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5.7 Removal of Dross
Dressing is carried out regularly before the bath is shut down and depending on the production. The amount of dross depends not only on the way galvanizing is carried out but also on the temperature of the inside kettle walls. Dross is not ony formed by the reaction of liquid zinc with the steel of the art1cles_b t also by the reaction with the steel kettle walls. The spec1f1c gravity of zeta crystals is only slightly higher than that of molten zinc. The crystals are carried al?ng with the streaming zinc and settle on the bottom of the bath. Due to the typical currents in the zinc they form a dross layer of unequal thickness. The dross layer thickness should not exceed 100 mm to
prevent the galvanized coating getting rough py captured dross and
also the depth of the kettle being reduced. When the dross layer
gets too thick it must be removed with a dressing grab..The time
interval for dressing depends on the throughput but 1s usually
once a week. To reduce dross production the dipping time of the
articles in the zinc must be as short as possible and all articles dipped into the zinc must be taken out of the bath as
soon as possible. Articles should not be left in the molten zinc
the end of the shift.
6. Kettle Serviceability
Damage by insufficient serviceability (plant reliability) manifest itself by the formation of cracks especially in the corner wall the bottom and in the middle of the long side of the heated ket le wall. Especially dangerous is the region of the lead layer. The cracks are intercrystalline and are mostly formed during or shortly after the first time the bath is put into use.
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6.1 Liquid metal corrosion
With liquid metal corrosion (LMC) brittle cracks and in certain cases brittle ruptures are formed due to an intercrystalline attack by lead not saturated with inc together with tensile stresses in the kettle walls. The stresses in the kettle can be caused by stresses introduced during manufacture, temperature differences during the heating up period and the hydrostatic pressure of the liquid zinc. Very important in causing LMC is the composition of the melt which comes into contact with the kettle. The element causing the damage is the zinc which, however, is only dangerous in melts of a certain composition. Pure zinc melts do not create brittleness, whilst zinc melts col"}taining lead can lead to LMC. Particularly intensive is the intercrystalline attack of zinc not saturated with lead (-lead layer).
Figure 7: ScheMa. tic lllustra tlon of the deMo.ge of the crystal boundo.rles by z;nc not so.turo.ted with leo.cl