ElectroPlating: from the Sixties to Today

We touched on plating a couple of articles ago when zinc and its coatings were discussed. This month I thought I'd say a bit more about this topic particularly since it has been brought to my attention recently that everybody's pocket contains plated items! Yes copper coins are NOT copper anymore. If you want to prove it take a magnet to a recently "minted" one (fig 1). I'll leave readers to decide what drove the change and when it occurred (the latter can be done experimentally!) but I will tell you that an Institute member (Dr David Cabe) was consulted.


Plated metals included nickel, cadmium, zinc (lots of it), silver, copper, gold, chromium (tricky bath) and, the most exotic of all, rhodium.


In relation to the previous article about cleaning, one reader (Ms X from CX) asked me whether it was true that coke could clean "copper" coins. Well I tried it in the lab just yesterday and sure enough it worked a treat (fig 2) (Coca-Cola has a very low pH of 2.5-due mainly to orthophosphoric acid). But back to plating.

My own experience started back in 1967 when I took an eight month job as an analyst for a plating shop (initial salary 10 guineas a week; going up to £11 on my Birthday!). The company was Standard Telephones and Cables based in New Southgate (long since demolished and turned into a leisure park or some such). There were probably twenty to thirty baths to analyse and it took me two to three days. Titrations (e.g. nickel, zinc, copper) several involving Ethylene diamine tetra acetic acid (EDTA), and gravimetric (gold, rhodium) methods were used. The method I used initially for rhodium was inaccurate and very time consuming and I was given the task of developing a new one which I did after routing around in inorganic (Vogel?) chemistry tombs. Checking thickness was important - there was one chap who did this most of the time: involved a lot of weighings if I remember aright - when I was at the BNF a few years later they did this coulometrically: controlling voltage and having a large array of chemicals enabled determination of the thickness of individual coats, e.g. chromium on say nickel, then nickel on copper and finally copper on steel.

Having done the analysis, the next job was to collect the chemicals and supervise the topping up; somebody more senior than me did the latter. Cyanide (zinc, copper and gold) baths were ubiquitous and there was aV/atts bath for Nickel. Brighteners were added on the basis not of analysis but of appearance/surface tension. There were ammeters/voltmeters beside all the baths. They probably set voltage although it is the number of coulombs (it) passed that determines the thickness bearing in mind that with zinc and chrome, hydrogen evolution would constitute part of the current (less of a problem with nickel and copper). So how have things changed? Some things are the same, some are different! To measure thickness X-Ray fluorescence is probably the preferred method but expensive, with probes (eddy current/ magnetic) the cheaper option. Atomic absorption spectroscopy (AAS) will determine the bath composition perhaps with a feedback system.

Some innovations are Pulse Plating (there has been a recent project at Northumbria on this), more environmentally friendly baths (cyanate, trivalent chrome rather than hexavalent chrome), alloy plating e.g. brass, gold alloys, cupronickel etc.The latter brings me back to the topic mentioned the start of this article. My students suggested that it would not be long before 5p, 10p and 20p coins are made from a base metal electroplated with copper nickel or nickel!

Returning to last time's column, the nasty aluminium failure reminded a reader of "Vulture" lectures delivered by Michael Clark in the 80s/early 90s. These were not for the faint hearted - if you were a metal they would carry an X certificate! But boy were they popular. A metallurgical society in the 1980s which typically would have 15-20 people present for a technical meeting - when Michael came it was more like 50, you could hardly get a seat! Does he still give them? Probably not! One can certainly still find nasty examples of corrosion but we have to be more positive these days.

Thanks are due to Noreen Thomas and Graham Buiman for contributing ideas for this article.

Comments on that (or anything else I've written here) are, as always, welcome douglas@harrbridge.freeserve.co.uk