what are the characteristics  of  galvanized steel coating ? 

It is important to understand that the coating characteristics of steel galvanized by the batchprocess, described above, and the coating characteristics of sheet products galvanized by thecontinuous process are not the same. Continuous galvanizing involves sheet steel that isgalvanized in coils and then fabricated into products. After-fabrication (“batch”) galvanizinginvolves plate, bar, and structural-shaped steel that has been fabricated into parts and thengalvanized. Because the two types of galvanized coatings have very different characteristics,they cannot be considered interchangeable for purposes of duplex-coating preparation.The most important component of coating batch hot-dip galvanized steel is understanding thezinc coating’s characteristics in each stage of its weathering. Although the zinc begins reactingwith the environment immediately upon removal from the galvanizing bath, the zinc coating cantake up to two years to weather completely, depending on the environment.For purposes of determining the stage of weathering, also called the “zinc patina” development(zinc oxides, zinc hydroxides & zinc carbonate), galvanized steel is divided into three categories:newly galvanized, partially weathered and fully weathered. Each stage of galvanized steelweathering must be prepared for duplex coating slightly differently because the galvanizedsurface has different characteristics and different surface materials.When steel is removed from the galvanizing bath, the zinc immediately begins to react with theenvironment. Zinc oxide corrosion products form 24 to 48 hours after galvanizing. Zinc oxide isa powdery, lightly adhering corrosion product and is the first step in the development of theprotective zinc patina. This newly galvanized steel (before the formation of zinc oxide) requires little or no surface preparation, perhaps simply slight profiling, to encourage paint or powdercoatadhesion.

When the zinc oxide is exposed to freely moving air, the surface reacts with moisture in the atmosphere, such as dew, rainfall, or even humidity, to form a porous, gelatin-type, grayishwhitemixture of zinc oxide and zinc hydroxide. This partially weathered galvanized steel formstypically between 48 hours and 2 years after galvanizing. The zinc has begun to form itsprotective zinc patina but has not completed the process. The zinc oxide and hydroxides are ballshapedparticles electrostatically connected to the steel surface and must be removed orneutralized using a sweep-blast and/or chemical cleaning. Removal by sweep-blasting (with amedium softer than zinc) or using an etch primer is suggested to slightly roughen the surface andimprove paint adhesion. Solvents alone will not remove zinc oxides and hydroxides.During continuation of the weathering process, the zinc oxides and hydroxides react with carbondioxide in the atmosphere and progress into a thin, compact, tightly adherent layer of basic zinccarbonate. This progression to zinc carbonate enhances the excellent barrier protection affordedby the galvanized coating. Because the zinc patina is relatively insoluble, it prevents rapidatmospheric corrosion of the zinc on the surface of galvanized steel. This fully weatheredgalvanized steel is between 8 months and 2 years old and has a completely formed zinc patina.The patina has a very stable and finely etched surface, providing excellent paint adhesion. Theonly surface preparation needed is a warm-water power-wash to remove loose zinc oxide andzinc hydroxide particles from the surface.