Metal Coatings

Nov. 15, 2002
Many new materials have been developed, but steel remains the principal construction material for automobiles, appliances, and industrial machinery.

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, coatings are necessary to provide various degrees of protection. They range from hot-dipped and electroplated metals 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 coating is as important as its chemical barrier properties in many applications. For instance, coatings 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 coatings on flanges and support brackets can be chipped or penetrated during assembly if impact strength is inadequate. Selecting the best coating for an application requires evaluating all effects of the specific environment, including thermal and mechanical conditions.

Zinc: One of the most common and inexpensive protection methods for steel is provided by 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.

Oxidation protection of steel by zinc operates in two ways -- first as a barrier coating, then as a sacrificial coating. 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 materials are intended specifically for stamped automotive components, which are usually joined by spot welding and which require a smooth, Class A painted finish.

Among today's improved galvanized steels is Inland Steel's Paint-Tite B family of three products. These galvannealed (heat-treated) steels have a hot-dipped zinc coating on one side and a light, gas-jet-wiped coating on the smooth side. They are thermally treated to provide a uniform, spangle-free, paintable, weldable, zinc-iron alloy coating. Because the zinc-iron crystals on the coated side are very fine, the sheet can be formed or rolled with no "print-through" in the top surface.

Another improved zinc-coated steel is Armco's Ultrasmooth, which has a hot-dipped zinc coating on both sides and surfaces comparable to a Class 1 cold-rolled steel finish. Jets of nitrogen, which surround the steel as it emerges from the molten zinc, control the coating thickness. The resulting surfaces are free from ripples and oxide patterns commonly found on conventional hot-dipped galvanized steels.

Armco's newest zinc-coated product, Electrasmooth, has an electrogalvanized coating on one or both sides. Made by the vertical-cell process, it has a smooth, uniform surface suitable for painted automotive panels, appliance wrappers, and architectural products. Electrasmooth coatings are available on the full range of commercial and drawing-quality grades.

Another new process for zinc coating of steels is Galfan, developed by the International Lead Zinc Research Organization (ILZRO). In addition to zinc, the coating contains mischmetal (a mixture of cerium, lanthium, and other rare-earth metals) and 5% aluminum. Galfan is claimed to provide improved formability and paintability, and to have two to three times the corrosion resistance of conventional, hot-dipped galvanized steel in salt-spray, sulfur-dioxide, and humidity tests.

Licenses to produce the material in the U.S. have been granted to Gregory Galvanizing & Metal Processing Inc., Canton, Ohio, and Weirton Steel Corp., Weirton, W.Va. Galfan has been used in Europe and Japan in applications such as building panels, automobile and appliance parts, and marine wire rope. U.S. applications include panels for washing machines, fence posts and related components, and garage-door hardware. Automotive applications include tubing for transmission cooling lines, various under-hood brackets and housings and, for the 1988 Ford T-Bird, a fuel-tank shield.

Another corrosion-resistant coated-steel product involving zinc is Zincrometal (Metal Coatings International), which is processed with a proprietary, two-coat system. The base coat is a chromium-base inorganic material called Dacromet. After the coating is baked, the coil is coated with a second material -- Zincromet, an epoxy-based, zinc-rich topcoat -- followed by another baking cycle.

Like the other zinc-coated materials, Zincrometal is used principally for truck and automobile exterior panels. Minimum dry-film thickness for such applications is 0.5 mil. With proper lubricants and drawing compounds, the formability of the material is comparable to that of cold-rolled steel. Welding characteristics are claimed to be similar to those of the uncoated steel.

Unlike hot-dip galvanized or electrogalvanized materials, Zincrometal provides barrier, rather than sacrificial, protection. Thus, it is recommended for use where corrosion protection is the primary need, where components are not subjected to abrasion or mechanical abuse. Zincrometal, which is manufactured on a toll basis by coil-coating companies, is essentially a one-sided product, however, and is being replaced in some automotive applications by two-side coated steels.

Aluminum: Two types of aluminum-coated steel are produced, each for a different kind of corrosion protection. Type 1 has a hot-dipped aluminum-silicon coating to provide resistance to both heat and corrosion. Type 2 has a hot-dipped coating of commercially pure aluminum, which provides excellent durability and protection from atmospheric corrosion. Both grades, developed by Armco, the producer of these aluminized steels, are usually used unpainted.

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.

