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WHAT IS ELECTROLESS NICKEL?
This volume is concerned with autocatalytic nickel plating, commonly referred to as electroless nickel plating. In contrast with electroplating, electroless nickel (EN) plating does not require rectifiers, electrical current or anodes. Deposition occurs in an aqueous solution containing metal ions a reducing agent, chelates, complexing agents and stabilizers. Chemical reactions on the surface of the part being plated cause deposition of a nickel alloy.
Since all surfaces wetted by the electroless nickel solution are plated, the deposit thickness is quite uniform. This unique property of EN makes it possible to coat internal surfaces of pipes, valves and other parts. Such uniformity of deposit thickness is difficult, if not impossible, to achieve by any other method.
The discovery of electroless plating is credited to Brenner & Riddell in the 1940×s. Today EN has grown into a very substantial segment of the metal products finishing industry.
Compared with plating of other metals, electroless nickel (EN) plating is relatively young being commercially available for less than 30 years; however, in the past decade the usage of the coating has grown to such proportions that electroless nickel plated parts are found underground, in outer space, and in a myriad of areas in between.
This guide seeks to provide the reader with more thorough understanding of the process. The volume includes de×ions of deposit properties, equipment required, process applicability and test procedures to the end that a high quality EN deposit can be achieved and maintained.
The chemical reactions that occur when using sodium hypophosphite as the reducing agent in electroless nickel plating are as follows:
H2PO~2 + H2O ---------› H2PO~3 + H2
Ni++ + H2PO20 + H2O Catalyst › Ni0 + H2PO~3 + 2H+ 
H2PO~2+ H+ ---------› P + OH~ + H2O
An electroless nickel coating is a dense alloy of nickel and phosphorus. The amount of phosphorus codeposited can range from less than 1% to 12%, depending upon bath formulation, operating pH and bath age. The deposition process is auto-catalytic; i.e., once a primary layer of nickel has formed on the substrate, that layer and each subsequent layer become the catalyst that causes the above reaction to continue. Thus, very thick coatings can be applied, provided that the ingredients in the plating bath are replenished in an orderly manner. In general commercial practice, thicknesses range from 0.1 mil to 5 mils but in some salvage operations 30 mil deposits are not uncommon.
Electroless nickel deposits are functional coatings and are rarely used for decorative purposes only. The primary criteria for using electroless nickel generally falls within the following categories:
1) Corrosion resistance.
2) Wear resistance.
3) Hardness.
4) Lubricity.
5) Solderability and bondability.
6) Uniformity of deposit regardless of geometries.
7) Nonmagnetic properties of high-phosphorus nickel alloy.
In the early years, platers en×ered many problems with electroless nickel because of poor formulations, inferior equipment, misapplications and a general misunderstanding of the process and the deposit. In the first decade and a half of its existence, electroless nickel plating had an aura of "black magic" attached to it. Modern bath formulations, however, use only the purest grades of chemicals, delicately balanced and blended to give the processor plating baths with long life, exceptional stability, consistent plating rates, self-maintaining pH and most importantly, reproducible quality. In addition, advancements in tank design, filtration systems, heating and agitation have virtually eliminated the problems that plagued the user years ago.
Furthermore, in the past decade, advancements have been made in auto-catalytic nickel plating solutions. Reducing agents other than sodium hypophosphite are used for special applications; composites of nickel with diamonds, silicon carbide and PTFE are available; and ternary alloys may be applied. Also, baths have been formulated to yield specific results; i.e., high corrosion resistance, brightness, high plating rate, improved ductility and low or high levels of magnetic response.
It has taken many years of hard work and cooperative effort by the suppliers and users to arrive at the present state of the art.
TYPES OF ELECTROLESS NICKEL
All electroless nickels are not the same. Different types have been developed to provide special properties, depending on the end-use requirement. 
Nickel-phosphorus Baths
Acid nickel-phosphorus. Deposits from these baths can be identified by phosphorus content which, in turn, determines deposit properties. 1-3%=Very low phosphorus; 3-6%=Low phosphorus; 6-9%=Mid phosphorus; 9-12%=High phosphorus.
Alkaline nickel-phosphorus. These baths plate at relatively low temperatures, making them suitable for plating on plastics. In addition, because of the low phosphorus content deposited (3 to 4 pct), they are used in many applications in the electronics industry, × enhanced solderability is often required.
Alkaline systems can be used as strike baths over zincated aluminum. This eliminates zinc buildup in the acid electroless nickel bath used for final buildup.
Some types of Alkaline EN are also used to strike zinc die cast alloys prior to buildup with acid EN.
Nickel-boron Baths
Nickel-boron baths are usually formulated using an amine borane as the chemical reducing agent. Alloy deposits can be plated from acid as well as alkaline baths and are harder, as plated, than nickel-phosphorus deposits. In addition, the melting point of nickel-boron alloys is higher.
