Many new materials have been developed, but steel remains the principal construction material for automobiles, appliances, and industrial machinery. Because of steel's vulnerability to attack by aggressive chemical environments or even from simple atmospheric oxidation, metal coating is necessary to provide various degrees of protection, ranging from hot-dipped and electroplated process to tough polymers and flame-sprayed ceramics. In general, corrosive environments contain more than one active material, and the coating must resist penetration by a combination of oxidizers, solvents, or both. Thus, the best barrier is one that resists "broadband" corrosion.
Physical integrity of the metal coating is as important as its chemical barrier properties in many applications. For instance, metal coating on impellers that mix abrasive slurries can be abraded quickly; coatings on pipe joints will cold-flow away from a loaded area if the creep rate is not low; and metal coating on flanges and support brackets can be chipped or penetrated during assembly if impact strength is inadequate. Selecting the best metal coating for an application requires evaluating all effects of the specific environment, including thermal and mechanical conditions.
One of the most common and inexpensive protection methods for steel is provided by metal coating with zinc. Zinc-coated, or galvanized, steel is produced by various hot-dipping techniques, but more steel companies today are moving into electrogalvanizing so they can provide both. Zinc works as a barrier metal coating and as a sacrificial metal coating to prevent oxidation. If the zinc coating is scratched or penetrated, it continues to provide protection by galvanic action until the zinc layer is depleted. This sacrificial action also prevents corrosion around punched holes and at cut edges.
The grades of zinc-coated steel commercialized in recent years have been designed to overcome the drawbacks of traditional galvanized steel, which has been difficult to weld and to paint to a smooth finish. The newer metal coatings are intended specifically for stamped automotive components, which are usually joined by spot welding and which require a smooth, Class A painted finish.
Two types of aluminum-coated steel are produced, each for a different kind of corrosion protection. Type 1 has a hot-dipped aluminum-silicon metal coating to provide resistance to both heat and corrosion. Type 2 has a hot-dipped metal coating of commercially pure aluminum, which provides excellent durability and protection from atmospheric corrosion.
Type 1 aluminum-coated steel resists heat scaling to 1,250°F and has excellent heat reflectivity to 900°F. Nominal aluminum-alloy coating is about 1 mil on each side. The sheet is supplied with a soft, satiny finish. Typical applications include reflectors and housings for industrial heater panels, interior panels and heat exchangers for residential furnaces, microwave ovens, automobile and truck muffler systems, heat shields for catalytic converters, and pollution-control equipment.
Type 2 aluminized steel, with an aluminum coating of about 1.5 mil on each side, resists atmospheric corrosion and is claimed to outlast zinc-coated sheet in industrial environments by as much as five to one. Typical applications are industrial and commercial roofing and siding, drying ovens, silo roofs, and housings for outdoor lighting fixtures and air conditioners.
Use of protective electroplated metal coating has changed in recent years, mainly because of rulings by the Environmental Protection Agency. Cyanide plating solutions and cadmium and lead-bearing finishes are severely restricted or banned entirely. Chromium and nickel plating are much in use, however, applied both by conventional electroplating techniques and by new, more efficient methods such as Fast Rate Electrodeposition (FRED). This latter method has also been used successfully by Battelle Columbus Labs to deposit stainless steel on ferrous substrates.
Functional chromium, or "hard chrome," metal coating is used for anti-galling and low friction characteristics as well as for corrosion protection. These coatings are usually applied without copper or nickel underlayers in thicknesses from about 0.3 to 2 mil. Hard-chrome metal plating is recommended for use in saline environments to protect ferrous components.
Nickel coating in thicknesses from 0.12 to 3 mil are used in food-handling equipment, on wear surfaces in packaging machinery, and for cladding in reaction vessels.
Electroless nickel metal coating, in contrast to conventional electroplating, operates chemically instead of using an electric current to deposit metal. The electroless metal coating process deposits a uniform coating regardless of substrate shape, overcoming a major drawback of electroplating -- the difficulty of uniformly plating irregularly shaped components. Conforming anodes and complex fixturing are unnecessary in the electroless process. Deposition thickness is controlled simply by controlling immersion time. The deposition process is autocatalytic, producing thicknesses from 0.1 to 5 mil. In general, nickel coatings are nodular. As coating thickness increases, nodule size also increases. Because the columnar structure of the coating flexes as the substrate moves, nickel-boron resists chipping and wear.
