Improve the compatibility of phosphating film with coatings
Text / Tang Chunhua
ã€Abstract】 In order to improve the compatibility of phosphating film and coating, the factors affecting the compatibility of phosphating film and coating were analyzed. The specific measures to improve the compatibility of phosphating film and coating were improved to improve oil removal and rust removal methods. The phosphating process, the rational design of the phosphating process equipment layout, the improvement of the self-control degree of the bath and the matching of the purification equipment.
Key words: phosphate film; coating; matching
Phosphating film as the bottom material of steel drum coating should have good coating property and can improve the decorative and corrosion resistance of the whole coating, and finally achieve the purpose of extending the service life of the steel drum. The main role of phosphating is reflected in the following aspects.
a. improve the corrosion resistance of the coating;
b. increase the adhesion of the coating to the surface of the steel drum;
c. Prevent chemical reaction between the paint film and the surface of the steel drum;
d. Slow down the expansion speed of the coating under the coating.
With the development of steel drum coating technology, the requirements for phosphating are getting higher and higher. If the phosphating film and the coating are poorly matched, it directly affects the protective effect of the coating on the steel drum. The effect of phosphating on electrostatic spraying is particularly obvious. It has been found that the non-phosphorized spray paint coating is far less corrosion resistant than the non-phosphorus spray paint coating. The reason is that the phosphate film does not fully meet the requirements of electrostatic coating. .
Now, the factors affecting the compatibility of phosphating film and coating are analyzed, and corresponding measures to improve the matching of phosphating film and coating are put forward.
1 Process factors affecting matching
1.1 The influence of the substrate
1.1.1 Steel plate composition
When the carbon content of the steel sheet is low, the phosphating film is uniform and dense; on the contrary, the film is thin, has many defects or does not form a film.
1.1.2 phosphating film metallographic structure
The orientation of the crystalline film also has a significant effect on the phosphating film and stability. The results of X-ray diffraction studies show that orientation (311) has higher alkali resistance (high pH conditions) than (020).
1.1.3 Surface state
The surface of the steel drum is rough, and the film forming speed is fast, but the film layer is loose, the uniformity is poor, and the corrosion resistance is low. The surface of the steel drum is flat, the film forming speed is slow, the phosphating film is dense and uniform, and the corrosion resistance is good. Therefore, it is not advisable to spray sand before powder spraying, otherwise rust spots will appear.
1.1.4 Geometry
The phosphating film has good performance in the flat part, while the corrosion resistance of the phosphating film in the corner part is relatively poor.
1.1.5 Product Structure
There are many corners of the bottom cover of the steel drum, which may cause the residual liquid at the corner to be cleaned, and the phosphating will easily cause yellowing or severe rust. If a lot of grooves, curls and top seams are formed in the ring and the corrugations of the barrel, improper protection of these parts, salt water, lime soil and water will enter it, so that it is in a long-term wet state, the corrosion rate of the steel plate It has accelerated dramatically.
1.2 The effect of strong alkali strong acid treatment
The steel drum is treated with strong alkali strong acid before phosphating, which brings serious damage to the coating, which not only affects the stability of the phosphating solution, but also accelerates the corrosion rate of the coating. Although the water is washed, neutralized, and tempered before phosphating, the coating is rusted from the inside out. If pickling causes intergranular or pore corrosion, some insoluble matter remains on the metal surface, making it difficult to form a film. Even if a complete phosphating film is formed, the corrosive medium is easily penetrated because the film itself is porous. Also, the residual acid cleaning is not completely carried into the phosphating solution, which reduces the stability of the bath. The residual acid remains in the weld, and as a result, the corrosion rate of the coated steel drum is greater than that of the unwashed, which greatly reduces the protective properties of the coating.
1.3 Effect of phosphate film
1.3.1 Membrane type
In general, both the common iron film and the zinc film can be used as the bottom layer of the coating. However, the phosphating film with two different components has different effects on the adhesion of the paint film. The iron-based film is finer and denser, the film has a small particle size, a large surface area, and a large contact area with the paint film. At the same time, the iron film is particularly resistant to bending, impact, abrasion and chipping, so it has good adhesion to the paint film, but its film is thinner and has poor rust resistance. It is used as a coating for powder coating, electrostatic spraying and epoxy lacquer. However, the light iron film is not suitable for nitrocellulose lacquer or perchloroethylene lacquer, and is not suitable for automatic line production. It has been studied to obtain a green iron-based film from a mixture of sodium molybdate and sodium phosphate, which has good corrosion resistance and can be used as a coating underlayer to improve the corrosion resistance and adhesion of the coating. It is worth noting that the iron-based film obtained from the three-in-one and four-in-one solution has complex composition, coarse and uneven film, white powder or other water-soluble salts and oil traces, and most of the coating performance is poor, sometimes lower than Do not process.
