Published on August 5, 2014

Author: arun.shrivastava07



Selecting Coating Thickness (Weight or Mass) for Galvanized Steel Sheet Product Arun Shrivastava CGL(Prod./Quality) ISCL,Gandhidham: Selecting Coating Thickness (Weight or Mass) for Galvanized Steel Sheet Product Arun Shrivastava CGL(Prod./Quality) ISCL,Gandhidham GENERAL DESCRIPTION: GENERAL DESCRIPTION The proper selection of coating thickness to meet a galvanized steel sheet user’s needs requires knowledge of the corrosiveness of the environment in which the product will be used. The thickness of the zinc coating largely determines its ultimate life, but it is not used directly to specify the amount of coating. Why galvanized sheet coatings are specified, not as thickness, but as coating weight or coating mass ? EFFECT OF ATMOSPHERIC CONDITIONS: EFFECT OF ATMOSPHERIC CONDITIONS The corrosion rate of a zinc coating varies widely depending upon many environmental factors. For example, “time of wetness” is an important issue that affects corrosion rate, i.e., outdoor applications in the dry States are very different from locations that experience high annual rainfall or extended foggy periods. Also, the presence of impurities such as sulfates, chlorides, and nitrates can dramatically affect the rate of corrosion. Other variables, including the amount of oxygen present in the electrolyte, and the temperature of the environment are important determinants for predicting product life. PRODUCT LIFE CONSIDERATION: PRODUCT LIFE CONSIDERATION The performance requirements, i.e., the desired product life, will be a factor in determining the required coating weight needed for a given application. For example, consider an application such as a metal building roof where the desire is for no red rust being visible for many years. In this case, the time to failure might be defined as the time for the onset of red rust (the time for the zinc coating to be consumed in a large enough area for rusting of the steel to be observed). This application requires a thick zinc coating. Another example is an application in which the time to failure is defined as the time when perforation of the steel sheet is observed. In this case, failure is affected by the thickness of the steel sheet (and the corrosion rate of the steel) as well as the thickness of the zinc coating. PowerPoint Presentation: Once the desired product life is determined, it is important to match the desired life with corrosion rate information for any specific application. By combining the rate of corrosion (zinc coating thickness loss/year) for a specific application with the desired life in years, one can then readily determine the zinc coating weight to specify. Designation System for Ordering a Specific Coating Weight [Mass]: Designation System for Ordering a Specific Coating Weight [Mass] For galvanized steel sheet products, the coating weight , and hence the thickness, is defined by the designator system in ASTM Specification A653/A653M. The inch-pound coating weight designators (as A653) range from designations G30 (0.30 oz/ft2 of sheet) to G235 (2.35 oz/ft2 of sheet), with many intermediate coating weights between these two. The equivalent SI coating mass designators (as A653M)are Z90 (90 g/m2 of sheet) to Z700 (700 g/m2 of sheet). This is almost an eight-fold difference in weight of zinc. These coating designations are total-both-sides, meaning that the coating weight on one side of the sheet is nominally one-half of the indicated value. PowerPoint Presentation: For many outdoor applications of bare (unpainted) galvanized sheet, the most common coating weight[mass] in use today is G90 [Z275]. This product is also specified for indoor applications where there is the potential for considerable amount of dampness due to condensation, etc. For other indoor applications where the environment is relatively dry, a G40 [Z120] or G60 [Z180] coating weight [mass] are usually sufficient. Outdoor applications such as corrugated steel pipe (CSP) for drainage applications require very heavy coatings. The most common coating weight [mass] for CSP is G200 [Z610]. Effect of Coating Weight [Mass] on Product Life: Effect of Coating Weight [Mass] on Product Life Although the corrosion rate can vary considerably depending on the environmental factors, as Figure 1 above shows, the life of a zinc coating is a linear function of coating weight [mass] for any specific environment. This means that to achieve twice the life for any specific application, twice the coating weight [mass] is required. PowerPoint Presentation: A G60 coating weight will exhibit approximately twice the life of a G30 coating weight. A G90 coating weight will exhibit about 50% longer life than a G60 coating weight CORROSION RATE DATA: CORROSION RATE DATA In addition to The Coating Life Predictor that is available , the following two reference books are excellent sources for additional and more detailed information on the corrosion behaviour of zinc-coated steel sheet products. These publications go beyond the information available using The Coating Life Predictor in that they contain information on corrosion rates in various aqueous solutions, as well as in organic and inorganic solutions, and in soils. PowerPoint Presentation: 1. Corrosion and Electrochemistry of Zinc, X. Gregory Zhang, Published by Plenum Press, 1996. 2. Corrosion Resistance of Zinc and Zinc Alloys, Frank C. Porter, Published by Marcel Dekker, Inc.,1994 PowerPoint Presentation: These publications document that corrosion can range from very low rates – in the order of less than 0.01mil*/year [0.254 μm /yr] – to much higher rates. If the rate of corrosion were, for example, 0.05 mil/year[1.25 μm /year], the life of a G90 coating would be approximately 16 to 17 years, since a G90 coating is approximately 0.83 mil [21 μm ] thick on each side of the coated steel sheet. In some environments, the rate of corrosion is so high that galvanized steel is not the preferred product. Generally, such applications are those that have either very acidic or very basic environments. THE END: THE END

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