Bright decorative nickel plating is one of the most common plating processes. Though today’s automobiles don’t feature as much nickel-chromium trim as they once did, bright work is experiencing something of a resurgence, especially on wheels and bumpers of passenger trucks. That’s not to mention the thousands of other products—somefunctional, some decorative—that are nickel-chromium plated.
Nickel plating isn’t always free of problems, as every plater knows. Without warning, the electroplate can begin to show roughness or cloudiness, to say nothing of pitting, adhesion problems, dullness and misses in low-current-density areas.
It’s a temptation to search for these problems in the nickel-solution chemistry. But you can waste a lot of time and encounter a lot of frustration if that’s where you begin your troubleshooting. It’s better to look for problems in the equipment and accessories on the plating line before beginning to search for what’s wrong in the nickel plating solution.
The Unusual Suspects
Are the problems occurring on every rack or might some racks be making poor contact? An electrical problem at the nickel station? Could this be a cleaning problem? Are cleaning stations up to spec for concentration, current density (in electrocleaning) and operating temperatures? Are parts properly activated before reaching the nickel station? Are rinses so contaminated that they allow dragin from cleaning and pretreatment stations to the plating tank? Check the whole line before you decide to tamper with the nickel-solution chemistry.
In the nickel tank, be sure your air agitation and solution filtration are working well. Agitation helps to prevent burning and assures proper diffusion of brighteners into the cathode film. Holes in air-agitation piping can become clogged, and while air agitation may appear to be okay, it is not functioning in certain areas of the tank if some of the holes are plugged. Inspection of piping on a regular basis is a good idea.
Filtration of the plating solution removes unwanted solid particles, keeping the solution clear and free of roughness-causing particles. Since most organic brighteners are not removed rapidly by carbon filtration, you can filter out brightener-decomposition products as well as particulate matter by maintaining adequate flow rates of the plating solution through your carbon-packed filter. Don’t add all the carbon at once. Applying smaller amounts of activated carbon at intervals keeps solution exposed to fresh carbon and minimizes the effects of “channeling” as the filter becomes loaded. Generally, it is best to maintain an hourly flow rate through the filter media of at least three times the volume of solution in the tank. You also should check your nickel anodes for polarization, especially bagged anodes. The bags may have become clogged and may be in need of replacement, or nickel anodes may be “hung up,” so that voids beneath the hang-up reduce the anode-to-cathode ratio.
Once you’ve satisfied yourself that your cleaning, pretreatment, agitation and filtration are functioning as they should, and that your racks, carrier bars and anodes are in good shape, it’s time to look at the nickel plating solution itself. It’s tempting to rush into analysis for impurities or even into solution treatment, but too often the problem is simply that solution composition is out of whack.
Inspecting the Chemistry
In your lab, check to be sure the concentration of nickel, chloride and boric acid are as specified and that pH (generally 3.5-4.2) and temperature (usually 135-145°F) are within range. Your supplier should have given you a data sheet showing concentrations, pH and other operating parameters of the solution. It is beyond the scope of this article to delve into all the concentrations, the use of sulfur and low-sulfur deposits for multi-layer nickel plating, etc. But suffice it to say that the composition and operating conditions should be within the specs given by the supplier.
It is normal for pH to rise during operation of a nickel-plating bath, and thus, acid should be added to keep pH within prescribed limits. As for concentrations of organics, analysis of organic impurities in the presence of brighteners and other organics is very difficult, if not impossible. So use a Hull cell or other methods of qualitative testing.
It’s obvious that filtration is a starting point if the problem is plate roughness. That out of the way, it’s better (and in the long run time-saving) to perform chemical analysis and Hull-cell tests to diagnose the problem before you attempt purification. For dullness or other problems related to the deposit, carbon treat first. Then carbon treat at high pH. It’s a temptation to jump in and treat with peroxide or permanganate as soon as you sense the need for bath purification. But often, simple carbon treatment will solve the problem. So carbon treat, filter and then determine whether oxidation is desirable.
While it’s true that oxidation with permanganate or peroxide alters organics in the solution and makes carbon adsorption more effective, oxidation also may change the organics to a more soluble form that has detrimental effects. If your tests show that oxidation is appropriate, begin with low peroxide concentrations and carbon treatment; then low-pH peroxide or permanganate followed by high-pH carbon treatment.
If that seems to help, verify your results by starting over, with a fresh sample. Remember that too many samples plated in the same test solution may in and of themselves have a purifying effect.
Check to see that the brighteners respond properly after treatment. Sometimes a treatment will appear to be helpful because it is removing a combination of impurities and brightener, only to reappear when brightener concentration is re-established.
Try dummying. Low-current-density (2-5 asf) plating on corrugated iron cathodes should remove zinc, cadmium, copper, lead and even some organic contaminants. First, nickel plate the cathodes at normal current densities to avoid iron contamination from the dummy cathodes. Use vigorous agitation, inspect the cathodes for powdery deposits, and occasionally raise the current density for a few minutes to seal in the contaminants. Complete the treatment at higher current density.
Rather than having to treat for removal of impurities, you may reduce the need for treatment by excluding contaminants. Inorganic contaminants may be introduced by hard water, airborne dust, corrosion of tanks through cracks in tank linings, corroding anode bars, dissolving parts that have fallen into the tank, and dirt falling from conveyors and supporting structures above the line. Analyze the bath for presence of copper, cadmium, zinc, lead and other inorganics.
Organics can be introduced by improperly cured rack coatings, wetting agents, maskants, lubricating oil on conveyors, buffing compounds, and paint spray drifting from adjacent areas. Taking steps to reduce or eliminate these sources is well worth the effort.
Is it the Solution?
Maybe. But first, check every step in pretreatment and inspect your equipment – from racks to anodes and anode bars, carriers, filtration and agitation. Often you’ll save time and correct the problem without meddling in the nickel solution. If none of that helps, then it is time to begin Hull-cell testing and analysis of the nickel bath.
blog comments powered by Disqus