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Metal finishing: an overview
INTRODUCTION METAL FINISHING: AN OVERVIEW by Leslie W. Flott Wabash, Ind. FINISHING BASICS A metal is any of a category of electropositive elements, usually having a dense shiny surface. Metals are, in most cases, good conductors of heat and electricity, and can be melted or fused, hammered into thin sheets, drawn into wires, or otherwise modified by machining or forming. Metals form salts with nonmetals, basic oxides with oxygen, and may also form alloys with two or more elements: metallic or nonmetallic. The word "metal" comes from Middle English by way of French and the Latin "metallum," which in turn comes from the Greek "metallon," meaning to mine a mineral or metal. The word "finishing" means to give a desired or particular surface texture. A finish is defined as the last treatment or coating of a surface or the surface texture produced by such a treatment or coating, or a material used in surfacing or finishing. Finishes are applied by "finishers." The word comes to us from the Middle English "finishen," which comes from the Old French "finir," "finiss" meaning "to complete" and in turn comes from the Latin "finire," from "finis," i.e., the "end." Metalfinishing thus means the application of a desired treatment, texture, or coating to the surface of a metal. Many common metals are unsuitable for a variety of applications in the form in which they are initially produced. Bare steel, raw aluminum, or copper--being quite susceptible to corrosion as they come from the mill--often have their surfaces altered in order to make them more useful. The metal finishing industry exists to supply these important surface alterations. This Guidebook is a general source of information about a variety of surface modifying processes, i.e., electroplating (see the section entitled Electroplating Solutions and Plating Procedures), anodizing (see tile section entitled Anodizing of Aluminum under Surface Treatments), nonmetallic finishes (see the sections entitled Chromate Conversion Coatings and Blackening and Antiquing), or mechanical finishing (see the section entitled Mechanical Surface Preparation). The Guidebook can also help inform end users of what information they should provide their suppliers to enable orders to be processed in a timely and effective manner. It is equally useful to assist purchasing, engineering, and production people to understand one another's needs about what properties are necessary for a particular part. This handy reference is a ready catalog of the characteristics of the processes available. Expanded descriptions are included in the body of the Guidebook to provide further information about these processes including some design guidelines. The information in this Guidebook should be of use to metal finishers and their customers for selection of an appropriate finish for metal parts.
General Engineering Considerations In broad engineering terms, metal finishing may be defined as any process done to protect, beautify, insulate, or increase the corrosion resistance, conductivity, or solderability of 19
metal objects, generally of iron, aluminum, or copper alloys. Metal finishing is not, however, a haphazard process. Modem metal finishing is every bit as much a skilled operation as any other manufacturing endeavor. Most finishing problems aren't finishing problems at all; they are usually people problems. Finishing solutions must be carefully controlled as to load size and current density, with every step done in a consistent manner. Those who design parts to be plated, anodized, or otherwise treated must seek to broaden their own knowledge of the finisbing processes in order to make informed decisions about finishing specifications. There are three general categories of metal finishing: plating (including electrodeposition, electroless, and mechanical methods), anodizing, and nonmetallic coatings. Throughout the following summary will be found references to appropriate sections covering details of specific processes found elsewhere in the Guidebook. There are, additionally, ancillary areas of interest to all three of the categories of metal finishing. These common areas include cleaning (refer to the sections on both mechanical and chemical surface preparation), process control (covered in the section entitled Control Analysis and Testing), and finally plant engineering, safety, and waste treatment (see the appropriate chapters under Finishing Plant Engineering). Every aspect of metal finishing worthy of concern after the parts have been finished is worthy of consideration before the parts are finished. This means that requirements must be clearly understood by the person who designs the part, the person who purchases the part, the person who finishes the part, the person who inspects the part, and the end user. ffthis is done, the customer will consistently receive those services needed, rather than those just thought to be wanted.
Nature of the Basis Metal Before finishers can begin to process a part, they must know what kind of metal they are dealing with. Are the parts to be finished made of cold-roll, hot-roll, or stainless steel? If they are made of a ferrous alloy, which alloy? Have the parts been stamped or otherwise machined from wrought material or are they castings or weldments? If the parts are stamped, what is the gauge of the metal? Have the parts been heat treated? If so, is the treatment carburization, nitriding, case hardening, or some other process? If the parts are heat treated, will they require baking for hydrogen embrittlement relief before or after plating? ff the parts are made of a nonferrous metal or alloy, they will require entirely different pretreatments. Are they made of aluminum, copper, or zinc? Knowledge of the exact alloy being used is critical to successful metal finishing. Of equal importance, especially for anodizing, is not to mix alloys in the same lot or within a single weldment. Mixing alloys will always result in a less uniform finish within a lot or a single weldment, and can result in parts being destroyed.
Metal Preparation In the preparation of metal parts for plating or anodizing, by far the most important consideration is how to clean the parts. Cleaning affects the adhesion, appearance, composition, and corrosion resistance of the final deposit. Even if the parts will receive no other finish than a dry-to-touch oil, cleaning is critical. It is, therefore, vital that the finisher be given as much information as possible about possible preexisting contamination such as inclusions in the base metal, the type and nature of the lubricants and cutting oils used in metalworking, the use or presence of synthetic organics or other coatings such as cured paints and the likely existence of inorganic films such as heat-treat scale. Before finishers can begin to process a part, they must know what kind of metal they are dealing with. When the finisher knows what material is being finished, intelligent choices can be made. Each basis metal requires a somewhat different pretreatment. Aluminum, for example, cannot be cleaned in solutions formulated for cleaning steel. Even variation within an alloy may require modification of the pretreatment process. A wrought alloy, such as 1100, 20
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ANODE
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CATHODE Fig. 1. How electroplating works: Metal balls or slabs are connected to the positive electrode (called the anode)in a weakly acidic or alkaline solution. The acid or base help to dissolve the metal at the anode. The solution contains the same metal as the anodes but in ionic form (that is they carry an electrical charge). As an electrical current passes through the solution, these ions migrate toward the negative electrode (called the cathode) where the object(s) to be plated is attached. The ions gain an electron at the cathode, which causes the object to attract a coating of the metal in solution. This coating builds one atom at a time in accordance with Faraday' s Law, which states that 1 Faraday (96,500 coulombs) of electricity will deposit 1 gram equivalent weight of any metal. The currentpassing through the solution is measured in Amperes (1 A equals 1 coulomb per second).
must be cleaned differently than 380 diecast alloy. Zinc-aluminum (ZA) alloys are especially difficult to clean. Any solution that cleans the aluminum will attack the zinc portion of the alloy, and anything that will clean the zinc is likely to attack the aluminum component. Environmental regulations (40 CFR, part 313 and others) have severely restricted the cleaning options available to metal finishers, and procedures that were common a few years ago may now be illegal or very severely restricted. ISO 14000, merely proposed at the time of this writing, may well further restrict options. Although the choices for cleaning are more limited today, most soils and contaminants can be safely removed if the finishers know what they are dealing with. Information supplied by the customer is the key to successful metal finishing. The sections on mechanical and chemical surface preparation and Mil-S-5002 and ASTM B 322 are excellent summaries of metal preparation processes. Once the basis metal has been determined, these references should be checked for suggestions, if not requirements, for cleaning the specific material being finished. -
THE ELECTROPLATING PROCESS Electroplating is a process by which metals in ionic form migrate from a positive to a negative electrode. An electrical current passing through the solution causes objects at the cathode to be coated by the metal in solution (see Fig. 1). The size, shape, and weight of the objects being plated determine how they will be plated. Electroplating is done to protect, beautify, insulate, or increase the corrosion resistance, conductivity, or solderability of metal objects, generally of iron or copper alloys. Plating protects by one of two mechanisms, first sacrificially and second mechanically. Zinc and cadmium protect the base metals they cover sacrificially. They are more reactive than iron or copper alloys and, therefore, corrode first, preferentially to those basis metals. Copper, nickel, 21
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Sl~ c o g e n t Fig. 2. A typical electroplating process: Objects to be plated are secured on racks or placed in barrels and dipped into a variety of solutions to prepare them for plating. The size, shape, and weight of the objects to be plated play a major role in determining how they will be plated. One thing is essential, however, regardless of what finish is to be applied: the parts must be "surgically" clean. Typically, more time is spent in preparing the parts to be plated than in plating them. For a typical plating line layout the steps would be: 1: 2: 3: 4:
Soak Clean Electroclean Cold water rinse Acid pickle
5: 6: 7: 8:
Cold water rinse Plating solution Drag-out rinse Cold water rinse
9: Chromate conversion coating 10: Cold water rinse 11: Hot water rinse 12: Dry
"B should not be determined from this figure that all electroplated metals require a chromate conversion coating; however, several may require one. These would include cadmium, zinc, and sometimes silver. chromium, tin, and most other metals provide merely mechanical protection. They protect the basis metal only so long as they themselves remain intact. If there are defects in the plating, the basis metal will corrode before the plating. Electroplating demands as much skill as any modern endeavor. Plating baths are more sophisticated today, as are customers. Solutions must be tested regularly and the results recorded. In addition, in this day of chemical hazard awareness, the need for platers to optimize performance is greater than ever. Platers immerse objects into a variety of chemical baths in order to change their surface condition (Fig. 2). The chemical make-up of the tanks is based on the desired result and every plating plant is different. Regardless of the finish being applied, the parts must be "surgically" clean. Typically, more time is spent in preparing the parts to be plated than in plating them. Not only must the baths be controlled, but every step of the process as well. Although this in itself may not be different from past practice, what is different is that you are now being asked to prove conformance to customers' requirements. Verification requires not only keeping track of what is happening, but maintaining records to prove that conformance.
Brass Plating Brass is an alloy of copper and zinc. Its origins can be traced to ancient times when the Egyptian pharaohs had death masks made from brass and gold using mercury amalgams and 22
the plating was done mechanically. The metals were dissolved in mercury and then applied directly to a dead pharaoh's face, and the mercury was then carefully separated by the process of placing papyrus mats over the amalgam and gently tapping until the mercury (a liquid metal) separated from the other metals and passed through the papyrus, leaving the other metals behind. Most brass plating today tends to be limited to decorative applications. See the chapter entitled Brass and Bronze Plating in the section Electroplating Solutions.
