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Cadmium vs. Zinc vs. Nickel Plating Comparison

 

Q. We are currently looking for an alternative to cadmium plated parts. The argument for an "equivalent" to cadmium plate has been going round and round. The parts are threaded assemblies and maintaining tight dimensions is critical to the operation of the assembly.

We were getting a .2 Cadmium plate (commercial grade). The alternatives appear to be Zinc plating or Nickel Plating. Tin-Zinc plating is not an option with our current vendor.

With Zinc, the interaction with metals and the sacrificial nature are issues. The corrosive by-products may cause operation issues in the threads and other tight areas. The base material is SA-1983-B7 Steel.

Can anyone offer any insight to this? The consensus opinion is that Nickel plating offers better corrosion resistance than Zinc. The plater says the Zinc doesn't offer any less protection than the Cadmium plate. I'm just shooting for something comparable to the Cadmium. Any advice/opinions are greatly appreciated.

Brian Bukoski
- Erie, Pennsylvania, USA


simultaneous

A. Here are two opposites to ponder, Brian:
1). there is no true substitute for cadmium plate because it offers sacrificial protection, malleability, freedom from sticky and bulky corrosion products, galvanic compatibility with aluminum, softness, and reliable torqueing through freedom from stick-slip, which no other metal offers.
2). the overwhelming volume of hardware worldwide is zinc plated rather than cad plated.

In many cases just switching to zinc will do fine; in other cases it could be a disaster. It depends on how critical the issues I mentioned are.

Nickel does not offer sacrificial protection, but sacrificial protection may not be required for your application. Please try to describe it. Thanks!

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


A. First, there is no direct equivalent of cadmium. If you have to go away from it, you need to decide what properties you need and in what order or amount of preference. Zinc and cadmium have similar but not equal corrosion resistances. One is better in industrial settings and the other is better in maritime settings. Tin-zinc is the accepted best substitute by most. Nickel offers better corrosion resistance, but has far less lubricity and is not as malleable. You might want to look into some of the other zinc alloys like zinc-nickel, zinc-iron, zinc-cobalt to see if one of those might work as there are a lot more plating shops with those than with tin-zinc.

James Watts
- Navarre, Florida


I think James' reply is excellent with one small footnote regarding "Nickel offers better corrosion resistance ..."That's true, but.

Zinc is a sacrificial coating which is anodic to steel, so it corrodes to protect the steel. Nickel is a barrier layer coating which is cathodic to steel, so if there is a scratch, pinhole, or porosity, the steel is forced to corrode to protect the nickel :-)

Regards,

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



A.

You would not get the corrosion protection you are looking for from Barrel plated Nickel; your best alternative would be Zinc plate to approx. 4 /10th of a thou and finish with a yellow / iridescent chromate; if the fasteners are any harder than about 28 Rockwell "C" then you might need to consider Mechanical Plating, or plating from a Chloride Zinc solution if the surface is high in carbon & Carbides In most cases Zinc is a better performer than Cadmium in practice however this is not shown in accelerated corrosion testing.

Regards,


John Tenison - Woods
- Victoria Australia

 

A. Brian,

You said that Tin / Zinc is out of the question. How about some of the other alloy deposits such as Zinc / Nickel which seems to be the best of your two worlds, will give you some of the best of both zinc and nickel deposits.

In my opinion Zinc is not as good as Cadmium for corrosion although the chromates are. The nickel deposit will give you superior corrosion but will loose lubricity, nickel deposits tend to be much harder.

It is impossible to replace all the properties of Cadmium with one deposit, but with some research you can get what you have need for.

Chris Snyder
plater - Charlotte, North Carolina


 

A. The finish that will work well for you is Alkaline Zn/Ni 12-16% Ni. This process with a yellow chromate will give at least 1000 hours NSS to white rust and 2500 hours to red rust (assuming 8 microns minimum). The product produces very little in the way of white corrosion products and has excellent slip properties.

Hope that helps.

