Quality Control of Nickel Plating

Achieving high quality involves controlling the bath composition, the purity of the plating solution, and the thickness and uniformity of the deposits. Eliminating rejects, troubleshooting, and the testing of deposits are important aspects of process and product control.

Control of Solution Composition. Control of the composition of the plating bath is one of the most important factors contributing to the quality of nickel deposits. At the outset, the bath must be prepared to the specified composition, adjusted to the proper pH, and purified before use. Thereafter, the composition and pH of the solution must be controlled within specified limits, and contamination by metallic and organic substances must be prevented.

Purification Techniques and Starting Up a New Bath. Before any freshly prepared nickel plating bath is used, contaminants such as iron, copper, zinc, and organics present in trace quantities in commercial salts must be removed to obtain the best results. Several treatments are available for purifying a freshly prepared nickel plating solution.

High-pH treatment consists of adding nickel carbonate to the hot solution until a pH of 5.0 to 5.5 is obtained. This precipitates the hydroxides of metals such as iron, aluminum, and silicon, which in turn frequently absorb other impurities. Addition of hydrogen peroxide oxidizes iron to the ferric state, making it more easily precipitated at high pH, and frequently destroys organic impurities.

Treatment with activated carbon removes organic impurities.

Electrolytic purification removes most of the harmful metallic and organic impurities. A complete purification procedure for a Watts solution would comprise the following steps:

1. Use a separate treatment tank (not the plating tank) to dissolve the nickel sulfate and nickel chloride in hot water at 38 to 49 °C (100 to 120 °f) to about 80% of desired volume.

2. Add 1 to 2 m/L (0.8 to 1.6 pints/100 gal) of 30% hydrogen peroxide (agitate briefly and allow to settle for 1 h).

3. Add 1.2 to 2.4 g/L (1 to 2 lb/100 gal) activated carbon and agitate thoroughly.

4. Heat to 66 °C (150 °F), then add 1.2 to 2.4 g/L (1 to 2 lb/100 gal) of nickel carbonate to the solution, with agitation to adjust the pH to 5.2 to 5.5. More nickel carbonate may be required and the mixture should be stirred to assist the dissolution of the carbonate. Allow to settle 8 to 16 h.

5. Filtering into the plating tank.

6. Add and dissolve boric acid; add water to bring bath up to its desired volume.

7. Electrolytically purify by using a large area of nickel plated corrugated steel sheets as cathodes. The average cathode current density should be 0.5 A/dm2 (5 A/ft2), and treatment should continue until 0.5 to 1.3 A • h/L (2 to 5 A • h/gal) have passed through the solution. The solution should be agitated and the temperature held at 49 to 60 °C (120 to 140 °F). It is useful to prepare deposits at normal current densities at some point to check appearance, stress, and sulfur content. If not acceptable, continue dummying until the properties are acceptable.

8. Remove the dummy cathodes and adjust the pH of the solution to the desired value.

Controlling the Main Constituents. The following basic constituents of nickel plating baths must be regularly controlled: the nickel metal content; the chloride concentration; the boric acid; and any organic addition agents. Nickel metal concentration is maintained between 60 and 80 g/L (8.0 and 10.5 oz/gal) in most commercial applications. It is desirable to have a minimum of 23 g/L (3 oz/gal) of nickel chloride in the solution to promote anode corrosion. (The chloride content is not critical for anode corrosion when sulfur-activated anode materials are used.) Boric acid is the most commonly used buffering agent for nickel plating baths. Boric acid is effective in stabilizing the pH in the cathode film within the ranges normally required for best plating performance. It is available in a purified form and is inexpensive. Organic addition agents must be controlled within the limits specified by the suppliers of proprietary processes, and they must be replenished due to losses from dragout, electrolytic consumption, and the effects of carbon filtration (or batch treatment).

Procedures exist for chemical analysis of nickel, chloride, boric acid, and organic addition agents in nickel plating solutions, and modern instrumental techniques are available to monitor the main ingredients on a regular basis with improved precision. High-performance liquid chromatography is one of the improved techniques for controlling organics that is growing in popularity.