Copper Plating Copper plating, exclusive of continuous strip plating and nickel, is the most common metal plated. It is a soft, red, ductile, and solderable surface. It is not often used as a final plate, however, because it tarnishes easily. Copper may be plated from a variety of baths depending on the final finish desired. Probably the most commonly used is still the copper cyanide bath, which is used for both a finish coat and as an underplate or "strike," followed by another finish coating of copper or some other metal. For details on the chemistry of the many copper plating baths in current use, see the chapter entitled Copper Plating in this Guidebook. The following are several good reasons for copper plating's popularity: 1. Copper is an excellent choice for an underplate, since it often covers minor imperfections in the base metal. It is easy to buff or polish copper, giving it a high gloss. This gloss greatly enhances the appearance of any subsequent overplate. 2. Copper plate is relatively inert in most plating baths of other common metals. It is about the only metal that will readily plate directly onto zinc diecasts. 3. Copper has a very high plating efficiency, resulting in excellent coverage, even on otherwise difficult parts. 4. Copper is less environmentally hazardous than many other common metals and can, under some circumstances, be recycled. 5. Copper is highly conductive, making it an excellent coating for printed circuit boards or as a coating on steel wire used to conduct electricity. Among the most common copper plating specifications are Mil- C-14550 and GM 4252 M (copper and tin plating), now superseded by ASTM B 734 and B 545. Typically, copper will be applied at 0.0001 to 0.0002 in. (2.5 to 5.0/xm) and finish coatings for "commercial" copper plate will run 0.0002 to 0.0004 in. (5.0 to 10 /zm). The upper limit of 0.0004 in. (10 /xm) applies generally to externally threaded parts, but is common to most applications as well. Some automotive applications require 0.0005 in. (12.5 /xm) minimum. Note that most plating specifications are one sided, i.e., they specify only a lower limit, the upper limit is one imposed by the plater for economic reasons. A rule of thumb, provided the specific process is statistically capable, is to take the minimum thickness specified and double it in order to determine a reasonable upper limit. The target thickness may then be taken as the midpoint between the minimum and the calculated maximum.
Chromium Plating Chrome plating is appfied as a blue-white color that may be applied over underplated layers of copper and nickel for decorative purposes (see the chapter entitled Decorative Chromium Plating) or directly on the base metal for engineering pro-poses (see the chapter entitled Functional Chromium Plating). Chromium plate may be either shiny or dull, and often tends to highlight imperfections in the base metal. In spite of the very simple chemistry of the bath (see the chapters just listed), chromium plating is more difficult to run than many common metals, because of its very poor throwing power. Chromium does not normally throw plating into sharp inside comers and holes. The parts must be carefully racked to avoid being shaded-out by the rack. In cases in which design does not allow complete coverage, the nickel underplate will show through the chromium with a slightly yellowish color. Among the most 23
common chromium plating specifications are ASTM B 456 (actually copper/nickel/chromium) and federal specification QQ-C-320. Decorative chromium is normally applied over copper and nickel [normally > 0.0002 in. (5 /xm) copper and > 0.0003 in. (7.5 /xm) nickel] as a very thin coating typically 0.000050 in. (1.0/xm). The appearance of the final parts is usually determined by the underptate and is not the exclusive result of chromium plating. Specification QQ-C-320--"Chromium Plate"--calls for either Class l-Decorative, Type I-Bright, or Type II-Satin. Hard chromium is applied for wear resistance or to restore an old worn part to its original dimensions. It is generally applied directly onto the base metal. The appearance of hard chromium varies with the substrate onto which it is plated and can range from semibright to dull gray. A common chromium plating specification is AMS 2406, "Chromium-Hard Deposit-On Ferrous Metal Parts," and QQ-C-320-"Chromium Plate" Class 2-Engineering, and ASTM B 177. Engineering (hard) chromium may be applied in thicknesses up to significant fractions of an inch, but the default thickness is 0.002 in. (50 /xm), unless the customer specifies otherwise. Other chromium plating specifications include Mil-C-14538, Mil-C-20218, and Mil-C23422, all of which specify essentially special purpose coating.
Nickel Plating Nickel plating is a yellowish white, hard, reflective finish used for wear resistance, solderability, or dimensional restoration. Nickel plate is often applied over copper and under chromium for a decorative finish. Nickel is a very hard metal with relatively poor ductility; consequently, parts to be nickel plated should be bent into final shape before plating whenever possible. The chapter entitled Nickel Plating covers both decorative and engineering applications of nickel. Bright nickel plating is a highly reflective finish, which often eliminates the need for subsequent polishing. Bright nickel may be applied as a single or mnltilayer coating, though caution must be taken when attempting multilayer nickel, as it may result in poor adhesion in some instances. Multilayer nickel is favored for its excellent corrosion resistance. For applications requiring bright nickel, there are other considerations. The brighter the nickel plating, the greater the internal stresses and the lower the ductility. The brightest finishes result from first polishing the base metal substrate. It is best to avoid specifying bright nickel if the parts are to be bent or crimped after plating. Semibright nickel has a more satiny finish than bright nickel and may be marginally more ductile. If heat shock or minor bending of the parts is anticipated it would be better to specify semibright nickel in order to reduce the risk of the plating flaking off. Nickel may be specified by QQ-N-290--Nickel Plate, Class 1-"Decorative," Types I to III are for steel parts and Types V to VII for copper-base parts and ASTM B 456, "Copper/Nickel/Chromium" for a variety of service classes (SCs), which vary with alloy and base metal. The thickness of nickel plating for most commercial applications will be 0.0002 to 0.0004 in. (5 to 10/xm), but since nickel plates readily, it may be plated to hundreds of an inch (500 to 600/xm is easily possible) in some engineering applications.
Silver Plating Silver plating, in addition to being decorative, has the highest electrical and thermal conductivity of any metal. Silver may be plated from a noncyanide bath, but more typically is plated from a cyanide bath. See the section entitled Electroplating Solutions, specifically the chapter entitled Silver Plating for details on silver plating baths. Silver plating is highly ductile, malleable, and solderable. Silver tarnishes easily, however, due to sulfides in the air and should be packaged with special tarnish-resistant paper. Silver can be plated to military specification QQ-S-365, "Silver Plate" as Type I (matte finish, the best conductor), Type II 24
(semibright) or Type III (bright, the worst conductor) and may call for Grade A (chromate treated) or Grade B (no chromate). Matte silver plate (QQ-S-365, Type I) is used extensively for finishing electronic components where silver's mechanical, electrical, and thermal properties offer distinct advantages over other metallic, even silver, finishes. Semibright silver plate (QQ-S-365, Type II) is often specified where the electrical and/or mechanical properties of silver plate alone may not be enough and the design engineer feels that appearance may also be an important consideration. Semibright silver does, however, tend to tarnish faster than matte silver. Typically, platers prefer to apply silver plate as thin as practical, because it is a relatively expensive metal. Designers, of course, specify the thinnest plating that will serve their purposes, but anything over that minimum is "free." Precious-metal plating is thus controlled much more closely than are the more common metals. Silver is typically applied in thicknesses as low as 0.00005 to 0.0005 in. (1.0 to 10/xm), although most applications call for 0.0002 to 0.0004 in. (5.0 to 10.0/xm).
Finishes for Stainless Steel Passivation of stainless steel is not electroplating, it is a nonelectrical process whereby the free iron is chemically removed from the surface of stainless steel. This prevents the formation of possible corrosion sites and the development of tightly adhering oxides. The 300 series alloys are generally preferred for passivation, as some of the 400 series alloys will actually be discolored by the passivation process. Passivation imparts a limited neutral salt spray corrosion protection to the stainless steel, usually not much over 2 hours. It is critical when making assemblies to be passivated that all component parts be made of the same alloy; different alloys may be indistinguishable before passivation, but may have a different appearance afterward. Since different solutions are used to passivate different alloys, they must be properly identified. Mixing alloys may not only result in differences in appearance, but may result in some parts being destroyed. Passivation of stainless steel is generally specified by QQ-P-35 or ASTM A 380. Plating on stainless steel may be done, after suitable pretreatment, by any method listed here. Stainless steels and high-nickel alloys form a tightly adhering passive oxide film within minutes of being plated. Stainless parts can be plated with other metals if a fresh active surface is provided for subsequent plating (see the section of the Guidebook entitled Preparation of Basis Metals for Plating. Plating on stainless steel is addressed by ASTM B 254.
Tin Plating Tin plating is normally done to impart solderability to a variety of base metal substrates~ Tin is a silvery blue-white metal that is ductile, solderable, and covers very well. The solderability of tin can be affected by the substrate, since several metals tend to react with and migrate into the tin forming relatively nonsolderabte intermetallic layers. Of particular concern is that if the tin is to be plated over brass, or zinc die casts, the zinc will migrate into the tin and severely limit'the shelf life of the finished parts. The migration can be mitigated by the common practice of applying an undercoating of copper or nickel through which the zinc cannot migrate. Tin is a fairly easy metal to plate and is approved for a variety of industrial applications and is even approved for food-container or food-contact applications. There are no known common tin salts that are extremely toxic or carcinogenic. Tin does not tarnish easily, making it a good choice as a low-cost decorative finish as well. Tin plating is addressed in Mil-T-10727. Although this specification calls for a fairly thin plating for solderability (0.0005 in. or 1 /~m), common practice is to apply much more. This thinner plate was commonly used when "stannate tin" baths were in common use and practice was to reflow the tin periodically. The tin plate from these older-style tin baths tended to 25
change spontaneously from "white" tin to "black or gray" after a few months, hence the need to reflow the tin. This process also caused not a few fires. Today, where long-term shelf life is a consideration, 0.0003 in. (7.5/xm) minimum tin is commonly used over an underplate of 0.0002 in. (5.0/xm) minimum copper or nickel. There are a great many other specifications calling for tin combined with other metals being plated as an alloy. These include Mil-L-46064 and Mil-P-81~728, both of which are for tin-lead, and Mil-P-23408, tin-cadmium. Tin may also be plated as a tin-nickel alloy, which is sometimes used as a substitute from decorative chromium. All of these alloy finishes generally offer a better shelf life than pure tin plate, but are considered environmentally unfriendly, and so their use is much diminished. The tin and tin-lead alloy plating baths are discussed in the chapter entitled Tin-Lead, Lead, and Tin Plating; arid tin-nickel is discussed in the chapter entitled Tin-Nickel Alloy Plating.
