Selective Plating: Superior Results at a Fraction of the Cost
What is Selective Plating?
Selective plating, also known as "brush plating" is a small but growing job shop technology with a rapidly expanding scope of applications.
Simply defined, it is a special electrochemical process used to deposit any of a wide range of metals or metal alloys onto almost any conductive material and virtually any type of surface. It offers several significant advantages over conventional plating processes, the most important of which is the ability to deposit metal precisely onto the area where it is required, and in varying and closely controlled thicknesses. Hence the name selective plating. What's more, in many cases it can offer superior results, and at a fraction of the cost. Thanks to advances in the tooling and plating solutions used, as well as in automating the process, it has become an important job shop technology.
Though selective plating is a long-established technology, only in the last decade has American industry started to notice its range of applications, primarily for two reasons. First, little about it and its most closely related plating technologies was taught in engineering schools until recently. Second, as the technology evolved over the years, it went by a number of names, making recognition difficult.
In trade circles, the technology has been most commonly referred to as brush plating largely because of its origins. The basic process dates back to before the turn of the century, when silverware makers used it to touch up the bowls of spoons, its first industrial application. They could get the silver to adhere to all but the bottoms of the spoon bowls, since they didn't yet have plating solutions with good "throwing power," meaning the solutions could not carry electric current far enough to reach the bottoms of the bowls. So they would take an old file, wrap a rag around it, dip it into the plating tank, and touch up the bowls.
The range of industrial applications didn't expand much until just prior to World War II, when organic-based plating solutions were developed in France. By comparison with the inorganic solutions previously used, like those used in most tank plating today, the new solutions "held" considerably more metal, making them more conductive. This made it possible to pass more current through the solutions, resulting in a faster rate of deposition and better adhesion. What's more, in many instances the quality of deposit was now far superior to that obtainable from other plating processes.
Most plating experts now consider the term brush plating a misnomer, since it no longer adequately describes the state of the art of the process. Once deemed practical only for touching up existing plating, it has evolved into a highly technical process used for repairing, finishing or improving the surface properties of everything from sophisticated component parts on the Space Shuttle to axles on the BART mass-transit trains. It is also being used as a cost-effective production technique, and thus has become an important factor in the engineering design stage. As a result, "brush plating" has given way to several new names for the process, the most prominent of which are stylus plating, contact plating, and electrochemical metallizing. Among plating experts, though, the process is now usually referred to as simply selective plating.
The fundamentals of the process are fairly simple. A fixtured or hand-held graphite anode designed to conform to the shape of the work piece is used to pass current through the plating solution flowed between it and the surface to be repaired or modified. The anode carries a positive charge and the work piece a negative charge via a negative contact, or cathode. When anode and work piece are brought into contact, completing the circuit, the current passed through the system takes the metal ions from the plating solution and deposits them only where the anode touches the surface of the work piece. Thus the size and shape of the anode determines the size and shape of the area built up.
The periphery of the anode is usually covered with an absorbent material to help hold the solution during the operation, while a special abrasive material affixed to the face of the anode abrades the surface being plated as the anode passes over it.
A DC rectifier manufactured specifically for selective plating is used to ensure accurate control of the working voltage and current density. Some rectifiers have a built-in microprocessor capable of gauging the amount of current actually passed through the system, making it easier for the operator to compute and monitor the metal buildup.
Throughout the process, either the anode or the work piece is in constant motion. With cylindrical pieces, usually the part is rotated. For pieces with plane surfaces, usually the anode is placed in motion, in linear rather than rotary motion.
Since the design and setup of the anodic tooling is critical to the success of the process, it is usually tailor-made of high-grade pure graphite for the specific application. Specially designed for solution flow and gas release, in some instances it is even cooled to ensure a metallurgically sound deposit. For applications requiring manual operation of the anode, it is mounted on an insulated handle.
The entire process is capable of being semi- or fully automated for volume applications. CNC lathes, conventional lathes and rotary machines with less expensive computer controls, linear-motion machines, and robotics are available within the industry.
Most selective plating companies maintain facilities for in-house work, capable of handling everything from single items to high-volume production work.
From repairing valves and sealing surfaces in-place in the bowels of Trident nuclear submarines to modifying miniature parts on intricate circuit boards, the range of applications has expanded tremendously over the last decade.
The process is used extensively to repair and resurface component parts for the marine, aerospace, aircraft, automotive, electronics and other industries. It can be used to restore the ODs, IDs and plane surfaces of such defective parts as shafts, bearings, bores and journals of any size and shape to the OEM's original specifications. In many cases, the surface properties of the finished part are improved because a more desirable metal was selected.
Selective plating can also be used for such applications as filling corrosion pits, scores and scratches, restoring conductivity to electrical components, providing corrosion protection and hard facing, making bi-metal surfaces more bondable or solderable, anodizing treatment, and redimensioning difficult-to plate design configurations like 0-ring grooves, where it is not possible to use other electroplating techniques.
