or die the bolts after galvanizing.

Hot dip galvanizing is one type of zinc coating, and the best type for many applications. But one limitation is that the thickness of the zinc in the finished galvanized product is directly related to the thickness of the base material, so thin material doesn't get much coating, and other processes give better protection.

Coatings commonly used for priming steel are inorganic zinc silicate, which requires near white metal preparation and provides excellent protection; and zinc filled epoxy primers, which are somewhat more forgiving of preparation, thus applicable to recoating projects.

Knowledge of coatings among structural engineers is in most cases woefully lacking. Most of us had no education about coatings, and have had little training since. Such an important facet of the work we do deserves better.

 

[PackerFan]

A typical zinc plating is only 5 microns thick, hot dipping is around 50 microns thick. There is a huge difference between the two and how long they will last in a corrosive environment. In exterior application, a good rule of thumb is one micron a year, interior 0.1 micron a year. So you can do the math to see if zinc or hot dipping is what you need.

 

[yahoo123]

How is the process difference?

 

[CoryPad]

Electroplating zinc uses a liquid electrolyte containing zinc ions. Then, electrodes create an electrical circuit that causes the dissolved zinc ions to migrate to the parts. The thickness can be controlled very tightly, with typical coating thicknesses of 5 [µ]m to 15 [µ]m.

Galvanizing involves immersing the parts in liquid zinc. This produces thick coatings (> 50 [µ]m).

Regards,

Cory

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[yahoo123]

How about the mechanically applied spray on zinc coatings?

 

[CoryPad]

Yes, mechanical plating is a third type (the least common of the three). Here is a description:

http://www.pfonline.com/articles/100103.html

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[yahoo123]

Can somebody explain the advantages/disadvantages of zinc coating, hot dip galvanizing, and mechanically applied.

How is zinc coating applied different from mechanical plating?

Thanks!

 

[hokie66]

Galvanizing is generally thicker zinc, thus the best.

Don't know whether coating or mechanical plating is better.

As to coating, think paint.

 

[CoryPad]

Zinc electroplating
Advantages: produces thin coatings (prevents problems associated with clearances between external and internal threads); many plating companies (low cost, high availability)
Disadvantages: produces hydrogen during processing, so high strength parts must go through deembrittlement process; toxic chemicals used in this process; only suitable for indoor/light duty applications

Hot dip galvanizing
Advantages: produces thick coatings needed for outdoor exposure
Disadvantages: requires more zinc (high cost); large thickness requires more care with thread dimensions before and after coating to insure no problems with mating parts; high temperature of molten zinc will temper (reduce strength) of high strength parts

Mechanical plating
Advantages: produces thin coatings similar to electroplating; does not produce hydrogen during plating, so it is preferred for higher strength parts (like hardened washers)
Disadvantages: fewer suppliers, which limits availability

There is a fourth type of coating: coatings containing zinc flakes/powder. These use either organic or inorganic binders, have thicknesses of approximately 5 to 30 micrometers, may contain friction modifiers to control the torque/tension performance, and have corrosion resistance greater than plating. These coatings are applied by immersion in liquid suspensions of the binder+zinc followed by spinning (to spread the coating evenly over the part surface) and then curing in a furnace. These generally do not produce hydrogen embrittlement.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[yahoo123]

Much appreciated for the explanation!

 

[HgTX]

ASTM A 563 (nuts for use with structural bolts) says:

Hot-dip zinc-coated nuts shall be tapped after zinc coating.

Mechanically deposited zinc-coated nuts for assembly with mechanically deposited zinc-coated bolts shall be tapped oversize prior to zinc coating and need not be retapped afterwards.

Threaded components (bolts and nuts) shall be coated by the same zinc-coating process.





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[yahoo123]

Can I safely assume that any steel that is silver/gray in color have some kind of zinc coating since plain uncoated steel is black?

 

[hokie66]

No, it could just be painted with a coating containing no zinc.

 

[CoryPad]

Uncoated steel is not black, it is gray. Black is magnetite, which is an iron oxide produced by exposure to high temperature.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[yahoo123]

So A36 steel is black because of exposure to high temperature?

 

[CoryPad]

Yes, when it was hot rolled.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[HgTX]

And if you knock off that black mill scale, it's grey. Likewise if you break open a black bolt, it's grey.

The first time I ever saw freshly blasted steel, I thought it had been painted silver.

Hg

Eng-Tips policies: faq731-376

 

[yahoo123]

cool thanks

 

[Tmoose]

http://www.holo-krome.com/pdf/techbk24-29.pdf

Table 24