Letter 13089

Cadmium vs. Zinc vs. Nickel Plating Comparison 

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 census 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, PA, USA

Here are two opposites to ponder: 1). there is no 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. 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 these issues are.

Nickel does not offer sacrificial protection, but it may not be needed.

Ted Mooney, P.E.  finishing.com Brick, NJ

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 offer 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
- FL

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

Brian,

You said that Tin / Zinc is out of the question. How 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 chromate 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 need for

Chris Snyder
- Charlotte, NC

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
Atotech Asia Pacific - Hong Kong

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. Metal Coatings 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
Metal Coatings International Inc. - Chardon, OH, USA

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 torqueing is applied to the metal part. Tin-zinc usually doesn't.

Roger Fasting
- Sacramento, CA, USA

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

If one coating were just plain better than the other, nobody would ever use the other, Bhavneet. As already mentioned, the corrosion resistance of zinc will be better in many instances. That's because zinc is anodic to steel and can sacrificially protect it, whereas nickel is not anodic to steel but cathodic. 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 can give you a realistic assessment based on the function and the environment it will be exposed to.

Ted Mooney, P.E. 
finishing.com
Brick, NJ

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 sutible replacement as far as friction properties are concerned?

Rick Leslie
Automotive - Troy, MI, USA

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