The use of metallic zinc pigmentation in todays

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The use of metallic zinc pigmentation in today's coatings effectively eliminates under-cutting corrosion and sub- film corrosion through galvanic action. Abrasive blast removal of mill scale in the fabrication shop improves long- term adhesion and helps the original coating tolerate maintenance overcoating without costly surface prepara- tion. With an intermediate coat and topcoat applied, the first required maintenance should occur after approxi- mately 25 years of service. At that time, with spot cleaning, spot priming and the addition of another topcoat (approximately 2-3 mils), you could expect another 15-20 years of service life. At the end of that period, the same process would be repeated with the same anticipated results. A shop may be either a permanent painting shop (which may be part of a steel fabricator's plant), a separate painting shop, or a temporary shop constructed at or near the building site to repaint the steel. A covered shel- ter does not necessarily constitute a "shop." The shop-applied coating may include an initial coat or multiple coats as specified by the owner, or, if accept- able to the owner, as selected by the contractor. New steel used as a construction item is the easiest to protect from corrosion because it probably has not been contaminated with salts that act as electrolytes for the corrosion cells. Because the salts may not be present, it will be easier to achieve the degree of surface preparation needed to protect steel. Older steel (and specifically corroded steel) may have soluble salts imbedded in the corroded pits and intergranular surfaces. Though the salts may be of a soluble type, they are difficult to remove even with the most rigorous cleaning procedures and tend to shorten the service life of coating systems when compared to the life of the same systems on new steel. Mill scale is a hard, smooth, blue-black layer of iron oxide (Fe 2 0 3 ) that forms on steel during the hot-rolling process. Mill scale is very inert. When intact, it forms a very efficient barrier to protect steel from corrosion. Unfortunately it has a different coefficient of expansion than steel and is very brittle. Because of this, it cracks and chips. The remaining mill scale then becomes cathodic with respect to steel, forming very efficient corrosion cells. The result is that mill scale must be removed before painting. Red rust, a form of mixed iron oxides, is a surface contaminant familiar to everyone. It varies in color form light red to dark brown and may be loose and powdery or hard and granular. Red rust provides a weak foundation for paint, contributes to the formation of corrosion cells, and contributes to the destruction of coatings. In the case of light superficial rust, there are surface-tolerant primers that can be used to provide future protection of the steel. For example, steel that has been prepared and cleaned in the fabrication shop may develop superfi- cial rust on the jobsite prior to the building being enclosed may be adequately protected by such primers.
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  • Fall '19
  • Structural steel

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