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Important things need to know about process control of phosphating pretreatment line

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Important things need to know about process control of phosphating pretreatment line

August 12
12:52 2022

1. Degreasing

The degreasing is to remove grease from the workpiece surface and transfer grease into soluble substances or emulsify and disperse grease to be evenly and stably in the bath fluid based on the saponification, solubilization, wetting, dispersion and emulsification effects on various types of grease from degreasing agents. The evaluation criteria of degreasing quality are: the surface of the workpiece should have no visual grease, emulsion or other dirt after degreasing, and the surface should be completely wetted by water after washing. The degreasing quality mainly depends on five factors, including free alkalinity, temperature of degreasing solution, processing time, mechanical action, and oil content of degreasing solution.

1.1 Free alkalinity (FAL)

Only the appropriate concentration of degreasing agent can achieve the best effect. The free alkalinity (FAL) of the degreasing solution should be detected. Low FAL will reduce the oil removal effect, and high FAL will increase material costs, increase burden on post-treatment washing, and even contaminate the surface activating and phosphating.

1.2 Temperature of degreasing solution

Every kind of degreasing solution should be used at the most suitable temperature. If the temperature is lower than the process requirements, degreasing solution can not give full play to degreasing; if the temperature is too high, energy consumption will be increased, and negative effects will appear, so degreasing agent evaporates fast and the fast surface drying speed, which will easily cause rust, alkali spots and oxidation, affect the phosphating quality of the subsequent process. Automatic temperature control should also be regularly calibrated.

1.3 Processing time

The degreasing solution must be in full contact with the oil on the workpiece for a sufficient contact and reaction time, to achieve better degreasing effect. However, if degreasing time is too long, the dullness of the workpiece surface will be increased.

1.4 Mechanical action

Pump circulation or workpiece movement in the degreasing process, supplemented by mechanical action, can strengthen the oil removal efficiency and shorten the time of dipping and cleaning; the speed of spray degreasing is more than 10 times faster than that of dipping degreasing.

1.5 Oil content of degreasing solution

The recycled use of bath fluid will continue to increase the oil content in the bath fluid, and when the oil content reaches a certain ratio, the degreasing effect and cleaning efficiency of the degreasing agent will drop significantly. The cleanliness of the treated workpiece surface will not be improved even if the high concentration of the tank solution is maintained by adding chemicals. The degreasing liquid that has aged and deteriorated must be replaced for the whole tank.

2. Acid pickling

Rust occurs on the surface of the steel used for product manufacturing when it is rolled or stored and transported. The rust layer with loose structure and can not be firmly attached to the base material. The oxide and metallic iron can form a primary cell, which further promotes metal corrosion and causes the coating to be destroyed fast. Therefore, rust must be cleaned before painting. Rust is often removed by acid pickling. With fast speed of rust removal and low cost, acid pickling will not deform the metal workpiece and can remove the rust in every corner. The pickling should meet the quality requirements that there should be no visually visible oxide, rust and over-etching on the pickled workpiece. The factors affecting the effect of rust removal are mainly as follows.

2.1 Free acidity (FA)

Measuring the free acidity (FA) of the pickling tank is the most direct and effective evaluation method to verify the rust removal effect of the pickling tank. If the free acidity is low, 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”; and it is difficult to clean the residual acid, resulting in the pollution of subsequent tank solution.

2.2 Temperature and time

Most pickling is conducted at room temperature, and heated pickling should be performed from 40℃ to 70℃. Although the temperature has a greater impact on the improvement of pickling capacity, too high temperature will aggravate the corrosion of the workpiece and equipment and have adverse impact on the working environment. The pickling time should be as short as possible when rust has been completely removed.

2.3 Pollution and aging

In the rust removal process, acid solution will continue to bring in oil or other impurities, and suspended impurities can be removed by scraping. When soluble iron ions exceed a certain content, the rust removal effect of the tank solution will be greatly reduced, and excess iron ions will be mixed into the phosphate tank with the workpiece surface residue, accelerating the pollution and aging of phosphate tank solution, and seriously affecting the phosphating quality of workpiece.

