Procedure For Zinc-Nickel Plating

A. Hello Joseph,
Zinc-nickel plating is a (essentially proprietary) replacement technology for cadmium plating, or a step up from zinc plating. It is a non-decorative sacrificial plating, and not a suitable replacement for nickel or chrome plating. There is just enough nickel in it to give it the desired cathodic potential to protect steel. It is offered as either an acid or alkaline, depending on which supplier you go with. It always uses chromate conversion coatings which are basically similar in application to those used on zinc or cadmium plating, but the chemistry is a little different. Good luck.

A. Hi Yalamanchili. If you are just looking for raw research results on what might work in formulating such a process, you might wish to see Brenner's "Electrodeposition of Alloys".
It includes tables for many different potential formulations, the alloy percentage each formulation was found to deposit, etc.

But the gap from research to commercially viable processes is a substantial one, bridged by years of development work by the suppliers. Realistically, zinc-nickel plating is a proprietary process. You decide whether to do acid or alkali plating, select a supplier, and get from them the technical data sheets which answer all your questions about the procedure. Usually you get hands-on help as well. None of this means that you're forbidden to attempt home brew of course -- but commercial success is based on trade secret development, so you won't find the kind of data you need from trade journals, websites, or other publicly accessible resources. Good luck!

A. Zinc-Nickel Plating is best done from an Alkaline bath which deposits evenly in low current densities. The end use, if automotive, will require a trivalent chromate to match SST specs. Zinc Nickel Alkaline bath is available in India and most GM, Ford, Japanese and German specs specify and accept a 12% Nickel content.

The procedure is simple. Soak followed by Anodic cleaning and pickling, followed by a second Anodic stage constitute pre-treatment. The Zinc Nickel Bath runs at 6-10 volts Rack/Barrel with filter and moving cathode for rack. Temperature 25 °C is essential. Controls include Caustic, Zinc as metal, and Nickel as metal as specified in the operation manual. The current is passed through inert Nickel anodes. Nickel is replenished chemically using proprietary concentrates while zinc is dissolved off-line in a separate zinc generator and replenished periodically. Parts must be clean before plating, rinsed well after plating, chromated in special chromates meant for Zinc-Nickel plating and a top coat sealant helps to achieve the required SST hours.

The process is a consolidated metal system which needs to be controlled periodically by analysis to achieve the right alloy composition thereby ensuring consistent SST.

Regards

A. Sir:

You have several decisions to make regarding bath selection. First, alkaline or acid. (I prefer Alkaline). Secondly, low nickel in the alloy or high nickel in the alloy. The amount of nickel will have an impact on the ultimate corrosion protection given to your parts. Remember that the higher the % Nickel, the more difficult it will be to put an acceptable chromate passivate on it. Next, consult with your vendors to find a source that will meet your exact needs. I recommend checking the Chemicals advertising section of www.Finishing.com to see if anyone there can be of assistance to you.

A. Hi Myron. My own personal take on this issue starts with the recognition that both zinc and zinc-nickel alloy are sacrificial (cathodically protective) coatings, whereas pure nickel is only a barrier layer coating, so zinc-nickel offers an important feature which pure nickel doesn't: it corrodes preferentially to steel to protect it. One of its advantages over pure zinc is that, with the right percentage of nickel, it is only slightly more active than steel, rather than greatly more active, so the sacrificial corrosion is slower, and the corrosion products are more effective towards retarding continuing corrosion.

So I wouldn't focus much on hoping that spending an exact amount of immersion time in a chromate tank of a particular composition, both of which are chosen for optimum chromating success rather than nickel enrichment, will incidentally re-optimize a zinc-nickel percentage which was already optimized :-)

And many people feel that "leaching" is just a specs game anyway, i.e., that if you leach out the color, you're leaching out the chromate, and losing almost all of its protective value.

Regards,

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Ed. note: Readers, please note the date of Myron's inquiry. We are now in a RoHS (European driven "Reduction of Hazardous Substances" ) age where colorless new technology trivalent chromates are widely used and 'leaching' is fading into history. Some of the following Q&A's may also precede the continuing switch to improved trivalent conversion coatings.

A. We are living in changing times, Joe. Hexavalent chromate is the traditional type and historically exhibited far better corrosion resistance than trivalent chromate.

But there has long been a general movement away from hexavalent chromates on leading edge products like aircraft due to toxicity of the waste products and concern that the material might eventually be determined to be carcinogenic for skin contact. More recently, by directive of the European Parliament, hexavalent chromates can effectively not be used on automobiles anymore because they are believed to interfere with recyclability. This ruling served to lend immediacy to the search for replacements.

A number of things have happened therefore: some work that was formerly zinc plated has moved to electroless nickel plating; new topcoat processes have become available to add the required extra corrosion resistance to trivalent chromates (although perhaps sacrificing conductivity); improved trivalent chromates have been developed.