Regards,

Jonathan Timms
- Hong Kong


 

A. The topic of Cadmium replacement finishes has been ongoing since '95, when the final regulations in the automotive came to light. Many finishes were tested, and very few offer all of the benefits of Cadmium. We just completed a Cad Replacement Study with the US Army, in which our coatings (both chrome-containing and chrome-free versions) were chosen as the best performing alternatives for Cad. They offer most, if not all, of the benefits of Cad, but they are water-based and environmentally friendly. The areas looked at were: cosmetic corrosion, galvanic corrosion (including pit depth and pitted volume), torque-tension (including clampload at 90 ft-lbs, break-loose torque, and torque range), thickness, conductivity and cost. Tin Zinc and Zinc + Yellow were tested in this study and did not compare to the Dacromet(r) and Geomet(r) products. They showed excessive white/red corrosion very early in testing, and there was a large amount of pitting caused by the bimetallic cell (the bolts were tested assembled to an Al fixture). Also, the Army didn't consider Zn Nickel for this study because it did not perform well in their preliminary studies. At the very base of the results of this study was to not consider Zinc Plating + Yellow Chromate as an alternative to Cadmium.

Andrew R. Pfeifer
- Chardon, Ohio, USA



i. Thanks Andrew.

Yes, zinc-rich dip-spin coatings are an excellent alternative finish for many applications in both automotive and military. There are several other well-known suppliers of dip-spin coatings including Magni and Dorken. Dip-spin coatings may be the best choice 3 out of ten times or even 7 out of ten times ... but I am shocked that the Army would call them the "best performing alternative for Cad" since most professionals feel that nothing should ever be substituted for cadmium plating except on a case-by-case basis after design review. The army, if not misquoted here, seems to be implying that dip-spin coated fasteners should be specified as a replacement for cadmium plated fasteners on aluminum aircraft? That sounds like a disastrous mistake; I think only aluminum plating or aluminum Ivadizing are considered satisfactory replacements for cadmium on aluminum airframes. Substitution based on what is best most of the time rather than on a design review is a very bad idea.

Regards,

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

+++++++

A. Cadmium is a great corrosion inhibitor, however we're finding that a properly plated piece of iron with tin-zinc outperforms the best cadmium plating job.

Tin-zinc is malleable, offers great ductility and torque characteristics without chipping or flaking off the plated steel part. It offers excellent secondary processing without cracking.

Nickel is terrible for all of the above characteristics and offers poor corrosion characteristics when even compared to plain old zinc.

Remember, for wear protection, hard chrome and nickel are the best. For corrosion protection, zinc and cadmium are the best. If you want ultimate corrosion protection, go with tin-zinc. Zinc-Nickel is okay for corrosion protection but is lacks in secondary processing characteristics. The plating cracks when torquing is applied to the metal part. Tin-zinc usually doesn't.

Roger Fasting
- Sacramento, California, USA


+++++++

Q. I would like to know which coating is better Zinc Coating or Nickel Coating. Which one of the two will provide better corrosion resistance, surface finish.

Bhavneet Singh
buyer - Shanghai, China


+++++++

A. If one coating were just plain better than the other, nobody would ever use the other, Bhavneet. As mentioned above, the corrosion resistance of zinc will be better in many instances. That's because zinc is anodic to steel and sacrificially protects it, whereas nickel is not anodic to steel but cathodic, so a pinhole or porosity, or a scratch, will lead to immediate failure. But nickel is self-leveling and will certainly give a better "surface finish". Please describe the actual component you are thinking about plating, and then people may be able to give you an assessment based on the function and the environment it will be exposed to. Thanks!

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

February 26, 2008

Q. We are looking for a suitable replacement for the zinc coating found on some of our parts. The reason being is zinc's reaction with Polyamides 6 or 66 (reference GM's Worldwide standard- GMW3044). Would nickel plating act as a suitable replacement as far as friction properties are concerned?

Rick Leslie
Automotive - Troy, Michigan

January 22, 2009

Q. I have a '98 Cadillac Deville...I'm looking to replace all rotors. It's just for daily driving and NOT very aggressive driving. Which plating is better for this application? zinc or cadmium?

Brian johnson
auto repair - Lancaster Texas


January 28, 2009

A. Hi, Brian. Cadmium is a cumulative biotoxin much like Lead and Mercury. It's use is forbidden in many countries, and it should only be used where it's unique combination of properties (most of which are listed above) is required.

Because rotors can be satisfactory without cadmium plating, they should not (in my opinion) be cadmium plated.

Regards,

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

August 9, 2010

Q. I was wondering what would be better concerning self drilling fasteners (Zinc Nickel or Zinc) - both would come with a top coat, but in your opinion which would serve as a more corrosive resistant undercoat?

many thanks

Donnie Peterson
- Melbourne Australia


August 10, 2010

A. Hi, Donnie
Nickel is primarily a decorative finish. It is a "barrier layer" finish: once compromised by porosity or a scratch, it accelerates the rusting of the fastener.

