Because of the poor adhesion of the coating on the surface of aluminum and aluminum alloy substrates, the surface must be pretreated to form a conversion film on the surface. This will not only enhance the adhesion of the coating, but also greatly improve the corrosion resistance. The surface treatment of aluminum and Aluminum Alloy general for alkali washing, cleaning, water cleaning, hot water, cold water, cold water to film cleaning cleaning, chemical conversion, water cleaning, water cleaning, cleaning, drying, draining, deionized water.
The surface treatment process of aluminum and aluminum alloy is different from that of iron and steel.
(1) degreasing: the main purpose of alkali washing is to remove the greasy dirt on the surface of aluminum alloy. The alkali solution formula commonly used by Na2CO3, Na3PO4, Na2SiO3 and additives, mainly with immersion and spraying, electrolytic cleaning and ultrasonic cleaning are also available.
(2): the removal of oxide film on aluminum surface natural oxide film formed is not uniform non continuous film, must be thoroughly cleaned before painting, the mixture of rolling parts available Aluminum Alloy chromic acid, sulfuric acid and hydrofluoric acid to remove the surface oxide film, and die casting with nitric acid as the main.
(3) chemical conversion: the purpose of conversion treatment is to form a dense and uniform continuous film on the surface of aluminum alloy. This process is equivalent to the role of iron and steel parts surface treatment process of phosphating process, but its solution is different, conversion film of aluminum and Aluminum Alloy based on chromic acid / hydrochloric acid and phosphoric acid / hydrochloric acid and hydrofluoric acid, the conversion film quality is usually 2.5g / m2 conversion coating, must use clean water to wash, then with hot air drying, in order to enhance the hardness of the film.
Coating process can be generally divided into: painting, dusting and electrophoresis. The workpiece after surface treatment, corrosion resistance of the workpiece according to the different appearance and requirements, selection of appropriate equipment and spraying process, also should pay attention to the control flow, drying and cooling process, otherwise it will have adverse effects on the quality of the products.
In general, the painting process can achieve better appearance quality. The paint coating has better gloss, color and weather resistance, and is usually used in automobile exterior coating and motorcycle oil tank. For occasions requiring high corrosion protection, such as motorcycle frame, refrigerator placed in the kitchen and so on, the general use of dusting process. Electrophoresis process is generally used in salt spray test, impact resistance and other requirements of high occasions, and act as primer. Of course, just like an automobile, a wiper, an expensive door lock and so on, only an electrophoretic paint can meet the requirement. Sometimes it can be applied to three processes of a product, which is determined by various factors.
Three kinds of technology have advantages and disadvantages, in terms of investment in equipment investment and electrophoresis equipment, key equipment mainly rely on imports; dusting equipment investment at least, but because the powder baking temperature is high, so the equipment (energy consumption) high operation cost. At present, in foreign countries have developed a good leveling property and low temperature curing powder coatings and corrosion resistance can be beautiful with those of powder coating paint.
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