Nickel-boron systems are most often used in electronic applications to provide specific deposit properties. They are sometimes used in industrial wear applications because of their high hardness levels. The chemical cost of these systems can range from five to 10 times that of nickel-phosphorus baths.
Low-boron-containing baths (less than one pct B) produce deposits having high electrical conductivity, good solderability and good ultrasonic bonding ×acteristics. Baths formulated to produce higher levels of boron (2 to 3 pct) in the deposit have very high hardness values as plated and tend to retard the formation of oxides on the surface of the deposit.
Sodium borohydride is sometimes used as the chemical reducing agent in nickel-boron systems. These baths codeposit higher levels of boron (5 to 6 pct), but are less stable than amine borane baths because of the high pH values required to prevent hydrolysis and solution decomposition.
Polyalloys
Several electroless nickel plating solutions produce deposits having three or four elements. These include nickel-cobalt-phosphorus; nickel-iron-phosphorus; nickel-tungsten-phosphorus; nickel-rhenium-phosphorus; nickel-cobalt-phosphorus; nickel-molybdenum-boron; nickel-tungsten-boron; and others.
Each of the above is designed to maximize qualities such as corrosion resistance, hardness, high-temperature resistance, electrical properties and magnetic or nonmagnetic ×acteristics.
Composite Coatings
The excellent wear resistance of electroless nickel can be further enchanced by codepositing hard particulate matter with the nickel-phosphorus alloy. Usually, particles of silicon carbide (4,500 VHN) or synthetic diamonds (10,000 VHN) are used in this process. A uniform dispersion of particles (20 to 30 pct by vol) is held in place in the deposit by the nickel-phosphorus matrix. These deposits are very brittle and require a sound substrate to prevent cracking in use. Composites containing silicon carbide are most often used in mold and die applications. Those containing diamonds have found use in textile applications.
Teflon (PTFE) can also be codeposited with electroless nickel to provide even greater lubricity than that which naturally occurs in the nickel-phosphorus alloy deposit. Alternatively, Teflon can be impregnated in the deposit as a post-plating operation. Both techniques produce an extremely slick surface which is useful in the packaging machinery industry, × minimum loads travel at maximum speeds.
Most conventional electroless nickel plating baths are not well suited to composite plating, as the stabilizer is affected by the high concentration of particulate matter.
APPLICATIONS OF ELECTROLESS NICKEL
Electroless nickel produces an alloy with truly unusual properties. These properties have made EN very useful in a broad range of functional applications. Most applications take advantage of the hardness, lubricity, corrosion resistance, electrical and magnetic properties of electroless nickel, as well as its ability to cover complex geometries and internal as well as external surfaces. Table II lists many of the common applications and indicates which properties of EN are of value in each of these applications.
PROPERTIES OF ELECTROLESS NICKEL
It is the properties of electroless nickel that are responsible for the rapid expansion of its use as a functional metallic alloy deposit in recent years. Truly no other coating has the combination of properties offered by electroless nickel.
Corrosion Resistance
One of the most common reasons for ×ion of electroless nickel coatings in functional applications is its excellent corrosion resistance. In the very corrosive conditions en×ered in drilling oil wells and pumping out the oil, for example, electroless nickel has shown its ability to withstand the combination of corrosive chemicals and abrasion.
The alloy content of the EN deposit influences its performance in a variety of environments. For example, low phosphorus deposits dramatically outperform high phosphorus alloys in corrosion resistance in highly alkaline, high temperature applications. In most chemical environments the high phosphorus alloys are superior in corrosion resistance.
Density
An electroless nickel deposit containing 3 pct phosphorus has a density of 8.52 g/cm3. An electroless nickel deposit with a 7.5 pct phosphorus content has a reported density of 7.92 g/cm3. These values are lower than those of pure metallurgical nickel (8.91 g/cm3).
The lower density of electroless nickel is caused by the presence of phosphorus as an alloying constituent. The most common range of phosphorus present in commercially applied deposits is generally 3 to 12 pct. Analysis has also shown minor levels of other elements present. These elements affect density and include hydrogen (0.0016%); nitrogen (0.0005%); oxygen (0.0023%); and carbon (0.04%).
Coefficient of Thermal Expansion
The coefficient of thermal expansion of a deposit containing 8 to 9 pct phosphorus is 13 to 14.5 x 10-6/°C. This compares to values for electrodeposited nickel of 14 to 17 x 10 -6/°C.
Heat of Conductivity
The heat of conductivity for an electroless nickel deposit containing 8 to 9 pct phosphorus is 0.0105 to 0.0135 cal-cm/sec/°C. Pure metallurgical nickel has a value of 0.198 cal.cm/sec/°C.
Melting Temperature
The melting temperatures of electroless nickel deposits vary widely, depending upon the amount of phosphorus alloyed in the deposit. A generally accepted melting point is about 1616°F (880°C) for deposits from processes with approximately 7 to 9 pct phosphorus. This temperature corresponds to the melting point of nickel phosphide (NiP3), which precipitates during heating of electroless nickel deposits.