Zinc is a finish which offers cathodic protection to steel. While zinc-nickel is indeed an alloy of zinc and nickel, and one might think it's chosen to get "the best of both worlds", that's not quite what it's about. Rather, it's a coating which offers a very specific electrochemical potential so that it cathodically protects steel while not rapidly corroding away itself. So it's a cathodic protection coating intended as a replacement for cadmium or an improvement upon plain zinc. If you are going to topcoat the fastener, zinc-nickel is almost surely a better choice than either zinc or nickel and may be a bit less expensive than nickel and more expensive than zinc..

Regards,

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

April 18, 2012

Q. Which plating would be a good alternative to cad plating suspension bolts for a classic car restoration?
The car won't be driven much, just basically need a plating that looks like cad zinc, and will be safe to use.

Mitch Frego
- Seattle Washington USA


April 19, 2012

A. Hi Mitch.

Zinc plating is probably okay, and is very widely available. But make sure the plater knows that these are suspension bolts (probably high strength that will require hydrogen de-embrittlement baking).

Regards,

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

December 17, 2012

Q. I'm looking for a suitable plating for the steel housing of an E85 compatible fuel pump. High ethanol means the potential for attracting water and the susceptibility to corrosion. The housing is basically tube steel which is crimped at the top and bottom holding the inlet and outlet caps in place. Are either of the plating options in this discussion suitable for this application?

Todd Wilson
- Jupiter, Florida, USA


April 20, 2013

Q. I am looking for intelligent opinions on plating hardware of all kind on a Porsche road/race car. This includes engine studs, nuts and bolts vital for engine operation, engine mounting bolts, etc. Also I am replating all fuel hard lines.
I have a selection of zinc platers and one cadmium locally, among others.
I have had cad plated in the past and some decorative nickel.
Please help if you can. It's greatly appreciated.

Marcus Herman
- Grain Valley, Missouri, USA


April 23, 2013

A. Hi. Even critical jetliner parts are no longer cadmium plated despite its advantages, because of its cumulative toxicity, so cadmium should not even be considered. Nickel is a decorative finish, not intended primarily for underhood components. Zinc plating should be good. A plating of zinc alloy like zinc-cobalt, zinc-iron, zinc-nickel, or my preference tin-zinc should be even better.

Regards,

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

October 23, 2013

Q. Hi, I read with interest your remarks regarding the options for plating nuts & bolts, etc., now that cadmium plating is no longer available.
I am renovating some old British motorcycles and would like to get the parts plated; and I see there are many kits such as zinc/cad or just zinc but what are the safety issues regarding hydrogen embritllement?

I understand that load bearing bolts or high stress hardened steel has to be de-embrittled after plating.
I have had my LOTUS front suspension trunion legs done some years ago, is it too late to bake them now or are they likely to fail ?

Les Anrew
- Hull, East Yorkshire, England


October 2013

A. Hi Les. Baking for hydrogen relief must be done within hours of the plating. Some say 12 hours is okay, some say much less, especially for the highest strength ones.

But the need for hydrogen de-embrittlement is not a recent discovery or an obscure fact. If they were plated by an established shop who knew what they were, I suspect that the parts for your Lotus were properly baked all those years ago..

Regards,

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


October 24, 2014

Q. As Cadmium Plating has been banned in India, I want to know which is would be the better equivalent? We are basically looking for protection from Corrosion only. Majority of components are not liable for any torque OR pressure. Only few components have threaded parts.

Need suitable alternate ... Is Zinc-Cobalt the better option as compared to simple Zinc plating?

Nilesh Mahajan
- Indore, Madhya Pradesh, India


October 2014

A. Hi Nilesh. If only corrosion resistance is important, zinc plating is probably fine; most inexpensive hardware is zinc plated. Zinc-cobalt or Zinc-nickel would offer additional corrosion protection at the same thickness for somewhat higher cost. Tin-zinc is more costly because tin costs a lot more than zinc.

But we do have to ask ourselves why, if only corrosion protection is required, cadmium was specified in the first place. It can be dangerous to assume that the people before us were dumber that we are. It's possible, but not a sure thing :-)

Luck & Regards,

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



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