Zinc Plating Zinc plating is a soft, ductile, decorative, marginally solderable, corrosion-resistant finish. Unlike most other commonly plated metals, zinc protects the substrate by sacrificing itself and thus corrodes before the base metal. This means that zinc will protect even if the zinc coating sustains minor damage, such as scratches of small punctures, which is often an advantage that other types of plating do not offer. Zinc is the most reactive of all common metals; however, it may be attacked or dissolved by ordinary liquids such as soft drinks and vinegar. The ultimate corrosion resistance of zinc is a function of the plating thickness. To increase the corrosion resistance of zinc, a conversion coating is usually added. In the chapter entitled Zinc Plating, the authors discuss the three most common zinc plating baths in use today. They also examine the relative merits of these popular zinc plating baths. Additionally, the chapter entitled Zinc Alloy Plating looks at zinc-iron, zinc-cobalt, zinc nickel, and tin-zinc alloy baths, which have become increasingly popular both because of their remarkable corrosion resistance and as a possible substitute for cadmium. Chloride zinc plating was introduced about 1980, when environmental pressures began to demand the replacement of cyanide plating baths with noncyanide baths. Presently, over 50% of all zinc deposited in this country is from acid chloride baths. By far, the majority of these are chloride baths with potassium chloride and are ammonia-free with a pH around 5 to 6. These baths have high cathode efficiency and excellent throwing power because of the high cathode efficiency over the entire current density range. The most commonly used specification for zinc plating is ASTM B 633, which replaced QQ-Z-325 in 1982, although a great many drawings still call for QQ-Z-325. One of the main advantages of the ASTM specification over QQ-Z-325 is that ASTM B 633 distinguishes between colored and clear chromate conversion coatings. It calls out four types of post finishes for zinc plating, Type I: as plated; Type II: colored chromate (default yellow, but olive drab or black may be specified); Type III: colorless chromate (these are often designated as clear or bright, blue bright, or blue); and Type IV: phosphate coating. Thicknesses are spelled out by an SC designation: SC1, 0.001 in., SC2, 0.0005 in., SC3, 0.0003 in., SC4, 0.0002 in. and SC5, 0.0001 in.
Chromate Coatings Chromate conversion coatings are chemical conversion coatings. The substrate metal participates in the coating reaction and becomes a component of the coating. The collaboration has a profound effect on the properties of the coating. Among the metals commonly chromated are zinc, zinc die castings, hot-dipped galvanized steel, aluminum, and sometimes copper and silver. Chromate films are typically very thin, on the order of 0.0000001 in. and contribute no measurable thickness to the overall coating. The chemistry involves a reaction between the metal surface and an aqueous solution 26
containing chromates (chromium salts, either hexavalent or trivalent) and certain activators or film formers. Activators may include sulfates, fluorides, phosphates, and sometimes complex cyanides. Normally, a given chromate is designed to work on a particular metal, but in a few cases will work on two or more. The chromate solution is normally acidic. About 0.0005 in. of plating thickness is dissolved during the chromating process. Clear chromate offers from 8 to 12 hours' resistance to white corrosion (zinc oxide or "white rust") and has a clear to slightly iridescent blue appearance. This is one of the earliest types of chromating solutions and is used today. Older style "clear" chromates often deposited as a golden yellow coating and were then bleached by immersion in a dilute alkali solution to obtain the clear appearance. More up-to-date blue bright chromates are single-dip solutions, often using only trivalent chromium salts and are, therefore, more environmentally friendly. Some varieties of this type of baths may be dyed a variety of colors. These colors are often used for identification purposes. Gold, or yellow, chromate coatings are deposited from baths that contain chromate, sulfate, or chloride activators and produce a distinct iridescent golden yellow color. Yellow chromates contain more hexavalent chromium than clear films, which accounts for their color. Yellow chromate offers in excess of 96 hours' neutral salt spray corrosion protection and is an excellent paint base. Olive drab, or forest green, chromate is the ultimate in commonly available conversion coatings, with neutral salt spray resistance in excess of 150 hours. This chromate is most commonly specified for military applications. It is generally not possible to apply to barrel plate work. The color and the corrosion resistance are due to the inclusion of an organic acid modifier to the chromate formula. Many customers find the color is not especially pleasing, but specify it for nonmilitary applications for its functional, rather than its decorative, value. Black chromate is usually achieved by incorporating a soluble silver salt in a golden chromate formula, which produces a deposit of black silver chromate. This coating offers excellent corrosion resistance and a jet black semimatte to matte appearance. Black chromate has even found some applications in the space program, in which it is used on solar collectors. The use of silver salts makes black chromate coating rather more expensive and is also sensitive to chloride contamination. This is especially a problem when black chromate is to be applied over parts plated in a chloride bath. Chromate conversion coatings are included in a great many specifications and are also covered in their own specification Mil-C-5541, although that specification applied exclusively to chromate conversion coatings on aluminum.
Special Processes Special processes include a variety of finishes that do not change the surface of the parts other than through chemical or mechanical means. Pickle and oil uses an acid solution to remove scale and rust from the metal surface, after which a special rust-inhibiting oil preservative is applied to prevent or retard further corrosion. Some steels with very high polish or microfinish may show a reduction in gloss as a result of the pickle and oil process. Caution is recommended. Rust can also significantly reduce the gloss or microfinish on steel parts. Chemical etch is used on iron, copper, and aluminum to remove metal chemically, thus creating a more uniform matte finish than is found on the as-produced material. Chemical etching has the disadvantage of preferentially attacking high-energy areas, such as shalT corners and threads. The result can be the rounding of these corners, etc., which may not always be desirable. Chemical etching is not a "polishing" process and cannot be used to remove deep scratches or tool marks from the metal surface. Typically less than 0.0005 in. of material is removed by the chemical etch process. Since these effects are different for different alloys and even different steel production processes (hot-roll versus cold-roll), it is important for the customer to advise which alloy is to be treated. It is equally important that alloys not be mixed, either within a lot or within a single weldment or assembly. 27'
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Fig. 3. The anodize process: The aluminum base metal, A, reacts with the oxygen at B, forming an aluminum oxide coating. This dense layer of newly formed aluminum, C, is dissolved by the acidic electrolyte, causing the layer to become porous at D. The result is a relatively large pore, E, and a fairly thin cell wall, E When the forward film building process is in equilibrium with the dissolution of the coating, the anodize coating stops getting thicker. Hydrogen embrittlement relief is done after plating to remove hydrogen from the intercrystalline interstices, which could otherwise result in brittleness and premature failure of the part. The process is typically applied to fasteners, springs, and other parts having a Rockwell C (Rc) hardness of 30 or higher. The process must also be done within 1 hour of the plating process. Hydrogen embrittlement relief is generally done at 190 + 5°C and is, therefore, achieved without annealing the parts.
ALUMINUM FINISHING Anodizing is an electrochemical process whereby the naturally occurring oxide coating on the outer surface of an aluminum part is changed to a tightly adhering layer of oxide of aluminum of a specified thickness. The natural coating is approximately 0.0000005-in. thick, but the anodizing process can increase the coating to a film thickness of 0.0005 to 0.003 in. This thicker coating is much more resistant to corrosion and abrasion than the underlying aluminum. Anodizing coats very uniformly and will, therefore, not fill or smooth out a rough or damaged surface. In anodizing, there are two competing processes that occur simultaneously. Aluminum parts are first made the positive pole, or anode, in an electrochemical cell using an acidic electrolyte. As current passes through the solution, oxygen is generated at the anode and immediately reacts with the aluminum parts to form an aluminum oxide coating. Simultaneously, the aluminum oxide coating is dissolving into the electrolyte and is becoming porous and less dense. When these two competing processes are in equilibrium, the anodized coating reaches its maximum and can no longer become thicker (see Fig. 3).
28
Fig. 4. Cross section of an anodized bolt: In the illustration, the bolt is made of aluminum, A, with its original surface at B. The anodize nat only builds to a new surface at D, but also penetrates A by a nearly identical amount. This penetration depth, C, plus the build up, equals the total anodize film thickness.