Magnetism
Electroless nickel deposits containing greater than 8 pct phosphorus are considered to be essentially nonmagnetic as plated. The coercivity of 8.6 pct and 7.0 pct phosphorus content deposits has been reported at 1.4 oersteds and 2.0 oersteds respectively. A 3.5 pct phosphorus content deposit produces a magnetic coating of 30 oersteds. When the phosphorus content is increased to 10 pct, the deposit is nonmagnetic.
Coating thickness measurements with devices which rely on the nonmagnetic ×acteristic of the coating may become inaccurate and require special calibration if phosphorus content is below 8 pct.
Heat treatment of electroless nickel will increase the magnetism of the deposit. Most deposits which contain above 9 pct phosphorus will become slightly magnetic when heat treated above 518 to 536 °F (270 to 280 °C); however, some will show lower remnant magnetism. It is at this temperature that the solid solutions of phosphorus in nickel which occur in the asplated deposit begin to form both nonmagnetic nickel phosphide (Ni3P) and magnetic nickel.
Electrical Resistivity
The electrical resistivity of pure metallurgical nickel has a value of 6.05 microohm-cm. Electroless nickel deposits containing 6 to 7 pct phosphorus have values (as plated) which range from 52 to 68 microohm-cm. A deposit with 2.2 pct phosphorus has electrical resistivity of 30 microohm-cm, while a deposit with 13 pct phosphorus has a significantly higher resistivity-110 microohm-cm.
Heat treating electroless nickel reduces its electrical resistivity. Heat treatment up to 302°F (150°C) produces changes in the deposit primarily attributed to structural averaging of the phosphorus content and liberation of absorbed hydrogen. Beginning in the range of 500-536°F (260-280°C), heat treating produces a further decrease in electrical resistivity. This change is attributed to the precipitation of nickel phosphide (NiP3) in the coating. An electroless nickel deposit with 7 pct phosphorus, heat treated to 140F (60C) was reportedly reduced from 72 to 20 microohm-cm. 
Solderability/Weldability
Electroless nickel-phosphorus alloys are easily soldered with a highly active acid flux. Soldering without a flux or with mildly active fluxes can be more difficult if the parts are allowed to form oxides by extended exposure to the atmosphere. The heat processing of electroless nickel plated parts can make soldering very difficult unless a highly active acid flux is used.
Welding of electroless nickel deposits is not commonly done. There is a tendency of phosphorus to migrate to grain boundaries during cooling of the weld. This results in "hot cracks" or disintegration of the weld.
Adhesion
Excellent adhesion of electroless nickel deposits can be achieved on a wide range of substrates, including steel, aluminum, copper and copper alloys. Typical bond strengths reported for electroless nickel on iron and copper alloys range from 50 to 64 kpsi (345 to 441 M Pa). (kpsi = 1000 pounds per square inch. M Pa = Mega Pascal). The bond strength on light metals, such as aluminum, tends to be lower, in the range of 15 to 35 kpsi (103 to 241 M Pa) for most alloys.
Heat treatment is commonly employed to improve adhesion of EN on all metals, particularly on light metals such as aluminum or titanium. During this heat treatment diffusion occurs between the atoms of the coating and the substrate. Heat treatment is detailed later in this book in the "Post Treatments" section.
Thickness
Electroless nickel can be deposited to produce a wide range of coating thicknesses, with uniformity and minimum variation from point to point. This uniformity can be maintained in plating both large and small parts and on components which are fairly complex, with recessed areas. Electroplating of such parts, on the other hand, would produce thickness variation and possible voids in the plating when coating holes and inside diameters. The range of thicknesses for electroless nickel in commercial applications is 0.1 mil to 5 mils (2.54 to 127µm), although deposits as thick as 40 mils have been reported. Normally thickness is built at the rate of 0.3 to 0.8 mil/hr (7.5 to 20 µm/hr).
The majority of commercial applications, except those involving corrosive service or heavy buildup of worn parts, utilize a thickness between 0.1 and 1.0 mil (2.54 and 25.4 µm). Thicknesses of 1.0 to 3.0 mils (25.4 to 78 µm) are common for corrosive service, while deposit thicknesses above 3.0 mils (78 µm) are typical of repair and rework applications. Deposition of these heavier coatings (3.0 mils) requires more careful attention to process control to avoid roughness and pitting.
Brightness
The brightness and reflectivity of electroless nickel vary significantly, depending on the specific formulation. The reflectivity is also affected by the surface finish of the substrate. Thus a very bright electroless deposit may appear dull if the substrate is rough.
The appearance of electroless nickel is similar to that of electrodeposited nickel.
GOOD EN PLATING PRACTICE
Achieving the full potential of electroless nickel plating requires that the finisher pay attention to the original metallurgical surface condition before he ever begins to put the part in an electroless nickel processing line. In a like fashion, the part must be cleaned properly; the right equipment must be available for precleaning the parts and for operating the electroless nickel plating solution; the solution best able to produce the required properties must be used; the finisher must recognize the common processing problems and be able to correct them; and he must know how to heat treat and provide other postplate treatments required to achieve certain properties. The following sections elaborate on the metallurgy and processing methods important in producing sound deposits.
