Content Source: Products Finishing, www.pfonine.com.
Q. We have been having some issues with our electroless nickel line. We want the bath to last longer, but are afraid of a decrease in quality. How can we help our EN bath last longer while improving our quality?
A. The electroless nickel (EN) plater commonly wants the bath to stay in the tank for long periods of time so they don't have to transfer, due to plate-out, usually because they don't want to deal with transferring/passivating, as it can be time consuming and even dangerous in the presence of nitric acid fumes.
Overly stable baths.Some EN suppliers increase stability with heavy metals or organic stabilizers. This keeps the bath in the tank for long periods, with some negative consequences.
Starting the bath with a pH of 4.8 to 4.9 and at 188°F to 195°F can cause problems as the bath gets older. You can’t raise the pH or the temperature much more because it will cause whiteouts and decomposition of the bath. As the bath ages, it slows down, and there is no way to raise the plating rate. Once the bath starts this way, you're stuck with those parameters for the life of the bath.
Heavy use of stabilizers will co-deposit onto the substrate causing microcracking. Since the bath is loaded with organic or metallic stabilizers, corrosion will reduce.
Staying in the tank for long periods of time can increase drag-in issues, roughness and contamination.
If the bath is in a tank for long periods of time, it will build up particulates on the bottom of the tank, especially without bottom plumbing or proper filtration.
Inconsistent EN tank design is also common. However, what I see most is lack of bottom plumbing, improper filtration and unsafe transferring and passivating of the EN tanks.
Bottom plumbing of EN tanks is not new. Tanks without bottom plumbing—just a pump inside the tank—are better than no pump at all, but this can be drastically improved.
Improper bottom plumbing. Lack of bottom plumbing mimics a beaker on a hot plate; eventually the bottom will start plating. You can use air and a stirrer, but it will eventually take off. Minimal circulation of the bath can cause roughness, plate-out and reduce bath life and deposit quality.
Improper filtration. Filtration cleans out particles falling into the bath from parts (barrel plating), racks or even ceilings. The two methods are the 5 micron filter bag or an encased filter in a cartridge.
Optimum filtration uses a filter bag in an open box so you can see if the bag begins to plate out. The EN will flow across the tank and help with hydrogen release.
EN will plate almost anything, so at some point it is going to plate your tank. Eventually you will need to transfer the nickel out of the tank and passivate it. Most platers avoid this because nitric acid is not the easiest to deal with. It takes time and exposes the plater to nitric acid fumes due to the use of hand pumps to get the nitric acid in and out of the tank. The same is true when a plater neutralizes the nitric with ammonia; sometimes if there is only one EN tank, the bath starts plating out and needs to be pumped out into a drum or tote, which takes more time and effort.
Improper transferring/passivation processes can leave nitric acid in the tank (also attributed to improper bottom plumbing.) Nitric acid will cause havoc on an EN bath with slow rates and deposit issues including adhesion, blistering, pull-away and streaking. Nitric acid can even cause the bath to stop, causing the parts to turn black.
The greatest danger is to the plater’s health such as respiratory problems and severe burns caused by nitric acid. The plater often needs to be inside the tank to remove nitric acid.
Here are some solutions.
If the tank is bottom plumbed, then the pump and the filter can be outside the tank. If you have electric heaters, they can be strategically placed in the tank to save space, and the flow of the filter bag coming from the bottom plumbing can help the heaters from taking off. If you have a boiler, the heat exchanger can be plumbed outside the tank into the bottom plumbing and it will help the heat exchanger from taking off. This ensures proper flow of the bath with turnover from the pump, and it greatly improves filtration because it is sucking off the bottom, increasing bath life, plating rates and deposit quality.
If performed correctly, you can then transfer safely and quickly (less than an hour). Having clean passivated tanks will save time and money and help EN bath performance.
If set up correctly, you do not need as stable of a bath because you can transfer easily, have proper flow and filtration and know that your passivation is done correctly. Instead of starting out at a pH of 4.9 and a temperature of 190°F, you can then begin at a pH of 4.4 to 4.6, 180°F to 185°F, and achieve a solid plating rate.
As the bath ages, you can increase the pH and temperature and continue to attain a solid plating rate, a better deposit, longer life and overall increased production.