3. Surface activating

Surface activating agent can eliminate the evenness of workpiece surface due to oil removal by alkali or rust removal by pickling, so that a large number of very fine crystalline centers are formed on the metal surface, thus accelerating the speed of phosphate reaction and promoting the formation of phosphate coatings.

3.1 Water quality

The serious water rust or high concentration of calcium and magnesium ion in the tank solution will affect the stability of the surface activating solution. Water softeners can be added when preparing the tank solution to eliminate the impact of water quality on the surface activating solution.

3.2 Use time

Surface activating agent is usually made of colloidal titanium salt that has colloidal activity. The colloidal activity will be lost after the agent is used for a long period of time or impurity ions are increased, resulting in the sedimentation and layering of the bath fluid. So the bath fluid must be replaced.

4. Phosphating

Phosphating is a chemical and electrochemical reaction process to form phosphate chemical conversion coating, also known as phosphate coating. Low-temperature zinc phosphating solution is commonly used in bus painting. The main purposes of phosphating are to provide protection to the base metal, prevent the metal from corrosion to certain extent, and improve the adhesion and corrosion prevention ability of the paint film layer. Phosphating is the most important part of the entire pretreatment process, and has complicated reaction mechanism and many factors, so it’s more complicated to control the production process of the phosphate bath fluid than other bath fluid.

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

Increased acid ratio can accelerate the reaction rate of phosphating and make phosphating coating thinner. But too high acid ratio will make the coating layer too thin, which will cause ash to phosphating workpiece; low acid ratio will slow phosphating reaction speed, reduce corrosion resistance, and make phosphating crystal turn coarse and porous, thus leading to yellow rust on the phosphating workpiece.

4.2 Temperature

If the temperature of the bath fluid is increased appropriately, the speed of coating formation is accelerated. But too high temperature will affect the change of acid ratio and the stability of bath fluid, and increase the amount of slag out of the bath fluid.

4.3 Amount of sediment

With the continuous phosphate reaction, the amount of sediment in the bath fluid will be gradually increased, and excess sediment will affect the workpiece surface interface reaction, resulting in blurred phosphate coating. So the bath fluid must be poured out according to the amount of workpiece processed and use time.

4.4 Nitrite NO-2 (concentration of accelerating agent)

NO-2 can accelerate the speed of phosphate reaction, improve the denseness and corrosion resistance of phosphate coating. Too high NO-2 content will make the coating layer easy to produce white spots, and too low content will reduce the coating formation speed and produce yellow rust on the phosphate coating.

4.5 Sulfate radical SO2-4

Too high concentration of pickling solution or poor washing control can easily increase sulfate radical in the phosphate bath fluid, and too high sulfate ion will slow down the phosphate reaction speed, resulting in coarse and porous phosphate coating crystal, and reduced corrosion resistance.

4.6 Ferrous ion Fe2+

Too high ferrous ion content in the phosphate solution will reduce corrosion resistance of phosphate coating at room temperature, make phosphate coating crystal coarse at medium temperature, increase sediment of phosphate solution at high temperature, make the solution muddy, and increase the free acidity.

5. Deactivation

The purpose of deactivation is to enclose the pores of phosphate coating, improve its corrosion resistance, and especially improve the overall adhesion and corrosion resistance. At present, there are two ways of deactivation, i.e., chromium and chromium-free. However, alkaline inorganic salt is used for deactivation and most of the salt contains phosphate, carbonate, nitrite and phosphate, which can seriously damage the long-term adhesion and corrosion resistance of coatings.

6. Water washing

The purpose of water washing is to remove the residual liquid on the workpiece surface from the previous bath fluid, and the quality of water washing directly affects phosphating quality of the workpiece and the stability of bath fluid. The following aspects should be controlled during water washing of bath fluid.

6.1 The content of sludge residue should not be too high. Too high content tends to cause ash on the workpiece surface.

6.2 The surface of the bath fluid should be free of suspended impurities. Overflow water washing is often used to ensure that there is no suspended oil or other impurities on the bath fluid surface.

6.3 The pH value of bath fluid should be close to neutral. Too high or too low pH value will easily cause the channeling of bath fluid, thus affecting the stability of the subsequent bath fluid.


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