Controlling pH, Temperature, Current Density, and Water Quality. The pH of the nickel plating solution will rise during normal operation of the bath, necessitating regular additions of acid to maintain the pH within the prescribed limits. A decrease in pH accompanied by a decrease in nickel ion concentration indicates that the process is not functioning properly.

The operating temperature may have a significant effect on the properties of the deposits, and it should be maintained within specified limits (±2 °C) of the recommended value. In general, most commercial nickel plating baths are operated between 38 to 60 °C (100 to 140 °F).

The nickel plating process should be operated at specified current densities by estimating the surface area of the parts and calculating the total current required. The practice of operating the process at a fixed voltage is not recommended.

Controlling cathode current density is essential for accurately predicting average nickel thickness, for achieving uniform coating thickness on complicated shapes, and for producing deposits with consistent and predictable properties.

Since current density determines the rate of deposition, it must be as uniform as possible to achieve uniformly thick nickel deposits. The nickel plating solution has an electrical resistance, and almost all components to be plated have prominent surfaces that are nearer the anode than recessed areas. The current density is greater at the prominences because the anode-to-cathode distance is shorter and therefore has less electrical resistance. The apportioning of the current in this way is called current distribution. This means that the recessed areas receive a thinner nickel deposit than the prominent ones. Current distribution is controlled by proper rack design and proper placement of components on those racks, by the use of nonconducting shields and baffles, and by the use of auxiliary anodes, when necessary. With care, relatively good thickness distribution can be achieved.

The quality of the water used in making up the bath and in replacing water lost by evaporation is important. Demineralized water should be used, especially if the local tap water has a high calcium content (greater than 200 ppm). Filtering the water before it is added to the plating tank is a useful precaution to eliminate particles that can cause rough deposits.

Controlling Impurities. Inorganic, organic, and gaseous impurities may be introduced into nickel plating solutions during normal operations. Continuing efforts to eliminate the sources of these impurities from the plating shop can improve the quality of the deposits, as well as productivity and profitability. The presence of small quantities of inorganic or organic contamination may result in plating defects.

Inorganic contaminants arise from numerous sources, including nickel salts of technical grade, hard water, carryover from acid dip tanks, airborne dust, bipolar attack of metallic immersion heaters, corrosion of the tank material through cracks in the lining, corrosion of anode bars, and dirt from structures above the tank and from parts that fall into the solution and are not removed. The following table lists maximum limits for metallic impurities in nickel plating baths:

Contaminant

Maximum concentration, ppm

Aluminum

60

Chromium

10

Copper

30

Iron

50

Lead

2

Zinc

20

Calcium

(a)

Note: The limits may be different when several contaminants are present at the same time, and complexing agents are a part of the solution formulation.

(a) pH-dependent; will precipitate at the saturation point

The degree of contamination by many inorganic materials may be controlled by continuous filtration and dummying, that is, by electrolysis of the plating solution at 0.2 to 0.5 A/dm2 (2 to 5 A/ft2). This may be accomplished on a batch basis or continuously by installing a dummy compartment and overflow dam at one end of the plating tank. Solution from the filter is pumped into the bottom of the dummy compartment, up past the corrugated cathode sheets, over the dam, into the plating section of the tank, out through a bottom outlet at the far end of the tank, and back to the filter. Solid particles and soluble metallic impurities (for example, copper, zinc, lead) are removed simultaneously by this procedure.

Organic contaminants may arise from many sources, including buffing compounds, lubricating oil dropped from overhead equipment, sizing from anode bags, weaving lubricants on plastic anode bags, uncured rack coatings or stopoff lacquers, adhesives on certain types of masking tape, decomposition products from wetting agents, organic stabilizers in hydrogen peroxide, paint spray, and new or patched rubber tank linings. Many organic contaminants can be effectively removed from nickel plating solutions by adsorption on activated carbon on either a batch or a continuous basis. On a batch basis, the solution is transferred to a spare tank, heated to 60 to 71 °C (140 to 160 °F), stirred for several hours with a slurry of 6 g/L (5 lb/100 gal) minimum of activated carbon, permitted to settle, and then filtered back into the plating tank. It is usually necessary to do a complete chemical analysis and adjust the composition of the solution after this type of treatment.