Substrate Effects
Substrate surface smoothness influences the protective value of electroless nickel deposits. The smoother the surface to be plated, the better the quality of the electroless nickel deposits.
Fabricating operations such as rolling, stamping, casting, shearing, lapping, drawing, machining and grit blasting can cause defects in the basis metal before it enters the electroless nickel process line. Inclusions in the substrate metal may cause the part to be hard to clean and not easily wetted. This will make uniform coverage with electroless nickel difficult even when thick deposits are applied. In addition, pores in the substrate can entrap preplate chemicals, which then "bleed out" during the plating cycle, causing inferior electroless nickel deposits at those sites.
When it is necessary to electroless nickel plate substrates with surface defects, the quality of plate can be improved by alternating hot and cold rinses during the preplate cycle, running the bath in a slow mode, increasing the rate of agitation and lowering the plating bath surface tension with an approved wetting agent.
Preparation of Metals for Electroless Nickel Plating
The importance of cleaning and activating metal surfaces prior to electroless nickel plating cannot be overemphasized. Many of the problems thought to be caused by improper electroless nickel plating are actually caused by failure to clean and pretreat surfaces adequately. Here are recommended preplate steps for each of the metals commonly plated with electroless nickel. To optimize the performance of the preplate line, proper temperature and concentration must be maintained. Filtration of the preplate chemicals will reduce the chance of drag-in of particulate matter.
Steel
1) Soak clean.
2) Rinses.
3) Anodic or periodic reverse electroclean.
4) Rinses.
5) Acid dip - 10% sulfuric acid.
6) Rinses.
7) Electroclean - anodic or periodic reverse.
8) Rinse.
9) Electroless nickel plate.
Copper
1) Soak clean.
2) Rinses.
3) Electroclean.
4) Rinses.
5) Dip in 5 to 10 pct sulfuric acid.
6) Rinses.
7) Contact the surface to be plated with a steel rod, or apply direct current momentarily to initiate deposition. Since copper is not catalytic, this is necessary to begin deposition.
8) Electroless nickel plate.
Aluminum
Single-zincate process.
1) Non-etch soak clean.
2) Rinse.
3) Alkaline or acid etch.
4) Rinse.
5) Desmut.×
6) Rinse.
7) Zincate.
8) Rinse.
9) Electroless nickel plate.
Double Zincate Process:
1) Non-etch soak clean.
2) Rinse.
3) Alkaline or acid etch.
4) Rinse.
5) Desmut.×
6) Rinse.
7) Zincate - long immersion.
8) Rinse.
9) Strip in 50 pct nitric acid.
10) Rinse.
11) Zincate - short immersion.
12) Rinse.
13) Electroless nickel plate.
×Desmut in nitric acid 50 pct, or nitric acid 50 pct plus sulfuric acid 15 pct, o nitric acid 50 pct plus sulfuric acid 25 pct plus hydrofluoric acid 5 pct.
Equipment
Tanks. The most basic item in a line ion plating electroless nickel is a clean polypropylene tank of a known and calibrated capacity. Before the tank is used a dilute caustic solution heated to 120°F (49°C) should be pumped into the tank and allowed to leach out impurities. Stainless steel tanks can be used provided an anodic ×ge is applied to prevent EN deposition onto the tank.
After the caustic has been drained out and the tank has been rinsed, a 30 pc solution of nitric acid should be pumped in and then recirculated through th tank and accessory equipment.
New stainless steel heaters should be immersed in the nitric solution overnight for adequate passivation. Either 304 or 316 stainless steel is acceptable. After use, the nitric acid solution should be pumped to a holding tank and saved for tank maintenance.
A good rinse should always follow each passivation period. A small amount of ammonia added to the final rinse and pumped through the system will neutralize residual acidity.
Filters. For the tank of 150 gallons or less, cartridge-type filter units will be adequate. Such units can be purchased in appropriate sizes, and can be connected to a distribution manifold. Filtration rate should be ten turnovers per hour minimum.
Before new filters are used they should be rinsed thoroughly with a five pct sulfuric acid solution containing 0.1 pct of a suitable wetting agent. After this leaching operation the filter tubes or bags should be rinsed in deionized water before being used to filter the EN solution.
If polypropylene filter bags have been used before and subsequently stripped in nitric acid to remove nickel particles, they should be rinsed in deionizc water containing a small amount of ammonia, to neutralize nitric residual before being used again. Periodic laundering of these bags is advisable, as the nitric does not remove dirt. This dirt will cause more rapid plate-out in the filters, and subsequently lower the metal content of the bath.
