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The service life of the sixth section zinc pot

Current position: Home >> technical information >> The service life of the sixth section zinc pot

As mentioned earlier, the joint of the wall and the bottom and the wall of the pot are the weak links of the pot, while the joint between the middle and the bottom of the long-side pot wall is the weakest. When the new zinc pot is put into use for the first time, the zinc liquid containing unsaturated lead will cause intergranular corrosion of the pot wall, and brittle cracking may occur under residual stress, thermal stress and hydrostatic pressure of zinc liquid. Zinc bath will cause erosion to the pot wall. Excessive uneven erosion may lead to zinc pot perforation. Only when we know and solve these problems can we improve the service life of zinc pots.

1. corrosion of liquid metals

The corrosion of pure zinc solution on the wall of zinc pot is uniform and does not cause intergranular corrosion, as shown in Fig. 1-3a. The Fe-Zn alloy layer formed by reaction with the wall of zinc pot has a certain protective effect on the wall of the pot. When the protective layer of ferro-zinc alloy has not been formed, do not add lead. It is necessary to ensure that a certain thickness of ferro-zinc alloy layer is formed on the inner wall of the pot after zinc melts completely for a period of time.

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2. stress

The main stress causing cracking or deformation of zinc pot is tensile stress. The tensile stress of the inner wall of the pot may be due to the following reasons:

(1) stress caused by thermal expansion. The coefficient of thermal expansion of zinc is much larger than that of steel. If the zinc pot is heated closely after stacking zinc ingots, from 20 C to 419.6 C before melting, the expansion of zinc is 12.8 m m/m larger than that of steel, the tensile stress will occur on the wall of the pot. This kind of tensile stress can be avoided only if enough zinc ingot expansion space is set in the zinc pot.

If the zinc pot is heated too fast, the expansion of the upper part of the higher temperature zinc pot is restrained by the bottom of the lower temperature, and the tensile stress will appear in the lower part. The temperature difference between the inner and outer walls of the zinc pot will cause the inner wall to be in tension state. The uneven temperature around the wall of the zinc pot results in internal stress and bending deformation of the pot, as shown in Fig. 1-6. Pressure of zinc ingot or liquid zinc on the bottom of the pot will also cause tensile stress at the transition between the bottom of the pot and the side wall.

Correct heating control, slow heating, burner in the right place, good insulation at the bottom of the zinc pot, zinc pot wall temperature difference control in all places below 50 C, through these measures as far as possible to minimize the thermal stress. The maximum allowable temperature difference is related to the size and wall thickness of the zinc pot. The larger the zinc pot, the smaller the allowable maximum temperature difference. For example, the most typical zinc pot depth is 2.2 meters, temperature difference is 100 degrees Celsius, zinc pot can not detect creep caused by macroscopic deformation.

In fact, thermal stress is not the only cause of damage to the zinc pot, which is often accompanied by corrosion of liquid metals.

(2) hydrostatic pressure of zinc solution. The hydrostatic pressure of zinc liquid always acts on the wall of zinc pot during the use of zinc pot, which can produce internal stresses such as tension, compression and bending. Therefore, the deeper wall of zinc pot needs support. The maximum stress area is on the bottom side of the zinc pot length direction. When the zinc pot runs at high temperature, the material of the zinc pot can creep under the action of stress, resulting in the overall deformation of the zinc pot. The critical stress of creep is related to temperature and time. After long use, the bending of the wall is the example of creep of zinc pot material. Because the tensile strength of steel at high temperature decreases with the increase of service time at high temperature, the long-term use of zinc pot (about ten years) will cause damage.

(3) the stress generated during the zinc pot manufacturing process. In the process of making zinc pots, the bending and welding of zinc plate will produce internal stress. However, modern production technology and methods can make these internal stresses very low, the use of zinc pot will not have any impact, no additional measures to eliminate the internal stress. After heating the zinc pot to the usual hot galvanizing temperature, the residual stress in the process will be reduced. The bending process of zinc pot steel plate must be hot bent to prevent strain aging and embrittlement.

3. service life of zinc pot

The service life of a zinc pot is determined by the rate at which the iron on the wall of the pot is eroded and dissolved by zinc. The bottom of the pot is eroded and dissolved by zinc at the minimum rate, so the weight lost is the smallest. The amount of iron that is attacked by zinc corrosion can be measured by iron loss. When the zinc pot is used, the reaction mechanism between the zinc solution and the wall of the pot is the same as that on the dip-plated parts, that is, the reaction between zinc and iron forms an alloy layer on the surface of the iron. As time goes on, the iron in the pot wall is dissolved by diffusion. The alloy layer produced by the wall can play a role of retard diffusion. The service life of the zinc pot is related to the iron loss, and the iron loss is related to the interfacial temperature between the inner wall of the pot and the alloy layer. Fig. 1-7 shows the relationship between the iron loss of zinc dipping / h and the temperature. When the heating temperature is below 485 C, the diffusion increases slowly with the temperature rising, and the iron loss increases according to the parabolic law. At the temperature range of 490~530 C, the iron loss increases with the increase of temperature. The interfacial temperature between the inner wall of the zinc pot and the alloy layer can not be measured by general method. It is the result of the balance between the heat input and the heat output of the zinc pot.

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The service life of a zinc pot can be estimated by the time when the thickness of the pot wall is eroded by zinc and 20 mm thickness is lost. The estimated results are listed in Table 1-1. The iron loss after long time zinc leaching is determined by the constant a, while the constant a is determined by temperature. When the temperature of the inner wall of the pot increases, for example, from 480 to 490, the service life of the zinc pot decreases from 6 years to 4.3 years; when the temperature of the inner wall of the pot is 500, the iron loss has a linear relationship with the time, and the service life is only 20 days.

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Within a certain period of time, there are generally two kinds of appearance of corrosion of the inner wall of the zinc pot: wavy and flat surface. If the temperature of the zinc pot wall is uniform, the thickness of the alloy layer on the pot wall is uniform, the appearance of the corrosion of the inner wall of the zinc pot should be flat.

The formation process of wavy surface topography can be illustrated in Figure 1-8. For some reason, high-iron corrosion begins somewhere, for example, where the temperature of the inner wall of the pot reaches the critical temperature at the point where the ferro-zinc alloy layer is damaged or where the flame is directly touched, the thickness of the pot wall decreases to form eroded pits, as shown in Fig. 1-8a. The decrease of the thickness of the pot wall in the pit leads to the increase of the temperature of the pot inner wall, the erosion accelerates the increase of iron loss, and the erosion pit becomes larger and deeper gradually. When the temperature in the center of the pit rises gradually and exceeds the critical temperature zone, the erosion will be slowed down, and the high-speed rail loss zone will be transferred to the adjacent annular region at the critical temperature, as shown in Fig. 1-8b. Need to emphasize again, must avoid the temperature of the inner pan wall is above 480 degrees Celsius, can not be used in this temperature range for a long time.

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