Process control elements on the pretreatment line

The degreasing mechanism is the saponification, solubilization, wetting, dispersing, emulsifying and the like of various fats and oils by the degreasing agent, so that the grease is detached from the surface of the workpiece, becomes a soluble substance or is emulsified, dispersed and uniformly and stably present in the groove. In the liquid. The evaluation of the degreasing quality is mainly based on the fact that the surface of the workpiece after degreasing cannot be visually contaminated with oil, emulsion, etc., and the surface should be completely wetted by water after washing. The quality of degreasing mainly depends on factors such as free alkalinity, temperature of degreasing liquid, treatment time, mechanical action and oil content of degreasing liquid:

1.1 free alkalinity (FAL)

The proper concentration of the degreaser will ensure the best results. Generally only need to detect the free alkalinity of the degreasing liquid, the FAL is too low, the degreasing effect is relatively poor; the FAL is too high, not only causes the material cost, but also increases the burden on the subsequent washing. In severe cases, it will also pollute the post-order and phosphorus. Turn.

1.2 temperature of the degreasing solution

Any degreasing solution has the best degreasing temperature, the temperature is lower than the process requirements, and the degreasing effect cannot be fully exerted; if the temperature is too high, not only the energy consumption is increased, but also some negative effects are brought about. If the degreasing agent evaporates too quickly and the workpiece is detached from the bath, the surface drying speed is faster, which may cause defects such as rust, alkali spots and oxidation of the workpiece, and affect the phosphating quality of the subsequent process. Automatic temperature control also needs to be checked regularly.

1.3 processing time

The degreasing liquid must be in full contact with the oil on the workpiece, and there is sufficient contact reaction time to ensure a good degreasing effect. However, the degreasing time is too long, which will increase the bluntness of the surface of the worker and affect the formation of the phosphorus film.

1.4 The influence of mechanical action

In the degreasing process, supplemented by mechanical action, pump circulation or workpiece movement can enhance the degreasing effect and shorten the time of immersion cleaning; the speed of spray degreasing is more than 10 times faster than the immersion degreasing speed.

1.5 degreasing liquid oil content:

With the recycling of the bath, the oil content will increase continuously in the bath. When a certain ratio is reached, the degreasing effect and cleaning efficiency of the degreaser will be significantly reduced, even if the high concentration of the bath is maintained by adding the agent, the surface of the workpiece to be treated The cleanliness will still not improve. Degreased liquid that has deteriorated in aging must be replaced in full tank. Product production is generally carried out according to the use time of the bath and the processing workload.

2, pickling

The surface of the steel for product manufacturing will be rusted during rolling, forming or storage. Due to the loose structure of the rust layer, the adhesion to the substrate is not strong, and the oxide and the metal iron can form a galvanic cell, which can further promote the corrosion of the metal and cause the coating to be destroyed quickly, so it must be removed before painting. The products are generally used for pickling and rust removal. Pickling and rust removal will not deform the metal workpiece. The rust of each corner can be cleaned, 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 visible oxide, rust and over-etching as the standard. The main factors affecting the derusting effect are:

2.1 free acidity (FA)

Determination of the free acidity FA of the pickling tank is the most direct and effective evaluation method for verifying the effect of the pickling tank. The free acidity is low and the rust removal effect is poor. When the free acidity is too high, the acid mist content in the working environment is large, which is not conducive to labor protection; the metal surface is prone to "over-etching" phenomenon; and the cleaning of residual acid is difficult, which may lead to contamination of the subsequent bath liquid.

2.2 temperature, time

Most pickling is carried out at room temperature. When using acid pickling, it is generally controlled between 40 °C and 70 °C. Although the temperature has a great influence on the pickling ability, the excessive temperature will increase the workpiece. The corrosion of the equipment is also very unfavorable to the working environment; and under the premise of completely removing the rust, the pickling time should be as short as possible to reduce the corrosion of the metal and the effect of hydrogen embrittlement.

Therefore, the temperature of the bath and the processing time of the workpiece should be strictly controlled during the process.

2.3 pollution aging

In the process of derusting, the acid will continuously bring in oil or other impurities, and the suspended impurities can be removed by scraping; when the soluble iron ions exceed a certain content, the derusting effect of the bath will not only be greatly reduced. And the excess iron ions are mixed into the phosphating tank with the residual liquid on the surface of the workpiece, accelerating the aging of the phosphating bath, seriously affecting the phosphating quality of the workpiece, and the iron ion content of the general acid should be controlled to not exceed 6%- 10m/m is suitable. The bath must be replaced when the control indicator is exceeded.