Racks, Barrels. Consideration must be given to rack and barrel design an maintenance to minimize costly problems resulting from cross contamination of baths due to defective rack coatings and undersized holes in plating barrel (limiting solution transfer). Racks should be designed to provide for good drainage of processing solutions from racked parts. Certain shapes tend to entrap solution, which then causes contamination by carryover or dragin.
Bath Makeup, Replenishment and Operation
The proprietary electroless nickel product is a blending of many ingredients delicately balanced to provide long bath life and consistent, reproducible quality deposits. It is important that the user follow instructions provided by the supplier so as not to inadvertently disturb this aforementioned balance. There follows a list of primary points of which the user should be cognizant.
1) Tank volume should be known. Tank volume (gallons) = length(in.) x depth(in.) x width(in.) + 231.
2) Deionized water should be used.
3) Temperature should be controlled.
4) The solution should be filtered at proper rates through appropriate media.
5) Makeup and replenisher components should be used according to supplier instructions.
6) Replenisher containers should be well marked.
7) The specified pH should be maintained.
8) A replenishment/control log should be maintained.
9) Standardized reagents should be used for analysis.
10) Bath should not be allowed to become too depleted, so that "shocking the bath" with large replenisher additions may be avoided.
11) Working solution volume level should be maintained, especially before analysis.
12) Racks, hooks, or barrels used for cadmium, chromium, lead, solder,
tin or zinc plating should not be used in the electroless nickel
solution.
13) Bath should not be heated for extended periods of time unnecessarily.
14) Chemical balance of the bath should not be up× by unauthorized additions by a "creative" operator.
15) Bath should not be used beyond its normal life.
Chemicals required for an electroless nickel bath should be mixed according to instructions. The pH should be checked and adjusted to suggested working The tank should then be marked for continuous level observation. Care should be taken, at this point, to allow for solution contained in the pump and in pipes outside the tank. Not taking this into consideration can cause serious dilution of the bath and subsequent titration errors.
At this point, a sample of bath should be taken and reserved for a lab standard for EDTA titrations. A graph should now be made of pct Ni content versus ml of EDTA. This will allow rapid checking of bath conditions during production hours, without going through a series of calculations × errors might occur. The bath is now ready to be brought to recommended operating temperature.
Automatic analyzer/feeder systems are available to maintain proper chemistry without manual additions. These should be calibrated and standardized periodically.
Bath pH ranges required vary with different bath formulations and different suppliers. Appropriate instructions should be followed. Since variations in pH can cause erratic operation, it is best to check pH frequently and make necessary corrections. Although most proprietary baths maintain pH within the desired range with proper replenishment, complex parts can cause excessive "dragin" to cause pH variations. When parts of this type are being processed, the pH should be checked continuously and appropriate adjustments should be made to keep the bath pH within the specified working range.
Some electroless nickel baths are extremely susceptible to failure after any substantial amount of dragin contamination; therefore, a concentrated effort must be made to minimize dragin.
Periodic titrations and accurate records of results are a must for any shop that wants to do government or other specification electroless nickel plating. Record keeping is very important.
It is not necessary to replenish every time a bath is titrated, unless the bath is out of optimum working range. Anything less than 85 pct could drop the deposition rate enough to slow production.
A good way to assure continued uniform deposition rates and eliminate unnecessary titrations is to measure thickness of plated panels. Panels of known, consistent original thickness may be hung on racks carrying parts through. Periodically, throughout the run, a panel may be removed from the rack and deposit thickness measured. Parts being plated will, of course, have this same deposit thickness. Also, deposition rate of the bath can thus be determined. A properly processed panel will demonstrate good plate adhesion.
Usually it is more economical to have various sizes of tanks containing electroless nickel solutions rather than to process all work in one large tank. Not only is it expensive to heat and maintain a large tank, but there is a detrimental effect on the bath itself if only a few small parts are being plated in a large solution volume.
EN solutions should not be heated unless they are to be used. If a bath is continuously heated and not used, or if just one or two small parts are to be plated daily, deposition rate will fall. Depending on the formulation, other problems also may occur.
An important item to be considered in production usage of electroless nickel is filtration. Continuous filtration using equipment able to filter at hourly rates equivalent to 10 times the solution volume is good practice. A 100 gal tank should have a 1,000 gph filter, for example.
Mechanical agitation is usually suggested for electroless nickel baths, but this is practical only in small tanks and then not without certain drawbacks. Most conventional baths will accept air agitation from an oil-free blower.
Good air agitation will eliminate many roughness and gas-pitting problems. It also will eliminate the need for continuous or periodic moving of parts hung in the bath. Better solution changeover will be produced in barrel plating, too.
Care must be taken in plating shapes which can trap air on entry into processing tanks. This air could block access of solution to areas of the part being plated. Wherever air can be trapped, hydrogen or oxygen gas may also accumulate during a cleaning or plating step.
Good rinsing is mandatory. Air-agitated ×erflowed rinses after all preplate steps are best. They may be supplemented with sprays × practical. All rinses should have overflows along the entire length of the tank to skim the whole surface of the tank continuously-not just at one corner. Rinses should be kept at or near room temperature. Very cold rinses will "×-up" certain cleaners on the part surface rather than remove them. Finally, a two-second dip in a rinse tank is not enough; rinsing should be thorough.