Anodizing does not simply add material to the base metal as does plating. The anodized film grows outward and also penetrates the aluminum base metal, each by approximately one-half of the overall film thickness. Since each side grows by an amount equal to one-half the anodized film thickness, the part grows an amount equal to the total anodized film thickness (see Fig. 4). Anodized aluminum has many useful engineering properties. For example, anodizing offers excellent corrosion resistance, typically over 336 hours' salt spray resistance (tested per ASTM B 117)and a surface that is second only to that of diamond in hardness. Note, however, that the anodized film is rather thin and doesn't have a great deal of puncture resistance. Anodize hardness is normally expressed in terms of its ability to resist abrasion (tested by the Tabor Abrasion Test). Anodize must be sealedin order to obtain optimum corrosion resistance. Frequently, designers of electronic devices are inspired by the thought of using anodized aluminum for heat sinks. The idea is very appealing, since aluminum is a very good heat conductor and anodize is a dielectric. The idea of a dielectric thermal conductor is too good to pass up. This never works, however, because of the nature of anodizing. Anodizing, unlike electroplating, works from the surface inward. The coating grows outward from the interface between the anodic coating and the raw aluminum. The growth is at fight angles to the surface of the base metal, i.e., perpendicular. The coating is forming around a three-dimensional object, but cannot itself grow in three dimensions. The result is cracking at sharp comers. A thin anodic film will not crack to any great extent; however, the thicker the film, the worse the cracking becomes. Hardcoat anodize tends to crack more than sulfuric anodizing, for two reasons: first the hardcoat tends to be denser than clear (sulfuric) anodizing, and hardcoat films are generally thicker than clear anodizing. In order the minimize this comer fault, all edges should be radiused at least 0.030 in. for each 0.001 in. of coating thickness (see Fig. 5). Steel or brass inserts, or any material other than aluminum, that are part of an assembly or weldment submitted to be anodized must be removed or masked before anodizing. Unless removed or masked, the inserts will probably be destroyed by the anodize process. Because most anodized coatings are porous, they are able to absorb dyes before they are sealed. This produces an attractive colored oxide coating integral to the metal itself, and is less likely to chip or peel than plated coatings. With proper care and handling, anodized articles can last a lifetime. On threaded parts, the pitch diameter is measured over four separate surfaces; the apparent growth of the pitch diameter is, therefore, approximately equal to the thickness of the anodized film. 29
A
B
C
O
Fig. 5. Comer faults: A plain piece of aluminum, A, may be formed with sharp corners without any problems. Corners, however, create a special problem when the aluminum is anodized because anodizing builds from the inside out. When a thin anodize film is formed, stresses build at the outside comers until eventually the corners crack, as shown at B. The thicker the anodize, the greater the severity of cracking, as shown at C. These corner faults can be minimized by radiusing the comers as shown in D.
Common Types of Anodizing Chromic anodize is the oldest type of anodizing and applies the thinnest film. Rarely will chromic anodize apply more than 0.0002 in. to wrought alloys, and less to a cast alloy. Chromic anodizing is normally a grayish coating formed from a chromic acid electrolyte. Chromic anodizing is most often used as a base for paint for aircraft and marine applications. It has been phased out of many operations because of environmental considerations. •Sulfuric anodizing, also called clear anodizing, is an excellent corrosion-resistant finish and, under the right circumstances, a good electrical insulator. This is the most common type of anodizing done today. Sulfuric acid anodizing may exhibit a silver, bronze, tan, or gray color, depending on the alloy used. The coating is commonly dyed, or otherwise colored, with a variety of organic dyes, mineral pigments, or precipitated metals. Typically, a sulfuric anodizing coating will be from 0.0005 to 0.0015 in. for wrought alloys and a bit less for cast alloys. Hardcoat anodizing is an highly abrasion-resistant, nonconductive coating of aluminum oxide (A1203.H20) that makes the aluminum surface harder than tool steel. Hardcoating is done in a sulfuric acid electrolyte modified by the addition of one or more organic acids and is run at very low temperatures. Typically, the R c hardness will be in the 65 to 70 range. To obtain maximum wear resistance, the hardcoat should not be sealed. The nominal thickness of hardcoat runs from 0.002 to 0.003 in. on wrought materials and less on most cast alloys.
Weldments and Welded Parts Welds on aluminum are often porous; acid is trapped in the welds and the narrow spaces between the parts. It may take upward of 24 hours for the acid to begin to leach out and attack the anodized coating or bleach the dye. This will continue until all of the entrapped acid has seeped out. Hollow parts (pipes, square tubing, etc.) can hold a lot of acid and act as a reservoir. The stored acid poses a potential danger to end users or others who' may handle the parts at a later time. It is, therefore, critical that 1/4-in. minimum drain holes be drilled into each chamber that might potentially store acid. Drains will allow the acid to bleed out and be replaced by clear water during the anodizing process. The high temperatures required by the welding process will affect an area around each weld, causing the color to be slightly lighter and causing the welded area to appear larger than it actually is. 30
Choice of Alloys for Anodizing Anodizing is recommended for virtually all aluminum alloys. It is critical, however, that alloys not be mixed in a weldment, or within a single order. Alloys such as 2024 if mixed or welded to 6061, for example, may well be totally destroyed during the anodizing process.
Wrought Alloys 1100 series alloys yield a bronze to gray coating at 0.002 in. The alloys are particularly soft and do not machine well. The maximum coating that can be expected is 0.0025 in., but 0.003 in. is possible. 2000 series, principally 2011, 2017, 2024, and 2618 (forgings), develop a gray-black to blue-gray color at 0.0025 to 0.003 in. 3000 series, most commonly 3003, develops a gray-black color at 0.002 in. They machine well and readily take dye. 4000 series is not commonly used for anodizing. 5000 series, primarily 5005 and 5052, machine well and are good for dyed work. They develop a black color at 0.002 in. This material when hardcoated has excellent dielectric properties. 6000 series, most commonly 6061 and 6063, form excellent anodized coatings. Hardcoat on these alloys may be ground, lapped, or honed. The alloys have excellent dimensional stability and machine well, though they can be a bit stringy under some circumstances. The 6063 is commonly used for extrusions and as rod for welding other alloys. The maximum practical coating thickness is 0.0025 in. 7000 series (usually only 7075 is commonly used) develop a blue-gray color at 0.002 in. It is not a good choice for grinding or lapping, since it has a tendency to be checky. 8000 series is not commonly used for anodizing.
Cast Alloys Alloys for sandcasting commonly include 319, 355, 356, and sometimes 40E (a ternalloy). The 356-T6 is the most commonly used sandcast alloy because it grinds and polishes well. Porosity is always a problem with sandcastings. Pores will cause the coating to appear to have pits. Anodizing will not fill the pores. Sandcast parts are commonly coated to 0.001 to 0.002 in., but some will develop thicker coatings. Alloys for diecasting include 218,360, and 380. Of these, only 218 produces an anodized coating comparable to wrought or sandcast alloys. The 218 is hard to diecast, however, so is rarely used.
Reanodizing Parts Anodized coatings of any type--chromic, sulfuric, or hardcoat--cannot be built up over existing coatings. Once electrical contact with the anodized part has been broken it is very difficult to reestablish it. Stripping and reanodizing works but removes metal and, therefore, affects critical dimensions. Stripping and reanodizing may produce parts that are dimensionally unusable as a result. As a matter of policy, stripping and reanodizing are done only with specific customer approval.
Coating Thickness The coating thickness for any type of anodizing is roughly 50% buildup and 50% penetration. In other words, a 0.002-in. coating thickness will build up only 0.001 in. per side. This means that the overall dimension of the part increases by an amount equal to the total film thickness. Holes are closed by a like amount, unless plugged or masked. Hardcoat may be provided in a thickness ranging from a few tenths of a thousandth of an inch up to about three thousandths of an inch. As stated above, hardcoating (and indeed all anodizing) changes the dimensions of a part by an amount equal to one-half of the combined 31
total buildup on opposite sides of a part. When machining parts to be hardcoated, or anodized, it is essential to allow for this change and to specify a coating thickness on blueprints and purchase orders.
Masking When it is necessary to exclude the anodize from specific areas, the parts can be masked. Areas to be masked, such as threaded holes, ground points, etc., must be clearly identified by the customer. Masking is a hand operation, which may add to the total cost of anodizing a part. Even if anodizing is required on only one area of a part, it may be cheaper to anodize the entire part rather than pay for the masking. On the other hand, it may be cheaper to tap holes and mask them, rather than purchase special drills and taps and try to do the job after anodizing.
Racking The anodized coating that builds up on the part is an excellent insulator. Firm electrical contact must be made with the parts to be anodized and these "rack marks" will not have any anodize on them. Proper racking is the key to economical and effective anodizing. It is essential that the rack marks be located in a noncritical area. Any guidance as to where to locate these small bare spots will aid in the proper processing of the parts and, in the absence of such guidance, good practice should be followed. Occasionally, good practice may place the rack marks in an unidentified critical area in spite of best intentions. Parts should never, however, be racked in threaded holes without specific instruction from the customer.
Polishing and Lapping Norbide (boron carbide) or equivalent abrasive grain using a heavy oil or petroleum jelly carrier cuts very fast with excellent results. The use of polishing sticks or brushes is recommended. A grit size of 400 to 1,200 works well depending on the finish required.
Grinding Norton Crystolon (silicon carbide) abrasive or its equivalent is most satisfactory for surface grinding hardcoat. Eighty to 100 grit will yield a microfinish of 2 to 8 microinches. Soft wheels in H, I, or J grades are preferable for fast stock removal and their use minimizes the danger of burning or cracking. Typical wheel numbers are 39C60-J8VK for finishes of 6 to 10 microinches and 39C 100-HSVK for finishes of 2 to 3 microinches. Cylindrical grinding is best done with a 39C 120-JSVK. This finer grit wheel will be free cutting and still produce a very fine finish. For internal grinding, the 39C100-J8VK gives the best results. In general, grinding hardcoat should be done wet, using water as a coolant and a good quality medium-priced soluble oil mixed approximately 100 to 1. Cincinnati Cimplus has been used at 200 to 1 with good results.
Hardcoat and Teflon Teflon may be added to hardcoat as a posttreatment to provide a hard, corrosion-resistant, dry-lubricated, low-friction surface.
Chromate Conversion Coatings Chromate conversion coatings may be applied to aluminum from a chemical bath. All chromates on the QPL list should be considered interchangeable, and the same materials are generally approved for both clear (used for low contact resistance) or golden yellow per Mi1-C-5541. 32
The clear chromate is a nearly colorless coating that has excellent salt spray corrosion resistance (168 hours tested per ASTM B 117), with a typical contact resistance of less than 500 micro-ohms. Yellow, or golden, chromate is used most often for corrosion resistance and to increase subsequent paint adhesion. Parts with yellow chromate are normally (more frequently when required) tested monthly by an independent laboratory by Federal Test Standard 141 A. By specification, the color of a nominally yellow chromate conversion coating can vary from very light yellow to brown. Some variation of color from part to part and even within a single part is normal and acceptable per the above specification.
