Difference between Electrogalvanized and Hot Dip Galvanized Steel

A. Hi Frank. I don't think I give either side a vote of confidence :-)

A spec calling for zinc electroplating of fasteners to a thickness of 30 microns doesn't sound very practical to me. I've seen fasteners plated to half that thickness and it took 75 minutes plating time and brought on some problems like nicked threads. 25 microns is a lot when it comes to zinc plating of fasteners and is the maximum for "very severe" conditions per ASTM B633. Maybe 12-13 microns of zinc-nickel plating would be a more reasonable spec?

I think hot dip galvanizing or mechanical plating would be more practical, but the ID of nuts is often not galvanized anyway, and mechanical plating of internal threads sounds problematic.

Just because we've never heard personally of something doesn't mean it's impossible, but I've certainly never heard of trying to hot dip galvanize onto a zinc electroplated and chromated surface, and my strong suspicion is that it cannot work. Your contractor may be planning to cold galvanize (spray or dip-spin with zinc-rich paint) on top of the plating. That's probably possible but doesn't sound like a very good idea or robust finish to me.

Regards,

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

A. Hot dip galvanizing is not a coating in the usually accepted understanding of the word, rather an alloying reaction between zinc and iron.
If there's no contact between the iron and the zinc then HDG doesn't happen.
When re-galvanizing a piece, the old galv has to be removed in order to reveal the steel (iron), and without that all that happens it heating of the piece due to its immersion in the molten zinc.
I've never tried galvanizing a zinc plated item, but can't see that working. I think it would be a mess.

Further, to claim a control over the process in order to state the thickness of galvanizing like that sounds suspicious (unless it were stated as "about 20µ" or similar). 20µ is very thin for HDG, and an unlikely thickness.

I think you are right to be concerned.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

A. Hi Jay. Geoff says no, and I totally believe him. Please explain your situation. Thanks.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 2014

A. Hi Jeremy. Neither process bonds by any type of "tooth" or mechanical "keying", but rather by metal-to-metal chemical bonds. So I would expect perfect adhesion if processed perfectly. I suspect that the parts are not 100% free of every trace of dirt, oil, rust, mill scale, and oxidation.

I assume that these components are then electrocoated and/or powder coated? -- because bare edges on exterior automotive parts doesn't sound satisfactory.

Regards,

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

A. Hi Rajani. Hot dip galvanize might or might not be acceptable, but I can't imagine electrogalvanizing lasting more than a few days under seawater.

Regards,

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

A. Hi Simon. The description of an item for sale is designed to entice you to buy, not to technically inform you. There is no such thing as "Hot dipped galvanised electroplated steel".

Regards,

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

A. Hi Kiran. ASTM B633 for zinc plating was already given on this page. If you are applying the plating for no reason except to be able to claim that you complied, then .00015" - .0002" is probably sufficient; but please try your best to clarify your situation because "the minimum thickness requirement" for satisfactory performance clearly depends entirely on the specific application. Thanks. Good luck.

Regards,

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

A. Surely not, Mohamed. Galvanizing is always applied to steel substrates -- and they are highly paramagnetic.

Regards,

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

A. Hi Friends,
Great difference between Zinc Electroplating & Hot Dip Galvanizing.
1. In Zinc Electroplating there is NO ALLOY layers in between Zinc & Steel whereas Hot Dip Galvanizing has an alloy layer in between.
2. Zinc Electroplating is normally thin plating and for cosmetic purpose while as Hot Dip Galvanizing is really thick plating and for metal protection.

Umesh Dalela
- Sana'a, Yemen

A. Hi Sandy. If the fencing is PVC coated, the underlying layer of zinc is much less critical and zinc electroplating should be sufficient. If it is not PVC coated then I think you will find anything less than hot dip galvanized to be unsatisfactory.

Just an opinion, but the people who can safely buy from China are the big purchasers who can visit the factory, maintain a QA system there, etc. Your approach is often cynically called "throwing it over the wall", and I personally think a consumer is usually going to find that it was a big mistake to try to purchase 100 meter rolls of fencing direct from China.

Regards,

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

A. Hi Reza. I've explained the usual differences in appearance, but sorry, I have no such set of pictures prepared, although there are numerous pictures of both finishes scattered across this site if you search for them. Readers are welcome to submit their picture sets if they wish. And you could start it off by uploading pictures of items on which you'd like to identify the coating.

