WHAT IS ELECTROLESS NICKEL?
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 descriptions 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
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:
In the early years, platers encountered 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
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, where 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 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 characteristics. 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.
Each of the above is designed to maximize qualities such as corrosion resistance, hardness, high-temperature resistance, electrical properties and magnetic or nonmagnetic characteristics.
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, where minimum loads travel at maximum speeds.
Most conventional electroless nickel plating baths are not well suited to composite plating, as the stablizer is affected by the high concentration of particulate matter.
APPLICATIONS OF ELECTROLESS NICKEL
PROPERTIES OF ELECTROLESS NICKEL
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.
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
Heat of Conductivity
Coating thickness measurements with devices which rely on the nonmagnetic characteristic 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.
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.
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.
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.
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.
The appearance of electroless nickel is similar to that of electrodeposited
GOOD EN PLATING PRACTICE
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
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
STRIPPING ELECTROLESS NICKEL
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.
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
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.)
Localized overheating of stripping solutions may occur near heating devices, where much higher temperatures are attained than those recommended for the bulk of the stripping solution. Agitation serves to prevent this disfunction.
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.
Brazed components often present unique problems with respect to substrate metallurgy. Although the electroless nickel stripper is designed to be selective for a particular brazing alloy, etching or pitting may occur. Etching or pitting usually begins where 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 selective 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 selection 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
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.
The microscopic examination of the cross-section of the article to be tested should be in accordance with ASTM B-478 "Standard Method for Measure-ments 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.
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 disappen 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.
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 materit 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".
Other tests are available for electroless nickel-phosphorus deposits,
and they should be agreed upon between the purchaser and the applicator
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