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Cadmium Coatings

Cadmium coatings are applied to iron, steel, brass and aluminium and give excellent resistance to corrosion in most conditions and especially in marine and alkaline environments. Cadmium, like zinc, also provides sacrificial protection to a substrate such as steel by being preferentially corroded when the coating is damaged and small areas of the substrate are exposed. In addition to corrosion protection, cadmium coatings provide a low coefficient of friction and therefore good lubricity, predictable torque characteristics, good electrical conductivity, protection from galvanic corrosion (in particular when in contact with aluminium), easy solderability, low volume corrosion products and reduced risks of operating mechanisms being jammed by corrosion debris for many components in a wide range of engineering applications throughout industry.

Cadmium coatings are particularly useful in the electrical, electronic, aerospace, mining, offshore, automotive and defence industries where they are applied to bolts and other fasteners, chassis, connectors and other components.

Electroplating accounts for over 90 per cent of all cadmium used in coatings but mechanical plating and vacuum or ion deposition have some commercial significance. The coating is normally specified in thickness' between 5 and 25 mm depending on the severity of the atmosphere. Chromate post-treatment of the coating can increase coating life.

(a) Electroplating

Cadmium is electrodeposited on the metal article from an electrolyte solution of cadmium salts in barrels or vats. These electrolyte solutions are nearly always based on the alkaline cyanide system. Other solution types are used, such as those based on fluoroborates, but these have not proved popular as they lack the excellent combination of brightness, covering power, throwing power and wide operating parameters of the alkaline cyanide system. When a current is passed through the electrolyte, cadmium is electrodeposited on the metal article at the cathode and cadmium from the anode goes into solution. Large or delicate articles are attached to racks and vat-plated whilst small components, such as bolts, washers, nuts, springs and clips can be vat-plated in drum cages or plated in a rotating barrel.

(b) Mechanical plating

This process uses mechanical energy to deposit metal coatings on small components by the impact of glass beads. Either cadmium or mixed-metal coatings of cadmium-tin or cadmium-zinc can be applied when glass beads, proprietary chemicals, water and metal powder are tumbled with the components in a rotating barrel. The process is suited to components such as fasteners and clips which are small enough to be plated in a barrel.

(c) Vacuum and ion deposition

Conventional thermal vapour deposition involves heating of cadmium in a vacuum until it vaporises. Cadmium atoms then condense on the substrate to form a thin high quality coating of

cadmium. Ion deposition in argon atmospheres adds more energy to this coating process and uses 'sputter cleaning' to clean the substrate surface. As a result, ion deposition is said to give improved coating adhesion, density and uniformity. Components such as undercarriage legs of transport aircraft, helicopter rotor parts (as shown in Fig. 1) and other high strength steel components have been successfully coated using this method.

Galvanized vs. electroplated bolts, nuts and washers

adv.

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Dear sir,

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

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

Thanking you

Velayudham Ravishankarstructural fabricator - Manama, Bahrain, Kingdom of Bahrain

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Hi, Velayudham.

Hot-dip galvanized coatings are far heavier than electroplated zinc and will exhibit greater corrosion resistance. It is never permissible to make a substitution without approval because one never knows the intent of the designer. For example, zinc electroplating is also susceptible to zinc whiskers whereas galvanized coatings are not believed to be. If these items are intended for a computer room or hospital life-support equipment, for example, you could have great liability if you substitute. Good luck.

Regards,

Ted Mooney, P.E. RETfinishing.comBrick, New Jersey

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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 structural 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

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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 resistance. These bear a look .

Regards,

Asif NurieAtlanta Global

New Delhi, India

+++++

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

http://www.finishing.com/consultants/atlantaglobal.shtml

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 Cohenplayground design - Ithaca, New York USA

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Zinc plating is fine for some applications, Michael, mostly indoors -- but it's corrosion resistance isn't enough for rough outdoor service. The ACQ manufacturers have highly specific recommendations about fasteners (most likely saying that electroplated fasteners are not acceptable but hot-dip galvanized fasteners may be); yes, copper in the wood will greatly accelerate corrosion due to the galvanic couple.


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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 FASTER. AWAITING YOUR EARLY REPLY WITH SOLUTION TO RECTIFY THE DEFECT.

N.JAYAKUMAR- TRIVANDRUM, KERALA, INDIA

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Investigate replacing it, N. "Electro galvanized" is a euphemism for zinc electroplated. This is usually inadequate for outdoor service. But the "cold galvanizing spray [linked by editor to product info]" that you mention (which is actually zinc-rich paint) is pretty good at deterring corrosion. It is not nearly

Cold Galvanizing Spray

as attractive as electroplating though.


