Surface World June 2020 | Page 12

INSTITUTE OF MATERIALS FINISHING The fascinating world of electroplating John Burgess FIMF Following on from my previous article, (IMFormation Feb 2020) I said that I would share with you about an experience I had with problematic nickel bath. At the time I was working for M & T Chemicals (as it was known), and I was sent to a customer who was experiencing issues with roughness on settling surfaces. The customer was nickel/chromium plating brassware for bathroom fittings and on all the settling surface there was roughness as bad as sandpaper. Now one of the problems with plating brass sand castings, especially where there were hollowed sections as in bath and wash basin taps is that the current density is so low up in the tubular part of the tap that plating does not take place but because of the acidity of the solution there is a chemical attack on the inner surface of the brass. This attack will cause the sand from the casting to leach out into the plating solution and subsequently contaminate the solution with sand. As plating continues the sand in the plating solution will settle upon horizontal surfaces leading to roughness. Other factors have to be taken into consideration when looking at roughness and that it to ensure that the make up of the plating solution is correct. Weak solutions can cause roughness through the fact that there is insufficient metal content to support the current density that the solution is running at. As the company did not have a laboratory as such and technical representatives were only equipped with a set of 3 off Twadell Hydrometers, a set of pH papers and a thermometer I set about assessing the concentration of the Watts nickel bath. Now some of you may not have heard of a Twadell hydrometer but it was a scale that 1 Two was equivalent to about 1 oz per gallon therefore a nickel plating solution should be about 48 ozs/gal (300 g/l) and require a No. 2 Twadell hydrometer. The bath volume was around 1000 litres so, taking the hydrometer and lowering it into the solution I carefully let it go only to see it disappear at a vast speed into the bottom of the tank. Hmm, I thought, that wasn’t a clever move, so carefully I lowered the No 1 Twadell hydrometer in which gave me a result of about 22 Two which was about 22 ozs/gal (140 g/l). Obviously the bath was weak in fact about half strength, so chemicals had to be added. I decided that 3 bags of nickel sulphate, 1 bag nickel chloride and 0.5 bag of boric acid should be added. At least this would bring the solution up to somewhere full strength, enough to get it going and follow up later with a full analysis. Now back in the good old days Nickel Sulphate and Nickel Chloride were supplied in 50Kg hessian bags which were lined on the inside with polythene and labelled “INCO MOND Nickel Sulphate or Chloride 50Kgs. INCO supplied nearly all the nickel-based products. Boric acid, like now, was put into paper bags so the operator at time said to leave it to him and I could follow it up the following day. The following day arrived, and I rushed to the nickel tank in the hope that I might see my No 2 Twadell bobbing up and down in the solution but alas it was not to be, so I ruled that out as a lost cause. It was decided to put a job round to see if there was an improvement and after about 2 hours it came off the line in a worse state than when I arrived the previous day. The roughness had now developed into elongated whiskers and completely useless with a technician that was lost for words. We decided to drain the tank and see what might be in the tank. I was guessing that the original roughness was due to the sand from the castings which was not being filtered out but, as the solution slowly emptied out of the tank the words “INCO MOND Nickel Sulphate and Nickel Chloride came into view together with a paper sack with the words Boric acid. Now I understood where the problem lay. It is always useful when wanting to increase the strength of a plating solution to open the bag and pour the contents in but because the operation was being carried out close to going home time (and it was a lovely day) it was decided to drop the bags in and hope that somehow the contents would find their way into the solution. Unfortunately, the inner polythene lining is not soluble, but the hessian bag certainly did decide to break up and cause untold problems. Obviously this was a one off experience in my career but it goes to show that when trouble shooting a problem you do sometimes have to think outside the box. The problem got resolved by passing the solution through very fine filtration and the Twadell Hydrometer….it was lying behind one of the anodes still intact considering the ordeal it had been through. John Burgess FIMF IMF Summer Enrolment 2020 Time on your hands? The IMF would like to invite you to study for one of our qualifications. You may enrol anytime between now and up to 5th June and pay just £150 deposit, with the balance due by 30th September 2020. Email: [email protected] for further details 10 JUNE 2020 twitter: @surfaceworldmag