Continuous hot dip processing

Cold-rolled sheet steel coated on both sides with a lead-tin alloy by a continuous hot-dip process. 

Figure 6.49 Typical process layout of a continuous hot dip galvanizing line...

S. Koeszegi, I. Felhoesi and E. Kalman, GD-OES and electrochemical study of Ce containing conversion layers of hot dip galvanised and by Sendzimir process continuously hot dip galvanised steel plates . Materials and Corrosion, 59, 744, (2008). 

Hot dipping is carried out by immersing the metal on which the coating is to be applied, usually steel, in a bath of the molten metal that is to constitute the coating, most commonly zinc, but also aluminum and aluminum-zinc alloys. Hot dipping can be either a continuous process, as in galvanizing steel sheet, or a batch process— for example, galvanizing fabricated parts, nuts, bolts, and fasteners [1]. 

The coating produced by hot dipping is bonded to the underlying steel by a series of Zn-Fe alloys, with a layer of almost pure zinc on the outside surface. The coating produced in the batch process is thicker than that produced in the continuous process, and it has clearly distinguishable alloy layers. The continuous process results in a thinner coating with a very thin alloy layer at the coating-steel interface [17]. 

AppUcation of PVD coatings to coils of steel sheet in a continuous process was studied extensively in the United States during the 1970s, but the process was found to be more costly relative to other coating methods, such as electroplating and hot dipping. More recently, efforts are underway in Japan and Europe to further develop such a process. 

In the plastisol hot-dipping operation, a preheated mold is dipped into the plas-tisol for a predetermined amount of time. At the end of dwell time, the mold is withdrawn and the part is removed. As the process continues, the viscosity of the plastisol reduces considerably. Such a change in the flow properties of plastisol can have a significant effect on the appearance and thickness of the fused coating. The viscosity of the plastisol is conveniently measured by using a flow cup and any necessary adjustments in the viscosity are made. 

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