Letter 34079

Galvanized vs. electroplated [Bahrain]  

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Dear sir,

We have to supply some fabricated structural members along with fasteners.The client's specification for the fastners says that HM 8.8 BOLTS, NUTS AND WASHER-Galvanized.

This instant could I supply these fastners which has been galvanised by electroplating method. And please advice what is the difference between the hot dip and electroplated.

Thanking you

Velayudham Ravishankar
structural fabricator - Manama, Bahrain, Kingdom of Bahrain

Hot-dip galvanized coatings are heavier than electroplated zinc and will exhibit greater corrosion resistance. It is never permissable to make a substitution without approval, however, because one never knows for sure the intents of the specifier. For example, zinc electroplating is also susceptible to whiskering whereas galvanized coatings are not believed to be. If these items are intended for a computer room, you could have great liability.

Ted Mooney, P.E.  finishing.com Brick, NJ

I agree with Ted.

Hot Dip coatings are usually thicker, better bonded and will probably perform better than electroplated zinc, which are commonly more used for cosmetic purposes only. We find these fasteners are only well protected in stored conditions. For structurable fabrications, if atmospheric corrosion is your concern, I would get the client to agree on a design change.

But does it ever actually rain in Bahrain?

Martin Rich
- Plymouth, Devon, UK

Is it really the case that electroplated zinc coating is mostly cosmetic?

Would you also offer your opinions on fasteners in ACQ-treated wood in outdoor structures? Does the particular copper-rich formula of ACQ accelerate corrosion of galvanized nails, screws, and bolts?

I use 1/4" to 3/4" diameter bolts, mostly 3/8". I use either hot-dipped or electroplated zinc bolts. Should I not use electroplated any more?

Thanks.

Michael Cohen
Community Built Playground Design - Ithaca, NY USA

Zinc plating is fine for some applications, mostly indoors, but it's corrosion resistance isn't enough to provide much corrosion resistance in rough outdoor service. The ACQ manufacturers have specific recommendations about fasteners but, yes, copper in the wood will accelerate corrosion due to the galvanic couple.

Ted Mooney, P.E. 
finishing.com
Brick, NJ

Mr Ravishanker

Hot dip galvanising puts a thick zinc layer all over the part and does protect well. There are Electro Galvanising ( electroplated ) Zinc iron alloys that perform very well and compete with Hot dip galvanised coatings for Corrosion tesistance. These bear a look .

Regards,

Asif Nurie Atlanta Global Limited New Delhi, India

I HAVE RECEIVED SOME 3 METER CHANNEL CROSS ARM WHICH IS ELECTRO GALVANISED. DUE TO HEAVY RAIN IT IS GETTING CORRODED. WHAT MEASURE I HAVE TO TAKE FOR STOPPING CORROSION AND ANY ZINC SPRAY WILL HELP TO STOP THE CORROSION. IS ELECTRO GALVANISED OR HOT DIP GALVANISED GETS CORRODED FAST. AWAITING YOUR EARLY REPLY WITH SOLUTION TO RECTIFY THE DEFFECT.

N.JAYAKUMAR
- TRIVANDRUM, KERALA, INDIA

Investigate replacing it, N. As already stated in this thread, electrogalvanized is usually inadequate for outdoor service.

Ted Mooney, P.E. 
finishing.com
Brick, NJ

I'm under the impression that the thickness of the zinc coating is based on the standard or procedures for the specific galvanizing process. For example, a hot dipped galvanized piece could be galvanized to a lesser zince coating thickness than an electrogalvanized piece. (Typically not the case, but as an example.) Are my assumptions correct? I've spent a little time researching this...Also, the thickness of the steel must have an effect on the thickness of the zinc coating, right? Is or are there any studies or information documenting this?

David Riley
Contracting - Piscataway, NJ

The thickness of electroplated zinc on hardware is typically controlled at anywhere from from .0002" to .0006" according to specific needs, David. The upper limit is usually based on practicality in that it takes over an hour of zinc plating time to reach .0006" in barrel plating and it becomes uneconomic. There is no real lower limit, but the sloppy and dangerous "specification" of "commercial zinc plating" is often interpreted to mean .0002". The thickness of the steel has no effect on the thickness of electroplated coatings.

Galvanizing is never that thin. Typical thicknesses are .0015" to almost .004". In general you can't really control the thickness; rather it depends on the surface condition of the steel. Thicker steel takes longer to heat up and usually takes a heavier coating.

In all, it may be theoretically possible for an exceptionally thick electrodeposited zinc coating to be as thick as a very thin hot dipped coating, but it is nearly unheard of.

Ted Mooney, P.E.  finishing.com Brick, NJ

Can we use electroplated Binding wires for construction purpose or is it better to use hot dipped only???

Prasad A. Haridas
- Pune, Maharashtra, India

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