Troubleshooting Watts Baths Roughness. Roughness is generally the result of particulate matter suspended in the solution and adhering to the work, especially on shelf areas. Gross roughness may be traced to improper cleaning, torn anode bags, airborne dirt, dropped parts, precipitated calcium sulfate, inadequate filtration or carbon and filter aid from an improperly packed filter. A very fine type of roughness may be caused by precipitation of metallic contaminants in the cathode film where the roughness may be confined to a particular current density region. Chromium, iron and aluminum can precipitate as hydrates in the higher-current-density areas, where the film pH is normally higher than that of the body of the solution. A lower operating pH may be helpful in such cases. On occasion, high-current-density roughness has also been traced to a magnetic condition of the work. Another source of roughness can be the air blower used for air agitation. Inspection of the filter on the air blower may reveal that it could be defective or missing, or the source of intake air is dirty. If an external cause of roughness is not apparent, the quickest remedy is to pump the solution to a spare tank and inspect the plating tank. The cause may be apparent; dropped parts and torn anode bags are the most common sources. Pitting. Pitting and fine roughness are easily confused unless viewed under magnification from several angles. Pits that are very round and bright are generally caused by hydrogen gas adhering to the surface during plating. Lack of agitation, excessive current density or a low boric acid concentration may be indicated. An addition of the supplier’s wetting agent may be helpful in these cases. Dispersed air will give similar results and an indication may be a creamy appearance of the solution. Shut off the filtration and heat exchanger pumps and check for leaks in the intake piping and around the pump seal, which is the likely source. Large, splotchy pits or pitted areas are usually an indication of grease or oil. This could be carried in on the work from poor cleaning or grease-contaminated acid dips, or it may be dripping from some piece of overhead equipment. Such contaminants are either liquid or semi-solids that may be dispersed to some degree by heat, agitation and wetting agents. Their presence may not be apparent on the surface of the solution. Some organic decomposition products, low-solubility additives and wetting agents may give a similar condition. In these cases, such gross contamination dictates carbon treatment and tank clean-out. Small, irregularly shaped and spaced pits are likely to originate in the basis metal. Too often, additions of brighteners, wetting agents or acid are made before the basis metal is inspected. Look at several parts carefully and scribe any suspected areas prior to plating. All forms of pits and roughness typically have a negative impact on the corrosion resistance of the decorative deposits. Adhesion. Poor adhesion appears in many forms: nickel from basis metal; nickel from nickel; or subsequent chrome plate from nickel plate. Separation from the basis metal generally indicates that undesirable surface films are present and thus surface preparation has been inadequate. Poor cleaning may be caused by improper chemical maintenance and control of cleaners and acid dips; contamination and deterioration from prolonged use; poor rinsing; acid dips contaminated with copper, chromium or oil; or an inadequate process cycle for a particular soil or basis metal. Surface contamination will often be clearly visible or may be indicated by water breaks after rinsing. Cleaning problems generally involve much trial and error to identify their source. Try hand scrubbing between and skipping certain operations, hand precleaning or hand dipping parts in buckets of fresh acid solutions. If poor adhesion to the basis metal is traced to the nickel solution, severe contamination is indicated. Chances are that other problems such as poor ductility and stress will have given prior warnings. Of course, this does not rule out accidental spills and additions of wrong chemicals. Inadequate surface preparation such as with plastic, aluminum and zinc usually results in poor metal-basis metal adhesion. Nickel peeling from nickel is commonly caused by complete or partial loss of contact during nickel plating. Total loss may result in an overall peeling condition. Momentary or partial loss creates a bipolar condition in which current flow is from the lesser negative (poor or no contact) rack to the more negative (good contact) rack adjacent to it, resulting in an anodic oxide film. This will normally be confined to one area, such as the trailing edges of parts plated in an automatic machine. Bipolarity toward the end of the nickel cycle may appear as though the chromium is coming off as a powder. A thickness check of the peeled versus the adherent portion will help locate the general area of the problem. If there is no