For higher temperature applications, Type 1 coatings are supplied on two new products. Aluma-Ti is a vacuum-degassed, interstitial-free steel containing columbium and titanium that can be used in cyclic service to 1,400°F. The second product, Aluma-Fuse, operates to a maximum metal temperature of 1,600°F. Its high-temperature properties come from a combination of the diffused coating and a low-alloy steel base metal containing chromium, aluminum, silicon, columbium, and titanium, plus a heat treatment.

Armco has also developed a process for aluminum coating of Type 409 stainless steel. This product, first tested in some 1987 automobiles, is expected to extend exhaust-system life to five years or more because of its superior resistance to corrosion by hot exhaust condensates and road salts. Another benefit of the aluminum coating is cosmetic. It prevents red rust staining indefinitely.

Electroplating: Use of protective electroplated metals 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 platings 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," plating is used for antigalling and low-friction characteristics as well as for corrosion protection. These platings are usually applied without copper or nickel underplates in thicknesses from about 0.3 to 2 mil. Hard-chrome plating is recommended for use in saline environments to protect ferrous components.

Nickel platings, in thickness 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 plating, in contrast to conventional electroplating, operates chemically instead of using an electric current to deposit metal. The electroless 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. Deposit thickness is controlled simply by controlling immersion time. The deposition process is autocatalytic, producing thicknesses from 0.1 to 5 mil.

Proprietary electroless-plating systems contain, in addition to nickel, elements such as phosphorus, boron and/or thallium. A relatively new composition, called the polyalloy, features three or four elements in the bath. These products, such as Nibron by Pure Coatings Div. of Pure Industries, and Niklad by Allied Kelite Div. of Witco Chemical Corp., are claimed to provide superior wear resistance, hardness, and other properties, compared to those of generic electroless-plating methods.

The Nibron polyalloy contains nickel, thallium, and boron. Originally developed for aircraft gas turbine engines, Nibron offers excellent wear resistance. Comparative tests show that relative wear for a Nibron-coated part, measured by the Dow Corning LFW-1 Alpha test, is significantly less than that for hard chromium and nickel-phosphorus coatings.

In general, nickel-boron 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.

Adhesion quality for Nibron depends on factors such as substrate material, part preparation, and contamination. Although it is excellent for tool steels, stainless steel, high-performance nickel and cobalt-based alloys, and titanium, a few metal substrates are not compatible. These include metals with high zinc or molybdenum content, aluminum, magnesium, and tungsten carbide. Modifications can, however, eliminate this incompatibility. For example, Nibron can be applied over chrome-plated aluminum.

Another trend in composite electroless plating appears to be toward codeposition 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.

One example of a proprietary composite coating incorporating PTFE is Nimet Industries' NiCoTef, which disperses submicron particles of PTFE evenly throughout a nickel/phosphorus matrix. PTFE is incorporated at 23 to 25% in the plated deposits, achieving an extremely low coefficient of friction. Because the PTFE is codeposited with the nickel/phosphorus matrix rather than applied to the surface in a subsequent operation, a continuous supply is present throughout the life of the coating.

NiCoTef wears well in sliding-wear applications where the load is mild and evenly distributed. It effectively extends part life in many applications by reducing friction. In addition, the high phosphorus content of the matrix yields a finish that is nonporous and highly resistant to corrosion in both alkaline and process acid media.

NiCoTef's combination of lubricity, uniform coating thickness, and corrosion resistance makes it especially suitable for valves; fluid-power systems; fasteners; precision electronic applications; carburetor, brake, engine, and fuel-injection components; pumps; bearing and material-handling surfaces; small hears; cylinders, molds and dies; mixing blades; and computer components.

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.

Conversion coatings: Electroless platings 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.

Sputtering: Formerly used primarily to produce integrated-circuit components, sputtering has moved on to large, production-line jobs such as "plating" of automotive trim parts. The process deposits thin, adherent films, usually of metal, in a plasma environment on virtually any substrate.

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 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. Finally, sputtering requires less energy than conventional plating systems.

Chrome coating of plastics and metals is only one application for sputtering. The technique is not limited to depositing metal films. NASA's Lewis Research Center has successfully sputtered PTFE on metal, glass, paper, and wood surfaces. In another application, cattle bone was sputtered on metallic prosthetic devices for use as hipbone replacements. The sputtered bone film promotes bone growth and attachment to living bone.

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 sputtering research at Lewis Research Center is aimed at producing solid-film lubricants and hard, wear-resistant refractory compounds. NASA is interested in these tribological applications because coatings 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 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.

Most research on sputtered solid-lubricant films has been done with MoS2. Other films that have been sputtered are tungsten carbide, titanium nitride, lead oxide, gold, silver, tin, lead, indium, cadmium, PTFE, and polyimide. Of these coatings, the gold-colored titanium nitride (TiN) coatings are most prominent.

