Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:email@example.com
ALLOY PLATING IN THE '90'S
Alloys have different properties and characteristics from those of the metals in pure form. The objective of alloy deposition is to create surface characteristics which enhance performance and allow new and different uses for deposited metals than can be achieved by depositing pure metals. The term "pure" is used to describe typical deposits of metals neglecting trace impurities and very minor amounts of other elements used to enhance brighteners or leveling.
Some of the characteristics sought from alloy plating are improved corrosion protection of basis metals, and improved corrosion resistance to a wide variety of corrosive environments including acids, alkalies and electrochemical corrosion conditions. Also, alloy plating can provide electrical and magnetic performance not achievable from single metal deposits; it can improve welding, brazing, bonding and soldering performance, increase hardness, wear resistance, ductility and strength; and product amorphous structures. These capabilities extend the range of uses for alloy deposition across many applications.
Electroless nickel phosphorus, nickel-boron, and phosphorus and boron alloys of cobalt, although they are alloys, have been discussed in previous issues of METAL FINISHING.
Zinc is sacrificial to steel and many of its alloys, thus provides good corrosion protection. Selected zinc alloys such as zinc-nickel, zinc-cobalt and zinc-iron remain anodic to steel and are less active than zinc alone. The alloy will corrode in preference to steel in a corrosive environment but at a much lower rate than that of zinc, and without the voluminous corrosion products associated with zinc alone.
Other alloys of zinc will find selective applications in the 90's, but zinc-nickel is likely to dominate. These alloys of 90% zinc/10% nickel(± 5%) will be used widely in the 90's to provide protection to steel. In combination with conversion coatings (presently chromates) these zinc-nickel alloys can provide in excess of 1000 hours salt spray (ASTM BI17) protection. Longer salt spray protection (up to 2000 hours) has been reported.'
Much of the automotive industry has adapted zinc-nickel, zinc-cobalt, and to some extent zinc-iron alloys for pre-coated electrogalvanize sheet steel. These alloys are also used for auto components. Other industries are likely to follow suit.
The plating solutions typically used are acid-chloride and alkaline (non-cyanide). These solutions have throwing power, cathode efficiency and deposition rates similar to those of conventional zinc plating solutions. The ductility of deposits is similar also, and the intrinsic stress is low for deposits up to 12 microns, then increases with thickness.'
As nickel content increases, the corrosion protection increases up to about 12%, then decreases after that.
Cadmium plate exhibits excellent corrosion protection to steel. But cadmium has a bad reputation as environmentally harmful, although a few ppb's are required daily for good human nutrition. It is highly toxic, however, and cadmium vapors should be entirely avoided. Zinc alloys are being considered as a substitute for cadmium plating, and the 90's should see increased use of zinc alloys for cadmium replacement.
Plated tin-lead alloys are used extensively for solder deposition primarily in printed circuits and other electronic devices requiring solder plate. Solder plate is used to join non-electronic devices as well. The lowest melting point (eutectic alloy) for tin and lead is approximately 63% tin and 37% lead. Compositions near the eutectic alloy are the most common. Since lead is a toxic metal, the 90's will likely see a trend away from tin-lead solders and toward alloys with less potential hazard. Tin-bismuth would be a possible alternative since both tin and bismuth are considered non-hazardous. The difficulties are in finding soluble bismuth compounds compatible with tin salts which result in a plating solution capable of producing a significant amount of bismuth in the deposit. And the melting points of various Sn-Bi alloys are different from Sn-Pb alloys which may make Sn-Bi alloys unsuitable for some applications.
Pure tin deposits are subject to what is known as "tin pest," a re-crystallization resulting in an unsatisfactory coating. The addition of a bismuth compound to a potassium stannate plating solution to yield about 0.1-0.2% Bi is sufficient to inhibit "tin pest."2
Adding a small amount of many different metals with tin in plating solution results in deposits which will not grow "whiskers." Pure tin, in the influence of electrical currents or fields, has a tendency to form long dendrites known as "whiskers" which can cause electrical short circuits or break off to interfere with other mechanical or electrical functions.
Lowenheim reports: "Tin-cobalt alloys, sometimes containing a small amount of a third metal and comparable in many ways to tin-nickel, have been promoted as a replacement for decorative chromium deposits."2 The 90's are likely to see a growth in these alloys since they are reported to have better throwing power than chromium. And because chromium is not considered environmentally safe, substitutes for chrome plating will be more widely used.