clear pattern and the condition is intermittent, a faulty rack is indicated. Knowledge of bipolarity and other electrically related problems is essential in nickel and chromium plating. Poor adhesion of bright nickel from semi-bright nickel or chrome plate from bright nickel, if not the result of electrical problems, can be caused by the nickel passivating during transfer. Long transfer times, drying of parts during transfer or warm rinses will increase the chances for nickel passivation. In these situations, the most common remedy is to activate the nickel prior to plating using an acid or acid salt. Ductility and stress. Poor ductility and high stress are primarily an indication of poor maintenance of the plating solution. These properties are influenced by metallic and organic contaminants, improper chemical or brightener balance and, in some cases, additive decomposition products. In all bright nickel processes, a balance of primary and secondary addition agents is required, as they function synergistically to maintain minimum stress and maximum ductility at the optimum degree of leveling and brightness. Many ductility, stress and chromium plating problems have been traced to out of balance secondary brightener levels. Abnormally high voltages resulting from a lack of anode area may result in oxidation or chlorination of some organic additives, which may not be removed by carbon. Check all materials that are to come in contact with the solution, such as filter aids and anode bags, for soluble organics that may be harmful. Good housekeeping, solution control, continuous carbon filtration and periodic batch carbon treatments are essential to control ductility problems. Dull deposits. Lack of brightness can be the result of rough base metal, poor cleaning, solution contamination, non-uniform agitation, improper chemical or brightener balance or failure to exercise proper control of operating conditions. A low pH or low temperature may cause an overall loss of brightness and poor leveling. Loss of brightness in a particular current density may be the first clue to organic or metallic contamination. Dullness from poor cleaning or organic contamination may appear in any current density area. Metallics generally exhibit their effects by either co-deposition in the low-current-density area or as hydrates in the high-current-density areas. Chemical analyses and plating tests will, in most cases, reveal the course of corrective action that should be taken if the problem is in the plating solution. Metallic impurities. Copper, lead, zinc, and cadmium, even in relatively small quantities (20–50 ppm), produce a dull, black or skip plate condition in the low-current-density areas. These metals may be removed by low-current-density dummy plating. Phosphates, silicates, aluminum, trivalent chromium and iron all tend to precipitate in the high-current-density areas. Their presence can result in a hazy, fine roughness or burned appearance. Their effects will, therefore, be less pronounced at the lower value of the operating pH range. These elements are best removed by high- pH treatment. Iron must be oxidized to the ferric state with peroxide before it can be removed. Iron contamination problems are minimized by the use of air agitation. Iron is oxidized by the air, precipitates around pH 4.0 and is removed by continuous filtration. A small amount of iron, precipitated in the body of the solution, appears to have far less effect on the deposit than when precipitated in the cathode film. However, lack of attention to dropped parts will result in frequent filter repacking. Iron and some other of the metallic impurities can be complexed to allow temporary relief from the problems associated with them. In the case of iron, control additives are available that allow the iron to be plated out in a controlled manner without seriously affecting appearance. In the case of other metallic contamination, proprietary additives are available to temporarily complex the metallics. However, the need for electrolytic purification still remains and must be performed at the earliest possible time. Hexavalent chromium causes highly stressed and non-adherent deposits in the high-current-density areas. Its removal is best accomplished by reducing it to the trivalent state, followed by high-pH precipitation. A predetermined amount of sodium bisulfite is an effective reducing agent, and, after high-pH precipitation and filtration, a small addition of peroxide will oxidize traces of excess sulfite to sulfate. A brief period of dummy plating would then be in order. The most common sources of chromium contamination are poorly maintained racks and spray (from the chromium plating tank) that accumulates on the conveyer members. Mist from the chromium solution and chromium drippings from the rack coatings or the conveyer are common sources of blistering when they contact the work, particularly on copper, copper alloys and plastic. Calcium. Calcium contamination can cause