Why Nickel Plate?
Nickel is electrodeposited (plated) for many reasons. First and foremost nickel provides a decorative appearance because of its ability to cover imperfections in the base metal (leveling). This deposit can be made brilliant and when covered by a thin layer of decorative chrome will maintain its brilliance even under severe conditions. When nickel is applied in "duplex" form, excellent corrosion protection can also be achieved. This requires plating two different kinds of nickel (semi-bright and bright nickel). Nickel deposits also offer more wearability than softer metals such as copper or zinc and thus can be used when wear resistance is needed. Because nickel is magnetic, nickel can sometimes be plated where the ability to be magnetized is needed. Finally, nickel can be made to plate with little or no stress and is therefore used for electroforming or for aerospace applications where stress needs to be held to a minimum. In many applications, many of these requirements are required simultaneously so nickel is often not plated for just one reason.
Bright nickel plating is used extensively in automotive applications such as on plated wheels, bright trim, truck exhausts and bumpers, and restorations. In other transportation areas, nickel is used for the bright work on motorcycles and bicycles. Nickel is used to achieve brightness on hardware, such as hand tools. In the home, bright nickel is used on plumbing fixtures, light fixtures, appliances, and wire goods (racks). Bright nickel is also used for tubing applications such as on furniture and wheel chairs. Most of these applications for bright nickel rely on the nickel for a decorative appearance with corrosion protection and wearability.
Nickel is also used for engineering purposes where brightness is not desired. Thus nickel is used on molds to provide wearability, on coins, in jewelry and circuit boards as a barrier layer, on strip steel, in aerospace for low stress or for re-sizing, and in composites where a dispersed inorganic is co-deposited (such as silicon carbide). Most engineering applications use sulfamate nickel although nickel plated strip steel uses a nickel chloride/nickel sulfate bath.
The most commonly used nickel baths are Watts baths which use a combination of nickel sulfate and nickel chloride. This combination of nickel salts allows for a variety of nickel electrodeposit characteristics. The remaining focus of this discussion will be on Watts-type nickel plating baths.
Watts Bath Components
A typical Watts bath contains nickel sulfate, nickel chloride, and boric acid. Table I presents typical ranges for the components.
Each component of the Watts formulation performs a very important and necessary role in the production of satisfactory deposits.
Nickel Sulfate. Nickel sulfate is the source of most of the nickel ions and is generally maintained in the concentration range of 20-40 oz/gal (150-300 g/liter). It is the least expensive nickel salt and the sulfate anion has little effect on deposit properties. Nickel sulfate is usually maintained at the high end of the range for very bright applications where throwing power is not a major consideration. It is maintained at the lower end of the range for applications where throwing power is needed, such as in barrel plating.
Nickel Chloride. Nickel chloride is essential for good anode corrosion and improves the conductivity of the plating bath. The typical operating range is 4-20 oz/gal (30-150 g/liter). A concentration of four oz/gal (30 g/l) nickel chloride is considered minimum for anode corrosion unless special forms of anode material that contain nickel sulfide or nickel oxide are used as depolarizers. Low nickel chloride concentrations are used when throwing power is not a major consideration or when low deposit stress is required. This is especially true in semi-bright nickel plating where high chloride would yield a highly stressed deposit. High concentrations of nickel chloride are used when greater throwing power is needed and increased stress can be tolerated; such as in barrel plating.
| Table I—Typical Components of Watts Baths |
| Component |
Bright Nickel
Plating |
Semi-bright Nickel Plating |
Nickel Sulfatea
Nickel Chloride
Boric Acid |
(oz/gal)
20 - 40
8 - 20
5 - 7 |
(g/liter)
150-300
60-150
37.5-52.5 |
(oz/gal)
30 - 40
4 - 6
5 - 7 |
(g/liter)
225-300
30-45
37.5-52.5 |
| By convention, the amounts of nickel sulfate refer to the hydrated salt, NiSO4· 6H2O. Likewise, nickel chloride amounts refer to NiCl2· 6H2O. |
Total Nickel. This is an expression used for the combined nickel ions from the nickel sulfate and nickel chloride. A typical Watts formulation of 36 oz/gal nickel sulfate (22.3 pct nickel) and 12 oz/gal nickel chloride (24.6 pct nickel) would have 11 oz/gal (82 g/liter) of total nickel. This concentration is generally adequate, but as current density requirements increase, the increased depletion rate should be offset by an increase in nickel ion concentration. The total nickel ion concentration is a significant factor in the limiting current density, or the point at which nodular or burned deposits become evident.
Boric Acid. Boric acid buffers the hydrogen ion concentration (pH) in the cathode film. If it were not for this buffering action, the cathode film pH in the higher current density regions would very quickly exceed 6.0 and nickel hydroxide would be precipitated and co-deposited along with hydrogen, resulting in a green nodulation or burned deposit. An indication of low boric acid concentration is the appearance of pitting or roughness in high current density regions. Boric acid, therefore, plays a very important role in establishing the upper limits of the applied current density.