But I don't think it will be easy! There are several different types of zinc electroplating (acid/alkaline, dull/bright, barrel plated/rack plated), and they nearly always get a subsequent chromate conversion coating (hexavalent / thin film trivalent / thick film trivalent with topcoat / no topcoat), available in a number of colors -- plus the coating looks different depending on the surface it is applied to. In a few cases it's lacquered. Hot dipped coatings may be before fabrication or after, sometimes wiped or spun thinner. Even an expert looking at the actual object rather than a photo would probably only know most of the time rather than all of the time :-(

Let me relate an anecdote. I was asked by one of my country's most famous art museums (a modern one) to consult on a couple of issues involving metal items in their collection. At one point I expressed my hesitation about visual identification; I noted that after reading the labels on their collections of nickel plated and silver plated holloware, I had lost confidence that I could reliably tell the difference between nickel and silver plating by eye anymore. To which the curator responded that if I felt a piece that was marked nickel looked like silver, or vice versa, they could just change the label. What? ... WHAT???!! I assumed there was a written history going back a hundred years or more, plus careful metallurgical examination to confirm ... only to learn that people had in many cases just guessed :-)

Guessing from appearance simply isn't the way!

Regards,

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

A. Hi Rafael. Powder Coating is probably better, assuming a water desalination plant has a salty air environment. But it wouldn't hurt to have electrogalvanizing under the powder coating to provide sacrificial protection in the event of scratches or porosity.

Regards,

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

A. Hi Pete. Good questions.

1 & 2. The plating shop will probably not scratch the components, but less care will be exercised before they are finished, so they may get to the plating shop with scratches -- and electroplating will not hide them. If they are highly decorative you can perhaps use those blue plastic films that are used on appliances for the brakeing application.

3. If you can get good appearance with reasonable economy from HDG, that's the way I'd go. When hot dip galvanizing is done on coils, as opposed to after fabrication, it is common to "wipe" it to a thinner smoother look. Zinc electroplating is usually less expensive.

4. Regarding the "self-healing", there may be two different aspects conflated together here:
a. Zinc coatings from any deposition method are cathodic and sacrificial to steel ... with the implication that even if there is a scratch, pinhole, or porosity, the zinc will protect the steel from rusting. Other coatings like paints are neutral in this regard, not protecting the scratched area. Still other coatings like nickel plating are anodic to steel and the steel will vigorously rust from a scratch in an attempt to protect the nickel plating.

b. Zinc coatings are often protected from white rust via a hexavalent chromium conversion coating. This is indeed "self-healing", but RoHS regulations are causing hexavalent chromate to fade into history and be replaced by trivalent chromium conversion coatings which, in the opinion of many (I don't know of any detailed study in the literature) do not provide self healing at this point. But I have heard presentations about nano coatings (still in development rather than production) that do self-heal. If my take on this is dated, I'd appreciate the update.

Regards,

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

A. Hi Arya. People in the general zinc electroplating business don't actually call the process "electrogalvanization", they call it "zinc plating" or "zinc electroplating". The "electrogalvanization" term is pretty much used only by continuous coil/strip platers.

In the old days of say 40 years ago, virtually all zinc electroplating was done from a cyanide-based formulation, and the variations were mostly restricted to high cyanide, low cyanide and mid cyanide :-)

Then in the 1970's "acid zinc" was developed and marketed, which had the advantage of less toxicity and greater brightness (acid zinc was occasionally substituted for nickel-chrome plating), but the disadvantages of requiring much better precleaning, not offering as good uniformity of coverage, and being quite corrosive to the building and equipment. Again there were variations like ammonium chloride vs. sodium chloride based. I believe ammonia -based formulations have largely fallen out of favor because their tendency to complex metals makes waste treatment difficult.

The most recently developed and marketed process was the alkaline non-cyanide process. This offers the more uniform coverage of the cyanide process, without the toxicity. These usually have the one extra complication of requiring "generation tanks" -- a side tank with zinc anodes in it (because if the anodes are left in the main tank they dissolve in excess, but a high concentration in the side tank is not a problem, because you can just feed as much or as little of this into the main tank as needed.

Both acid zinc and alkaline non-cyanide are widely used today, with cyanide zinc plating being used in ever less amounts. Acid zinc tends to be used more for barrel plating, and alkaline non-cyanide for rack plating. I don't get out in the field as much as I used to, so I always appreciate updates if anyone disagrees.

Regards,

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