Zinc-nickel Plating, misc. Q & A's

A. Hi Vasco,
Proper plating can only occur on pure clean metal, and I suspect that welding slag and oxidation is not fully and reliably removed.

Before sending the components through the line, thoroughly scrub those welding lines on a half-dozen or so with a scrub brush ⇦ this on eBay or Amazon [affil links] well wetted with pumice ⇦ this on eBay or Amazon [affil links] and Spic and Span ⇦ this on eBay or Amazon [affil links] or similar detergent. Ask the person who is doing this to really overdo it, and if they are satisfied that the weld line is truly clean. Then rack the parts for the regular cleaning and plating cycle.

When you are satisfied that this solves the problem or does not solve the problem, please get back to us. Luck & Regards,


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

A. Hi again, Vasco.
You may have misunderstood the intention of my posting. I am not suggesting that parts be manually precleaned as a permanent solution, I was suggesting that you try this as a way of determining whether the problem results from your existing cleaning process being inadequate -- and you apparently proved that it is inadequate.

Sorry, I have no welding expertise and cannot predict the effect of those welding changes. All I can say based on the results of your experiments is that your current cleaning process is not adequate for the current condition of the components.

So it is time to talk about your mechanical and then your chemical cleaning/pretreatment steps before zinc plating and how to improve them.

I have no idea how large those parts are, but is vibratory finishing done? If not, is it a possibility? Luck & Regards,


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

A. I have heard of Zinc/Nickel being tack welded, but not soldered. Also, you can apply the alloy to Aluminum if needed. Please remember that the purpose of Zinc/Nickel originally was to serve as an alternative to Cadmium plating on steel. Both are sacrificial, but the Cadmium is both toxic and a carcinogen. On Aluminum use good preparation including a good Double Zincate.

You may wish to investigate Tin/Zinc plating. It can be applied to aluminum, is solderable, and can be plated from either a neutral or alkaline solution. You can check your vendors for names of products.

A. Hi, Russell. Indeed the older grades of pressure treated wood have been banned and the newer grades are heavy in copper, which causes very rapid corrosion of zinc plated nails.

As a personal anecdote the deck/dock boards where I live always get too rough and splintery after several years and rather than redoing the entire deck I am in the habit of replacing the worst dozen or so boards each year. The nails I had long been using did not survive even a single year in the new pressure treated boards.

I don't know for a fact that zinc-nickel plated nails will be any better because this is not atmospheric corrosion but galvanic corrosion. The most common satisfactory nails, other than stainless steel, are surely hot dip galvanized. Good luck.

Regards,

A. Pat, zinc nickel electroplated steel is currently being used for fuel tanks on automobiles. The zinc nickel coated steel typically has an epoxy coating applied prior to the parts being formed. It is the combination of the pretreatment, epoxy coating and zinc nickel substrate which provide the resistance to the fuel and the corrosive environment, not just the zinc nickel.

A. Hi,

While Roy answered Pat's question very well, and I learned a lot from it, it should not be interpreted that zinc-nickel plating is never used "stand alone". To the contrary, almost anything that is zinc plated can probably benefit from the additional corrosion resistance of zinc nickel plating instead.

Regards,

A. Hi Ramesh. There are no proper standards except those which were written into the purchase order :-)

So if you are in QA, that's what you'll need to find. but if you are writing a purchase order and spec'ing parts, standards are available from ISO, ANSI, military, etc., and a starting point might be ASTM B253 "Standard Guide for Preparation of Aluminum Alloys for Electroplating" and ASTM B841 "Standard Specification for Electrodeposited Coatings for Zinc Nickel Alloy Deposits".

Good luck!

Regards,

A. Alkaline Zn-Ni is a semi-bright coating. Take a look at aftermarket parts made for VW, Mercedes, or some other European automotive manufacturer, they commonly use this coating on fuel filters, etc.

Don't expect the "jewelry" bright look you get from zinc plating/clear chromate, particularly acid zinc. (I'm not familiar with the acid Zn-Ni process; it seems to me that the alkaline bath is more common and easier to run) But, as an old plating foreman I used to commiserate with, when his OC inspectors rejected parts for dull appearance, "the protection is there." Alkaline Zn-Ni is not the easiest process to run, nor is it much cosmetically, but it is a great finish when it comes to preventing rust.

That's the whole idea, at least it ought to be.

A. Hi Cee. If you were a production plater, the answer would be yes you can proceed directly to plating over what you've got AND yes you have to strip them ... because sending the parts through an electroclean tank will strip the chromate anyway, and sending them on to the HCl acid dip will strip the zinc anyway.

I don't know exactly what you intend to do for pretreatment, but the chromate must be removed, and the existing zinc plating must be at least acid activated.

Luck & Regards,

A. Hi again, Cee. The basic issue is that you can only successfully electroplate onto a waterbreak-free clean metal surface, and a chromate conversion coating is not metal.

The problem that arises for a hobby plater is that the simple way to remove a chromate conversion coating is to make it anodic in a strong caustic solution (similar to an electrocleaner) but this definitely generates hexavalent chromium -- the toxic, carcinogenic thing that made Erin Brockovich famous. A plating shop with a wastewater treatment system and employees with haz-mat training can easily deal with this -- a hobbyist not so well.

I guess what I would so is, while wearing a good dust mask, sand off as much of the thick chromate as practical, then immerse the part in an inhibited acid pickle, and assume that undercutting will occur, so as the zinc plating is removed, with it will go the chromate.

This site has approximately a quarter million questions posted, the majority having to do with plating defects, so please understand that it is rarely the case that anyone can get a quick answer to "this is what I see, what caused it?"   :-)

Usually we'll need good well-focused, representative, photos of the defect (attached in e-mail to mooney@finishing.com for posting here) together with some detailed data about what you are doing (concentration, temperature, time, surface area, volts & amps, sketches or photos of the arrangement) to move the guessing game along productively. 'Burning', if that's what it actually is, is caused by excessive current density for the situation, and is usually most pronounced at corners & edges because those are the areas which often receive the most current. 'Electricity follows the path of least resistance' is usually a helpful truism. But if your kit doesn't include a cleaning step, or at least cleaning instructions, you need to start there. You need exceptional cleanliness of the surface for successful plating.

Luck & Regards,

A. Hi Reuben. Although your question involves nickel plating, which is a very different thing than zinc-nickel plating, we added it to this thread because C B Evans just went through the same question as you about stripping the existing zinc plating, so that discussion should be applicable for you.

Although you're perfectly welcome to attempt zinc plating, zinc-nickel plating, or nickel plating there is one very big difference between them which refutes your "nickel coat is more durable than that of zinc, I presume?. That difference is that zinc and zinc-nickel are sacrificial coatings offering cathodic protection to steel, whereas nickel plating is at best a barrier-layer plating, but at worst it's anodic to steel and promotes the rapid rusting of steel. You are probably aware that steel boats and ships are often equipped with zinc anodes, the purpose of which is to sacrificially corrode to protect the iron/steel; ship anodes cannot be made of nickel because nickel is more noble that iron and the system would work exactly oppositely: the iron would corrode to protect the nickel.

Thus, imperfect zinc or zinc-nickel plating with a pinhole or porosity still offers substantial protection to the fasteners. Nickel plating must be perfect, completely free of pinholes & porosity, to function as a barrier coating; if a pinhole exists, the presence of the nickel will cause the fastener to sacrificially corrode. Thus it's usually not the right coating for a beginning hobbyist to apply if he is looking to add corrosion protection. If you want nickel plating, either buy nickel plated fasteners, or get very good at nickel plating :-)

Luck & Regards,