Preparation before hot-dip galvanizing of strip steel_FAQ

The brush strip must have a good surface condition and cleanliness before hot-dip galvanizing. On the surface of the strip steel with oil stains, it is impossible to obtain a bright, good bonding force, and superior corrosion resistance coating. Therefore, the strip must pass through the cleaning section of the scrubbing unit before entering the annealing furnace. The cleaning section mainly includes chemical alkaline washing, electrolytic alkaline washing and brush roller cleaning. (1) Spray cleaning and circulation Spray cleaning is mainly to complete the heating and rough washing of strip steel. It is carried out in the form of spraying, using the chemical action of hot lye to remove thick rolling oil and iron powder on the surface of cold rolled strip steel. These rolling oils are often contaminated by lubricating oil and hydraulic oil, and must be removed by chemical alkaline washing first, and then electrolytic alkaline washing. Grease is divided into two categories: saponified oil and non-saponified oil. The main component of saponified oil is glyceride containing various higher fatty acids, its general formula is (RCOO)3C3H3, which can react with alkali to produce soap and glycerin. Non-saponified oil refers to mineral oil, which cannot react with alkali, but they can form an emulsion under certain conditions, and the non-saponified oil can be removed from the surface of the steel strip by emulsification. The function of chemical alkaline washing is to use hot lye to remove saponified oil and emulsifier to remove non-saponified oil. The important thing in the spray cleaning section is to control the concentration and temperature. If the concentration is too high, the solubility of the soap and the stability of the emulsion will decrease, and the cleaning effect will be reduced. At the same time, the degreaser will be wasted. If the concentration is too low, the cleaning will not be effective. Generally, the concentration of lye is controlled at 2~3%. From the perspective of energy conservation, the oil removal temperature should be as low as possible, but lowering the temperature and improving the oil removal effect are contradictory. The higher the temperature, the faster the physical and chemical reaction between the grease on the surface and the degreasing agent, and the easier it is to remove the oil. To lower the temperature, a surfactant must be added to the degreasing agent. For the cleaning section of this unit, the temperature should be controlled at 60~80℃. (2) Alkaline scrubbing Because there are a large amount of grease and iron powder on the surface of the rolled hard coil, only the chemical caustic scrubbing effect is not very good, and the chemical caustic scrubbing and brush roller cleaning must be used for treatment together. Brush roller cleaning can mechanically remove oil and dust to improve the cleaning effect. For the brushing of this unit, in addition to controlling the concentration, temperature and spray pressure, the brush roller current must also be controlled. The brush roller current reflects the degree of compression of the brush roller on the strip steel, and the reduction of the brush roller can be adjusted by the motor. During the brushing process, the bristles of the brush roller will continue to wear out, and the length of the bristles of the brush roller must be checked frequently. When the bristles are too short, the brush roller should be replaced in time. (3) Electrolytic cleaning The mechanism of electrolytic cleaning is that when strip steel is used as a cathode or anode, hydrogen bubbles or oxygen bubbles are correspondingly precipitated on the surface of the strip steel due to the electrolyzed water, which destroys the oil film attached to the surface of the strip steel. Small bubbles break away from the steel strip to which the oil droplets are attached and stay on the surface of the oil droplets, and stay at the interface between the oil and the degreasing liquid. As new bubbles continue to precipitate out, the small bubbles gradually become larger, so the oil droplets stay in the bubbles. It is separated from the surface of the steel strip under the action of, and is carried into the degreasing liquid by air bubbles. At the same time, the generated small bubbles can bring out the oxide scale on the surface and some dirt in the voids. The commonly used methods of electrolytic degreasing are: the intermediate conductor method and the conductive roller method. This unit uses the intermediate conductor method. That is, the strip steel is not directly connected to the power source. The power source is connected to the electrode plates located above and below the strip steel. When the upper electrode plate is connected to the anode, the lower electrode plate is connected to the cathode. Compared with the anode plate, if the strip steel potential is negative, it is the cathode. At this time, the reaction on the strip steel surface is: 4OH- — 4e u003d O2 + 2H2O, which means that oxygen is precipitated on the strip steel. Compared with the cathode plate, if the potential of the strip steel is positive, it is the anode. At this time, the reaction of the strip steel is: 4H2O + 4e u003d 2H2 + 4OH- that is, hydrogen is precipitated on the strip steel. In this way, water is decomposed through an electrochemical reaction to generate hydrogen and oxygen. In the electrolytic cleaning section, the concentration and temperature of the lye should be slightly increased to enhance conductivity. Since a large amount of hydrogen and oxygen are produced in the electrolysis process, and hydrogen is a flammable and explosive gas, the electrolysis cleaning section must have an exhaust device to extract the generated gas in time. (4) Hot water scrubbing tank Hot water scrubbing uses mechanical methods to remove dust and dirt attached to the steel plate after electrolytic cleaning, further enhancing the cleaning effect. The difference from the previous alkaline spray operation is that the spray liquid is hot water, and the roller is brushed. The operation is similar. (5) Hot water rinsing After the strip is washed with hot water, it is squeezed dry by the squeezing roller, but there is still a certain amount of degreasing liquid on the surface. If it enters the annealing furnace directly, it is easy to form stains on the surface of the strip. Therefore, it must be rinsed with hot water to clean the surface of the strip. Hot water rinsing adopts three-stage overflow, in which the last stage is supplemented with new water, and the temperature, overflow flow and circulation volume must be controlled during operation. (6) Strip drying Remove the moisture on the surface of the strip. During operation, the circulation of hot air and the temperature of the hot air should be controlled, and the nozzle should be checked frequently to prevent it from clogging. (7) Inspection after cleaning For the effect of cleaning, the strip steel that has just been dried by hot air can be observed with the naked eye. If the surface of the board is black and the surface is black and gray, it means that the cleaning is not good. Check the concentration, temperature and the condition of the brush roller; if the surface of the board is stained, it means that the rinsing is insufficient, and the amount of supplementary water and circulating water should be increased. And overflow. If there are water marks on the surface of the board, it means that the drying is not good. The temperature of the hot air should be adjusted and the nozzle condition should be checked. Welcome to www.mxbrushmachinery.com