What to specify probably depends on the life cycle of your parts and designs, and your companies attitude toward environmentalism versus affordability. The replacement technologies are more expensive, more complicated, and consequently more subject to error in processing--but you can have a process that imparts sufficient corrosion resistance without using hexavalent chromates.

Mr Mooney, thanks very much for the reply. I really appreciate it! We want to be environmentally safe so we are looking into the hex alternatives. Unfortunately, our first quote using zinc/Nickel with Trivalent and top coat is about 6 times more expensive than zinc/Nickel with Hex. That would pretty much put our end price way past reasonable.

The search continues....

Thanks Again,

A. Hi again. All I can say is that the price differential will rapidly fall and will eventually become a minor issue as trivalent chromating becomes a necessity rather than an option. Good luck.

Regards,

A. Most likely that white substance is either white corrosion or pooling of the topcoat/sealer. Many topcoats for trivalent chromate systems pool up and turn white over a few days' time. You may get the same white stuff if you let the parts sit around untested.

I've heard trivalent chromates don't have as good synergy with zinc-nickel alloys. You may want to compare your zinc-nickel with similar samples of zinc-iron.

Tim,

I appreciate your response. We do not have a topcoat on trivalent chromate zinc-nickel. You probably don't see this condition on zinc iron deposits because the chromate is hex.

A. Cliff:

Our experience with trivalent yellow passivation on Zn/Ni has given us results similar to yours. That is to say, the appearance of a white or gray veil, that according to the analysis that we have made does not correspond to an oxidized part, but to the decomposition of a by-product that has nothing to do with oxidization of the part.

One observation that supports this thesis is that a Zn/Ni [part I tested] oxidized white in 24 hours, but never came to red corrosion in 1000 hours. I hope to continue speaking of the subject with you.

A. Hi, Tony. Very interesting question. Maybe the plating along the edges and at cut-outs is a substantially different alloy composition because of the higher current density there? Or maybe the edges were cut in a way that left a scale on them that the plating shop could not fully remove, so the spots didn't plate or plated differently. Any chance of getting a reading on the alloy composition before the salt spray test, or seeing the parts before they are plated, or learning the mechanism of the shearing and punching (maybe it was by laser or other cutting method rather than typical shear and punch press)?

Regards,

A. Unless ASTM B117 has changed, you mask the outer 1/4 inch of the panel.
For the type of metal finishing that we did, we chose to ignore the outside 1/4 inch rather than to do the masking step and it worked very well for our needs.

Hello Tony.

Unless I am mistaken the problem lies in the PLATING TECHNIQUE. I presume the bath is Alkaline Zinc Nickel?

1)Alkaline Zinc Nickel Plates at half the speed of regular Alkaline zinc. Pushing up Current density causes edge burn. The edge burn then translates to premature failure at the edges. Reducing the Current density could be considered to prevent edge burn which is sometimes not immediately visible.
2) Solution agitation using eductors [on eBay or Amazon] can help you increase current density without the burn.
3) Cathode rod agitation too will help increase the current density.
4) Operating at the recommended bath temperature between 20 and 28 °C too will help keep the metal distribution in order.

Solution circulation using eductors or Cathode rod agitation helps with distribution and uniform alloy composition.

Hope this helps.

Regards,

A. I agree with James. The sides of the sample have to be masked and not considered for the test.

A. Hi Noya. Asif Nurie notes above that Zinc Nickel plates at half the speed of alkaline zinc. Could the burning be resulting from trying to hold too high a current density?

To my limited knowledge the chromate will be more purple and red than the usual iridescent yellows because it doesn't take as thickly and uniformly on this nickel-bearing alloy as on zinc.

People will need a good deal of data from you to help you troubleshoot. Good luck.

Regards,

A. Hi Suriani. The plating goes where the current goes, and the current takes the shortest path, so there is always much less plating on the inside of a nut than on the outside. But there are a hundred different possible reasons why you are getting no plating on the inside, including the threads not being properly cleaned or activated, or the nuts being racked in a fashion whereby the rack hook steals all the current. So what we'd like to do along with you is to eliminate some of those possible reasons.

Please start by telling us what size these nuts are, whether you are barrel plating or racking them, and whether or not you are doing any other successful plating of these nuts or similar parts. Thanks!

Regards,

A. Hi Joel. As you will learn from reading the earlier entries on this thread, practical zinc-nickel production plating is a rather different thing than laboratory work...

For your lab studies you can start with Brenner's Electrodeposition of Alloys, which has a long and detailed chapter on the subject; it will give you dozens of potential formulations, and several good starting points for lab experiments with various organic and inorganic additives.

Actual production plating, however, will almost surely require proprietary products -- and no one can or will give you the fruits of those years of development work. So, if your lab work is about trying to study how addition agents work and can be improved, start with Brenner. But if your need is to put useful zinc nickel plating onto parts in preparation for using those parts to meet some need, you will want to purchase a proprietary process. Good luck.

Regards,