Plating on Aluminum Plating on aluminum is different from conventional electroplating because of the unique nature and reactivity of aluminum. The principal pretreatment involves the removal and subsequent prevention of the natural surface oxidation. Since an oxide film forms in seconds on a freshly cleaned aluminum surface, it is necessary to apply an immersion coating of zinc, temporarily, to prevent this oxidation. This coating, called "zincate," is removed in the next plating step (usually copper plating) after which the aluminum can be plated the same as any other metal.
Electroplating and Related Processes byJ.B. Mohler 311 pages $65.00 T h i s is a g o o d i n t r o d u c t o r y v o l u m e t o t h e i n d u s t r y . T h e e x p l a n a t i o n s are clear a n d t h e m a t e r i a l is u n c l u t t e r e d w i t h v e r b i a g e o r m e a n i n g l e s s illustrations. The reader wastes no time while preparing the groundwork for further study. S e n d O r d e r s to:
M E T A L FINISHING 660 W h i t e P l a i n s Rd., T a r r y t o w n , N Y 10591-5153 F o r f a s t e r service, call (914) 333-2578 o r F A X y o u r o r d e r t o (914) 333-2570 All book orders must be prepaid. Pleaseinclude $5.00shippingand handlingfor deliveryof each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and $20.00 for each book shipped expressto all other countries.
33
General Engineering Considerations In broad engineering terms, metal finishing may be defined as any process done to protect, beautify, insulate, or increase the corrosion resistance, conductivity, or solderability of 19
metal objects, generally of iron, aluminum, or copper alloys. Metal finishing is not, however, a haphazard process. Modem metal finishing is every bit as much a skilled operation as any other manufacturing endeavor. Most finishing problems aren't finishing problems at all; they are usually people problems. Finishing solutions must be carefully controlled as to load size and current density, with every step done in a consistent manner. Those who design parts to be plated, anodized, or otherwise treated must seek to broaden their own knowledge of the finisbing processes in order to make informed decisions about finishing specifications. There are three general categories of metal finishing: plating (including electrodeposition, electroless, and mechanical methods), anodizing, and nonmetallic coatings. Throughout the following summary will be found references to appropriate sections covering details of specific processes found elsewhere in the Guidebook. There are, additionally, ancillary areas of interest to all three of the categories of metal finishing. These common areas include cleaning (refer to the sections on both mechanical and chemical surface preparation), process control (covered in the section entitled Control Analysis and Testing), and finally plant engineering, safety, and waste treatment (see the appropriate chapters under Finishing Plant Engineering). Every aspect of metal finishing worthy of concern after the parts have been finished is worthy of consideration before the parts are finished. This means that requirements must be clearly understood by the person who designs the part, the person who purchases the part, the person who finishes the part, the person who inspects the part, and the end user. ffthis is done, the customer will consistently receive those services needed, rather than those just thought to be wanted.
Nature of the Basis Metal Before finishers can begin to process a part, they must know what kind of metal they are dealing with. Are the parts to be finished made of cold-roll, hot-roll, or stainless steel? If they are made of a ferrous alloy, which alloy? Have the parts been stamped or otherwise machined from wrought material or are they castings or weldments? If the parts are stamped, what is the gauge of the metal? Have the parts been heat treated? If so, is the treatment carburization, nitriding, case hardening, or some other process? If the parts are heat treated, will they require baking for hydrogen embrittlement relief before or after plating? ff the parts are made of a nonferrous metal or alloy, they will require entirely different pretreatments. Are they made of aluminum, copper, or zinc? Knowledge of the exact alloy being used is critical to successful metal finishing. Of equal importance, especially for anodizing, is not to mix alloys in the same lot or within a single weldment. Mixing alloys will always result in a less uniform finish within a lot or a single weldment, and can result in parts being destroyed.
Metal Preparation In the preparation of metal parts for plating or anodizing, by far the most important consideration is how to clean the parts. Cleaning affects the adhesion, appearance, composition, and corrosion resistance of the final deposit. Even if the parts will receive no other finish than a dry-to-touch oil, cleaning is critical. It is, therefore, vital that the finisher be given as much information as possible about possible preexisting contamination such as inclusions in the base metal, the type and nature of the lubricants and cutting oils used in metalworking, the use or presence of synthetic organics or other coatings such as cured paints and the likely existence of inorganic films such as heat-treat scale. Before finishers can begin to process a part, they must know what kind of metal they are dealing with. When the finisher knows what material is being finished, intelligent choices can be made. Each basis metal requires a somewhat different pretreatment. Aluminum, for example, cannot be cleaned in solutions formulated for cleaning steel. Even variation within an alloy may require modification of the pretreatment process. A wrought alloy, such as 1100, 20
+
ANODE
+
CATHODE Fig. 1. How electroplating works: Metal balls or slabs are connected to the positive electrode (called the anode)in a weakly acidic or alkaline solution. The acid or base help to dissolve the metal at the anode. The solution contains the same metal as the anodes but in ionic form (that is they carry an electrical charge). As an electrical current passes through the solution, these ions migrate toward the negative electrode (called the cathode) where the object(s) to be plated is attached. The ions gain an electron at the cathode, which causes the object to attract a coating of the metal in solution. This coating builds one atom at a time in accordance with Faraday' s Law, which states that 1 Faraday (96,500 coulombs) of electricity will deposit 1 gram equivalent weight of any metal. The currentpassing through the solution is measured in Amperes (1 A equals 1 coulomb per second).
must be cleaned differently than 380 diecast alloy. Zinc-aluminum (ZA) alloys are especially difficult to clean. Any solution that cleans the aluminum will attack the zinc portion of the alloy, and anything that will clean the zinc is likely to attack the aluminum component. Environmental regulations (40 CFR, part 313 and others) have severely restricted the cleaning options available to metal finishers, and procedures that were common a few years ago may now be illegal or very severely restricted. ISO 14000, merely proposed at the time of this writing, may well further restrict options. Although the choices for cleaning are more limited today, most soils and contaminants can be safely removed if the finishers know what they are dealing with. Information supplied by the customer is the key to successful metal finishing. The sections on mechanical and chemical surface preparation and Mil-S-5002 and ASTM B 322 are excellent summaries of metal preparation processes. Once the basis metal has been determined, these references should be checked for suggestions, if not requirements, for cleaning the specific material being finished. -
THE ELECTROPLATING PROCESS Electroplating is a process by which metals in ionic form migrate from a positive to a negative electrode. An electrical current passing through the solution causes objects at the cathode to be coated by the metal in solution (see Fig. 1). The size, shape, and weight of the objects being plated determine how they will be plated. Electroplating is done to protect, beautify, insulate, or increase the corrosion resistance, conductivity, or solderability of metal objects, generally of iron or copper alloys. Plating protects by one of two mechanisms, first sacrificially and second mechanically. Zinc and cadmium protect the base metals they cover sacrificially. They are more reactive than iron or copper alloys and, therefore, corrode first, preferentially to those basis metals. Copper, nickel, 21
2: $
4
5
6
10 1 1
Sl~ c o g e n t Fig. 2. A typical electroplating process: Objects to be plated are secured on racks or placed in barrels and dipped into a variety of solutions to prepare them for plating. The size, shape, and weight of the objects to be plated play a major role in determining how they will be plated. One thing is essential, however, regardless of what finish is to be applied: the parts must be "surgically" clean. Typically, more time is spent in preparing the parts to be plated than in plating them. For a typical plating line layout the steps would be: 1: 2: 3: 4:
Soak Clean Electroclean Cold water rinse Acid pickle
5: 6: 7: 8:
Cold water rinse Plating solution Drag-out rinse Cold water rinse
9: Chromate conversion coating 10: Cold water rinse 11: Hot water rinse 12: Dry
"B should not be determined from this figure that all electroplated metals require a chromate conversion coating; however, several may require one. These would include cadmium, zinc, and sometimes silver. chromium, tin, and most other metals provide merely mechanical protection. They protect the basis metal only so long as they themselves remain intact. If there are defects in the plating, the basis metal will corrode before the plating. Electroplating demands as much skill as any modern endeavor. Plating baths are more sophisticated today, as are customers. Solutions must be tested regularly and the results recorded. In addition, in this day of chemical hazard awareness, the need for platers to optimize performance is greater than ever. Platers immerse objects into a variety of chemical baths in order to change their surface condition (Fig. 2). The chemical make-up of the tanks is based on the desired result and every plating plant is different. Regardless of the finish being applied, the parts must be "surgically" clean. Typically, more time is spent in preparing the parts to be plated than in plating them. Not only must the baths be controlled, but every step of the process as well. Although this in itself may not be different from past practice, what is different is that you are now being asked to prove conformance to customers' requirements. Verification requires not only keeping track of what is happening, but maintaining records to prove that conformance.
Brass Plating Brass is an alloy of copper and zinc. Its origins can be traced to ancient times when the Egyptian pharaohs had death masks made from brass and gold using mercury amalgams and 22
the plating was done mechanically. The metals were dissolved in mercury and then applied directly to a dead pharaoh's face, and the mercury was then carefully separated by the process of placing papyrus mats over the amalgam and gently tapping until the mercury (a liquid metal) separated from the other metals and passed through the papyrus, leaving the other metals behind. Most brass plating today tends to be limited to decorative applications. See the chapter entitled Brass and Bronze Plating in the section Electroplating Solutions.
