Galvanized steel is prized all over the world for its durability and corrosion-resistance. But for as strong as the finished material is, the process is surprisingly simple and straightforward. Galvanizing steel consists of just three steps:
- Surface Preparation.
Like most things, the quality of a finished galvanized product is directly influenced by the amount of effort put into the preparation. Poor surface prep can cause the galvanization to fail, but galvanization has built-in quality control of sorts. The zinc used in galvanizing won’t react with an unclean steel surface. This makes it easy to see poorly coated areas as soon as the piece is pulled out of the galvanization tank. Unclean areas remain uncoated, allowing technicians to correct the problem right away.
Surface preparation consists of three steps:
- Degreasing/Caustic Cleaning. An acidic “bath” removes organic contaminants from the steel surface. Dirt, paint, grease, and oil can be removed this way. Materials that cannot be removed via the bath include: epoxies, vinyls, asphalt, and slag. These contaminants are removed by degreasing, grit-blasting, sand-blasting, or other mechanical means.
- Picking. A dilute solution of acid is used to remove mill scale and rust from the steel. Instead of or in place of the acid, an abrasive cleaner or air blasted sand could be used.
- Fluxing. Fluxing is a zinc ammonium chloride solution that removes remaining rust particles and adds a protective layer to the steel, helping to prevent any further oxides from forming on the surface before it can be galvanized.
Once the steel is ready for galvanizing, it is immersed in molten zinc. The chemistry of the bath must meet certain standards, requiring at least 98% pure zinc and a temperature of 815-850 F. During the bath, the zinc reacts and bonds with iron in the steel to form an extremely hard alloy layer that strengthens and protects the steel. Excess zinc is removed from the galvanized piece by draining, vibrating and/or centrifuge but the metallic reaction continues as long as the steel remains near bath temperature. Galvanized pieces are cooled by immersing them in a passive solution, in water, or by being left in open air.
The last step, the inspection process, is also the quickest. Coating thickness and surface condition are both closely scrutinized for integrity. Several tests are used to determine coating thickness, uniformity, adherence, and appearance. Results are compared to long-established and accepted standards set by the ASTM, the International Standards Organization (ISO), the Canadian Standards Association (CSA), and the American Association of State Highway and Transportation Officials (AASHTO).
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