Another trend in composite electroless metal coating appears to be toward co-deposition of particulate matter within a metal matrix. These coatings are commercially available with just a few types of particulates -- diamond, silicon carbide, aluminum oxide, and PTFE -- with diamond heading the list in popularity. The coating can be applied to most metals, including iron, carbon steel, cast iron, aluminum alloys, copper, brass, bronze, stainless steel, and high alloy steels.
Electroless metal coatings are more accurately described as conversion coatings, because they produce a protective layer or film on the metal surface by means of a chemical reaction. Another conversion process, the black oxide finish, has been making progress in applications ranging from fasteners to aerospace. Black oxide is gaining in popularity because it provides corrosion resistance and aesthetic appeal without changing part dimensions.
On a chemical level, black oxiding occurs when the iron within the steel's surface reacts to form magnetite (Fe3O4). Processors use inorganic blackening solutions to produce the reaction. Oxidizing salts are first dissolved in water, then boiled and held at 280 to 285°F. The product surface is cleaned in an alkaline soak and then rinsed before immersion in the blackening solution. After a second rinse, the finish is sealed with rust preventatives, which can produce finishes that vary from slightly oily to hard and dry.
Black oxiding produces a microporous surface that readily bonds with a topcoat. For example, a supplemental oil topcoat can be added to boost salt-spray resistance to the same level as that of zinc plate with a clear chrome coating (100 to 200 hr).
Black oxide can be used with mild steel, stainless steel, brass, bronze, and copper. As long as parts are scale free and do not require pickling, the finish will not produce hydrogen embrittlement or change part dimensions. Operating temperatures range from cryogenic to 1,000°F.
Formerly used primarily to produce integrated-circuit components, metal coating by sputtering has moved on to large, production-line jobs such as coating of automotive trim parts. The metal coating sputtering process deposits thin, adherent films in a plasma environment on virtually any substrate.
Metal coating by sputtering offers several advantages to automotive manufacturers for an economical replacement for conventional chrome plating. Sputtering lines are less expensive to set up and operate than plating systems. And because sputtered metal coatings are uniform as well as thin, less coating material is required to produce an acceptable finish. Also, pollution controls are unnecessary because the process does not produce any effluents. Metal coating by sputtering requires less energy than conventional plating systems. Metal coating by sputtering is the only deposition method that does not depend on melting points and vapor pressures of refractory compounds such as carbides, nitrides, silicides, and borides. As a result, films of these materials can be sputtered directly onto surfaces without altering substrate properties.
Much of the research in metal coating by sputtering is aimed at producing solid-film lubricants and hard, wear-resistant refractory compounds. There is interest in these tribological applications because metal coating can be sputter-deposited without a binder, with strong adherence, and with controlled thickness on curved and complex-shaped surfaces such as gears and bearing retainers, races, and balls. Also, because metal coating by sputtering is not limited by thermodynamic criteria, unlike most conventional processes that involve heat input, film properties can be tailored in ways not available with other deposition methods.
Research on sputtered solid-lubricant metal coating has been done mainly with MoS2. Other metal coating that has been done with sputtering includes tungsten carbide, titanium nitride, lead oxide, gold, silver, tin, lead, indium, cadmium, PTFE, and polyimide. Of these coatings, gold-colored titanium nitride coatings are most prominent. TiN coatings are changing both the appearance and performance of high-speed-steel metal cutting tools. Life of TiN-coated tools, according to producers' claims, increases by as much as tenfold, metal-removal rates can be doubled, and more regrinds are possible before a tool is discarded or rebuilt.
The basic difference between metal coating by sputtering and by ion plating is that sputtered material is generated by impact evaporation and transferred by a momentum transfer process. In ion plating, the evaporant is generated by thermal evaporation. Ion plating combines the high throwing power of electroplating, the high deposition rates of thermal evaporation, and the high energy impingement of ions and energetic atoms of sputtering and ion-implantation processes.