Copper Plating Copper plating, exclusive of continuous strip plating and nickel, is the most common metal plated. It is a soft, red, ductile, and solderable surface. It is not often used as a final plate, however, because it tarnishes easily. Copper may be plated from a variety of baths depending on the final finish desired. Probably the most commonly used is still the copper cyanide bath, which is used for both a finish coat and as an underplate or "strike," followed by another finish coating of copper or some other metal. For details on the chemistry of the many copper plating baths in current use, see the chapter entitled Copper Plating in this Guidebook. The following are several good reasons for copper plating's popularity: 1. Copper is an excellent choice for an underplate, since it often covers minor imperfections in the base metal. It is easy to buff or polish copper, giving it a high gloss. This gloss greatly enhances the appearance of any subsequent overplate. 2. Copper plate is relatively inert in most plating baths of other common metals. It is about the only metal that will readily plate directly onto zinc diecasts. 3. Copper has a very high plating efficiency, resulting in excellent coverage, even on otherwise difficult parts. 4. Copper is less environmentally hazardous than many other common metals and can, under some circumstances, be recycled. 5. Copper is highly conductive, making it an excellent coating for printed circuit boards or as a coating on steel wire used to conduct electricity. Among the most common copper plating specifications are Mil- C-14550 and GM 4252 M (copper and tin plating), now superseded by ASTM B 734 and B 545. Typically, copper will be applied at 0.0001 to 0.0002 in. (2.5 to 5.0/xm) and finish coatings for "commercial" copper plate will run 0.0002 to 0.0004 in. (5.0 to 10 /zm). The upper limit of 0.0004 in. (10 /xm) applies generally to externally threaded parts, but is common to most applications as well. Some automotive applications require 0.0005 in. (12.5 /xm) minimum. Note that most plating specifications are one sided, i.e., they specify only a lower limit, the upper limit is one imposed by the plater for economic reasons. A rule of thumb, provided the specific process is statistically capable, is to take the minimum thickness specified and double it in order to determine a reasonable upper limit. The target thickness may then be taken as the midpoint between the minimum and the calculated maximum.
Chromium Plating Chrome plating is appfied as a blue-white color that may be applied over underplated layers of copper and nickel for decorative purposes (see the chapter entitled Decorative Chromium Plating) or directly on the base metal for engineering pro-poses (see the chapter entitled Functional Chromium Plating). Chromium plate may be either shiny or dull, and often tends to highlight imperfections in the base metal. In spite of the very simple chemistry of the bath (see the chapters just listed), chromium plating is more difficult to run than many common metals, because of its very poor throwing power. Chromium does not normally throw plating into sharp inside comers and holes. The parts must be carefully racked to avoid being shaded-out by the rack. In cases in which design does not allow complete coverage, the nickel underplate will show through the chromium with a slightly yellowish color. Among the most 23
common chromium plating specifications are ASTM B 456 (actually copper/nickel/chromium) and federal specification QQ-C-320. Decorative chromium is normally applied over copper and nickel [normally > 0.0002 in. (5 /xm) copper and > 0.0003 in. (7.5 /xm) nickel] as a very thin coating typically 0.000050 in. (1.0/xm). The appearance of the final parts is usually determined by the underptate and is not the exclusive result of chromium plating. Specification QQ-C-320--"Chromium Plate"--calls for either Class l-Decorative, Type I-Bright, or Type II-Satin. Hard chromium is applied for wear resistance or to restore an old worn part to its original dimensions. It is generally applied directly onto the base metal. The appearance of hard chromium varies with the substrate onto which it is plated and can range from semibright to dull gray. A common chromium plating specification is AMS 2406, "Chromium-Hard Deposit-On Ferrous Metal Parts," and QQ-C-320-"Chromium Plate" Class 2-Engineering, and ASTM B 177. Engineering (hard) chromium may be applied in thicknesses up to significant fractions of an inch, but the default thickness is 0.002 in. (50 /xm), unless the customer specifies otherwise. Other chromium plating specifications include Mil-C-14538, Mil-C-20218, and Mil-C23422, all of which specify essentially special purpose coating.
Nickel Plating Nickel plating is a yellowish white, hard, reflective finish used for wear resistance, solderability, or dimensional restoration. Nickel plate is often applied over copper and under chromium for a decorative finish. Nickel is a very hard metal with relatively poor ductility; consequently, parts to be nickel plated should be bent into final shape before plating whenever possible. The chapter entitled Nickel Plating covers both decorative and engineering applications of nickel. Bright nickel plating is a highly reflective finish, which often eliminates the need for subsequent polishing. Bright nickel may be applied as a single or mnltilayer coating, though caution must be taken when attempting multilayer nickel, as it may result in poor adhesion in some instances. Multilayer nickel is favored for its excellent corrosion resistance. For applications requiring bright nickel, there are other considerations. The brighter the nickel plating, the greater the internal stresses and the lower the ductility. The brightest finishes result from first polishing the base metal substrate. It is best to avoid specifying bright nickel if the parts are to be bent or crimped after plating. Semibright nickel has a more satiny finish than bright nickel and may be marginally more ductile. If heat shock or minor bending of the parts is anticipated it would be better to specify semibright nickel in order to reduce the risk of the plating flaking off. Nickel may be specified by QQ-N-290--Nickel Plate, Class 1-"Decorative," Types I to III are for steel parts and Types V to VII for copper-base parts and ASTM B 456, "Copper/Nickel/Chromium" for a variety of service classes (SCs), which vary with alloy and base metal. The thickness of nickel plating for most commercial applications will be 0.0002 to 0.0004 in. (5 to 10/xm), but since nickel plates readily, it may be plated to hundreds of an inch (500 to 600/xm is easily possible) in some engineering applications.
Silver Plating Silver plating, in addition to being decorative, has the highest electrical and thermal conductivity of any metal. Silver may be plated from a noncyanide bath, but more typically is plated from a cyanide bath. See the section entitled Electroplating Solutions, specifically the chapter entitled Silver Plating for details on silver plating baths. Silver plating is highly ductile, malleable, and solderable. Silver tarnishes easily, however, due to sulfides in the air and should be packaged with special tarnish-resistant paper. Silver can be plated to military specification QQ-S-365, "Silver Plate" as Type I (matte finish, the best conductor), Type II 24
(semibright) or Type III (bright, the worst conductor) and may call for Grade A (chromate treated) or Grade B (no chromate). Matte silver plate (QQ-S-365, Type I) is used extensively for finishing electronic components where silver's mechanical, electrical, and thermal properties offer distinct advantages over other metallic, even silver, finishes. Semibright silver plate (QQ-S-365, Type II) is often specified where the electrical and/or mechanical properties of silver plate alone may not be enough and the design engineer feels that appearance may also be an important consideration. Semibright silver does, however, tend to tarnish faster than matte silver. Typically, platers prefer to apply silver plate as thin as practical, because it is a relatively expensive metal. Designers, of course, specify the thinnest plating that will serve their purposes, but anything over that minimum is "free." Precious-metal plating is thus controlled much more closely than are the more common metals. Silver is typically applied in thicknesses as low as 0.00005 to 0.0005 in. (1.0 to 10/xm), although most applications call for 0.0002 to 0.0004 in. (5.0 to 10.0/xm).
Finishes for Stainless Steel Passivation of stainless steel is not electroplating, it is a nonelectrical process whereby the free iron is chemically removed from the surface of stainless steel. This prevents the formation of possible corrosion sites and the development of tightly adhering oxides. The 300 series alloys are generally preferred for passivation, as some of the 400 series alloys will actually be discolored by the passivation process. Passivation imparts a limited neutral salt spray corrosion protection to the stainless steel, usually not much over 2 hours. It is critical when making assemblies to be passivated that all component parts be made of the same alloy; different alloys may be indistinguishable before passivation, but may have a different appearance afterward. Since different solutions are used to passivate different alloys, they must be properly identified. Mixing alloys may not only result in differences in appearance, but may result in some parts being destroyed. Passivation of stainless steel is generally specified by QQ-P-35 or ASTM A 380. Plating on stainless steel may be done, after suitable pretreatment, by any method listed here. Stainless steels and high-nickel alloys form a tightly adhering passive oxide film within minutes of being plated. Stainless parts can be plated with other metals if a fresh active surface is provided for subsequent plating (see the section of the Guidebook entitled Preparation of Basis Metals for Plating. Plating on stainless steel is addressed by ASTM B 254.
Tin Plating Tin plating is normally done to impart solderability to a variety of base metal substrates~ Tin is a silvery blue-white metal that is ductile, solderable, and covers very well. The solderability of tin can be affected by the substrate, since several metals tend to react with and migrate into the tin forming relatively nonsolderabte intermetallic layers. Of particular concern is that if the tin is to be plated over brass, or zinc die casts, the zinc will migrate into the tin and severely limit'the shelf life of the finished parts. The migration can be mitigated by the common practice of applying an undercoating of copper or nickel through which the zinc cannot migrate. Tin is a fairly easy metal to plate and is approved for a variety of industrial applications and is even approved for food-container or food-contact applications. There are no known common tin salts that are extremely toxic or carcinogenic. Tin does not tarnish easily, making it a good choice as a low-cost decorative finish as well. Tin plating is addressed in Mil-T-10727. Although this specification calls for a fairly thin plating for solderability (0.0005 in. or 1 /~m), common practice is to apply much more. This thinner plate was commonly used when "stannate tin" baths were in common use and practice was to reflow the tin periodically. The tin plate from these older-style tin baths tended to 25
change spontaneously from "white" tin to "black or gray" after a few months, hence the need to reflow the tin. This process also caused not a few fires. Today, where long-term shelf life is a consideration, 0.0003 in. (7.5/xm) minimum tin is commonly used over an underplate of 0.0002 in. (5.0/xm) minimum copper or nickel. There are a great many other specifications calling for tin combined with other metals being plated as an alloy. These include Mil-L-46064 and Mil-P-81~728, both of which are for tin-lead, and Mil-P-23408, tin-cadmium. Tin may also be plated as a tin-nickel alloy, which is sometimes used as a substitute from decorative chromium. All of these alloy finishes generally offer a better shelf life than pure tin plate, but are considered environmentally unfriendly, and so their use is much diminished. The tin and tin-lead alloy plating baths are discussed in the chapter entitled Tin-Lead, Lead, and Tin Plating; arid tin-nickel is discussed in the chapter entitled Tin-Nickel Alloy Plating.
