AWS WHC-4.03:2010

Coated Steels
Scope : One of the most active segments of the steel industry is the fabrication of metal-coated and prepainted steel products made of sheet steel. Coated sheet steel is supplied by manufacturers in coils produced on continuous coating lines. Products made of coated steels provide the following advantages over sheet steels that are metallic-coated or painted after fabrication: lower production costs; more consistent product quality; less energy usage; easier compliance with environmental, health, and safety concerns; and quick response to market changes. Metallic or paint coatings usually are applied to provide a protective surface to improve resistance to oxidation and corrosion or to provide a decorative finish to a product. Welding coated steels is the major topic of this chapter. Metallic coatings are applied by hot dipping, thermal spraying, or electro-deposition. The five common types of metal-coated steels discussed in this chapter are the following: 1. Terneplate, coated with lead alloyed with a small amount of tin; 2. Tinplate, coated with tin; 3. Galvanized steels, coated with zinc or zinc alloys; 4. Aluminized steels, coated with aluminum or aluminum alloys; and 5. Chromized steels, coated with chromium or chromium alloys. Coating designations for continuous hot-dipped sheet metals generally are based on the total mass (weight) of the coating on both sides per unit area of sheet; however, electrogalvanized sheet is designated by mass per unit area of each surface. Paints used for coatings on steel may be either organic or inorganic and may have a zinc fill (zinc powder added to the paint). Organic paints include epoxies, lacquers, acrylics, polyurethanes, polyesters, and polyvinyl chloride. Inorganic paints may be sodium silicate, ethyl silicate, or another inorganic material, and usually have a zinc fill. Products or components made from coated steel are joined by fusion welding processes (e.g., resistance, arc, arc stud, friction, and laser beam), brazing, braze welding, soldering, adhesive bonding, and mechanical joining or interlocking. These processes can be used alone, or in combination with another process, such as weld bonding. The welding processes are discussed individually under the heading of each the five types of coated steel described in this chapter. (Mechanical joining and adhesive bonding processes are not included.) When coated steels are fusion welded, the effectiveness of the coating adjacent to the weld often is significantly degraded by the heat of welding. The metallic coating melts and then may alloy with the steel, or it may oxidize or volatilize. An organic (resinous) coating usually is scorched, burned or charred. The result is a coating of reduced usefulness adjacent to the weld. A reconditioning operation often is required to restore corrosion resistance or the desired visual appearance to the affected areas. These topics and others are discussed in this chapter