Tin-nickel exhibits a high degree of corrosive chemical resistance. Although it is not sacrificial to steel and many other common metals used for parts fabrication, there are still some attractive uses which may find wider application in the 90's. At present tin-nickel alloys are plated primarily from fluoborate electrolytes. Since fluorides are hazardous and difficult to waste-treat, new non-fluoride electrolytes would make the use of tin-nickel alloys more attractive.
Nickel-cobalt alloys will find wider application in electroforming due to higher strength than electroformed nickel. Dini & Johnson report the optimum tensile properties of electrodeposited nickel-cobalt occur between 40 and 60 weight percent cobalt where it is approximately2i times as strong as electrodeposited nickel alone.3 "Excellent ambient and cryogenic tensile strengths and ductilities are produced through long-duration 204°C (400°F) heat treatment," according to R. J. Walter.4 In addition, heat treatment to 482°C (900°F) results in "superplastic" deformation, allowing final shaping of electroforms with complex shapes and internal channeling.4
Nickel-palladium alloys will continue to be used in electronic applications for some contacts as a gold replacement. Although the contact resistance is higher than gold, Ni-Pd is harder than gold and avoids polymer formation associated with pure palladium and some gold plating processes. "The alloy is superior to pure palladium deposits, because of absence of intermetallic compound formation at solder joints, resistance to organic vapors, higher ductility,..."5
Nickel-chromium-iron plated alloys (stainless steel deposits), now enhanced by pulse plating rectifiers and improved techniques for control, will likely find application.6
Electrodeposited amorphous alloys of nickel and cobalt with phosphorus show promise for good corrosion resistance, wear resistance, and in some cases ductile deposits. These alloys will likely be useful for special applications.?
Specialized surface properties and characteristics will be needed to keep pace with technical developments. Numerous other alloys not mentioned in this paper have been developed and properties reported. Some of these may find application in the 90's.
Electroless alloys will find wider use in the 90's because of their ability to deposit uniformly over all surfaces and their special deposit properties, some of which are unattainable by electrodeposition.
As in the case of electrodeposited alloys, some metals which cannot be deposited as a single metal from an aqueous solution (or as the phosphorus or boron alloys) can be deposited along with another metal. Molybdenum is an example of such a meta1.8
Nickel Cobalt Alloys
Nickel-cobalt phosphorus deposits covering a wide range of composition find a number of interesting applications. Pearlstein reports: "Electroless nickel-cobalt-phosphorus deposits are more electrochemically active (more negative in mixed potential in neutral salt solutions) than electroless nickel-phosphorus deposits, and thus double-layer deposits of conventional electroless nickel-phosphorus on steel followed by electroless nickel-cobalt-phosphorus provides enhanced basis metal corrosion protection in marine environments. The outer electroless nickel-cobalt-phosphorus deposit apparently provides sacrificial protection to the inner electroless nickel-phosphorus layer, thereby preserving the inner layer's protective barrier."9,10
Nickel-cobalt-phosphorus, along with cobalt-phosphorus, and alloys containing other metals such as zinc, molybdenum and tungsten are used to produce magnetic deposits suitable for use in computer memory discs. Electroless magnetic deposition methods are more economical than sputtered deposition techniques. Savings in process time, equipment and materials are significant. High coercivity and other desirable magnetic parameters are attainable.
Electroless nickel-cobalt-boron deposits exhibit high permeability and very low coercive force magnetic characteristics, suggesting its use for thin film read/write devices for memory disc systems.
Electroless nickel-iron-boron alloys show similar characteristics. Electroless nickel-tungsten-phosphorus alloys have high temperature resistance. Alloys containing about 20% tungsten were resistant to attack by concentrated nitric acid and 7N hydrochloric acid."9 These deposits are also somewhat ductile compared to electroless nickel-phosphorus deposits. Co-deposited molybdenum and tin allow copper and some other non-catalytic metals and alloys to be plated directly without sensitizing or catalytic pre-treatment steps.11
Electroless nickel-phosphorus-boron alloys are reported to have superior wear, hardness and corrosion protection characteristics compared to electroless nickel-phosphorus alone."
Electroless palladium-nickel-boron and phosphorus alloys of palladium-nickel are used for electronic contacts to provide good wear resistance and because the moderating catalytic properties of palladium, when used alone, can cause polymerization of organic vapors that product resistive films.9 Expansion of the use of these alloys is expected in the 90's. The addition of a third metal such as nickel, cobalt or zinc to these plating solutions can produce ternary alloys with modified and useful characteristics.
There are many other electroless alloys which have been reported.12 Some significant uses may emerge from this list in the 90's.
Alloy plating processes offer versatility to the plating industry, and thus offer great opportunity for new uses for plated deposits.
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