Zinc Plating Zinc plating is a soft, ductile, decorative, marginally solderable, corrosion-resistant finish. Unlike most other commonly plated metals, zinc protects the substrate by sacrificing itself and thus corrodes before the base metal. This means that zinc will protect even if the zinc coating sustains minor damage, such as scratches of small punctures, which is often an advantage that other types of plating do not offer. Zinc is the most reactive of all common metals; however, it may be attacked or dissolved by ordinary liquids such as soft drinks and vinegar. The ultimate corrosion resistance of zinc is a function of the plating thickness. To increase the corrosion resistance of zinc, a conversion coating is usually added. In the chapter entitled Zinc Plating, the authors discuss the three most common zinc plating baths in use today. They also examine the relative merits of these popular zinc plating baths. Additionally, the chapter entitled Zinc Alloy Plating looks at zinc-iron, zinc-cobalt, zinc nickel, and tin-zinc alloy baths, which have become increasingly popular both because of their remarkable corrosion resistance and as a possible substitute for cadmium. Chloride zinc plating was introduced about 1980, when environmental pressures began to demand the replacement of cyanide plating baths with noncyanide baths. Presently, over 50% of all zinc deposited in this country is from acid chloride baths. By far, the majority of these are chloride baths with potassium chloride and are ammonia-free with a pH around 5 to 6. These baths have high cathode efficiency and excellent throwing power because of the high cathode efficiency over the entire current density range. The most commonly used specification for zinc plating is ASTM B 633, which replaced QQ-Z-325 in 1982, although a great many drawings still call for QQ-Z-325. One of the main advantages of the ASTM specification over QQ-Z-325 is that ASTM B 633 distinguishes between colored and clear chromate conversion coatings. It calls out four types of post finishes for zinc plating, Type I: as plated; Type II: colored chromate (default yellow, but olive drab or black may be specified); Type III: colorless chromate (these are often designated as clear or bright, blue bright, or blue); and Type IV: phosphate coating. Thicknesses are spelled out by an SC designation: SC1, 0.001 in., SC2, 0.0005 in., SC3, 0.0003 in., SC4, 0.0002 in. and SC5, 0.0001 in.
Chromate Coatings Chromate conversion coatings are chemical conversion coatings. The substrate metal participates in the coating reaction and becomes a component of the coating. The collaboration has a profound effect on the properties of the coating. Among the metals commonly chromated are zinc, zinc die castings, hot-dipped galvanized steel, aluminum, and sometimes copper and silver. Chromate films are typically very thin, on the order of 0.0000001 in. and contribute no measurable thickness to the overall coating. The chemistry involves a reaction between the metal surface and an aqueous solution 26
containing chromates (chromium salts, either hexavalent or trivalent) and certain activators or film formers. Activators may include sulfates, fluorides, phosphates, and sometimes complex cyanides. Normally, a given chromate is designed to work on a particular metal, but in a few cases will work on two or more. The chromate solution is normally acidic. About 0.0005 in. of plating thickness is dissolved during the chromating process. Clear chromate offers from 8 to 12 hours' resistance to white corrosion (zinc oxide or "white rust") and has a clear to slightly iridescent blue appearance. This is one of the earliest types of chromating solutions and is used today. Older style "clear" chromates often deposited as a golden yellow coating and were then bleached by immersion in a dilute alkali solution to obtain the clear appearance. More up-to-date blue bright chromates are single-dip solutions, often using only trivalent chromium salts and are, therefore, more environmentally friendly. Some varieties of this type of baths may be dyed a variety of colors. These colors are often used for identification purposes. Gold, or yellow, chromate coatings are deposited from baths that contain chromate, sulfate, or chloride activators and produce a distinct iridescent golden yellow color. Yellow chromates contain more hexavalent chromium than clear films, which accounts for their color. Yellow chromate offers in excess of 96 hours' neutral salt spray corrosion protection and is an excellent paint base. Olive drab, or forest green, chromate is the ultimate in commonly available conversion coatings, with neutral salt spray resistance in excess of 150 hours. This chromate is most commonly specified for military applications. It is generally not possible to apply to barrel plate work. The color and the corrosion resistance are due to the inclusion of an organic acid modifier to the chromate formula. Many customers find the color is not especially pleasing, but specify it for nonmilitary applications for its functional, rather than its decorative, value. Black chromate is usually achieved by incorporating a soluble silver salt in a golden chromate formula, which produces a deposit of black silver chromate. This coating offers excellent corrosion resistance and a jet black semimatte to matte appearance. Black chromate has even found some applications in the space program, in which it is used on solar collectors. The use of silver salts makes black chromate coating rather more expensive and is also sensitive to chloride contamination. This is especially a problem when black chromate is to be applied over parts plated in a chloride bath. Chromate conversion coatings are included in a great many specifications and are also covered in their own specification Mil-C-5541, although that specification applied exclusively to chromate conversion coatings on aluminum.
Special Processes Special processes include a variety of finishes that do not change the surface of the parts other than through chemical or mechanical means. Pickle and oil uses an acid solution to remove scale and rust from the metal surface, after which a special rust-inhibiting oil preservative is applied to prevent or retard further corrosion. Some steels with very high polish or microfinish may show a reduction in gloss as a result of the pickle and oil process. Caution is recommended. Rust can also significantly reduce the gloss or microfinish on steel parts. Chemical etch is used on iron, copper, and aluminum to remove metal chemically, thus creating a more uniform matte finish than is found on the as-produced material. Chemical etching has the disadvantage of preferentially attacking high-energy areas, such as shalT corners and threads. The result can be the rounding of these corners, etc., which may not always be desirable. Chemical etching is not a "polishing" process and cannot be used to remove deep scratches or tool marks from the metal surface. Typically less than 0.0005 in. of material is removed by the chemical etch process. Since these effects are different for different alloys and even different steel production processes (hot-roll versus cold-roll), it is important for the customer to advise which alloy is to be treated. It is equally important that alloys not be mixed, either within a lot or within a single weldment or assembly. 27'
D E
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Fig. 3. The anodize process: The aluminum base metal, A, reacts with the oxygen at B, forming an aluminum oxide coating. This dense layer of newly formed aluminum, C, is dissolved by the acidic electrolyte, causing the layer to become porous at D. The result is a relatively large pore, E, and a fairly thin cell wall, E When the forward film building process is in equilibrium with the dissolution of the coating, the anodize coating stops getting thicker. Hydrogen embrittlement relief is done after plating to remove hydrogen from the intercrystalline interstices, which could otherwise result in brittleness and premature failure of the part. The process is typically applied to fasteners, springs, and other parts having a Rockwell C (Rc) hardness of 30 or higher. The process must also be done within 1 hour of the plating process. Hydrogen embrittlement relief is generally done at 190 + 5°C and is, therefore, achieved without annealing the parts.
ALUMINUM FINISHING Anodizing is an electrochemical process whereby the naturally occurring oxide coating on the outer surface of an aluminum part is changed to a tightly adhering layer of oxide of aluminum of a specified thickness. The natural coating is approximately 0.0000005-in. thick, but the anodizing process can increase the coating to a film thickness of 0.0005 to 0.003 in. This thicker coating is much more resistant to corrosion and abrasion than the underlying aluminum. Anodizing coats very uniformly and will, therefore, not fill or smooth out a rough or damaged surface. In anodizing, there are two competing processes that occur simultaneously. Aluminum parts are first made the positive pole, or anode, in an electrochemical cell using an acidic electrolyte. As current passes through the solution, oxygen is generated at the anode and immediately reacts with the aluminum parts to form an aluminum oxide coating. Simultaneously, the aluminum oxide coating is dissolving into the electrolyte and is becoming porous and less dense. When these two competing processes are in equilibrium, the anodized coating reaches its maximum and can no longer become thicker (see Fig. 3).
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Fig. 4. Cross section of an anodized bolt: In the illustration, the bolt is made of aluminum, A, with its original surface at B. The anodize nat only builds to a new surface at D, but also penetrates A by a nearly identical amount. This penetration depth, C, plus the build up, equals the total anodize film thickness.
Anodizing does not simply add material to the base metal as does plating. The anodized film grows outward and also penetrates the aluminum base metal, each by approximately one-half of the overall film thickness. Since each side grows by an amount equal to one-half the anodized film thickness, the part grows an amount equal to the total anodized film thickness (see Fig. 4). Anodized aluminum has many useful engineering properties. For example, anodizing offers excellent corrosion resistance, typically over 336 hours' salt spray resistance (tested per ASTM B 117)and a surface that is second only to that of diamond in hardness. Note, however, that the anodized film is rather thin and doesn't have a great deal of puncture resistance. Anodize hardness is normally expressed in terms of its ability to resist abrasion (tested by the Tabor Abrasion Test). Anodize must be sealedin order to obtain optimum corrosion resistance. Frequently, designers of electronic devices are inspired by the thought of using anodized aluminum for heat sinks. The idea is very appealing, since aluminum is a very good heat conductor and anodize is a dielectric. The idea of a dielectric thermal conductor is too good to pass up. This never works, however, because of the nature of anodizing. Anodizing, unlike electroplating, works from the surface inward. The coating grows outward from the interface between the anodic coating and the raw aluminum. The growth is at fight angles to the surface of the base metal, i.e., perpendicular. The coating is forming around a three-dimensional object, but cannot itself grow in three dimensions. The result is cracking at sharp comers. A thin anodic film will not crack to any great extent; however, the thicker the film, the worse the cracking becomes. Hardcoat anodize tends to crack more than sulfuric anodizing, for two reasons: first the hardcoat tends to be denser than clear (sulfuric) anodizing, and hardcoat films are generally thicker than clear anodizing. In order the minimize this comer fault, all edges should be radiused at least 0.030 in. for each 0.001 in. of coating thickness (see Fig. 5). Steel or brass inserts, or any material other than aluminum, that are part of an assembly or weldment submitted to be anodized must be removed or masked before anodizing. Unless removed or masked, the inserts will probably be destroyed by the anodize process. Because most anodized coatings are porous, they are able to absorb dyes before they are sealed. This produces an attractive colored oxide coating integral to the metal itself, and is less likely to chip or peel than plated coatings. With proper care and handling, anodized articles can last a lifetime. On threaded parts, the pitch diameter is measured over four separate surfaces; the apparent growth of the pitch diameter is, therefore, approximately equal to the thickness of the anodized film. 29
A
B
C
O
Fig. 5. Comer faults: A plain piece of aluminum, A, may be formed with sharp corners without any problems. Corners, however, create a special problem when the aluminum is anodized because anodizing builds from the inside out. When a thin anodize film is formed, stresses build at the outside comers until eventually the corners crack, as shown at B. The thicker the anodize, the greater the severity of cracking, as shown at C. These corner faults can be minimized by radiusing the comers as shown in D.