For solutions in which organic contamination is a recurring problem, continuous circulation of the solution through a filter, coated at frequent intervals with small amounts of fresh activated carbon, is recommended. When continuous carbon filtration is used, the wetting agent in the solution must be replenished and controlled more carefully, to prevent pitting of the nickel deposits.

Gaseous contamination of nickel plating solutions usually consists of dissolved air or carbon dioxide. Dissolved air in small amounts may lead to a type of pitting characterized by a teardrop pattern. Dissolved air in the plating solution usually can be traced to entrainment of air in the pumping system when the solution is circulated. If this occurs, the circulating pump and valves should be checked and modified, if necessary. Nickel plating solutions can be purged of dissolved air by heating to a temperature at least 6 °C (10 °F) higher than the normal operating temperature for several hours. The solution is cooled to the operating temperature before plating is resumed. Dissolved carbon dioxide in a nickel plating solution is usually found after nickel carbonate has been added to raise the pH, and it is liberated from warm nickel plating solutions after several hours. If solutions containing carbon dioxide are scheduled for immediate use, they should be purged by a combination of heating and air agitation for approximately 1 h at 6 °C (10 °F) or more above the normal plating temperature.

Effects of Impurities on Bright Nickel Plating. The presence of impurities is especially troublesome in decorative nickel plating. Contamination by zinc, aluminum, and copper is most often caused by the dissolution of zinc-base die castings that have fallen from racks into the plating tank and have been permitted to remain there. Inadequate rinsing before nickel plating increases the drag-in of metallic elements. The presence of cadmium and lead may be attributed to a number of sources, including lead-lined equipment and tanks, impure salts, and drag-in of other plating solutions on poorly rinsed racks. Chromium is almost always carried into the nickel solution on rack tips that have not been chromium stripped, or on poorly maintained racks that have been used in a chromium plating tank and have trapped chromium plating solution in holes, pockets, and tears in the rack coating.

Metallic contaminants affect bright nickel deposition in several ways. Aluminum and silicon produce hazes, generally in areas of medium to high current density. Aluminum and silicon may also cause a fine roughness called "salt and pepper" or "stardust." Iron produces various degrees of roughness, particularly at high pH. Calcium contributes to needlelike roughness as a result of the precipitation of calcium sulfate when calcium in solution exceeds the saturation point of 0.5 g/L (0.06 oz/gal) at 60 °C (140 °F). Chromium as chromate causes dark streaks, high-current-density gassing, and possibly peeling. After reduction to the trivalent form by reaction with organic materials in the solution or at the cathode, chromium may produce hazing and roughness effects similar to those produced by iron, silicon, and aluminum. Copper, zinc, cadmium, and lead affect areas of low current density, producing hazes and dark-to-black deposits.

Organic contaminants may also produce hazes or cloudiness on a bright deposit, or they can result in a degradation of mechanical properties. Haze defects may appear at any current density, or they may be confined to narrow current density ranges.

Mechanical defects producing hairline cracks, called macrocracking, may be encountered if the coating is sufficiently stressed as a result of solution contamination. These cracks usually appear in areas of heavier plating thickness (higher current density) but are not necessarily confined to those areas.

Eliminating Rejects/Troubleshooting. The production of defective plated parts or rejects may be associated with the presence in solution of soluble and insoluble impurities. The nature of the coating defect is often an indication of the source of the problem. Common defects include roughness, pitting, blistering (often associated with poor preparation of the surface prior to plating), high stress and low ductility, discoloration, burning at high-current-density areas, and failure to meet thickness specifications.

Roughness is usually caused by the incorporation of insoluble particles in the deposit. In bright nickel baths, chlorine generated at an auxiliary anode that is close to the cathode can react with organic additives to form an insoluble material that is incorporated in the deposit. Insoluble particles may enter the solution from many sources: incomplete polishing of the base metal so that slivers of metal protrude from the surface, incomplete cleaning of the surface so that soil particles remain on the surface, detached flakes of deposit from improperly cleaned racks, dust carried into the tank from metal polishing operations and other activities, insoluble salts and metallic residues from the anode, and others.

