Pretreatment process control of aluminum material spraying

[China Aluminum Network] aluminum coating process before the spray process, according to the specific circumstances, such as plant conditions that the size of space, workpiece material (cold rolled sheet, hot rolled plate, galvanized sheet, aluminum alloy, glass steel, etc.) The surface conditions of the workpiece (corrosion, anti-rust oil, debris), production volume, quality requirements and other options. Different manufacturers use different processes. For example, the overall flow and quality requirements for passenger car production are basically the same. The pre-treatment process generally has the following two types:

1, the workpiece processing typical process flow:

Pre-degreasing→degreasing→water-washing I→pickling→washing II→neutralization→washing III→external conditioning→phosphating→washing IV→passivation Manufacturers can make appropriate adjustments according to the actual conditions of the factory:

As a part of the pre-treatment, pickling rust is essential. However, in the pre-treatment process, it is better to separate the corroded parts from the non-corrosive parts, and the corroded parts are pickled, and the non-corrosive parts are preferably not pickled.

2. Pre-treatment of products The pre-treatment of products is to immerse the entire product in the bath for surface treatment, which has been adopted by many manufacturers. There are two kinds of manual control and automatic control between processes. More advanced is the use of PLC program automatic control, can achieve automatic transfer between processes. The general process is: degreasing → washing → surface modulation → phosphating → washing.

The common equipment for pre-spray treatment is a spray-type combined cleaning machine. The cleaning principle is to complete the processes of oil removal, phosphating, passivation, and cleaning by means of jet mechanical force and chemical action.

The typical process of spray combined washing machine is: degreasing, washing, surface adjustment, phosphating, passivation, water washing, pure water washing.

Its structural features are: the spray pipe of each process of the pre-treatment production line is arranged in the tunnel, and the spray is provided by the pumps of the liquid tanks. All work processes are separated by a door plate to prevent sputum. Each heating tank is heated in and out of the tank. The inner heater has a row of tubes, a serpentine tube, a corrugated plate, etc.; the outer heater has a column tube, a plate heater, and the like. The heating medium includes hot water, steam, and heat transfer oil. In the degreasing and phosphating processes, there are a degreasing system and a deslagging system, respectively. In order to prevent the overflow of steam at the top of the tunnel and at the door opening of each heating section, an exhaust fan is provided at the top of the tunnel.

The process control elements are described as follows:

1, degreasing and degreasing mechanism is through the degreaser on various types of grease saponification, solubilization, wetting, dispersion, emulsification and other effects, so that the grease off the surface of the workpiece, into a soluble material or emulsified, dispersed and evenly and stably Exist in the bath. The evaluation of degreasing quality is mainly based on the fact that the surface of the workpiece after degreasing can not have visible grease, emulsion, etc., and the surface should be completely wetted by water after washing. The quality of degreasing depends mainly on the free alkalinity, the temperature of the degreasing bath, the processing time, the mechanical action, and the oil content of the degreasing bath.

(1) Free Alkalinity (FAL)

Degreasing agent concentration can ensure better results. Generally only need to detect the free alkalinity of the degreasing solution, FAL is too low, the degreasing effect is relatively poor; FAL is too high, not only cause material wastage, but also increase the burden on the subsequent washing, severe cases will also contaminate the order of the table and phosphorus Change.

(2) The temperature of the degreasing liquid Any kind of degreasing fluid has a better degreasing temperature, the temperature is lower than the process requirements, can not fully play the role of degreasing; the temperature is too high, not only increase energy consumption, but also bring about some side effects. For example, if the degreasing agent evaporates too fast and the workpiece detaches from the bath solution, the surface drying speed will be faster, which may cause the defects such as rust, alkali spots, and oxidation of the workpiece and affect the quality of the phosphating process. Automatic temperature control also requires regular checks.

