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HOT DIP GALVANIZE VS. ZINC PLATE

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Q. CAN YOU PLEASE GIVE ME AN IDEA ON WHICH IS MORE CORROSION RESISTANT AND THE DIFFERENCE BETWEEN THE TWO PLATINGS: HOT DIP GALVANIZE AND ZINC PLATE?

Richard D. Ochotorena
- Hong Kong


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A. This question has been asked and answered many times, Richard, so I'll keep my answer brief; you can search the site for longer answers if it's too brief. Essentially, "zinc is zinc", and the corrosion resistance of zinc coatings is roughly proportional to their thickness. When you want a thin coating, electroplating is usually the most cost effective way to get it; when you want a thick coating, galvanizing is usually the most cost effective way to get it. So ... galvanizing is considerably more expensive and corrosion resistant than zinc electroplating in most cases because it is significantly thicker (say 5-10x as thick, maybe more).

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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Q. At this facility we produce Safety Relief Valves. As part of this assembly we use carbon steel bolts with fine threads which we now currently protect with an electrodeposited zinc cobalt alloy with a yellow chromate conversion coating. Standard coating thickness for our fine thread fasteners is 0.00015 - 0.00020 inches. This coating must have a neutral salt spray per ASTM B117 [link is to spec at TechStreet] resistance as follows:
No white corrosion(except at corners & edges), nor red rust visible to the unaided eye at normal reading distance for the following test times, white corrosion - 140 hrs, red rust - 300 hrs.

Background: The above process was selected to replace our previous electrodeposited cadmium plating process back in 1994 when EPA & OSHA regulations regarding the handling, use and disposal of cadmium products became cost prohibitive. We also selected this process so as not to require any changes in machining tolerances due to excess plating build up or thickness as is normally the case with hot dip zinc galvanized coating.

Question: We have customer now requesting use of Hot dip Zinc Galvanized coating on bolts. We have supplied our Zinc Cobalt Plating process for their review and now they are asking for direct corrosion resistance comparison between our Zinc Cobalt process and their requested hot dip Zinc Galvanized coating. Do have any such comparison data or can you direct me to someone who may have ASTM B117 salt spray results for hot dip zinc galvanized coating at the 0.00015 -0.00020 inch thickness we now use for our Zinc Cobalt plating process? I am not sure if hot dip Zinc galvanized coating can be applied, even with the new "spin" application, at the 3.1 to 5.1 micron thickness we use for the Zinc Cobalt plating.

Any information you can provide on above specific comparison will be appreciated.

Thanks,

James Lee Luksa
valves & controls - Stafford, Texas

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A. Please sit down with your customer because I think it's more likely that they are confused than enlightened. A galvanized coating of that thickness, if it could be achieved, will not compare favorably with the corrosion resistance of your zinc cobalt plating. I don't have salt spray ratings for galvanized coatings in my library, and certainly not for coatings of such unheard of thinness, but galvinfo.com/GINotes/G_Note15.PDF [Ed. note: that URL is no longer functional] says 10 hours per micron, making it 30 to 50 hours to red rust.

The advantage that galvanizing usually has over electroplating is thickness. Take that away and I can't imagine hot-dip galvanizing competing against zinc-cobalt as a thin coating. Zinc-cobalt is an alloy coating with a carefully chosen potential designed to protect the steel while corroding less than plain zinc; a plain zinc coating of the same thickness won't not match it in corrosion resistance.

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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February 11, 2009
Sherardizing is a thermochemical process where the steel parts to be treated are heated in contact with zinc dust and thereby allowing the zinc to be absorbed onto the steel substrate via diffusion to form a sherardized coating. Based on metallography, sherardized coating consists of zinc-iron alloys enriched in zinc in the outer layers that decreases with increasing iron content closer to the coating to substrate interface. The primary role of sherardized coating for the protection of steel from the corrosive environment is to form a protective film and act as a sacrificial anode. The sherardizing provides excellent corrosion resistance to a variety of metal parts including those with fairly tight tolerances.
Dmitry Dzhurinskiy
- Windsor, Ontario, Canada


Thanks for reminding the readers that electroplating and hot-dip galvanizing are not the only two ways to apply zinc-rich coatings to a steel substrate, Dmitry, although they are the most common. There is also the sherardizing that you mention, plus mechanical plating, dip-spin zinc-rich paint coatings, and flame spraying.
Regards,
pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


March 30, 2009

A. Hot-dip galvanizing (HDG) bolts with fine thread is not recommended. The zinc will fill in the threads and likely cause great problems in assembly. The mating internal threads will have to be tapped oversize to accommodate the heavy galvanize thickness, but how much is anyone's guess. HDG of fine thread bolts is not a common practice and is typically applicable to coarse thread bolts/studs 1/4" diameter and larger.

Bill King
fastener company - Carson, California



August 27, 2009appended

Q. Our company has been in the stainless fabrication business for 40 years. We are competing against a metal part that is not stainless steel.
I am trying to understand the difference between bright zinc electroplating and galvanizing. Is there a difference? My understanding is that the process is basically the same for both except that galvanization is a dull finish and for thicker parts coating and zinc electroplating is a brighter finish for thinner parts coatings.
Is this correct?
Thank you in advance for your help.

John Coiro
manfacturer - Allentown, New Jersey


September 4, 2009

A. Yes, John, that is correct, but to summarize in case you've made any assumptions that you haven't stated:

- Galvanizing is done by immersing the parts in molten zinc; whereas electroplating (sometimes called electrogalvanizing) is done in an aqueous bath by applying electricity to the parts such that zinc is dissolved into solution from zinc anodes and reduced onto the parts as zinc metal.

- Although the thickness of galvanizing varies depending on the thickness of the steel and some compositional factors, and although electroplating thicknesses can be varied by keeping the parts in solution a shorter or longer time, galvanizing tends to be maybe 5 to 10 times as thick as electroplating.

- The corrosion resistance is closely proportional to the thickness of the zinc coating.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


April 25, 2012

Q. Why is zinc coating called Galvanizing?

Rashid Ali
- Lahore, Punjab, Pakistan


April 26, 2012

A. Hi Rashid.

SOME zinc coating is called galvanizing. Not all zinc coating is called galvanizing.

Sorry, I don't know the origin of the term, but my guess would be that it's derived from "galvanic protection processing"

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

April 14, 2013

Q. Would you compare Hot dip galvanizing and zinc electroplating process for the purpose of steel sheet used in car (auto body)

Milad Fadavi
- New York, USA


April 14, 2013

A. Hi Milad. Since you are speaking of autobody sheet metal, you are probably speaking about something that will be painted, rather than remain bare and unpainted. In this case, less galvanic protection is required because the whole car body is electroprimed and painted.

And then the equation starts to shift away from what coating offers the best corrosion protection toward what coating can be most successfully painted. And at that point car manufacturers moved to "galvanneal" which means electrogalvanizing followed by an annealing process which renders the coating more paintable.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

June 16, 2013

Q. Dear sir,
I want to electroplate zinc on copper wire which should be white enough either by hot dip or electroplate, kindly send me both methods along with recipe.

Shakeel Khan
- Lahore, Pakistan


June 17, 2013

A. Hi Shakeel. I have not personally heard of hot dipping of zinc onto copper wire, although I have seen tin hot dipped onto copper wire. Electroplating of zinc onto copper is certainly possible, but again I must wonder because it is tin that is usually electroplated onto copper wire. Each subject fills entire library shelves, cousin, so no one can possibly put sufficient info into an internet forum response to enable you to do the process. But you can view the Metal Finishing Guidebook online. Best of luck.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
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