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Zinc Coatings - Zinc Coatings Microstructures of Various...

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Zinc Coatings Microstructures of Various Zinc Coatings H OT -D IP G ALVANIZED M ETALLIZED Z INC P AINT G ALVANIZED S HEET E LECTROPLATED
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Contents Introduction ....................................................................................................... 1 Production Processes for Zinc Coatings .............................................................. 2 Batch Hot-Dip Galvanizing ............................................................................ 2 Continuous Sheet Galvanizing ........................................................................ 3 Electrogalvanizing .......................................................................................... 3 Zinc Plating ................................................................................................... 3 Mechanical Plating ........................................................................................ 3 Zinc Spraying (Metallizing) ............................................................................. 4 Zinc Painting ................................................................................................. 4 Characteristics of Zinc Coatings .......................................................................... 5 Batch Hot-Dip Galvanizing ............................................................................ 5 Continuous Sheet Galvanizing ........................................................................ 5 Electrogalvanizing .......................................................................................... 5 Zinc Plating ................................................................................................... 6 Mechanical Plating ........................................................................................ 6 Zinc Spraying (Metallizing) ............................................................................. 6 Zinc Painting ................................................................................................. 6 Selection of Zinc Coatings .................................................................................. 7 Coating Thickness vs. Coating Weight ............................................................ 7 Economic Considerations .............................................................................. 7 The material included in this publication has been developed to provide information regarding zinc coatings on iron and steel products. Every effort has been made to ensure that the information is technically correct. This material is for general information only and is not intended as a substitute for competent professional examina- tion and verification as to accuracy, suitability and/or applicability. The publication of the material contained herein is not intended as a representation or warranty on the part of the American Galvanizers Association. Anyone making use of this information assumes all liability arising from such use. © 2000 American Galvanizers Association 6881 South Holly Circle, Suite 108 Englewood, CO 80112 1-800-468-7732 FAX 720-554-0909 E-mail: [email protected] Web site: www.galvanizeit.org
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I NTRODUCTION Zinc metal has a number of characteristics that make it well-suited for use as a coating for protect- ing iron and steel products from corrosion. Its excellent corrosion resistance in most environments accounts for its successful use as a protective coat- ing on a variety of products and in many exposure conditions. The excellent field performance of zinc coatings results from their ability to form dense, adherent corrosion product films and a rate of cor- rosion considerably below that of ferrous materials, some 10 to 100 times slower, depending upon the environment. While a fresh zinc surface is quite reactive when exposed to the atmosphere, a thin film of corrosion products develops rapidly, greatly reducing the rate of further corrosion. Figure 1 shows the expected service life to first maintenance (5% red rust) of iron and steel based on the zinc coating thickness and the environment. In addition to creating a barrier between steel and the environment, zinc also has the ability to cathodically protect the base metal. Zinc, being anodic to iron and steel, will prefer- entially corrode and protect the iron or steel against rusting when the coating is damaged (see Figure 2). A number of dif- ferent types of zinc coatings are com- mercially available, each of which has unique characteris- tics. These character- istics not only affect applicability but also the relative econom- ics and expected ser- vice life. The method of processing, adhe- sion to the base A MERICAN G ALVANIZERS A SSOCIATION Z INC C OATINGS Service Life* vs. Thickness of Zinc 11 21 32 43 54 65 75 86
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