Stressing problems with watts nickel solution

A. I agree with the other responders, but would like to add a few comments. Firstly, there is very little detail as to the actual process being carried out. What is the volume of your electroforming tank and what are your deposition conditions? How hard is "very hard"? What is your mandrel made from and is it adequately cleaned and activated? Is there any possibility of drag-in from the upstream stages? You also say the problem has been ongoing for some months, but is it since you began this job or has it always been there and with other jobs? You do not give details of your present additives; some organic additives, such as naphthalene trisulphonic acid (NTS), are extremely difficult to remove from plating baths and peroxide treatments will not shift them. Why did you fit a polyethylene liner to the tank - did you suspect contamination from the tank lining? Using an activated carbon filter all the time can result in the continual removal of organic additives, so perhaps you have been taking out your stress relievers. Personally, I like to run electroforming tanks at filtration rates of about 9-10 turnovers per hour with a 1 µm or less filter; I only use a carbon filter as a by-pass and use experience of the job and tank to judge if it is on the whole time or only some of the time. I would suggest you limit the flow through the carbon to about 10% of the total flow through the filter; this helps keep the bath in spec. I also wonder why you have not tried a low current dummy, as this can often take out unwanted contaminants.

Watts nickel baths are usually tensile stressed, so to begin with you are not giving yourself much leeway. Your description definitely infers tensile stress in the deposit. I would also agree with the idea of using a sulphamate based bath, and even go as far as suggesting a Hi-Speed bath, which uses a nickel sulphamate concentration of about 600-650 g/l. This was originally developed by INCO and has been marketed by at least one international supply house. I like to run sulphamate baths at between pH 3.8 and 4.2, which is lower than a Watts bath, but I see no major problem with your present pH range; however, I am not sure whether you feel a pH shift from 4.2 to 4.6 in 12 hours is expected or not. This could be symptomatic of a too heavy workload for you tank volume. Personally I like to run nickel baths at the top end of the boric acid range as this buffers the pH; however, everyone to their own!

Your symptoms tend to suggest there is a drag-in of an impurity as the first electroforms after your treatments are OK, but then deteriorate; hence, changing the bath composition may not solve the long term problem. I would suggest you carry out Hull cell tests to verify the performance and composition of the bath; you should also do some stress tests to determine exactly when the bath starts to fail. Are you using the bath for this dedicated job or are you running other jobs in parallel? If you are, this could increase the drag-in situation. The addition of saccharin, preferably as sodium saccharin as it is more soluble, will reduce stress, but it will also harden and brighten the nickel. I believe the addition of more than 0.3 g/l sodium saccharin will make the Watts bath slightly compressive, but this addition has other problems. Firstly it will harden your deposit even more and secondly it will put sulfur into the electroform. This can have problems as if the nickel electroform is heated to over about 170 °C it will become brittle. This may or may not be a problem to the finished electroform. If you need a hard deposit, try adding cobalt to the solution; this has the advantage that it will increase the hardness to about 550-600 Hv with no detrimental affects to use at high temperatures. I would seriously consider changing to a sulphamate bath and run with about 0.2-0.5 g/l sodium saccharin, but there are other alternative stress relievers such as coumarin, p-toluene sulfonamide and if you can live with a slight tensile stress (ca 140 MN/m2), you could consider glycine. One benefit of a sulphamate bath is that, if you use S-nickel rounds, you don't need any chloride in the solution, so this will further reduce the stress; however, many electroformers still continue to use about 10 g/l chloride just to be on the safe side.

I think you should also be aware that trying to coat electrodeposited nickel that has come from a bath that is heavily loaded with a lot of organics can be problematical and needs good activation. This is being discussed elsewhere on this site.

I hope these ramblings are of some use and I apologize to anyone for the length of this reply, but with the shortage of original problem details, one can only speculate on causes and make suggestions based on these speculations.

A. Hi Chris, you do not remove metallic contamination with carbon treatment or H2O2. You could have a metallic contamination. We run a modified Watts bath with Pulse Reverse without problems. Layer thickness up to 5 mm. Have you considered using Pulse Reverse?