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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 zinc 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 Rileycontracting - Piscataway, New Jersey

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Hi, David. The thickness of electroplated zinc on hardware is typically controlled at anywhere from from .00015" on up to a practical limit of about .0006" according to specific needs. The upper limit is essentially based on practicality in that it takes over an hour of zinc plating time to reach .0006" in barrel plating and it becomes uneconomic, and you're approaching beating the fastener to death in the plating tank :-) The thickness of the steel has no effect on the thickness of electroplated coatings; it's a function of the electroplating current and time.

Hot dip galvanizing is never that thin. Typical thicknesses for fasteners are .0015" to almost .004". In general you can't readily 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 might be theoretically possible for a very thick electrodeposited zinc coating to approach the thickness of a very thin hot dipped coating, but it is rarely heard of. Rather, a "rule of thumb" might be that galvanizing is about 5x as thick.


May 25, 2008

What is the difference between Galvanised Iron and hot dip galvanised iron. Are both the same or electroplated iron is called as GI ?

Syed Mohamed Gani Ganicoating inspector - India

May 27, 2008

Hi, Syed.

The phrase "electrogalvanized", which means zinc electroplated, has become pretty common usage. But to my mind GI means "galvanized iron" and "galvanized" means hot dipped. Unfortunately, it's hard to put a fine point on words like this. Because although I can say "electroplated" is not "galvanized", a supplier of the electroplated finish can always say "I've always called it galvanized; try to stop me!"

Regards,


June 25, 2008

Hello Ted,

it looks like you are very knowledgeable in this field.I heard of the stop of production in China during the Olympic games, due to heavy pollution. There they say that electroplated zinc method is of far higher pollution than the galvanised method. Can you tell me why there is so big difference in pollution?Thanks for your help and best regards from Hamburg.

Reinhold

Reinhold Becker- Hamburg, Germany

June 25, 2008

Hi, Reinhold. Sorry but I have no idea what production China will halt during the Olympics. But here in the USA, and probably in Germany as well, it has been many years since an electroplating plant has been permitted to pollute.

It is difficult or impossible to calculate the environmental impact of one process against any other. So I would turn the question around and ask the person who claims that electroplating is more polluting than galvanizing to present any calculation whatsoever to promote his case and give me a shot at tearing it apart.

In the best light most such claims are based on highly subjective and transient interpretations of environmental impact.

Regards,


July 1, 2008

Hi Ted, thanks for your reply. Surely there are, as you mentioned, many points of view "what is pollution" (in general speaking) and what kind, style of pollution we talk about. I was just hoping it was easy to get get a reply like " method A is clearly considered to be more harmful because of ..... Therefore method B came into the picture". Best regards from Hamburg.Reinhold


July 2, 2008

Can you give me a URL for a news report about this? I haven't heard about a shutdown, let alone one targeted against electroplaters. And I don't know who "they" are who claim that electroplating pollutes more than galvanizing, or what parameters they use in making such a claim.

I don't think method G came into existence as a result of pollution or perceived pollution by method E (they've both been around for over a hundred years) so I can't enunciate the relevant list of alleged pollution grievances :-)

But rather than belabor the point, I will say that E is a "wet process". Ions of the metal to be electroplated are dissolved into aqueous solution, and those ions are washed off with water. That introduces the potential of discharging water that may contain an excessive amount of the metal in question.

>Regards,


July 16, 2008

Hello Ted,sorry for late reply. Here the URL you asked for

http://www.nytimes.com/2008/07/07/sports/olympics/07china.html

It looks like the main point they focus on is the air pollution. Is 1 of the 2 ways more harmful there. As it appears other pollution forms are not so much under focus.

Have a great dayReinhold


July 2008

Hi, Reinhold. I saw mention of "coal mines, coal-fired power plants, steel mills, cement factories and other clusters of heavy industry" and "coking operations, cement factories and small power generators", but not electroplating or galvanizing.

No metal is released into the atmosphere by zinc electroplating or galvanizing, and it is relatively easy and inexpensive to scrub the acid and alkali fumes, so I doubt that either contributes substantially to air pollution.

Regards,


July 1, 2010

Dear Sir

what is difference between galvanizing and electroplated washer? I will use washers for U BOLT together. Which can I prefer?

Serkan CAMALmachinery - Turkey

July 6, 2010

Hi, Serkan. I'm sorry but I can't figure out what parts of this thread need clarification. We've said several times that galvanized coatings are applied by hot dipping and are much thicker and more corrosion resistant than electroplated zinc. We've said that they are less susceptible to metal whisker growth. Is that important for your application? They are not as attractive and they are significantly more expensive that electroplating. Is that important to you? Thanks.