The excellent adherence of ion-plated metal coating is attributed to the formation of a graded interface between the film and substrate, even where the two materials are incompatible. The graded interface also strengthens the surface and subsurface zones and increases fatigue life. High throwing power and excellent adherence makes possible metal coating of complex three-dimensional configurations such as internal and external tubing, gear teeth, ball bearings, and fasteners. Gears for space applications, for example, have been ion plated with 0.12 to 0.2 µm of gold for lubrication and to prevent cold welding of the gear pitch line. Ion plating has also been used, on a production basis, to plate aluminum on aircraft landing-gear components for corrosion protection.
Metal coating by arc spraying, a thermal form of spraying of metals, is done to metal surface using a wire arc gun. The coating metal is in the form of two wires that are fed at rates that maintain a constant distance between their tips. An electric arc liquefies the metal, and an air spray propels it onto the substrate. Because particle velocity can be varied considerably, the process can produce a range of metal coating finishes from a fine to a coarse texture.
Arc sprayed metal coating is somewhat porous, being composed of many overlapping platelets. Used in applications where appearance is important, thermally sprayed metal coating can be sealed with pigmented vinyl copolymers or paints, which usually increase the life of the metal coating. Arc sprayed metal coating is thicker than those applied by hot dipping, ranging from 3 to 5 mil for light-duty, low-temperature applications to 7 to 12 mil for severe service.
Because zinc and aluminum are, under most conditions, more corrosion resistant than steel, they are the most widely used metals used in arc sprayed metal coating. In addition, since both metals are anodic to steel, they act galvanically to protect ferrous substrates. In general, aluminum is more durable in acidic environments, and zinc performs better in alkaline conditions. For protecting steel in gas or chemical plants, where temperatures might reach 400°F, an aluminum metal coating is recommended. A zinc metal coating is preferred for protecting steel in fresh, cold water. In aqueous solutions above 150°F, aluminum metal coating is the usual choice.
For service to 1,000°F, a thermally sprayed aluminum coating should be sealed with a silicone-aluminum paint. Between 1,000 and 1,650°F, the aluminum coating fuses and reacts with the steel base metal, forming a coating that, without being sealed, protects the structure from an oxidizing environment. And, for continuous service to 1,800°F, a nickel-chrome alloy is used, sometimes followed by aluminum.
In Europe, where thermally sprayed metal coatings for corrosion protection have been far more widely used than in the U.S., many structures such as bridges are still in good condition after as long as 40 years, with minimum maintenance. Other applications include exhaust gas stacks, boat hulls, masts, and many outdoor structures.
Metal coating by thermal spraying has become much more than a process for rebuilding worn metal surfaces. Thanks to sophisticated equipment and precision control, it is now factored into the design process, producing uniform metal coating onto metal and ceramic. With some processes even gradient coatings can be applied. This is done by coating the substrate with a material that provides a good bond and that has compatible expansion characteristics, then switching gradually to a second material to produce the required surface quality such as wear resistance, solderability, or thermal-barrier characteristics.
Plasma spray metal coating relies on a hot, high-speed plasma flame (nitrogen, hydrogen, or argon) to melt a powdered material and spray it onto the substrate. A direct-current arc is maintained to excite gases into the plasma state. The high-heat plasma (in excess of 15,000°F) enables this process to handle a variety of metal coating materials as well as ceramics, carbides and plastics. Although most coating materials are heated to well beyond their melting points, substrate temperatures commonly remain below 250°F.
The plasma spray metal coating process has found wide acceptance in the aircraft industry. Plasma-sprayed metal coatings protect turbine blades from corrosion, and sprayed ceramics provide thermal-barrier protection for other engine parts.
Refinements in plasma spray metal coating technology include a wear resistance coating material that lends itself to forming amorphous/microcrystalline phases when plasma sprayed. The resultant coating provides excellent corrosion resistance with minimal oxidation at higher temperatures. This promises to eliminate problems of work-hardened crystalline coatings that chip or delaminate in response to stress, which have previously been taken care of by expensive alloying elements.