Roughness from incomplete polishing, cleaning, and inadequate rack maintenance is avoided by good housekeeping, regular inspection, and control. Roughness caused by dust can be controlled by isolating surface preparation and metal polishing operations from the plating area, by providing a supply of clean air, and by removing dirt from areas near and above the tanks. Roughness caused by the precipitation of calcium sulfate can be avoided by using demineralized water. Continuous filtration of the plating solution so as to turn over the solution at least once an hour is important for minimizing roughness problems. Anode residues must be retained within anode bags, and care should be taken not to damage the bags or allow the solution level to rise above the tops of the bags.

Pitting is caused by many factors, including adhesion of air or hydrogen bubbles to the parts being plated. Air should be expelled as already mentioned. Pitting from adherent hydrogen bubbles can result from a solution that is chemically out of balance, has too low a pH, or is inadequately agitated. Other sources of pitting include incorrect racking of complicated components, too low a concentration of wetting or antipitting agents, the use of incompatible wetting agents, the presence of organic contaminants, the presence of copper ions and other inorganic impurities, incomplete cleaning of the base material, and incomplete dissolution of organic additives that may form oily globules. Pitting is therefore avoided by maintaining the composition of the plating solution within specified limits, controlling the pH and temperature, and preventing impurities of all kinds from entering the solution.

Blistering may be associated with poor adhesion resulting from poor or incorrect surface preparation prior to plating. Nickel can be deposited adherently on most metals and alloys, plastics, and other materials by following standard methods of preparation and activation, including the proper use of intermediate deposits such as cyanide copper, acid copper, and acid nickel chloride strikes. Standard procedures for the preparation of materials prior to electroplating can be found in handbooks and in the Annual Book of ASTM Standards, Volume 0.205. Blistering may also be related to incomplete removal of grease, dirt, or oxides, formation of metal soaps from polishing compounds, or formation of silica films from cleaning solutions. In the case of zinc-base die castings or aluminum castings, blistering during or immediately after plating may be due to surface porosity and imperfections that trap plating solution under the coating.

High stress and low ductility usually occur when organic addition agents are out of balance, and also because of the presence of organic and inorganic impurities. Solutions must be maintained in a high state of purity.

Discoloration in low-current-density areas is most likely the consequence of metallic contamination of the plating solution. The effects can be evaluated systematically by plating over a reproducible range of current densities on a Hull cell cathode. Hull cells are available from plating supply houses and are shaped so that nickel can be deposited onto a standard panel over a predictable range of current densities. The variation in current density over the face of the panel is achieved by placing the panel at a specified angle to the anode. Bent panels that are L-shape and plated with the recessed area facing the anode can also be used to assess discoloration at low-current-density areas, and they may provide information on roughness problems.

Burning at high current densities can be caused by applying the full load on the rectifier to the lowest parts on a rack as it is lowered into the tank. This can be controlled by applying a reduced load or ramping the current during immersion of the rack. Burning can also be caused by exceeding the recommended maximum cathode current density, the presence of phosphates in solution introduced via contaminated activated carbon, or incorrect levels of organic additives.

Failure to meet thickness specifications is most frequently due to the application of too low a current and/or too short a plating time. This can be avoided by measuring the area of the parts to be plated, then calculating the total current required for a specified current density and plating for the appropriate time (see Table 1). Another major cause of failure to meet thickness requirements is nonuniform distribution of current leading to insufficient deposition in low-current-density areas. Poor electrical contacts and stray currents can also cause thin deposits, and anode and cathode bars, hooks, and contacts should be kept clean.

Controlling and Testing Deposit Properties. The requirements for testing electrodeposited nickel coatings may vary significantly, depending on the application. In almost all decorative applications, the appearance and the thickness of the deposit should be controlled and monitored on a regular basis. The plated surface must be free of defects such as blisters, roughness, pits, cracks, discoloration, stains, and unplated areas. It must also have the required finish--bright, satin, or semibright. Quality can only be maintained by checking the thickness of a specified number of plated parts. In decorative, electrodeposited multilayer coatings, it is also important to control the sulfur contents of the deposits, the relative thicknesses of individual layers, the ductility of the semibright nickel layer, and the difference in the electrochemical potentials between individual layers. Requirements for corrosion performance and adhesion may also be specified and may require additional testing.