Organic Brighteners. In order to obtain uniform, bright deposits it is necessary to add brighteners to a Watts bath. These are typically organic compounds which modify the nickel electrodeposit to achieve a desired appearance. Brighteners for semi-bright nickel are designed to give a uniform nickel deposit where sulfur does not co-deposit with the nickel (see below). Brighteners for bright nickel typically include a carrier additive which adds sulfur to the deposit, provides ductility, and gives a uniform grain structure. A secondary brightener is included which works with the carrier to provide a high degree of luster. Finally, in some formulations, a leveler is added to provide extreme brilliance through a leveling mechanism. It is critical that these organic components remain in balance and therefore they are provided in proprietary packages from metal finishing suppliers. These packages are formulated to achieve the best combination of stability, brilliance, ductility, and ease of use for various applications.
Bright and Semi-bright Nickel. Most nickel plating requires a bright deposit and therefore most applications use the bright nickel plating formulations given in Table 1. However, bright nickel deposits contain enough co-deposited sulfur that they are more electrochemically active than nickel without sulfur and therefore corrode at an increased rate. In severe environments, this can lead to an early penetration of the bright nickel deposit and the subsequent rapid corrosion of the base metal. In order to solve this problem, a layer of nickel which does not contain sulfur can be deposited prior to the bright nickel deposit. This sulfur-free deposit is called semi-bright nickel and is electrochemically less active than the bright nickel. The corrosion protection afforded to the base metal is increased significantly by the use of semi-bright nickel. Special additives are used in semi-bright nickel to achieve a uniform deposit without adding sulfur to the deposit. The combination of plating a semi-bright nickel layer followed by a bright nickel layer is called "duplex" nickel with the semi-bright nickel layer typically at least twice as thick as the bright nickel layer for best corrosion protection.
Special Nickel for Microporous Chromium To achieve the best possible corrosion protection, a "special" nickel can be deposited following the duplex nickel. This special nickel layer is very thin and contains co-deposited particles. These particles act to create micropores in the subsequent decorative chrome layer. This "microporous chrome" provides added corrosion protection because the corrosion cell has spread out over the entire surface rather than being focused in isolated spots. The special nickel bath used for this application is normally a Watts-type bath with minimal amounts of organic brighteners and the suspended particles. This special nickel is usually only required for outdoor exposure applications, such as exterior automotive parts. A good article which summarizes exposure testing of nickel deposits is given by Snyder in the June 1992 issue of Plating & Surface Finishing.
Operating Conditions
The operating conditions for almost all Watts-type nickel baths are similar. These typical parameters are given in Table II.
pH. Bright or semi-bright baths are generally operated between pH 3.5 and 4.2. Most organic addition agents give optimum brightness and leveling in this range. Higher pH values always present the danger of adverse effects from the precipitation of metallic contaminants and increased consumption of brightener components.
The pH should rise slowly during operation, since the cathode efficiency is slightly lower than the anode efficiency. Sulfuric acid should be used for pH adjustment, although hydrochloric acid may also be used with the added advantage of maintaining the chloride ion concentration. However, the disadvantages of using hydrochloric acid include not only the higher amounts required but the escaping hydrogen chloride gas, especially from a hot, air agitated solution. Nickel carbonate is preferred for increasing the pH. It dissolves quite readily below a pH of 4.0. Very small adjustments to air agitated solutions can be made below this value by adding a water-carbonate slurry while the tank is not in operation. Larger adjustments are best made in a treatment tank, followed by filtration.
If the pH requires no adjustment or if it decreases, look for anode problems. Insufficient anode areas, the overuse of inert auxiliary anodes, plugged anode bags or poor anode contact might be the cause. If not eliminated, these problems can quickly lead to salt depletion, poor plate distribution and off-color deposits from brightener decomposition. If the pH rises abnormally, it is rarely a cathode efficiency problem, provided the solution is in chemical balance. It is more likely that the acid is reacting with dropped parts, a portion of the tank wall, or alkaline cleaner solution carried in on poorly maintained racks.
Agitation and Temperature. Agitation and temperature increase the diffusion rate of ions into the cathode film. This is required in order to prevent burning and also to allow the brightener additives to reach the cathode film.
Air agitation from a low-pressure blower has been universally accepted and is a contributing factor in many improvements in nickel plating, especially in the decorative area. Air agitation has broadened operating ranges of bath ingredients, reduced the required concentrations of addition agents and minimized the use of wetting agents and hydrogen pitting problems. Note that the use of air agitation will cause particulate matter to become suspended in the solution, thus resulting in rough deposits unless good filtration practices are used.
| TABLE II—Typical Operating Parameters for Watts Nickel |
Operating Parameters
pH
Temperature
Agitation
Filtration |
Typical Range
3.5 - 4.2
135 - 145F (57 - 63C)
Aira
Continuous with packed carbon |
| a Air is by far the most usual form of agitation. In its place, mechanical agitation can be tried but (except for barrel) this is not often used. |
The temperature range is important in terms of physical properties and, along with agitation, aids in keeping the bath components mixed and solubilized. The temperature range is also an important factor in addition agent response. If the temperature is too high, the addition agent consumption is increased adding to the expense of operation and possible plating problems. If the temperature is too low, the boric acid will begin to precipitate and the brighteners will not respond efficiently.