The commonly used zinc-based film has better corrosion resistance and adhesion than the iron-based film, especially in combination with electrostatic spraying. For a long time, most of the low-zinc phosphating process has been implemented in the steel drum industry. However, the zinc film is greatly affected by the pretreatment.
In addition, the zinc-calcium film has good heat resistance and is more brittle than the zinc film, and its corrosion resistance, especially salt spray resistance, is not as good as that of the zinc film. Its film thickness will affect the film's resistance to bending and impact. It is generally only used as the bottom layer of high-solvent paints and heat-resistant paints. It is not suitable for high-viscosity paints, powder coatings and electrostatic coatings.
1.3.2 Film thickness
Generally speaking, the phosphating film is thin, the absorption coating is less, and the gloss of the paint film is good; on the contrary, the film layer is thick, the absorption coating is large, the brittleness is large, the glossiness of the paint film is poor, and the flexibility and impact resistance are poor. Therefore, the phosphating film is thick, and although it has good corrosion resistance, it does not necessarily improve the corrosion resistance of the steel drum after being matched with the paint film. Electrostatic coatings require a thin, dense phosphating film. Other coatings require a thicker film. For example, electrostatic coatings require a slightly thicker film. Solvent-based paints, water-soluble paints, solid anti-corrosion and anti-corrosion agents require thicker films.
1.4 Effects of phosphating conditions
1.4.1 Temperature
The temperature has a great influence on the quality of the phosphate film. The phosphating film obtained by high temperature and medium temperature treatment has coarse, thick and porous crystals, which directly affects the performance of the coating. Conversely, the temperature is too low, the film is thin, and the corrosion resistance is poor. The phosphating temperature to be used is good, depending on the requirements of the steel drum coating and the storage period after phosphating. For example, if the steel drum is stored for 3 to 5 days after phosphating, it can be phosphatized at room temperature; it can be stored for more than 10 days, and it is suitable for tempering at medium temperature.
1.4.2 Phosphating water
The phosphating water should be pure. Turbid water containing insoluble materials or water having too high hardness and alkalinity may become crystal nuclei in phosphate hydrolysis precipitation or consume a large amount of phosphating solution.
1.4.3 Phosphating slag
A certain amount of phosphating slag does not affect the phosphating effect, but it should not be too high, otherwise the residue adheres to the phosphating film to produce powder, which can not be washed off, which affects the coating quality. Especially in the manganese-containing low-zinc phosphating process, the amount of sediment should be strictly controlled, otherwise it is not conducive to the formation of high-quality phosphating film.
1.5 Effects of phosphating treatment
1.5.1 Passivation
In order to improve the corrosion resistance of the thin phosphating film, especially after the tempering of the steel plate, the passivation treatment must be carried out, which can greatly reduce the porosity and improve the rust prevention property, thereby improving the compatibility of the phosphating film with the electrostatic spraying.
After passivation, it should be fully rinsed with deionized water to prevent the surface of the steel drum from being contaminated by foreign ions, meeting the requirements of high corrosion resistance and paint adhesion.
1.5.2 Drying
After the phosphating film is baked at a high temperature, the film crystal is finer and denser, and the adhesion to the surface of the steel drum is greatly increased, and the corrosion resistance is better. At the same time, it can avoid the formation of rust spots in the joints and welds. However, the temperature should not be too high, otherwise the phosphate film will be white, causing powdering when it is severe. Generally, those who apply organic solvent-based coatings have a drying temperature of 100 to 130 °C.
1.6 Influence of impurities
Al3+, Cu3+, and Pb2+ metal ions play an adverse role in the phosphating film formation process. For example, when the content of Al3+ in the manganese-containing low-zinc phosphating solution reaches 3g/L, Al3+ will cause the phosphating film to appear bright particles in the form of crystals until the flower is discolored, so that the phosphate film is incomplete. When the content of SO42- and Cl- ions is too high (>0.5g/L), the membrane layer is porous and rust-prone.