Regards,


October 12, 2010

Dear Sir,

Is there a general understanding in using different types of coating on bolts and nuts ? i.e. an electroplated bolt (zinc coated) with a galv. or black nut. Obviously this is not the common practice but I came across a case that may be required since the material is already purchased. Further, what would the affects be if an electroplated bolt is used with a galv. or black nyloc nut ?

Thanks Savas

Savas OnalConstruction - Melbourne/Australia

October 12, 2010

Hi, Savas.

Mixing hardware like that is difficult and may not be useful. It's difficult because the dimensions on galvanized fasteners are different and they tend to not join up well with black or electroplated fasteners.

The reason that it may not be useful has to do with the protection mechanism. Zinc plating or galvanizing does two things symbiotically. First, it protects the steel cathodically, with the zinc

corroding/dissolving to keep the steel from corroding/dissolving. Secondly it protects the steel by coating it: while the corrosion products of steel are a fluffy non-adherent rust that does not deter continued rusting, the corrosion products of zinc can be highly adherent and build a good impervious skin against continuing corrosion. But if you have black hardware in contact with the zinc plated hardware, the zinc cannot protect the black steel by coating it, only by quickly dissolving away. Regards,


April 1, 2011

please, I need advise on process for cermet base coating and fluoropolymer topcoating of thread, how different is it from hot dip galvanizing.

Adeniyi Egbemode- Osogbo, Osun and Nigeria

April 2, 2011

Hi, Adeniyi.

Cermet means ceramic-metal, like titanium nitride, but not necessarily titanium nitride. Most cermets are probably applied in PVD (physical vapor deposition) vacuum chambers. Fluoropolymer means an organic coating similar to Teflon. If only the threads are topcoated, I'd have to guess that the topcoating is a Teflon-like lubricant rather than a corrosion protection barrier.

It is difficult to answer your question properly in the abstract, though. What is your situation?

Regards,


April 5, 2011

Thank you for your assistance

My situation is that I want to supply fasteners to an oil company in Nigeria; part of their

requirement is have the fastener to be cermet base coat and fluoropolymer top-coating; at the same time some requirements is to have other sets of fasteners with bichromate treatment.

Adeniyi Egbemode- Osogbo, Osun and Nigeria

April 5, 2011

Hi, again. Steel does not accept bichromate treatment, so the fasteners you are speaking of must be zinc plated or cadmium plated, or something similar. It seems to me that you are trying to quote or supply fasteners when you have only vague instructions as to what is needed. There should be tight and definite specifications for what they want. Good luck.

Regards,


May 24, 2011

Hello;

Does hot-dip galvanized nuts, washers, and bolts change color over time when exposed to the elements? If so what might the color and timeframe be?

Thanks;Chris

Chris Au- Vancouver, BC, Canada

May 31, 2011

"Nothing lasts forever but the earth and sky" -- Kansas, 'Dust in the Wind'

Hi, Chris. In normal and proper application, Galvanized surfaces may take on a rust stained look relatively early on because of the iron content in the galvanized coating. This is not indicative of true rusting, which hopefully does not set in for a few decades. But galvanized coatings can also get "wet storage stain", which is white and crusty, and not considered proper. They can also get black spots, which some say is just an early manifestation of wet storage stain.

Please try your best to put your question into the full context of your own situation; otherwise the answers you get can be more misleading than informative. Thanks.

Regards,


December 6, 2011

Hello Ted,Thanks for sharing with us your knowledge!I have it clear that hot dip galvanizing gets me a thicker layer than electroplated. I wonder if this may become a problem on small diameter bolting material, due to thread tolerance? Is there a minimum diameter to use hot dipped on bolting material?

Concerning the pollution issue, a found the following in this site: http://www.galvanizeit.org/aga/about-hot-dip-galvanizing/what-is-hot-dip-galvanizing/the-hdg-coating/zinc-coating/

"Zinc plating is identical to electrogalvanizing in principle in that both are electrodeposition processes. Zinc plating is used for coatings deposited on small parts such as fasteners, crank handles, springs, and other hardware items. The zinc is supplied as an expendable electrode in a cyanide, alkaline non-cyanide, or acid chloride salt solution. Cyanide baths are the most operationally efficient, but they potentially create a pollution and hazardous material problem."

Brazilian regulation on electroplating has became very strict, but many enteprises are still offering services as they adapted to more strict pollution control, even with chrome. I guess the problem in China might be a pollution control not suited to electroplating.

Dion Teles- Piracicaba , SP - Brazil

February 21, 2012

Hi, Dion. Yes, galvanizing interferes with thread clearance, but galvanized screws are spun to limit the thickness, and often the inside of the nuts is not plated; and I believe that sometimes the threads may be cut deeper in advance if the hardware is destined for galvanizing. Sorry, I don't know the minimum size of galvanized hardware.

Regards,


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