Engineers have in recent years started to recognize its manifold uses as a production technique, reviewed later.
Though metal deposited by selective plating has all the same metallurgical characteristics as that deposited by tank plating and other conventional electroplating processes, it also has several unique molecular features: lower porosity, greater hardness, and superior bonding strength. Together, they make the deposits significantly more wear and corrosion resistant.
- Lower Porosity: Selective plating deposits are more dense and fine grained and less porous compared with deposits applied by other electroplating processes. Generally, they are 75 percent less porous than deposits of similar dimensions applied by conventional tank plating, and 95 percent less than those applied by metal powder or wire spray. As a result, they can offer appreciably greater corrosion resistance, as well as hardness, depending upon the type of metal deposited and the type of base metal.
- Greater Hardness: The deposits generally are much harder than those obtained through other electroplating processes, making them more abrasion resistant and less susceptible to fatigue loss. Example: A hardness of Rockwell 'C' 40 to Rockwell 'C' 68 can be achieved with nickel and nickel-based alloys. In many cases, the exceptional hardness of the deposits obviates the need for expensive and time-consuming heat treating and postmachining sometimes necessary to achieve a hard surface.
- Superior Adhesion: A recent test conducted by the U.S. Navy Quality Control Laboratories showed that selective plating deposits have more than double the bond strength of chrome plating and four times that of metal spray.
The reason for the better bond: Since inorganic solutions are used in most tank plating and other conventional plating processes, typically the plating is done at 100 to 500 amps per square foot. With the higher conductivity of the organic solutions used in selective plating, however, metal is deposited at a far greater current density, usually 1000 to 3000 amps per square foot. This is similar to the current density range of arc welding. As a result, the metal is virtually driven into the microscopic valleys of the surface structure of the base metal, locking them into place. By comparison, photomicrographs of metal deposited by other conventional plating processes reveal that the deposit lies flat across the peaks of the surface structure, meaning there are open areas between the deposit and the base metal.
The bond strength varies with the type of base metal. Generally, metal deposited onto high-alloy steels, chromium, nickel-chrome alloys, aluminum, and refractory metals has far superior bond strength to that produced by other conventional plating processes. With mild steels, copper and brass, the results are equivalent, and in most cases better.
The ability to control the amount of metal deposited as well as the area the metal covers, precisely is far superior with selective plating than with that of tank plating, metal spraying and welding. Succinctly, accurate control of the metal buildup is accomplished by controlling the rate at which it is deposited.
As a general rule, if the process is semi- or fully automated, it can consistently maintain a tolerance of 10 percent of the thickness of the metal being applied, even when used in high volume production runs. That is, if 0.002" of metal is being deposited, the operator can hold a tolerance of 0.0002" with minimal effort. The ability to plate to such precise tolerances in many cases obviates the need for postmachining or grinding. Other than rinsing any chemical residue off the work piece with cold tap water, it is ready for use or sale once the process is completed.
What's more, the cost of using selective plating to maintain precision tolerances in high-volume work compares very favorably with that of CNC and conventional machining methods: the percentage of rejects, as well as machinery costs, are usually much lower.
Since the equipment used in selective plating is completely portable, it can be brought on-site to repair defective parts in place, ranging in size from large turbine shafts to small electrical components. The ability to do field repairs is particularly important in cases where it was too costly or too difficult to dismantle and ship a part or where expensive and intricate parts are involved. Nicks, gouges, corrosion pits and other wear-related defects can often be repaired without reworking the entire part. A bearing area on a military helicopter was redimensioned in-place with a deposit of nickel. A sampling of some of the common field-service applications: turbine couplings, large crankshafts and driveshafts, large valve seats, offshore marine components, offshore oil rig components, printing and calendar rolls, large cylinders and Yankee Dryers, hydraulic cylinders and rods, bus bars and circuit breakers.
As a general rule, because of the higher conductivity of the organic solutions used in selective plating, metal can be deposited at a much faster rate than with most tank plating processes. The deposition rate is a function of the area to be covered, the thickness of the deposit required, the calibration factor of the plating solution used, and the number of amp hours required. It can vary considerably with operating conditions, as well as with the types of plating solutions and base metals.
Selective plating is particularly useful for cases where the base metal has a bimetal surface, like defective nickel plating on an aluminum substrate. With this example, since the deposit can bond to both metals there is no need to strip the nickel.
It is also useful for making difficult-to-wet metals, like stainless steel and aluminum, solderable and bondable. A solderable or brazeable metal can be deposited onto the surface of the base metal to enable easy and efficient joining. In addition, the metal is deposited onto only the area where the actual joint is to be formed. In this situation, selective plating can bond to both metals without the need to strip the nickel.