Filtration. The value of adequate continuous filtration for prevention of roughness and pitting cannot be overemphasized. Because most bright nickel addition agents are not removed to any great degree by activated carbon. Good filtration over an activated carbon pack tends to keep concentrations of foreign organics, brightener decomposition products and particulate matter at a minimum. A well maintained, carbon packed filter of adequate capacity tends to keep the physical properties of the deposit near optimum and minimizes the need for frequent batch treatments. (It is better to apply smaller amounts of carbon at regular intervals over the normal repacking cycle than to add the total amount in one charge. This maintains the efficiency of the carbon pack by keeping fresh carbon on the surface and minimizing the tendencies of channeling of the solution through less-restricted areas.) A suggested rate of use for carbon packing filters is one to two pounds of carbon per 1000 gallons of nickel solution per 40-80 hours of operation. The rule of thumb is that the minimum hourly discharge rate of the filter should equal the volume of the solution. To achieve this with a carbon pack and as insolubles are collected, it is obvious that the filter should have two to three times the capacity in order to avoid frequent repackings.
Problems and Remedies
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 it 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.
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. Pitted 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 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.
Adhesion. Poor adhesion appears in many forms: nickel from base metal; nickel from nickel; bright nickel from semi-bright nickel; or subsequent chrome plate from nickel plate. Separation from the base 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 perhaps 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 pre-cleaning 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.
Nickel peeling from nickel is generally caused by complete or partial loss of contact during nickel plating. Total loss for a period may result in an overall peeling condition. Momentary or partial loss creates a bipolar condition in which the 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. A knowledge of bipolarity and other electrically related problems is essential in nickel and chromium plating. An excellent discussion by Guernsey can be found in the February and March 1976 issues of Plating & Surface Finishing.
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 or warm rinses will increase the chances for nickel passivation. The most common remedy is to activate the nickel prior to plating by the use of an acid or acid salt.
Ductility and stress. Poor ductility and high stress are primarily an indication of a poor condition of the plating solution. These properties are influenced by metallic and organic contaminants, improper chemical or brightener balance, and in some cases brightener 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 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 repackings.
Iron and many 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 which allow the iron to be plated out in a controlled manner without serious 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 is allowed to accumulate 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 and copper alloys.
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, 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/liter 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 disposal of the solution.
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. 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 treatment is usually better as the first step. First carbon treat, filter, and 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 milliliter bath sample to 100-150 milliliters, adjust to a pH of 3.0 to 3.5, heat to 150F 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.
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 amp/ft2 of cathode surface will be effective. 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.
Troubleshooting
When problems arise, the first place to look is the plating line. Look for recurring patterns in regards to the problem. Do only the top parts on a rack show the problem? Does every rack show the problem? Can you see the problem on parts prior to the plating tank? Could this be an electrical problem? If you hand wipe a part, is it better or worse? The vast majority of "plating" problems are really mechanical problems or cleaning problems. Look at the line before spending a lot of time in the lab trying to "fix" the plating bath. Once you have narrowed the problem to the bath, then proceed to the lab to find a way to solve it.
Analytical measurements of organic impurities in the presence of the many organic materials needed to give brightness is non possible. Therefore, qualitative plating tests must be relied upon. The use of a Hull cell or jiggle cell for qualitative testing of nickel baths is practically mandatory for maintaining and troubleshooting nickel plating baths.
In all troubleshooting cases, the first thing to check is the basic bath chemistry; the nickel metal, chloride, boric acid, pH, and temperature. These simple checks are easily overlooked in the rush to find the cause of problems.
In the laboratory, one may start with a simple filtration if the problem is roughness. For dullness or poor physical properties, after analysis and adjustment of the basic components, try the following treatments in the order listed by running a plating test: carbon treatment; carbon treatment at high pH; low pH peroxide carbon; and low pH peroxide or permanganate followed by high pH carbon. Some improvement in plating results may indicate the need for more carbon, peroxide or electrolysis. Initially, start with a larger volume of plating solution than required so that division is possible at some point as a step saver to the next alternative treatment. A treatment that appears effective should be duplicated with a fresh sample and one plating test. Too many panels plated in one solution may have a purification effect. Check to see that the correct brightener response can be obtained after treatment. Very often, a treatment will appear to be effective due to a combination of some brightener and contaminant removal, only to reappear when the brightener is added.
Prevention. Problems occur in the best of shops, but by knowing the function of each chemical in the bath, the effect of operating conditions and proper methods of maintaining equipment and solutions, you can stay out of trouble most of the time. You must have a conscientious program for keeping all nickel plating variables within the limits required. PFD
