Pretreatment and coating of steel drum surface (also on the development environment of steel drum industry in India) [India] SS Arumali [Translation] Yang Wenliang Foreword 1. India's closed economy 2. Opened from 1991 to 1992 India’s economy was once extremely closed and affected by the worldwide economic environment. It has just opened up soon. The new economic system has made consumers the protagonist of the market, so consumers are beginning to have higher and higher requirements for products and have their own choices. India’s industrial economy has finally recovered from sleep. From 1991 to 1992, consumers began to change their previous concepts, and they have more demanding requirements for industrial products. Steel drum products must meet their basic requirements: a) through leak test b) internal cleaning c) good adhesion of the paint film Under the premise of meeting the above quality requirements, the price of steel drums has shown a downward trend, while the price of steel products has risen by about 25%, and the shortage of steel can only meet the demand of 70-80%. In India, the total annual demand for steel drums is about 5.2 million, and the type and capacity of steel drums are relatively simple. Even so, its quality level is far from meeting international standards. Raw material 80% of the cost in our steel drums is steel, and the quality of the steel also determines the quality of the steel drum. The main factors affecting the quality of the steel drum are: 1, the chemical composition of steel 2, hardness 3, ductility 4, thickness 5. Lubricating oil (on the surface of the steel plate) 6, surface quality 7, packaging In the above aspects, there are problems in our country's steel drum raw materials. These problems must be raised and hope to be resolved. The chemical composition, hardness and thickness of steel are the three major factors that directly affect the quality of steel drum forming and welding. ——The surface quality of the steel and the lubricating oil are the main factors that directly affect the brightness of the surface of the steel drum and the adhesion of the paint film. -- The quality of the packaging affects the quality assurance of the steel during transportation and determines whether the material will withstand the wind and rain. Because in a steel transportation journey, a place may rain, the next place may be sunny, there may be storms in the next place, or the place may be full of dust... So, the packaging is not vague. a) The surface of the steel drum is not damaged b) external packaging c) use high quality lubricants Because steel is chemically active in the air, it can rust anytime, anywhere, especially in humid air. The surface coating prevents the surface of the steel drum from remaining rusting for a certain period of time. The steel plate that has just been produced from the rolling mill seems to be smooth and clean with our eyes, but if observed under a microscope of 100 times, we will see: 1, some parts of rust have begun to generate 2, some parts of the lubricant has begun to dry up 3, some parts of fresh oil have begun to become dirty oil Let's take a closer look at the finished steel drum coated with a 120-150 micron film thickness and look at the gullies and mountains that are not flat on the outer surface. Many mountains are like the Himalayas or the Alps. . Let us take a closer look at the microscopic surface of the steel drum that has begun to have a small area of ​​corrosion. 1, there is no proper amount of lubricant 2. Uneven distribution of lubricating oil 3, the lubricant has deteriorated We visited one or two rolling mills. They applied lubricant to the steel plate and sprayed it onto the surface of the steel plate with a spray nozzle. Due to the different distances and distances between the nozzle and the steel plate, it is inevitable that some dead angles will not be sprayed or Insufficient oil. In this way, the steel plate will first rust during the storage and transportation process with less oil or no oil. After applying the paint to the surface of such a steel sheet, we found that the rust continued, and the paint film was easily peeled off. When the paint film with poor adhesion is torn off, the surface under the paint film will be rusted as before the coating. Let us look at the third question above - the deterioration of lubricating oil. We visited the rolling mills. The lubricants they used were all low-priced inferior oils. Because of the large amount, the high-quality oils were too expensive. In order to reduce costs, they had to use them. It is said that the use of high quality lubricants is not easy to deteriorate and dry. These inferior lubricating oils are undoubtedly low-energy to prevent re-rusting of the steel sheet because it cannot withstand the erosion of the steel sheet by moist air. It deteriorates into an emulsion in a short period of time, and rust starts to form. Dry lubricant For dry oil, we are difficult to remove by mechanical means, only by solvent degreasing. We have not used a brush to clean it, because it is difficult to reach because it penetrates into the fine surface gully. When we apply I paint on the surface of a steel plate with dry oil, the dried oil will re-dissolve under the action of the paint. The oil that becomes liquid is formed between the paint and the steel plate, which causes the adhesion of the paint film to be poor. Extremely easy to fall off. Fresh lubricant For lubricating oils with a relatively fresh steel surface, it can be mechanically erased by about 90-95%. The rest of the oil remains in the fine gully on the surface of the steel sheet and is difficult to remove. When the surface of the steel sheet is painted, the remaining oil will continue to ooze out, causing the paint film to fall off. When we studied the fine structure of the surface of the steel plate, we can conclude that the oil on the surface of the steel plate cannot be completely mechanically cleaned by tools such as a brush, and only the solvent degreasing treatment can achieve the desired effect. According to reliable data, the US military has a paint called DQS-A that is compatible with the lubricating oil on the steel plate, but it is said to be very rare. Degreasing treatment The operation of removing surface grease from the substrate is generally referred to as degreasing treatment. There are several different methods for degreasing: l, seven-slot processing 2. Alkalinization treatment 3, water vapor degreasing treatment 4, ultrasonic degreasing treatment In view of the limitations of our industry, in general, we use the second or third method described above for degreasing, depending on the size of the output. Considering factors such as environmental protection and personal health, Option 3 has more advantages than Option 2, so we generally recommend the third option. The treatment method of scheme 2 is: a) Soaking treatment - completely soak the steel drum in the alkaline degreasing solution, dissolve the oil stain on the surface of the steel sheet with alkali solution, and prepare for the next process. b) In the step - after the steel drum is soaked in the lye for a while, the steel drum is taken out into another lye tank by means of an operating tool, and moved up and down or left and right in the lye to make the lye deeper. Dissolve grease and eliminate the surface tension of grease. c) Step 2 - Change to the next lye tank and repeat step 1 until there is no oil on the surface of the drum. The lye tank is alternately placed in two lye: l, cold lye 2, hot lye In our country, the temperature is hot most of the year, so we generally do not consider the heating of the lye. In this case, the performance of the lye is still 40%. Typically, the cold lye is treated with a 5-6% lye, each time in the tank for 10 seconds. The hot lye is used in a concentration of 2-3% lye, the temperature is 55-65 ° C, and the treatment time is 5-6 seconds. Water cleaning When the steel drum is washed in the above hot and cold lye, we need to use the spray method to clean the steel drum with cold water or hot water to completely remove the oil and the lye removed on the steel drum to facilitate the Painting of the subsequent process. The operation method of this step can also be the same as the cleaning of the upper lye. The steel drum after water washing should be baked into the oven in time to dry the surface and then paint. The residence time of the steel drum in the air before drying should be as short as possible to prevent the surface from rusting again. There are two types of steel drum moisture drying ovens, which can be used. l, vertical drying oven 2, horizontal drying oven If the steel drum enters the oven in an upright manner, the furnace temperature must be at least 140-160 ° C, and if the steel drum is placed in the oven, the furnace temperature should be at least 75-85 ° C. When drying, the steel drum should keep rolling continuously in the furnace, and the roasting time is not less than 5-6 minutes. The surface of the steel drum after drying should be very clean, but in order to get a better coating effect, the surface temperature of the product should be lower than 45-50 °C when the steel is baked out of the oven. In India, we usually use the following two types of paint drying to dry steel drums: l, air drying oven - this is our country-specific type, seems to be rare in other parts of the world. 2, general dry type - this is a universal class in the world. 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