In engineering and electroforming applications, it may be necessary to monitor the mechanical properties, including hardness, tensile strength, ductility, and internal stress, as well as wear resistance and other properties. Some of the more important test methods are briefly outlined below. Additional details can be found in the standard test methods collected in the Annual Book of ASTM Standards.

Thickness may be measured using a variety of techniques. The coulometric method described in ISO 2177 and ASTM B 504 can be used to measure the chromium and nickel thicknesses, as well as the thickness of copper undercoats, if present. The coulometric method measures the quantity of electrical energy required to deplate a small, carefully defined area of the component under test. A cell is sealed to the test surface and filled with the appropriate electrolyte, and a cathode is inserted. The component is made the anode, and the circuit is connected to the power supply via an electronic coulometer. By integrating time in seconds with the current passing, the electronic coulometer provides a direct reading in coulombs; modern instruments provide a direct reading of thickness. The completion of the deplating is shown by a marked change in the applied voltage. For routine control of production, it is convenient to monitor nickel thickness nondestructively by means of a magnetic gage, calibrating the gage at intervals with standard samples. Instruments for measuring thickness by beta backscatter, X-ray spectrometry, and eddy current techniques are also available. The traditional method of measuring thickness by microscopic examination of a metallographically prepared cross section of the plated part is still employed, but it is time-consuming, expensive, and destructive.

The simultaneous thickness and electrochemical potential (STEP) test was developed to measure the difference in electrochemical potential between semibright and bright nickel layers in multilayer nickel deposits on parts that are plated in production. It is similar to the coulometric method just described. By including a reference electrode in the circuit, however, it is possible to measure the electrochemical potential of the material being dissolved at the same time that the thickness of the individual layers is being measured. For example, with a double-layer nickel coating, a relatively large change in potential occurs when the bright nickel layer has dissolved and the semibright nickel layer begins to be attacked. The potential difference is related to the overall corrosion resistance of the double-layer coating and should be greater than 100 mV. Details can be found in ASTM B 764.

Corrosion testing may be specified and may require the plater to perform accelerated corrosion tests on a specified number of production parts as part of an overall quality assurance requirement. Three accelerated corrosion tests are recognized internationally: the Copper-Accelerated Acetic Acid Salt Spray (CASS), the Corrodkote, and the Acetic Acid Salt Spray tests. The CASS test is the one most widely used. The CASS and Corrodkote tests were developed when conventional chromium was the only type of chromium available; when the accelerated tests are used to evaluate microdiscontinuous chromium coatings, the surface appearance deteriorates more rapidly than in real-world environments. Details of these three tests can be found in ISO 1456 as well as in ASTM standards. The salt spray tests involve the application of the corrosive solution in the form of a spray or fog inside a fog cabinet or room made or lined with glass, rubber, or resistant plastics. The Corrodkote test involves applying a corrosive slurry to parts and exposing them to high, controlled humidity in a suitable chamber or cabinet; the slurry is formulated to simulate road mud containing corrosive salts. The CASS and Corrodkote tests were developed to control the quality of decorative, electrodeposited nickel-chromium-plated parts for exterior automotive use under severe conditions of corrosion and abrasion. CASS and other corrosion test requirements are specified in ASTM B 456 for nickel-plus-chromium coatings applied to steel, zinc alloys, or copper alloys. Similar information for nickel-plus-chromium coatings on plastics is given in ASTM B 604.

Ductility testing is used in decorative nickel plating to test that the percent elongation of semibright nickel deposits is greater than 8, and to verify that bright nickel solutions are in good working condition. The simple test described in ISO 1456 and in ASTM B 489 is based on bending a test strip of the deposit over a mandrel of specified diameter until the two ends of the strip are parallel. Other tests based on hydraulic or mechanical bulge testing are available. The percent elongation can also be determined by traditional mechanical testing by machining a test sample from relatively thick electroformed nickel and subjecting it to a tensile test. Because ductility is affected by the thickness of the coating, ductility should be measured at the actual thickness specified in a specific end use.

Other useful tests described in ASTM standards include adhesion (B 571), internal stress measurements with the spiral contractometer (B 636), and microhardness testing (B 578), among others.

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