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After a certain point in the guide, it may become necessary to know exactly what is wrong with the deposit, but there is a good chance that as you adjust the solution and make improvements in the physical plant, the problem will disappear before you know what caused it.
A Note on Organic Brighteners
Most bright nickel solutions contain a stress reliever and an auxiliary brightener whose concentration can be determined by simple analytical techniques. The leveling portion of the brightener is best determined by the appearance of the deposit, either from the work coming off of the line, or via Hull Cell analysis.
Check the following, adjusting as you go:
The solution contains excessive amounts of the leveling component or is contaminated with breakdown products. Brighteners function by breaking down at the cathode surface and interfering with the deposition of metal ions. The result is the formation of organic compounds that are different from the original chemicals that were present in the addition agent. The brighteners in modern formulations do not form particularly harmful breakdown products, as evidenced by the long life of plating solutions. The baths will, however, eventually contain products that have to be removed either by dragout or adsorption on activated carbon.
Brittle deposits are also sometimes characterized by poor leveling. Carbon treatment can be very effective in restoring the ductility and leveling properties of a plating solution. Carbon treatments can be effective at rates of 1 to 6 grams of carbon per liter of solution volume. The use of carbon filtration (i.e., merely filtering the solution through much smaller amounts of carbon) is also worthwhile if the contamination is minor. Most plants utilizing a preventative maintenance scheme of control will filter continuously through fresh carbon to prolong the useful life of a plating solution between batch treatments. This procedure saves downtime and is probably cheaper in the long run.
Bench testing of different concentrations of carbon is the most satisfactory method for deciding if carbon treatment will improve the deposit. A panel is plated at 2 A for 10 minutes and the high-current-density corner of the panel is bent to get a good idea of deposit ductility before and after the contemplated treatment. A few minutes of laboratory work can save untold hours in the plant. There is no point in treating the whole plating solution if improvements cannot be detected in the Hull Cell. If an impurity does not respond to carbon treatment, try activated clay, peroxide or potassium permanganate treatment in addition to, or in combination with carbon.
A lab test should always be performed to determine the efficacy of the treatment. Allow the powdered carbon to react with the solution for 30 minutes with agitation at 140° F.
A deposit that has excellent leveling and brightness but is brittle may just be too high in organics due to addition of excessive amounts of brightener. To test this hypothesis, plate two or three consecutive Hull Cell panels in the laboratory, adjusting pH before each test. If the deposit becomes progressively more ductile, the same thing will happen in the tank if you shut off the brightener feeder and continue to plate.
Darkness in the Recesses
Darkness in the recesses can be caused by metallic impurities, high concentrations of the leveling agent, or interactions between the two. Analyzing the solution for copper, zinc, lead or cadmium by atomic absorption (AA) spectroscopy may give some indication of the type and extent of contamination. Compare the AA data to the baseline concentration of these elements in your solution. In other words, it is possible to have a certain level of metallic contaminants in the bath and still be able to plate without difficulty. A one-time analysis can be misleading due to the interacting effect of brighteners and metallics.
Darkness can be eliminated most easily and completely by dummy electrolysis. Both the metallic contaminants and organic brighteners are plated at 2 to 4 Amperes/foot of cathode area. Assuming the brightener feeder is turned off, and the source of metallics is eliminated, the levels of both bath constituents will drop to allow satisfactory plating. Prior to electrolysis, the tank should be dragged to remove dropped parts and equipment.
The harmful concentrations of metallic impurities and breakdown products from brightening agents is usually in the range of tenths of a gram/Liter. Because they are preferentially plated at the low current density being employed, these materials will be depleted rapidly from the cathode film. Therefore, agitation of the solution is imperative for efficient removal of the materials from the solution. It is important to start out with smooth, clean, nickel-plated steel dummy panels (preferably corrugated) and continue electrolysis until the deposit has a light gray color at the higher current-density edges.
Poor adhesion is in most cases due to a cleaning or activation problem of the basis metal or previous deposit. Some addition agents of copper cyanide and acid copper plating baths will have to be removed from the surface of the copper before nickel plating. An acid dip will sometimes help. In general, the pH of the last rinse prior to nickel plating should be 4 to 7.
High brightener concentration in the nickel bath can sometimes cause poor adhesion. The use of live entry has often been successful in preventing peelers. The reason may be because these addition agents are surface active and can passivate the basis metal before plating is initiated.
The plating of nickel over nickel is difficult. It may be necessary to activate the base nickel with several steps such as cathodic cleaning, rinsing, cathodic acid salt treatment, and hydrochloric acid dip. A Wood's nickel strike may be required between nickel layers in order to obtain perfect adhesion. Sometimes, it is better to use a very low voltage in the strike&emdash;just enough to get a very mild gassing at the nickel surface for 15 to 30 sec. Experiment with different voltages and concentrations to see what works best for you.
