Cadmium vs. Zinc vs. Nickel Plating Comparison

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.

A. A 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,

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.

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!! 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 very bad idea.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002

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.

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!

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; plus probably one where the corrosion products are tighter and more encapsulating than plain zinc. 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,

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,

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,

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,

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 than we are. It's possible, but not a sure thing. Zinc generates bulky, sticky, corrosion products … are we sure that's okay?

Luck & Regards,

? Dear Nilesh Mahajan

Where is it mentioned that the cadmium plating is banned in india?

A. Hi Vini. Nickel plating is certainly not "better" than zinc plating although there may be a few cases where it's more appropriate; yes it is shinier. Hydrogen embrittlement relief baking is required for all high strength components, but not for mild low carbon steel components.

If you want to play around with plating as a hobby, you could start with our FAQ, "How Plating Works", and gradually start learning. But for an amateur with no experience, no baking oven, and no notch testing equipment, it would be a mistake to try to plate high strength fasteners. Good luck.

Regards,

A. Hi Brent. Unless you know something really important about the application that you haven't told us yet, don't even dream of adding cadmium flakes! Cadmium is a biocumulative toxin that the world has been struggling mightily to get out of the environment.

Because you are mixing the flakes into an adhesive, you are apparently calling the flake material "the substrate", but that is the opposite sense to the way finishers use the word "substrate", so it could become a source of confusion :-)

To us, the substrate is the material you are applying the adhesive onto. Whether a metal is anodic or cathodic must be said with regard to another metal. Nickel flakes will be cathodic to a steel substrate; cadmium flakes will be anodic to a steel substrate. Other common metals which are anodic to steel are zinc and aluminum, which sound like much better choices than cadmium.

But just so we don't mislead other readers, we should mention that zinc and/or aluminum flakes are commercially mixed with oil based or water based solvents and available as products like "cold galvanizing" spray paint, and "zinc-rich dip-spin coatings". Good luck.

Regards,

A. Hi Peter. I'm not sure that plating them at all is a good idea since the base coat of your "black chrome look" paint may not adhere properly to zinc or nickel. I think buffing the steel would be a better idea.

Regards,

A. Hi Jose. I rather doubt that any plating will prove satisfactory for 'porosity filled pores'. Please look into 'vacuum impregnation' before the plating.

Regards,

A. Vacuum impregnation is a good idea. An anaerobic sealant may also work.

A. Hi Andrew. Although I am not personally familiar with plating cadmium on top of electroless nickel, thread 44575 talks about it, from which I infer that it's doable and practical. And I see nothing in your description that seems to contra-indicate it.

While cadmium and hexavalent chromate conversion coating are undesirable environmentally, no alternatives offer anything approaching their successful application history as a robust solution ... including against the unanticipated. I think your solution 2 sounds very good. Best of luck.

Regards,

A. Cadmium and electroless nickel, and most other nickel platings will most definitely cause galvanic corrosion. For most applications the standard galvanic potential charts on google can be used and the difference between nickel (ignoring the phos) and cadmium will be great enough to cause a cell. In most expected cases the cadmium will corrode as it is less noble. Of cause what you see on the parts may be variable on the chromate coating that's on the cadmium part, but I don't have enough information to comment on that.

If this test is for a lifetime of a part anyways. The current shift is towards zinc based products as zinc is sacrificial to cad, can be chromated and is cheap.

A. Hi Doug. Judging solely from appearance is never sure, but if the parts are strongly yellow (more or less brass tone), they're not nickel, they're zinc or cadmium with a yellow conversion coating. If they are dotted with tiny pits of rust on them, looking something like volcanos when viewed with a magnifying glass, it indicates that the base metal is being anodically sacrificed to the plating instead of the other way around, and the plating is probably nickel or nickel-chrome. How do you know they're not tin, or terne though?

There are chemical spot tests that could be tried, or a local scrap yard may have an instrument for this.

Regards,

A. Hi Robert. You're probably right that cadmium is the ideal coating for your application; it often is -- because nothing can match it's combination of lubricity, corrosion resistance, freedom from gummy corrosion products, compatibility with aluminum, and other properties.

Although cadmium is toxic and bio-accumulative, I personally find it ridiculous & tragic that we're spending years and fortunes trying to remove it from critical aerospace components where it has proven itself for decades, while every few years we have yet another instance of it being found in children's jewelry and that being considered no big deal, just a routine recall to try to catch the bulk of it. Even still, there are those who feel it must be eliminated, so you may be pursuing a dead end in specifying it for a new part.

When looking for an alternative -- and I think you realize this -- nothing offers ALL of the properties of cadmium; instead, you have to decide which of its properties are essential for your application and which are unimportant or of lower importance.

We cannot publicly suggest shops except for our supporting advertisers, but potential vendors can contact you.

Regards,

A. Robert Galinski:

While there is concern about cadmium toxicity, it is still fairly widely available.

We aren't supposed to recommend specific businesses, but if you will look around Ohio...

A. I have heard that Zinc Nickel is being looked at as a replacement for Cad, as well as IVD aluminum; neither are as lubricious as Cad, but do seem to offer better corrosion protection.