Mechanism and prevention of edge over coating in continuous hot-dip galvanizing

In order to clarify the mechanism of edge over coating (EOC) for continuous hot-dip galvanizing, a visualization test of the gas flow on strip and a c old model test to measure the profile of the coating thickness at the strip edge were carried out. Outward deflected gas flow was observed at the stri p edge and EOC developed in the absence of gas wiping. With gas wiping, EOC developing below the wiping position is reduced by the impinging pressure of the gas wiping jet, and the film thickness becomes approximately uniform at the gas wiping position. However, upward of the gas wiping position, EO C increases again and the outward deflected gas flow on the strip edge swee ps the liquid film to the strip edge. EOC is considered to develop at the l ocation where the dynamic pressure of the outward deflected gas flow balanc es with the surface tension. For the prevention of EOC, edge masking was devised and the effects which r educe EOC were measured in the cold model test and on a commercial line tes t. The edge mask which can be kept farther away from the strip edge is more effective for preventing EOC than the edge plates. The optimum dimension o f the edge mask is 30 mm in width and 75-100 mm in depth, and installing it at 4-10 mm away from the strip edge is most effective. It was confirmed by the commercial line test that the edge mask can reduce EOC from 45% to les s than 10%.