Many users of electroless nickel use one tank of electroless nickel solution day after day. Even though an adequately filtered bath usually will last through this punishment, daily transfer is preferable.
Electroless nickel tanks are best × up in pairs. One is operated one day and one the next. Nitric solution (30 pct) is left overnight in the previously used tank to remove any particulate matter and to passivate the heaters and other equipment. This procedure should be followed even though the tank is clean. While the working tank is being heated and the first load of parts is in the cleaning cycle, the nitric solution should be pumped to a holding tank for storage, and the plating tank rinsed out. Now the plater has a working tank and a clean spare.
If for some reason the solution has to be drained immediately, it can be pumped to the clean tank, the heater turned on, and production may continue. This happens sometimes when a critical part is dropped, or when a triggering action has started and must be stopped by filtering the entire bath quickly. Another reason for daily transfer, even when a high filtration rate is used, is that the geometry of a tank does not allow every bit of solution to be filtered, no matter how many times per hour the bath is turned over.
If heaters become plated, they should be stripped in the stronger concentration of nitric acid. In most instances, 30 pct nitric is not strong enough to remove large quantities of nickel in a short period of time, especially after heaters have been used for a while.
Usually, a poly-lined drum of stronger solution can be kept without too much inconvenience. This will save the tank from unnecessary exposure to a strong nitric solution. Even with daily transfer practices and nitric exposure, these tanks will last two to five years.
This may seem to be a complicated procedure, but if practiced daily, it is a fast and inexpensive one. It will eliminate costly downtime and loss of baths by plate-out or triggering.
POST TREATMENTS
Heat Treatment
Heat treatment is frequently used to improve adhesion or to modify properties in order to satisfy the needs of a particular application. As a result of heat treatment, hardness, corrosion resistance, wear resistance, ductility and stress, fatigue properties, magnetic properties, and other qualities of the deposit can be affected. Figure 1 indicates changes of EN as a result of heat treatment. Figure 2 graphs changes in hardness and wear resistance of EN following heat treatment at different temperatures.
Heat treatment is normally performed at temperatures of 200 to 750°F (93 to 400°C) for 30 minutes to several hours. Maximum hardness is produced by heating at 750°F(400°C), followed by cooling slowly to 390°F (200°C) or lower. The higher temperatures are less likely to be used in commercial practice, since most processors prefer to treat at lower temperatures for longer time periods. The normal range is 200 to 300°F (93 to 149°C) for 30 minutes to several hours. Heat treatment at above 500 to 550°F (260 to 288°C) will change physical, mechanical and protective properties of EN. This is due to precipitation of nickel phosphide, which begins to occur in this range.
Heat treatment at temperatures and times beyond those required to develop maximum hardness increases the ductility of the deposit. Typical coatings will withstand six pct elongation without failure, provided that the basis metal is not stressed beyond its elastic limit.
Heat treating should be carried out in an inert atmosphere such as one of
argon or nitrogen, in order to minimize oxidation. If the temperature is increased beyond 500°F (260°C) in air, a highly discolored surface results. In addition to the poor appearance of such oxides, there are problems in soldering heavily oxidized surfaces.
Chromate Coatings
Proprietary chromating solutions are sometimes used to passivate and help protect the substrate from corrosive attack.
STRIPPING ELECTROLESS NICKEL
Although modern techniques for electroless nickel plating have greatly reduced the need for stripping of unsatisfactory deposits, ×ive stripping of these metallic coatings may be required, either immediately after plating, or after plated parts have been in service and require rejuvenation.
Most electroless nickel deposits are highly resistant (passive) to chemical attack. The most prevalent basis metal on which electroless nickel is deposited is steel, usually of complex geometry, and seldom resistant to chemical attack.
Cleaning and activation of the electroless nickel surface is necessary before stripping can begin. Components which have seen severe service may require rigorous cleaning to remove organic soils, carbonaceous deposits, and other incrustations. Thorough cleaning by vapor degreasing, followed by alkaline soak and electrocleaning is recommended.
Electroless nickel deposits which have aged or have been heat-treated should be activated in acid between the cleaning and stripping cycles. Inhibited hydrochloric acid (30 to 50 pct by volume), mixed acids (40 pct by volume hydrochloric acid and 10 pct by volume sulfuric acid), or proprietary acid salts may be used for activation.
Stripping Solutions
Electroless nickel deposits are commonly stripped by immersing them in aqueous chemical solutions. Since in most instances, electroless nickel is applied to components which do not lend themselves to electroplating because of their complex geometry, electrolytic stripping or "deplating" has rarely been employed for stripping these deposits.