TiN coatings are changing both the appearance and performance of high-speed-steel metalcutting 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.

Ion plating: The basic difference between sputtering and 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 film adherence of ion-plated films 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.

The high throwing power and excellent adherence makes possible the plating 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 ∝m 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.

Ion plating is also one of the two methods used to deposit diamondlike coatings (DLCs). A relative newcomer to the coatings field, DLCs are commonly made from hydrocarbon (often methane) and hydrogen gases heated to 2,000°C. The carbon coatings are prized for their wear resistance, as well as electrical and optical properties. Although they represent a huge potential, present DLCs are at the earliest stages of commercialization. However, their wide range of properties, along with their relatively low cost, leads many to predict huge growth in DLCs.

At Battelle Laboratories in Columbus, Ohio; Geneva, Switzerland; and Frankfurt, W. Germany, development work is continuing on DLCs. Battelle researchers propose the coatings be used to improve wear resistance in tool bits, as electronic heat sinks, and to boost wear and corrosion resistance in optical materials.

Chemical vapor deposition (CVD) is the method most often used to deposit DLCs. Adjusting deposition conditions allows the processor to change the coating from graphite to diamondlike. One process used at Battelle deposits the DLC in a gas atmosphere at reduced pressure without a fixed target. This plasma-assisted CVD allows large workpieces to be coated on all sides without turning. However, substrates must be heated to roughly 800°C when using CVD.

Reduced substrate temperatures are offered by dual ion-beam enhanced deposition, a process developed by the BeamAlloy Corp. Substrate temperature reaches only 150°F, and the dual ion-beam process does not rely on epitaxial growth for its formation as CVD does. Epitaxial growth requires a crystalline substrate; because dual ion-beam processing is free of this need, it enables amorphous materials to be coated as well.

Materials that are compatible with BeamAlloy's Diond process include ferrous and nonferrous metals, glasses, ceramics, plastics, and composites. In addition to the Diond coating, dual ion-beam enhanced deposition can apply metallic coatings to fiber-reinforced carbon/carbon materials.

The basic ion-implantation process sends beams of elemental atoms (produced in a particle accelerator) into the surface of the target component. With dual ion-beam enhanced deposition, two simultaneous beams are used. One beam continuously sputters carbon onto the surface, providing the carbon material necessary to grow a diamond film. A second beam, consisting of inert gas at higher energy, drives some of the diamond layer into the interface zone. Then, the energy of the second beam is reduced to allow diamond growth. Implanting diamond material within the interface zone optimizes adhesion.

Thermal spraying: Arc spraying, a form of thermal spraying of metals, is done on a prepared (usually grit-blasted) 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 coating finishes from a fine to a coarse texture.

Arc-sprayed coatings are somewhat porous, being composed of many overlapping platelets. Used in applications where appearance is important, thermally sprayed coatings can be sealed with pigmented vinyl copolymers or paints, which usually increase the life of the metal coating. Arc-sprayed coatings are 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 spray-coating metals. 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, aluminum is recommended. Zinc is preferred for protecting steel in fresh, cold waters; in aqueous solutions above 150°F, aluminum 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.

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 coatings of metals and ceramics. With some of the processes, even gradated 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 spraying: Plasma-spray 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 coating materials -- most metals, ceramics, carbides, and plastics. Although most coating materials are heated to well beyond their melting points, substrate temperatures commonly remain below 250°F.

This process has found wide acceptance in the aircraft industry. Plasma-sprayed metallic coatings protect turbine blades from corrosion, and sprayed ceramics provide thermal-barrier protection for other engine parts.

Proprietary refinements in plasma-spray 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.

Another amorphous alloy development involves a crystalline material that, upon abrasive wear, transforms to an amorphous hard-phase alloy. The top layer, three to five microns thick, results in hardness levels over 1,300 Vickers. Wear tests have indicated this material is superior to more expensive tungsten carbide coatings.

Detonation gun coatings, developed by Union Carbide and considered by many to be an industry standard, use a detonation wave to heat and accelerate powdered material to 2,400 fps. In the line-of-sight process, each individual detonation deposits a circle of coating with a 1 in. diameter and 2 ∝m thickness. Coatings, thus, consist of multiple layers of densely packed lenticular particles tightly bonded to the surface.

The Super D-Gun, Union Carbide's next generation, has been developed to increase particle velocities. New coatings (the UCAR 2000 Series) applied with the gun offer improved wear resistance without affecting fatigue performance. The system has been targeted for fatigue-sensitive aircraft components.

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