3, table tune

The surface conditioner can eliminate the unevenness of the surface state caused by the degreasing or pickling of the surface of the workpiece, and form a large number of extremely fine crystal centers on the surface of the metal, thereby accelerating the speed of the phosphating reaction and facilitating the phosphorus. The formation of a film.

3.1 Impact of water quality

If the water used in the bath is severely rusted and the content of calcium and magnesium ions is large, it will affect the stability of the surface conditioning solution. The softener can be added before the preparation of the bath to eliminate the influence of water quality on the surface conditioning.

3.2 use time

The general surface adjustment agent uses a colloidal titanium salt, which has colloidal activity. When the use time is long or the impurity ions are contained, the colloidal activity is lost. At this time, the stable state of the colloid is destroyed, and the bath liquid is precipitated and layered, and is flocculated. At this time, the bath must be replaced.

4. Phosphating

Phosphating is a chemical and electrochemical reaction to form a phosphate chemical conversion film. The phosphate chemical conversion film formed is called a phosphate film. Commonly used for passenger car coating is low-temperature zinc-based phosphating solution. The main purpose of phosphating is to provide protection to the base metal, to prevent the metal from being corroded to a certain extent; to be used for primer before painting, to improve the adhesion of the paint layer and Corrosion resistance. Phosphating is the most important part of the whole pretreatment process. The reaction mechanism is complex and there are many influencing factors. Therefore, the phosphating bath liquid phase is much more complicated to control the production process of other bath liquids.

4.1 acid ratio (ratio of total acidity to free acidity)

Increasing the acid ratio can speed up the phosphating reaction, making the phosphating film thin and detailed, but the acid ratio is too high, the film layer is too thin, and the phosphating workpiece is easily ashed; the acid ratio is too low, and the phosphating reaction rate is slow, phosphorus The crystal is coarse and porous, and the corrosion resistance is low, and the phosphating workpiece is prone to yellow rust. In general, the phosphating solution system or formula has different acid ratio requirements.

4.2 temperature

The bath temperature is appropriately increased, and the film formation speed is increased. However, if the temperature is too high, the acid ratio will be changed, which will affect the stability of the bath, and the crystal nucleus of the membrane will be coarse, and the amount of slag from the bath will increase.

4.3 sediment amount

As the phosphating reaction continues, the amount of sediment in the bath will gradually increase. Excessive sediment will affect the interfacial reaction on the surface of the workpiece, resulting in phosphating film blooming, serious ash, or even film formation. Therefore, the bath must be According to the amount of processed workpiece and the time of use, the tank is poured in time, and the slag is removed.

4.4 nitrite NO-2 (accelerator concentration)

NO-2 can accelerate the phosphating reaction speed and 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; too low, the film forming speed is slow, and the phosphating film is easy. Raw yellow rust.

4.5 sulfate SO2-4

If the concentration of the acid washing liquid is too high or the washing control is not good, the sulfate ion in the phosphating bath liquid will increase, and the excessively high sulfate ion will slow down the phosphating reaction speed, making the phosphating film grains coarse and porous, and the ash ash is serious. The corrosion resistance of the phosphate film is lowered.

4.6 Ferrous ion Fe2+:

When the amount of ferrous ion contained in the phosphating solution is too high, the anti-corrosion ability of the normal temperature phosphating film will decrease; the grain of the intermediate temperature phosphating film will be coarse, the surface will be white ash, and the anti-corrosion ability will decrease; the amount of high-temperature phosphating solution will increase. Large, the solution becomes cloudy and the free acidity increases.

5, passivation (closed)

The purpose of passivation is to close the pores of the phosphating film, improve the corrosion resistance of the phosphating film, and in particular improve the overall adhesion and corrosion resistance of the film. At present, chromium-containing treatment and chromium-free treatment are generally used. However, some are inactivated with alkaline inorganic salts (mostly containing phosphates, carbonates, nitrites, phosphates, etc.), which seriously damage paint. Long-term adhesion and corrosion resistance of the film.

6, wash

The purpose of water washing is to remove the residual liquid from the surface of the workpiece, and the quality of the water washing directly affects the phosphating quality of the workpiece and the stability of the entire bath. The washing bath generally controls the following:

6.1 The sludge residue content should not be too high. If the content is too high, the surface of the workpiece may be ashed.

6.2 The surface of the bath should be free of suspended impurities. Generally, the water washing method is covered with overflow water to ensure that there is no suspended oil or other impurities on the surface of the bath.

6.3 The pH of the bath should be close to neutral. If the pH value is too high or too low, it will easily cause the tank liquid to be grooved, thereby affecting the stability of the subsequent bath.

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