(3) Treatment time The degreasing liquid must be in full contact with the oil on the workpiece, and there must be sufficient contact reaction time to ensure a good degreasing effect. However, if the degreasing time is too long, it will increase the bluntness of the workpiece surface and affect the formation of the phosphating film.

(4) Effect of mechanical action In the degreasing process, supplemented by mechanical action, the use of pump circulation or workpiece movement can enhance the degreasing effect and shorten the time for immersion cleaning; the speed of spray degreasing is 10 times faster than that of impregnation and degreasing. .

(5) Degreasing fluid oil content:

With the recycling of the bath solution, the oil content will continuously increase in the bath. When a certain proportion is reached, the degreasing effect and cleaning efficiency of the degreasing agent will be significantly reduced. Even if the high concentration of the bath solution is maintained by adding the medicament, the surface of the workpiece will be treated. The cleanliness level will still not improve. Deteriorated degreasing fluid must be replaced by full tanks. The production of the product is generally based on the use of the bath time and the amount of processing time to change the tank.

2. Corrosion may occur on the surface of the steel used in the manufacture of pickled products during roll forming or storage. Due to the loose structure of the rust layer, it is not adhered to the substrate, and the oxide and aluminum iron can constitute the primary battery, which further promotes corrosion of the aluminum material and causes the coating to be destroyed quickly. Therefore, it must be removed before painting. The product is generally used for pickling and rust removal. It does not deform the aluminum workpieces, and the corrosion of each corner can be removed. The rust removal speed is fast and the cost is relatively low. The quality of pickling is mainly based on the fact that the workpiece after pickling should not have visually visible oxides, rust and over-etching as the criteria, and the factors that affect the derusting effect are:

(1) Free Acidity (FA)

Determining the free acidity FA of the pickling tank is a more direct and effective evaluation method to verify the level of rust removal in the pickling tank. Low free acidity, poor rust removal. When the free acidity is too high, the acid mist content in the working environment is relatively large, which is not conducive to labor protection; the surface of the aluminum material is prone to "over-etching"; and the cleaning of the residual acid is difficult, and it may easily lead to contamination of the subsequent bath liquid.

(2) Temperature and time Most picklings are performed at room temperature. When using heated pickling, they are usually controlled at temperatures between 40°C and 70°C. Although the temperature has a great influence on the pickling ability, the temperature is too high. The high corrosion will exacerbate the corrosion of workpieces and equipment, and the impact on the working environment is also very unfavorable; and under the premise of completely removing rust, the pickling time should be as short as possible in order to reduce the influence of aluminum corrosion and hydrogen embrittlement. Therefore, the temperature of the bath and the processing time of the workpiece should be strictly controlled during the processing.

(3) Contaminant aging The acid will continue to carry oil or other impurities during the rust removal process. The suspended impurities can be removed by scraping; when the soluble iron ion exceeds a certain level, the rust is removed from the bath. The effect is not only greatly reduced, but excessive iron ions are mixed into the phosphating bath with the residual liquid on the surface of the workpiece, accelerating the pollution aging of the phosphating bath, and seriously affecting the phosphating quality of the workpiece. Generally, the iron ion content of the acid liquid should be controlled at Not more than 6% to 10% is appropriate. The bath solution must be changed when the control index is exceeded.

3, surface conditioning agent can eliminate the surface of the workpiece due to lye degreasing or pickling rust caused by the uneven surface state, so that the aluminum surface to form a large number of very fine crystallization center, thus speeding up the phosphating reaction Speed, conducive to the formation of phosphate film.

(1) Influence of water quality If the water quality used in the bath solution is severe, the content of calcium and magnesium ions is large, which will affect the stability of surface adjustment. When the bath solution is prepared, a softener can be added to eliminate the water quality. Impact.

(2) Use time Generally, the surface adjustment agent adopts colloidal titanium salt, which has colloidal activity. When the use time is longer or the impurity ions are contained in more colloidal activity, the colloidal activity will be lost. At this time, the stable state of the colloid is destroyed and the bath liquid precipitates. The layer is flocculous and the bath must be changed.