Immersion strippers may be classified in two chemical categories: alkaline (cyanide and non-cyanide), and acid. Alkaline solutions incorporating nitro-organic compounds and cyanide are recommended for stripping electroless nickel deposits (containing up to approximately 7 pct by weight phosphorus) from steel and steel alloy substrates. For maximum bath life and efficiency, alkaline cyanide strippers should not be operated at temperatures greater than 140°F (60°C).
Alkaline non-cyanide solutions utilize nitro-organic oxidizers, and replace cyanide with amino compounds. These solutions are recommended for stripping electroless nickel deposits with higher phosphorus content (8 to 14 pct by weight) from steel and steel alloy substrates. Proprietary processes of this type are available which incorporate inhibitors and permit the solutions to strip these high-phosphorus nickel deposits from certain copper and copper alloys. Alkaline non-cyanide electroless nickel strippers are more stable than the cyanide type, and may be operated at temperatures up to 200°F (95°C).
Solutions of nitric acid (40 to 50 pct by volume) are recommended for stripping electroless nickel from aluminum and most aluminum alloys.
Stripping Bath Operation
Concentration. Proprietary electroless nickel stripping solutions must be made up at the concentrations recommended by the supplier. Overly concentrated solutions may lower the solubility of the deposit being stripped and cause localized and/or overall etching of the basis metal. Conversely, dilute solutions may reduce the rate of stripping, and cause localized solution depletion and inadequate inhibition. Organic and metallic impurities may adversely affect the performance of the stripping process.
When immersion nickel strippers become saturated with dissolved metal, they should be discarded (in accordance with pertinent regulations governing the disposal of such chemicals). Since the stripping rate of these strippers is reduced as the concentration of dissolved metal rises, the economies of disposal when the dissolved metal reaches the saturation point are far greater than those to be realized by trying to prolong bath life by chemical replenishment beyond the saturation point. (Saturation is reached when further additions of chemicals do not significantly increase the stripping rate.)
Temperature
Certain types of electroless nickel strippers are designed to operate within specific temperature ranges. Temperature control, therefore, is of paramount importance for good stripping results.
Agitation. 
Electroless nickel stripping solutions may require agitation when in use. In most cases, agitation of the solution or the parts to be stripped will increase the rate of stripping. The primary function of agitation is to keep fresh stripping solution moving past the work surface. Air or gas bubbles adhering to the surfaces of the parts being stripped result in non-uniform stripping. More importantly, as the metal stripping reaction commences and continues, a localized depletion of the chemical reactants (e.g., oxidizers, complexers, inhibitors) occurs at the interface of the solution and the metal surface being stripped. Agitation of the solution or the parts serves to displace these reacted products with fresh reactants.
Localized overheating of stripping solutions may occur near heating devices, × much higher temperatures are attained than those recommended for the bulk of the stripping solution. Agitation serves to prevent this dysfunction.
Equipment
The importance of equipment specifications relating to electroless nickel stripping cannot be overstated. Specifications must take into ac×:
a) resistance to chemical attack from the operation of the stripping process.
b) accommodation of temperature extremes in the operating solution.
c) operator safety and prevention of health hazards.
d) adequacy of materials of construction.
e) prevention of electrical problems.
The dimensions of the vessel containing the stripper solution must provide clearance between the parts being stripped and the tank bottom, with an additional 12-inch clearance at the bottom for sludge accumulation. Heating and/or cooling coils, and the parts to be stripped, must be electrically insulated from the tank. All other equipment such as mechanical agitators and temperature- controlling devices in contact with the stripping solution should also be insulated to prevent stray currents from entering the tank.
Since fumes often evolve from solutions being used to strip electroless nickel, ventilation is required. Suitable corrosion-resistant materials, should be used for ducts and exhaust systems.
Substrate Metallurgy
Since mechanical working, prior heat treatments, and exposure to severe service environments may result in stressed or otherwise deformed basis metal components, the metallurgy of the basis material is an important consideration in the prevention of subsequent problems during the stripping operation.
Brazed components often present unique problems with respect to substrate metallurgy. Although the electroless nickel stripper is designed to be ×ive for a particular brazing alloy, etching or pitting may occur. Etching or pitting usually begins × the braze interfaces with the basis metal. It is believed that these sites are vulnerable to intergranular attack because of diffusion alloying. Preliminary tests to determine the rate of attack should be made on any brazed component before deciding on any method of or solution for stripping electroless nickel deposits.
High-strength, hardened and tempered steels may also be subject to pitting and etch. Machining, drawing, stamping, and other mechanical working operations affect substrate metallurgy. Components made from these types of steels and alloys usually experience wear, damage, and corrosion in service. Consequently, the substrate may be non-heterogeneous with respect to its electrochemical activity in an electroless nickel stripping solution.
Once all of the electroless deposit has been removed, the stripping solution may preferentially attack certain areas along grain boundaries in the substrate, even though the stripper has been properly formulated and maintained. The ×ive stripping of electroless nickel deposits should be regarded as a finishing process. The course of the stripping action must be monitored. It is good practice to remove the components from the solution as soon as the electroless nickel deposit has been stripped. With good equipment, careful ×ion of the stripping material, and controlled operation of the stripping solution, valuable components, which would otherwise be scrapped, can be salvaged.