Some base metals, for instance the leaded steel alloys used for screw machine parts, may need special activation steps in order to get good adhesion. In this example, the use of fluoboric acid or other treatments as a predip has proven useful.
Every plated item will have an upper and lower surface in relation to its placement on the rack and in the plating tank. Roughness of the deposit is often due to particulate matter that settles on this upper surface during plating. The term "shelf roughness" was coined to describe this condition. Roughness is usually caused by precipitated iron, carbon or shop dirt. These precipitants can be detected either by passing a finger over the surface to feel for roughness or rubbing a cloth over the surface to see if the cloth threads become caught on it. Filtration is the cure, with additions of peroxide needed in the case of iron.
To remove iron, very cautiously use 1 to 2 pints of 30 percent hydrogen peroxide/1000 gal of solution. Premix the peroxide with 5 parts of water. If iron is a constant problem, run the bath at a pH of 4.5 to help precipitate the iron. You can also check for ripped anode bags or other obvious paths of entry of particulate matter.
Pitting and/or Blue Haze
While the two topics may not seem related, they often are caused by the same factors. Even though the surface tension may be between 40 and 45 dynes/cm, the use of additional wetting agent can sometimes eliminate pitting and hazes caused by high levels of brightener or impurities dragged into the bath.
There are three types of wetting agents. Low-foaming wetting agents are required for air-agitated baths. Wetting agents for still or cathode rocker baths can be high foamers, and, indeed, these are the only types that work in solutions that are not air agitated. The third type does not lower the surface tension of the bath to a great extent but is specially formulated to emulsify and negate the effects of dragged-in shop oils. The wetting agents should be added in increments of approximately 1/4 of a full charge or less.
Compressed air should not be used for agitating the plating tank because introduction of finely divided oil droplets or air into the tank can cause pitting of the deposits. A low-pressure blower should be used instead.
Black Spots on Barrel Work
Black spots are caused by ferrous iron dissolved in the bath. The spots correspond to the holes in the barrel. Add 1 pint of hydrogen peroxide/1000 gal of solution and run the filter to remove precipitated iron. Note that only dissolved iron, not the iron that is being picked up on the filter, will cause the spots.
While on the subject of barrel plating, it is useful to note that particulate matter really does no harm in barrel plating, and, for this reason, it is usually not necessary to bag your anodes. Removing the bags will allow improved corrosion of the anode and decrease the voltage requirements of the tank. The only reason to filter precipitated iron out of the solution is to prevent the iron from being dissolved when acid is added to the tank to adjust the pH.
This phenomenon is usually caused by the precipitation of aluminum, calcium, or iron in the cathode film. The pH of the film is higher than that of the solution, and iron may precipitate and become incorporated into the deposit.
Aluminum and calcium may precipitate as an extremely fine material in the body of the solution and cause what may be described as a microroughness showing up at the areas of high current density. The best way to remove these impurities is to filter the solution with an addition of peroxide in the case of iron. Calcium sulfate should be filtered at 155° F because it is less soluble at this high temperature. The filter should be cleaned after this treatment to prevent the calcium from redissolving at normal operating temperature.
The source of the impurities should be eliminated. Calcium is present in hard water and accumulates when tap water is added to make up evaporation losses. A sign that calcium is a problem is the clogging of air lines, which must be cleared by running water through them every morning.
A troubleshooting guide would not be complete without mentioning that many rejects encountered in nickel plating have very little to do with the bath itself. It is hoped that the information in this article can be used to help eliminate the nickel bath as the source of the problem, so the plater can concentrate his efforts on finding the actual source. The problems associated with preplate operations and defects due to the basis metal are subjects for another guide.
Hull Cell Tests
The Hull Cell can give misleading results but is an invaluable tool when used in combination with other laboratory equipment. While no one would suggest that a fully equipped laboratory brimming with technicians is all that is necessary to assure satisfactory plating, it is part of a successful plating operation.
A statesman of the plating industry, Dr. Don Swalheim, in an introduction to a lecture on the Hull Cell (part of the AESF Intensive Training Course series) said, "any plating foreman who does not know how to plate a Hull Cell panel should be fired on the spot." While many people in charge of plating operations have no business actually plating Hull Cell panels, they should be able to intelligently discuss and interpret laboratory results because thousands of dollars for man hours and materials can be conserved by testing theories in the laboratory rather than on the production floor. An extension of this idea is that many plating solutions can be saved from the waste-treatment pit if laboratory personnel can prove, via Hull Cell testing and other types of analyses, that there is nothing wrong with the solution or that an economical treatment is available for the problem.
Environmental concerns will make it imperative that all of our plating solutions last as long as possible. Laboratory control will become ever more important as the industry is forced to approach zero discharge of hazardous chemicals.