Phosphating phosphating is a chemical and electrochemical reaction that forms a phosphate chemical conversion film. The phosphate chemical conversion film formed is called a phosphating film. Passenger car painting is commonly used low-temperature zinc phosphating solution. The main purpose of phosphating is to provide protection to the aluminum body of the substrate, to prevent the aluminum material from being corroded to a certain extent; to paint before the primer and to improve the adhesion of the paint film layer. Efforts and corrosion resistance. Phosphating is an important part of the entire pretreatment process. Its reaction mechanism is complex and there are many influencing factors. Therefore, the control of the phosphating bath liquid phase for the production of other baths is much more complicated.

(1) Acid ratio (ratio of total acidity to free acidity)

Increasing the acid ratio can speed up the phosphating reaction rate and make the phosphating film thin and detailed. However, if the acid ratio is too high, the film layer will be too thin, and the phosphatized workpiece will be ashing. The acid ratio is too low and the phosphating reaction is slow. Phosphorus The crystal is coarse and porous, and its corrosion resistance is low. The phosphatized workpiece is prone to yellow rust. In general, the phosphating solution system or formula has different acid ratio requirements.

(2) The temperature of the bath temperature is properly increased, and the film formation speed is increased. However, if the temperature is too high, the change of the acid ratio will be affected and the stability of the bath liquid will be affected. At the same time, the nucleation of the film layer will be coarse and the amount of slag in the bath will increase.

(3) Slag amount As the phosphating reaction continues, the amount of sediment in the bath will gradually increase. Excessive sediment will affect the interface reaction on the surface of the workpiece, causing the phosphating film to cause serious flowering and ashing, and may not even form a film. Because of this, the bath must be inverted at the appropriate time according to the amount of work to be handled and the time of use.

(4) Nitrite NO-2 (Accelerator Concentration)

NO-2 can speed up the phosphating reaction speed, improve the compactness and corrosion resistance of the phosphating film. When the content is too high, the film layer is prone to white spots or coloring; if it is too low, the film formation speed is slow, and the phosphating film is easy. Yellow rust.

(5) Sulfate SO2-4

Excessively high concentration of acid solution or poor washing control can easily lead to the increase of sulfate ion in the phosphating bath. Excessive sulfate ion can slow down the phosphating reaction rate and make the phosphate film coarse and porous. , Phosphate film corrosion resistance decreased.

(6) Ferrous ion Fe2+:

When the phosphating solution contains too high ferrous ions, the anti-corrosion ability of the phosphating film at room temperature will be reduced; the grain size of the middle-temperature phosphating film will be coarse, the surface will float white ash, and the anti-corrosion ability will decline; the amount of high-temperature phosphating solution will increase the amount of slag Large, the solution becomes turbid, and the free acidity increases.

5, passivation (closed)

The purpose of passivation is to seal the pores of the phosphating film and improve the corrosion resistance of the phosphating film, especially to improve the overall adhesion and corrosion resistance of the paint film. At present, chromium-containing and chromium-free treatments are generally used. However, some are inactivated with alkaline Inorganic Salts (most of which contain phosphates, carbonates, nitrites, phosphates, etc.), and these substances seriously damage paint. Long-term adhesion and corrosion resistance of the membrane.

6. The purpose of washing and washing with water is to remove the residual liquid from the surface of the workpiece. The quality of the washing can directly affect the quality of the phosphate and the stability of the entire bath. Washing bath solution generally controls the following:

(1) The sludge residue content should not be too high. Excessive content is prone to ash on the surface of the workpiece.

(2) The surface of the bath should be free of suspended impurities. The general washing method adopts overflow water washing to ensure that there is no suspended oil or other impurities on the surface of the bath.

(3) The pH of the bath should be close to neutral. If the pH value is too high or too low, the trough trough can be easily caused, which will affect the stability of the subsequent bath liquid.

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