No "heat" is produced during selective plating process. That is, the temperatures produced during the operation, typically between 140 - 150F, are never high enough to cause thermal distortion of any kind, a big advantage over metal spraying and welding. The maximum operating temperature of most plating solutions used is 140F. Though the temperature of the work piece may go above that, it never reaches a distortion temperature.
Aside from degreasing the work piece with a suitable solvent, like trichlorethylene or methyl ethyl ketone (MEK), or removing oxide with an activating or etching solution, other special preparation is seldom required. If a part has been working or running in oil for long periods, it may have to be baked or vapor degreased.
Metal can usually be deposited directly onto the base metal. If not, striking solutions are used to apply a pre-plate, usually of copper or nickel, before the desired metal is applied.
What's more, in many instances, premachining is unnecessary, an advantage over metal spraying.
The ability to deposit metal precisely onto the area it is required minimizes the amount of masking necessary. With tank plating, in which the entire work piece is immersed in the bath, extensive, and often expensive, masking is usually required.
Hydrogen embrittlement can usually be prevented more easily with selective plating than with other electroplating processes.
Despite continued advances in automating the process, and in the anodic tooling and plating solutions, the entire operation is still highly operator dependent. Tests have shown operational techniques have a considerable effect on the metallurgical and bonding characteristics of the metal deposited. Such variables as the relative motion of the anode and cathode, working temperatures, working voltage, current density, current flow, solution temperature, and gas release all must be accurately controlled to ensure uniformity of metal buildup and a metallurgically sound deposit with excellent adhesion. What's more, operational techniques vary considerably depending upon the type of metal repaired or modified and the type of metal deposited. Acquiring the skills essential to perform these techniques requires extensive training and hands-on experience.
More than 70 organic-based metal or alloy plating solutions are now available for commercial use. Most can be deposited onto nearly any conductive material, as well as some metals considered difficult to bond to with other electroplating processes, like beryllium, aluminum, stainless steel and refractory metals, though some base materials are easier to plate onto than others.
Though nearly all the metals plated by conventional electroplating processes can be deposited by selective plating: no hard-chrome selective plating solutions are currently available. In cases calling for a hard-chrome deposit, nickel is usually plated instead. A nickel-tungsten alloy of Rockwell C62, one of the hardest selective plating deposits available, is usually the preferred choice. Though nickel is not as hard as chrome, and doesn't have as a low a friction coefficient, it does have many of the same properties, as well as some important advantages. It can be plated thicker than hard chrome, and more important, since it is less porous it gives significantly more corrosion protection to the base metal.
Literally hundreds of specifications for selective plating, as well as approved operational systems and procedures, have been delineated by different industries, the U.S. Armed Forces, and various government agencies. Westinghouse, IBM, and General Electric, for example, have their own specifications for the use of selective plating not only in repairing electrical component parts, but also in producing them. Excerpting Federal QQ-C-320, "for hard chrome, selective plating deposits, properly applied, meet all Mil Specs, as well as those of the FAA and NASA and other government agencies".
For small- to large-volume work, or even for single items, whatever their size or design, selective plating should be considered a viable alternative to other methods of metal deposition like tank plating, metal spraying and welding. In many cases, it can not only be a more economical alternative, but offer superior results as well.
The cost-effectiveness of the process varies considerably from application to application. Among the important factors: the volume of work, whether the work is to be done in the field or in-house, the cost of the tooling, the purpose for which the metal is being deposited, the size of the area to be covered, and the required thickness of the deposit.
In addition, with large component parts for such things as marine engines and hydroelectric turbines, the cost of dismantling and removing the part and downtime become important considerations. The same considerations are often important to the electronics industry, which also faces shipping problems with such things as expensive, intricate circuit boards and laser and optical equipment.
Though there is no limit to the thickness of metal buildup possible, as well as the size of the surface area to be covered, there are also practical limitations to be taken into account, as well.
As a general rule, though, for most small-volume applications, the economics of the process are most favorable if the parts can be salvaged for less than half their original cost. Of course, the more parts to be repaired or resurfaced, the better the economics.
While selective plating has been widely used for redimensioning and resurfacing small to large volumes of worn or mismachined parts and the like, its cost benefits as a production technique have gone largely unnoticed by engineers until recently. Many are unaware of its ability to maintain precision tolerances during high-volume production runs, as well as its concomitant ability to improve the surface properties of the base metal. It allows engineers to select surface metals that are more wear- or corrosion-resistant, solderable or brazeable, or have better conductivity or other specific properties. Consider one of the possibilities: should a part design call for precision tolerances as well as a special coating, such as a corrosion-resistant coating like nickel, selective plating can be used as an efficient production method for meeting both requirements cost effectively.
At a time when cost efficiency and high quality are the watchwords of American industry, you may want to investigate further the benefits and myriad applications of this growing job shop technology.
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