Common Types of Anodizing Chromic anodize is the oldest type of anodizing and applies the thinnest film. Rarely will chromic anodize apply more than 0.0002 in. to wrought alloys, and less to a cast alloy. Chromic anodizing is normally a grayish coating formed from a chromic acid electrolyte. Chromic anodizing is most often used as a base for paint for aircraft and marine applications. It has been phased out of many operations because of environmental considerations. •Sulfuric anodizing, also called clear anodizing, is an excellent corrosion-resistant finish and, under the right circumstances, a good electrical insulator. This is the most common type of anodizing done today. Sulfuric acid anodizing may exhibit a silver, bronze, tan, or gray color, depending on the alloy used. The coating is commonly dyed, or otherwise colored, with a variety of organic dyes, mineral pigments, or precipitated metals. Typically, a sulfuric anodizing coating will be from 0.0005 to 0.0015 in. for wrought alloys and a bit less for cast alloys. Hardcoat anodizing is an highly abrasion-resistant, nonconductive coating of aluminum oxide (A1203.H20) that makes the aluminum surface harder than tool steel. Hardcoating is done in a sulfuric acid electrolyte modified by the addition of one or more organic acids and is run at very low temperatures. Typically, the R c hardness will be in the 65 to 70 range. To obtain maximum wear resistance, the hardcoat should not be sealed. The nominal thickness of hardcoat runs from 0.002 to 0.003 in. on wrought materials and less on most cast alloys.
Weldments and Welded Parts Welds on aluminum are often porous; acid is trapped in the welds and the narrow spaces between the parts. It may take upward of 24 hours for the acid to begin to leach out and attack the anodized coating or bleach the dye. This will continue until all of the entrapped acid has seeped out. Hollow parts (pipes, square tubing, etc.) can hold a lot of acid and act as a reservoir. The stored acid poses a potential danger to end users or others who' may handle the parts at a later time. It is, therefore, critical that 1/4-in. minimum drain holes be drilled into each chamber that might potentially store acid. Drains will allow the acid to bleed out and be replaced by clear water during the anodizing process. The high temperatures required by the welding process will affect an area around each weld, causing the color to be slightly lighter and causing the welded area to appear larger than it actually is. 30
Choice of Alloys for Anodizing Anodizing is recommended for virtually all aluminum alloys. It is critical, however, that alloys not be mixed in a weldment, or within a single order. Alloys such as 2024 if mixed or welded to 6061, for example, may well be totally destroyed during the anodizing process.
Wrought Alloys 1100 series alloys yield a bronze to gray coating at 0.002 in. The alloys are particularly soft and do not machine well. The maximum coating that can be expected is 0.0025 in., but 0.003 in. is possible. 2000 series, principally 2011, 2017, 2024, and 2618 (forgings), develop a gray-black to blue-gray color at 0.0025 to 0.003 in. 3000 series, most commonly 3003, develops a gray-black color at 0.002 in. They machine well and readily take dye. 4000 series is not commonly used for anodizing. 5000 series, primarily 5005 and 5052, machine well and are good for dyed work. They develop a black color at 0.002 in. This material when hardcoated has excellent dielectric properties. 6000 series, most commonly 6061 and 6063, form excellent anodized coatings. Hardcoat on these alloys may be ground, lapped, or honed. The alloys have excellent dimensional stability and machine well, though they can be a bit stringy under some circumstances. The 6063 is commonly used for extrusions and as rod for welding other alloys. The maximum practical coating thickness is 0.0025 in. 7000 series (usually only 7075 is commonly used) develop a blue-gray color at 0.002 in. It is not a good choice for grinding or lapping, since it has a tendency to be checky. 8000 series is not commonly used for anodizing.
Cast Alloys Alloys for sandcasting commonly include 319, 355, 356, and sometimes 40E (a ternalloy). The 356-T6 is the most commonly used sandcast alloy because it grinds and polishes well. Porosity is always a problem with sandcastings. Pores will cause the coating to appear to have pits. Anodizing will not fill the pores. Sandcast parts are commonly coated to 0.001 to 0.002 in., but some will develop thicker coatings. Alloys for diecasting include 218,360, and 380. Of these, only 218 produces an anodized coating comparable to wrought or sandcast alloys. The 218 is hard to diecast, however, so is rarely used.
Reanodizing Parts Anodized coatings of any type--chromic, sulfuric, or hardcoat--cannot be built up over existing coatings. Once electrical contact with the anodized part has been broken it is very difficult to reestablish it. Stripping and reanodizing works but removes metal and, therefore, affects critical dimensions. Stripping and reanodizing may produce parts that are dimensionally unusable as a result. As a matter of policy, stripping and reanodizing are done only with specific customer approval.
Coating Thickness The coating thickness for any type of anodizing is roughly 50% buildup and 50% penetration. In other words, a 0.002-in. coating thickness will build up only 0.001 in. per side. This means that the overall dimension of the part increases by an amount equal to the total film thickness. Holes are closed by a like amount, unless plugged or masked. Hardcoat may be provided in a thickness ranging from a few tenths of a thousandth of an inch up to about three thousandths of an inch. As stated above, hardcoating (and indeed all anodizing) changes the dimensions of a part by an amount equal to one-half of the combined 31
total buildup on opposite sides of a part. When machining parts to be hardcoated, or anodized, it is essential to allow for this change and to specify a coating thickness on blueprints and purchase orders.
Masking When it is necessary to exclude the anodize from specific areas, the parts can be masked. Areas to be masked, such as threaded holes, ground points, etc., must be clearly identified by the customer. Masking is a hand operation, which may add to the total cost of anodizing a part. Even if anodizing is required on only one area of a part, it may be cheaper to anodize the entire part rather than pay for the masking. On the other hand, it may be cheaper to tap holes and mask them, rather than purchase special drills and taps and try to do the job after anodizing.
Racking The anodized coating that builds up on the part is an excellent insulator. Firm electrical contact must be made with the parts to be anodized and these "rack marks" will not have any anodize on them. Proper racking is the key to economical and effective anodizing. It is essential that the rack marks be located in a noncritical area. Any guidance as to where to locate these small bare spots will aid in the proper processing of the parts and, in the absence of such guidance, good practice should be followed. Occasionally, good practice may place the rack marks in an unidentified critical area in spite of best intentions. Parts should never, however, be racked in threaded holes without specific instruction from the customer.
Polishing and Lapping Norbide (boron carbide) or equivalent abrasive grain using a heavy oil or petroleum jelly carrier cuts very fast with excellent results. The use of polishing sticks or brushes is recommended. A grit size of 400 to 1,200 works well depending on the finish required.
Grinding Norton Crystolon (silicon carbide) abrasive or its equivalent is most satisfactory for surface grinding hardcoat. Eighty to 100 grit will yield a microfinish of 2 to 8 microinches. Soft wheels in H, I, or J grades are preferable for fast stock removal and their use minimizes the danger of burning or cracking. Typical wheel numbers are 39C60-J8VK for finishes of 6 to 10 microinches and 39C 100-HSVK for finishes of 2 to 3 microinches. Cylindrical grinding is best done with a 39C 120-JSVK. This finer grit wheel will be free cutting and still produce a very fine finish. For internal grinding, the 39C100-J8VK gives the best results. In general, grinding hardcoat should be done wet, using water as a coolant and a good quality medium-priced soluble oil mixed approximately 100 to 1. Cincinnati Cimplus has been used at 200 to 1 with good results.
Hardcoat and Teflon Teflon may be added to hardcoat as a posttreatment to provide a hard, corrosion-resistant, dry-lubricated, low-friction surface.
Chromate Conversion Coatings Chromate conversion coatings may be applied to aluminum from a chemical bath. All chromates on the QPL list should be considered interchangeable, and the same materials are generally approved for both clear (used for low contact resistance) or golden yellow per Mi1-C-5541. 32
The clear chromate is a nearly colorless coating that has excellent salt spray corrosion resistance (168 hours tested per ASTM B 117), with a typical contact resistance of less than 500 micro-ohms. Yellow, or golden, chromate is used most often for corrosion resistance and to increase subsequent paint adhesion. Parts with yellow chromate are normally (more frequently when required) tested monthly by an independent laboratory by Federal Test Standard 141 A. By specification, the color of a nominally yellow chromate conversion coating can vary from very light yellow to brown. Some variation of color from part to part and even within a single part is normal and acceptable per the above specification.
Plating on Aluminum Plating on aluminum is different from conventional electroplating because of the unique nature and reactivity of aluminum. The principal pretreatment involves the removal and subsequent prevention of the natural surface oxidation. Since an oxide film forms in seconds on a freshly cleaned aluminum surface, it is necessary to apply an immersion coating of zinc, temporarily, to prevent this oxidation. This coating, called "zincate," is removed in the next plating step (usually copper plating) after which the aluminum can be plated the same as any other metal.
Electroplating and Related Processes byJ.B. Mohler 311 pages $65.00 T h i s is a g o o d i n t r o d u c t o r y v o l u m e t o t h e i n d u s t r y . T h e e x p l a n a t i o n s are clear a n d t h e m a t e r i a l is u n c l u t t e r e d w i t h v e r b i a g e o r m e a n i n g l e s s illustrations. The reader wastes no time while preparing the groundwork for further study. S e n d O r d e r s to:
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