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Aluminum Anodizing

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September 27, 2008

Solution of Hot Dipping

During galvanizing the parts are taken from the fluxing solution and well drained, or even dried off, without rinsing, and immersed into the clean molten zinc. They are then withdrawn as quickly as possible, and encouraged to drain by shaking of brushing, special attention being given to the lower edges. The parts are then put aside to cool or quenched in an air blast or in water. The thickness of the zinc coating can be controlled are usually expressed by the weight of zinc and the period of immersion. Coatings are usually expressed by the weight of zinc per unit area; 2 oz zinc per ft2 = 0.0018 in thickness is a usual coating, but thickness of 4 to 6 oz are possible. As a result of surface tension forces, small re entrant crevices and angles, such as screw threads, tend to become clogged, whilst the coating may be thinner than average on sharp edges, except at the bottom edge where may be a thickening.

The surface appearance is irregularly crystalline because the solidification of the zinc proceeds from nuclei, and causes characteristic and decorative spangles. Small additions of aluminum increase the fluidity of the molten bath, and restrain the growth of the alloy layer. Dross and oxide films floating on the bath surface are liable to become entangled in the molten zinc coating and impair its appearance. The zinc use is often rather impure (about 99%). Moreover, it reacts with the iron immersed in it and with the pot; eventually it becomes saturated with the iron-zinc inter-metallic compound, which crystallize out. These crystals tend to sink to the bottom of the zinc pot, but some may get caught up in coating causing unsightly lumps.

Steel sheets are galvanized mechanically by processing them through the molten zinc by power driven rolls. Sheet wire is passed mechanically through the zinc bath, and is usually drawn through a wiper on the exit side.

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