1.7 Effect of cleaning between processes
1.7.1 Cleaning after degreasing and derusting
Thoroughly clean after degreasing and descaling, otherwise it will cause various adverse effects. If the cleaning is not clean after degreasing, the surface of the steel drum is covered with a layer of alkaline substance. The phosphating film is embossed and rough, and the corrosion resistance is low. If the degreaser is brought to the steel drum, the shrinkage hole of the paint film is easy to occur. Pinholes, etc. Another example is that if the rust remover is brought into the phosphating solution, the free acid is raised, the ionization of the phosphate is inhibited, the filming ions are reduced, the phosphate film is rough, uneven, and even a rust film is generated. In addition, they also destroy the composition of the phosphating solution.
1.7.2 Cleaning after phosphating
After phosphating, the residual soluble salt on the phosphating film should be thoroughly washed. Otherwise, the soluble salt will affect the adhesion of the coating. Under humid and hot conditions, it may cause early foaming and detachment of the coating, and may also contaminate the electrophoresis tank. Therefore, after phosphating, it is best to use 1 to 2 tap water immersion and 1 deionized water spray. It is required that the conductivity of tap water must be less than 500 μS/cm and the conductivity of deionized water be less than 25 μS/cm. This is good for reducing the contamination of the steel drum surface and obtaining a good coating.
1.8 The effect of rust on the barrel between processes
In the production, it is found that the barrel parts are more likely to rust after pickling and washing, causing the phosphating film to ash and yellow, and cannot form a complete phosphating film. The reason is that the empty time of the barrel after pickling and washing is too long or the interval between the steps is too long.
2 Measures to improve the matching
2.1 Improved oil removal and rust removal methods
2.1.1 Neutral degreasing
In order to eliminate the harm caused by strong alkali degreasing to phosphating, the application of neutral degreasing, oil removal effect is good. It reduces the surface tension of the oil of the barrel by the emulsification function, and separates the oil from the barrel by the powerful auxiliary agent. The treatment temperature is applicable at both normal temperature and medium temperature, which creates favorable conditions for obtaining high quality coating. The strong alkali degreasing is easy to cause the barrel to be mixed, because the emulsifier of the degreasing liquid acts to reduce the surface tension of the oil, and the hydrophilicity is improved. In fact, the oil does not leave the surface of the barrel, and the barrel is washed out by water. The surface is completely wetted by water, causing the illusion that the oil in the barrel is removed. As a result, after the coating, after a period of time, the adhesion was poor, the coating peeled off, and the rust was observed. It is best to use a clean toilet paper to check whether the oil on the surface of the barrel is removed. If the toilet paper does not see black oil, it means the oil is clean. In order to prevent the oil in the degreasing liquid from being deposited on the surface of the barrel, 0.5 g/L of polyvinyl alcohol may be added to the degreased liquid.
2.1.2 Neutral derusting
Some steel drums use a large area of ​​rust, and the barrels of this material must be rust-removed.
In view of the shortcomings in strong acid derusting, in recent years, neutral descaling methods have been studied. For example, the neutral descaling agent originally developed by Hunan University, after the barrel parts are treated by the descaling agent, removes the surface corrosion and generates one. The layer is similar to the protective layer of the phosphating film, and the film has good corrosion resistance and adhesion to the paint film, and can be used as a primer layer for painting. For example, the implementation of phosphoric acid or citric acid degreasing and rusting in one, such as the Ch9301 two-in-one treatment agent produced by Liuyang Chemical Plant in Hunan Province, the barrels are smooth after treatment, there is no corrosion residue, and it is not easy to rust. If the treatment agent can be combined with ultrasonic pickling, the purpose of efficient degreasing and derusting can be achieved.
2.2 Optimize the phosphating process
2.2.1 Adjusting the composition of the phosphating solution
Metal ions such as Ni2+, Mn2+, and Mg2+ are added, and these ions can replace the iron ions to form a polycrystalline phosphorous film on the surface of the steel sheet. The film layer is fine, dense, low in porosity, and has high alkali resistance and corrosion resistance under the film to improve the appearance of the coating.
Adding F-ion 0.2-0.5g/L can eliminate the influence of Al3+ on the phosphating of hot-dip galvanized steel sheet and maintain the complexing ability of Al3+.
