CoatingsPro Magazine

JAN 2013

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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understanding when examples of PMCF involving duplex coating systems are presented in this two-part series. UnDerstAnDing BAtCh hot-Dip gAlvAnizeD proCess The batch hot-dip galvanizing process consists of a series of sequential preparatory tanks into which steel is dipped prior to immersing it into molten zinc. In order to transport the steel between the tanks, each piece must incorporate a means of hanging it on a rack. To ensure proper drainage during the dipping processes, complex pieces are designed with weep holes. An often overlooked, but important, step in the HDG process is selecting properly sized pieces for the rack of each HDG batch. Since molten zinc does not begin accumulating on steel until the steel reaches the bath temperature in the kettle, it is imperative to place pieces of steel with similar thickness on the same rack. When thin steel pieces are placed on a rack alongside thick steel pieces, one of two results occurs. If the steel is kept in the zinc kettle long enough for the thicker steel to achieve its required thickness, the thinner pieces often warp. If the steel is only kept in the kettle long enough for the thinner pieces to attain their specified thickness, the thicker pieces will contain less than its required thickness. The thickness of HDG directly correlates with anticipated life expectancy according to numerous published articles; therefore, reduced zinc thickness reduces HDG's useful life expectancy. After selecting pieces of similar steel thickness and placing them on racks in the correct orientation to allow proper drainage, the steel begins its journey through several dip tanks. The first tank, hot alkaline, removes oil, grease and dirt. The second tank, water rinse, is followed by a third tank, typically sulfuric or hydrochloric acid, which removes mill scale and rust. After another water rinse in the fourth tank, the steel is dipped into the final tank in the preparation process, flux solution (typically zinc chloride and ammonium chloride) to remove oxides. (Note: In some plants the flux solution is floated on top of the zinc kettle.) After the final preparation tank, the rack heads to the kettle or galvanizing bath, which contains zinc in a molten (liquid) form. The steel is immersed into the zinc bath and remains in the kettle, usually less than ten minutes, until it accumulates the specified thickness. Water or chromate quenching after galvanizing is standard practice in most galvanizing shops to facilitate rapid handling of the steel and to reduce wet storage stains. However, quenching after galvanizing is NOT recommended when applying a duplex coating system as quenching often leads to subsequent delamination. Since quenching is the default mode, it is imperative to notify the galvanizer in writing NOT to quench the steel when a duplex system is specified. It is extremely important for HDG plants to sample and analyze the contents of all tanks on a regular basis, preferably several times a week, and to adjust the composition of each tank as needed in order to maintain the proper solution in each tank. The frequency of adjustment depends upon the quantity of steel processed through the plant. This author has visited HDG plants on three continents and found, unfortunately, that all HDG plants do not place the same emphasis on regular monitoring and adjustment of the tanks, especially the composition of Above Quenching the HDG steel in the shop, after galvanizing it, left chromates on the surface that were not removed prior to coating. This resulted in delamination of the coating system. the kettle. Inadequate monitoring of the kettle composition can result in an excessive amount of aluminum in the zinc bath, causing bare spots and black marks on the steel after galvanizing. trAnsforMAtion of hDg oUter lAyer In the hot-dip galvanizing process, three distinct alloy layers (gamma, delta, and zeta), and one pure zinc layer (eta) are formed. The gamma layer, the thinnest, is formed direct ly on the steel and consists of 75% Zn and 25% Fe. Each subsequent layer contains progressively increasing amounts of zinc and decreasing amounts of iron, (e.g. 90%Zn and RIGHT Severe corrosion and loss of galvanic protection occurred with this duplex coating system as a result of improper surface preparation prior to coating. January 2013 g www.coatingspromag.com 81

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