problems at concentrations of about 500 ppm. Typically the problem will be a fine roughness, which is often mistaken for pitting. If calcium contamination is causing the problem, its presence can be confirmed by shining a light through the hot (160°F) plating solution and looking for fine, needle-shaped crystals. Although the roughness could disappear when cooled, experience dictates that a removal treatment is needed. The preferred method is to precipitate calcium as the fluoride salt by the addition of 1.0–1.5 g/L of sodium bifluoride, followed by high-pH treatment and filtration. Calcium is not completely removed but is reduced to a more desirable level (100–200 ppm). Calcium problems are best avoided by using deionized water make up. Phosphoric and nitric acids. Contamination from these acids is unusual but has occurred. Both acids cause an extremely stressed and non-adherent deposit at high current density and removal is difficult. High-current-density dummy plating is effective in remedying low levels of this contaminant, but severe contamination could require solution disposal. Purification of nickel solution. There has been so much progress in nickel plating, and especially bright nickel, that prolonged and frequent purification treatments are rare. A simple carbon treatment, which may include peroxide, is generally sufficient and can be performed at some convenient production interval. Continuous treatment with a resin unit can be performed while plating. When the need for purification is indicated and the cause of the problem is not readily apparent, chemical analyses and plating tests should always be performed to determine the best course of action. If the tests duplicate the plating results, the task is somewhat easier, but, if they do not, further investigation in other areas would be in order. Too often, oxidation with peroxide or permanganate is tried without sufficient investigation. Commonly, one hears that these oxidizers “burn out” organics and oxidize them to carbon dioxide and water. In fact, sometimes the organic material is altered structurally, making carbon adsorption more efficient, or it may be oxidized to a more soluble form that has less deleterious effects on the deposit. But the oxidation could also result in a more soluble product that has a greater detrimental effect. Carbon (or resin) treatment is usually better as the first step. First carbon treat, then filter, then determine if an oxidization treatment is required. Permanganate is a more powerful oxidizer than peroxide, but its use as a treatment must include increasing the solution pH to precipitate and remove the manganese dioxide. This, coupled with unreacted carbonate and carbon, may result in filtration difficulties and abnormal solution losses. To avoid using excess permanganate, which can result in serious loss of ductility and other deposit properties, dilute a 25–50 ml bath sample to 100–150 ml, adjust to a pH of 3.0–3.5, heat to 150°F and titrate with a standard permanganate solution to a pink endpoint. Calculate the amount of permanganate reacted; then try about one-half of this amount in the plating bath in the lab. This technique is also useful in checking the effectiveness of other organic removal treatments. Several suppliers offer equipment that purifies nickel and dye-free acid copper plating solutions that operate like an ion-exchange unit. Like ion-exchange, this purification system can be regenerated giving the purifying material many years of useful life. These units can replace batch carbon treatments by keeping the plating solutions at the purity level of almost new solutions for optimum plating performance. Some of these purification units remove more organic contaminates than carbon (even with peroxide/permanganate) and some have additional columns to remove metallic impurities. A reduced but continuous filtration with carbon is still recommended. Copper, lead, zinc, cadmium and some organics can be removed by low-current-density electrolysis. The most efficient current density may vary to some degree for each metal, but 2–5 asf of cathode surface can be tried first. Corrugated iron is ideal for cathodes since it will provide a favorable distribution of current density. The pieces of corrugated iron should be as long as the plating rack and should be cleaned, pickled and nickel plated first at normal current density in order to avoid additional contamination of the solution being dummied. The cathode area should be as large as possible and good circulation or agitation of the solution should be employed. Inspect the cathodes for flaky or powdery deposits and occasionally raise the current density a few minutes as a seal. When finished, be sure to raise the current density again to seal in the contaminants. Frequent stripping of the dummy sheets or using new nickel-plated ones is important to not contaminate the solution.
Posted on: Sun, 11 Aug 2013 15:53:18 +0000
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