Metal Coatings

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2. METAL COATINGS • 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.

3. • 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.

4. • 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.

5. • 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.

6. • 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.

7. • 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.

8. • 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.

9. • 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.

10. • 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.

11. • 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.

12. • 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