Typical stripping rates are 0.4 to 0.6 mil/hr except for very high phosphorus alloyed deposits and certain heat-treated or aged deposits. These will strip at a rate of 0.2 to 0.4 mil/hr.
SPECIFICATIONS AND TESTS FOR
ELECTROLESS NICKEL PLATE
There are a number of specifications and test methods commonly used to judge the quality of electroless nickel. The tests described below apply to nickel-phosphorus deposits. They do not cover testing of deposits from solutions reduced by borane, hydrazine or other reducing agents. The tests mentioned do not cover all possible physical properties, but do cover those normally of interest to users of electroless nickel: hardness, thickness, porosity, corrosion resistance, solderability and phosphorus content. A number of the tests are those developed by the American Society for Testing and Materials (ASTM), 916 Race Street, Philadelphia, PA 19103 (tel. 215-299-5400). Further information is available from ASTM.
Hardness
Hardness may be determined by the method outlined in ASTM B-578 "Microhardness of Electroplated Coatings", using a 100-gram load and a deposit thickness of two mils unless otherwise specified.
Thickness
Thickness of deposits may be determined by microscopically examining a cross-section, by beta backscatter methods, or by x-ray fluorescence. The deposit thickness also can be measured by using a micrometer before and after processing the article or a test specimen. Magnetic test methods may be used but are affected by the magnetics in deposit. Electroless Nickel standards must be used.
The microscopic examination of the cross-section of the article to be tested should be in accordance with ASTM B-478 "Standard Method for Measurements of Metal and Oxide Coating Thicknesses by Microscopical Examination of a Cross-section." Deposits on metals which have an atomic number less than 18 or greater than 40 can be measured by the use of a beta backscatter device. This test should be in accordance with ASTM B-567 "Standard Method for Measurement of Coating Thickness by the Beta-backscatter Principle."
ASTM B659-90, "Standard Guide for Measuring Thickness of Metallic and Organic Coatings" illustrates the use of beta backscatter, coulometric, eddy current and magnetic methods of measurement. Some methods are sensitive to alloy composition.
Porosity
Plated parts may be inspected for pits and porosity by a number of methods.
Ferroxyl test. This test is for use with EN on steel and iron basis metals. Prepare a test solution by mixing 25 grams of potassium ferricyanide and 15 grams of sodium chloride in one liter of deionized water. Clean the article and immerse in test solution for five seconds. Blue spots visible on the surface indicate pore sites.
Copper sulfate test. This test is also for deposits on iron and steel basis metals. Immerse or swab the deposits for 15 seconds using 60 g/liter copper sulfate acidified to pH 3.0 with sulfuric acid. Pore sites will be indicated by copper-colored spots on the deposit.
Alizarin test. This test for deposits on aluminum alloys is performed wiping a test specimen with a 10 pct sodium hydroxide solution. After the minutes, rinse and apply the solution of Alizarin sulfonate.
After four minutes, apply glacial acetic acid until the violet color disappears. Red spots indicate pore sites. The Alizarin sulfonate solution is prepared dissolving 1.5 grams of methyl cellulose in 90 milliliters of boiling deionization water, to which, after cooling, a solution of 0.1 gram of Alizarin sulfonic are dissolved in five milliliters of ethanol is added.
Hydrochloric spot test. This test is for deposits on aluminum alloys. It performed by immersing the article which has been plated into a solution 50 pct hydrochloric acid at room temperature for two minutes. Black spots the surface indicate pore sites.
Five pct neutral salt spray test. This test may be used on all alloys a should be in accordance with ASTM Standard B-117, "Method of Salt Spring (fog testing)."
Electrochemical pitting test. This test also can be used on any basis materials, in accordance with ASTM G-61, "Standard Practice for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion.
Corrosion
Corrosion test methods may be used to determine the corrosion rate of 1 deposit in various environments.
The immersion weight loss test is performed on all types of basis materials accordance with ASTM G-1, "Standard Recommended Practice for Preparing Cleaning, and Evaluating Corrosion Test Specimens."
Electrochemical test methods also can be used on all types of basis material in accordance with the following methods: ASTM G-3, "Standard Recommended Practice for Electrochemical Measurements and Corrosion Testing ASTM G-5, "Standard Recommended Practice for Standard Reference Methods for Making Potentiostatic and Potentiodynamic Anizatodic Polarization Measurments"; ASTM G-59, "Standard Practice for Conducting Potentiodynamic Polarization Resistant Measurements".
Solderability
Solderability tests may be performed by heating a plated article to 450°F (232°C) and applying a 60-40 tin-lead solder. This solder shall wet the surfaces indicating that the deposit is solderable.
Other tests are available for electroless nickel-phosphorus deposits, and they should be agreed upon between the purchaser and the applicator of coatings.