2.2.2 Grasp the specific application of phosphating solution
The basic requirements for fast, low slag and high quality phosphating solution are inseparable from the specific application conditions. The compatibility of phosphating solution with coating, the characteristics of the product and the equipment conditions of the relevant site should also be considered. For example, a steel drum factory produces 208 liter steel drums with large batch size and high production efficiency. Therefore, the tank type multi-gun spray treatment is employed, and a high-speed phosphating solution (a 2-4 μm phosphating film can be formed in 15 to 30 s at 50 to 55 ° C). Later, the PA377-3 zinc-based high-speed phosphating solution developed by Wuhan Material Protection Co., Ltd. was used, and the phosphating speed was fast, the film layer was uniform and meticulous, and it had excellent matching with the paint. Another example is the small and medium barrels produced by a factory. Because of the wide weld seam on the barrel, the rust is severe after phosphating (the surrounding surface of the weld). The reason for the analysis is that the concentration of zinc-calcium phosphating solution is too high, and the phosphating solution retained on the surface of the weld is difficult to clean. When the weld is not cleaned, the phosphate film in the joint will be corroded. Inflated and contracted, the acid oozes out the phosphating film on the surrounding surface from the weld. According to the above reasons, take corresponding measures. The first is to appropriately reduce the ratio of total acidity to free acidity within the allowable process range, so that the film formation speed at the gap is fast, the film layer is thick, the crystal is fine, the corrosion resistance is improved, and the weld seam is easy to clean; the second is phosphorus Neutralization after the treatment, even if a small amount of acid is retained in the weld, it is neutralized by alkali, and the effect is good.
2.2.3 Strict operating conditions
All equipment should be inspected before production, such as whether the pipeline or nozzle is clogged, whether the water in the cleaning tank is clean, whether the tank temperature and spray pressure meet the requirements.
2.3 Reasonable design of the layout of the phosphating process equipment
When designing the layout of the phosphating process equipment, it is necessary to consider the requirements of the coating process and the connection relationship of the key processes of phosphating. If the process is not well connected, it may affect the quality of phosphating. According to reports, a barrel factory designed a coating production line with an annual output of 300,000 steel drums. There is a problem that the adjustment tank is too long apart from the phosphating tank (6 to 7 m apart), and the barrels are adjusted from the table to the phosphating tank. It takes about 8 to 12 minutes. The barrel parts are in contact with the severe acid gas in the pre-treatment chamber for a long time during the movement, so that the surface of the barrel table is corroded and destroyed, which is not conducive to phosphating film formation and formation. Once the meter liquid is out of control, the barrel moves in the corrosive gas for a long time, and the surface often appears yellow and produces a large amount of water rust, which seriously affects the effect of the phosphate film. There is also a phenomenon of coarse grains, which becomes an important factor affecting the quality of the coating. A reasonable design should be that after the phosphating tank is next to the metering tank, in case of a cathodic turn that needs to be connected at a relatively long distance, the general process arrangement should be considered, such as using a cold water tank and a hot water tank for the best transition.
2.4 Improve the degree of self-control of the tank and the advanced nature of the conveyor
Foreign countries use computers to control phosphating, from degreasing, surface conditioning to phosphating, all of which have been automatically analyzed and automatically added. There is still a big gap in China, and the current backward situation of this component analysis should be changed as soon as possible. In addition, in order to avoid the dripping of liquid on the catenary and affect the quality of phosphating, foreign pretreatment mechanization has evolved from the commonly used push rod catenary to the double rail swing rod conveyor.
It is worth pointing out that the cleaning equipment for the bath should follow up. At present, the country has made great progress in this respect, paying attention to the research of tank liquid separation technology. It has promoted modified arma filter and drum type ultrafilter, which can automatically remove the oil and mechanical impurities of the degreasing liquid and ensure the degreasing liquid. Long-term use, do not change the bath, extend the life of the bath. The belt type automatic slag filter is used to control the automatic slag removal process of the filter and the chain net, the degree of automation is high, the phosphating solution is completely slag-removed, and the maintenance is convenient.
I believe that with the further emphasis on China's environmental protection industry, in the case of increasingly tight water resources and increasing phosphating requirements for coating, the implementation of tank purification equipment has obvious economic and social benefits.
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