Metal Finishing Guide Book

2013

Issue link: https://metalfinishing.epubxp.com/i/218436

Contents of this Issue

Navigation

Page 575 of 843

small rectifier for 15 seconds (maybe less). Just a quick note on electroless nickel over 100% tungsten: It should be activated with a diluted percentage of palladium chloride. We are not doing so here as we have 25% copper in the substrate. Q: We have a sink that is hand hammered copper, plated with nickel and brushed. We are experiencing a problem with the sink discoloring and turning black. It can be cleaned and polished, but the black returns. I understand this comes from a problem in the plating process. Can you help me understand that? A: I have actually faced this exact issue in the past. (I am assuming that you are  referring to electroless nickel when you mention nickel?) A great way to resolve it is by using an electroless nickel with a slightly higher phosphorous content. It sounds like you are using either a low phosphorous or "low-medium" phosphorous electroless nickel on these sinks. What is happening is that the low pH(a.ka. acidness) of the water or tomato juice – or whatever else you are putting in the sink – is reacting with the low-phos of the electroless nickel, causing it to turn black. By increasing the phos content to over 7% you should be able to eliminate the problem. Either way, the plating process sounds fine – you just need a bath with higher phos content. Q: We are having pitting/porosity issues with our high-phos electroless nickel process. After extensive testing, we found the pitting begins after 0.004" of deposit. This is a problem when we need to build up to around 0.020" . I feel like we've tried everything. Do you have any suggestions on how to stop this? A: Please check the following: pH, temp, Ni concentration, and plating rate. Assuming that you are using a standard semi-bright, high-phos electroless nickel, I would like to see your temp dropped to 183°F, pH at 4.7, and nickel concentration at 100% of manufacturer's recommendation. This will slow your plating rate, which will help increase high build without stressing the surface. We might even let the pH drop to 4.5 later. It is also possible that you have contamination in the bath or in your pretreatment. Please check for nitric in the electroless tank with nitric test strips (make sure they are not old). Also, shut off all agitation to the pretreatment baths (including rinse tanks), wait 5 minutes, and check for oil slicks at the solution surface. If you have oil, you will have pits. Try "spray rinsing" to avoid the rinse tanks and see if this helps as a test. Also check your concentrations while plating. It sounds like the pitting is happening at around the first hour mark. Do you replenish while in production? If not, you are plating too much of the wetter out of the bath while plating. You should be replenishing while in production for thicknesses which require over 1 hour of plating. TIP: Make sure you add the "A" or nickel component first, then the "C" or hypo/stabilizer component. If the substrate is aluminum: Are you using a caustic or acid-based etch? I would recommend an acid etch for most high build/high-phos applications to avoid porosity at the part's surface, which can lead to hydrogen "leakage" at around 1 hour of plating. Q: What is the ideal post-plating treatment over high phosphorus EN plated regions ( apart from local removal of phosphorus) that need to receive tin-copper solders? Should regionspecific bronze plating be tried? 566

Articles in this issue

view archives of Metal Finishing Guide Book - 2013