by Dan Schafer
In past months we have written several articles about the problems encountered in trying to plate to a European standard of "Nickel Free" for jewellery, or jewellery worn next to the skin in particular. In those articles we more than once contrasted the difficulty of plating nickel free with the ease of plating with nickel. Nickel is a metal used widely in the metal finishing industry to plate a bright, hard, corrosion resistant layer. On the basis of those features, jewellery manufacturers have been able to plate precious metals like gold over nickel beautifully, durably, and affordably. With nickel free plating we were attempting to achieve something close to those results by a much more complicated and sensitive process.
However, not everything needed to be plated nickel free. A nickel basis was fine on any of our products not worn next to the skin. So fortunately our nickel free teething pains did not affect the other half or more of our product line that continued to use nickel. Ironically, about the time we seemed to be winning in regard to plating nickel free, our old stand by, the easy and sure nickel bath, began to let us down.
Nickel was not such a standby that we never had trouble, but usually there was some fairly simple procedures to follow to bring it round to healthy plating again. This time the jewellery still looked fine. We plated gold and it came out looking beautiful -- only one difficulty. When we put it down on the quality control table we saw little flecks of gold lying about next to the jewellery. Where was it coming from? Looking closely we saw both the nickel and gold layer were flaking off from the tiny wires that make up the pattern on the front of our jewellery. We couldn't send it out that way. If we couldn't solve this problem and quickly it would mean a more major disruption to production than our wearisome nickel free failures.
How to solve it? By now I had learned at least this much, "Go to the experts." I read the books and called our plating consultant, Norman Wang. I got the same answers, poor adhesion is caused by one of two things: either the base metal has not been cleaned properly or the plated metal, in our case the nickel, is too brittle. I had also learned another thing, "Start acting. Don't just theorize about what might be the trouble." So first we looked to see if there was anything wrong in our cleaning process. Everything looked OK, but the last acid dip before going into the nickel tank was not up to the supplier recommended strength. So we mixed up a new acid solution and bravely tried again, hoping to have licked the problem. The first few racks looked fine, but when we got them on the QC table there were still those tell tale flecks of gold lying about and coming off on our fingers.
Sure then that the problem lay in the nickel plate being too brittle, we had to ask, "why"? The only reason I could imagine was that somehow the nickel bath was contaminated. I also questioned Norman, our plating expert. He mentioned things to me like chemical imbalance and too much current, but those explanations didn't make sense to me since we had been adding chemical and plating at the same current levels for over five years with excellent results. I was so sure of myself that Norman went along with me. What else could he do? He was on the other end of the telephone line either in Bangkok or Hong Kong and only had what I told him what to go by. So we operated on my suspicion of contamination.
Now, normal contamination occurs in an acid based bath like nickel because it dissolves what you put into it just like water dissolves salt or the way our stomach acids dissolve the food we eat. So if we are plating copper pieces and for some reason there is not an electric current to keep depositing nickel on it, the action will reverse and the base metal, copper, will "undeposit", that is, dissolve into the nickel solution, thereby contaminating it. If the operator has his wits about him that won't happen under normal plating conditions because he always keeps electrical current flowing. But if a piece falls off a rack into the solution, and goes unnoticed there, it starts to feed the bath with what the acid likes to eat in its unmanaged state, copper ions. Another metal that the nickel bath will happily consume to its own detriment is lead. And we give it the occasion to do that if something soldered with lead solder slips away from its electric saviour, and lies uncharged in the acid.
Copper or lead can cause trouble in a nickel bath, but usually the symptoms like darkness and dullness are the more pronounced and noticeable. The copper and lead contamination can actually be plated out onto "dummy" cathodes, large accordion like sheets of stainless steel, that we connect to a current in the same way we connected the racks of jewellery we plated. Given enough time and the right current, temperature and pH levels a nickel bath can be cleaned up quite well by the dummying technique. We did our dummying, and it seemed to help, but only for a short time.
In the books I read, another contaminant was occasionally mentioned. That one was very serious because, unlike copper and lead, there was no practical decontaminating process for it. I became increasingly suspicious that it might be this other contaminant, nitric acid, because it was what we used to clean our plating racks and we rinsed in some of the same rinses as for plating. Norman, still by phone, somewhat sceptically went along with this analysis. So on that basis we did the only thing practical to get us back in production. We dumped the old nickel bath and started over with a new one. Needless to say it worked beautifully, but six months later something happened again with our new bath. Next issue I will explain what and some of the humbling lessons I learned.
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