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Phosphating technology

The early application of the phosphating process is anti-rust, and the steel parts are phosphatized to form a layer of phosphate film, which acts as a rust preventive. The rust-preventing period of the phosphating anti-rust treatment can reach several months or even several years (for oil-coated workpieces), and is widely used for rust prevention and rust-proof phosphating during process, transportation, packaging, storage and use. There are three major varieties of iron phosphating, zinc phosphating and manganese phosphating.

The iron-based phosphating main bath component is a ferrous phosphate solution, does not contain an oxidizing accelerator, and has a high free acidity. The iron phosphating temperature is higher than 95 ° C, the treatment time is longer than 30 min, the phosphating film is more than 10 g / m 2 , and has the dual functions of derusting and phosphating. This high-temperature iron phosphating is currently used in applications because the phosphating rate is too slow.

Manganese phosphating has the best performance as rust-proof phosphating. The microstructure of phosphating film is densely packed and is the most widely used rust-proof phosphating. Adding and no accelerator can increase the speed of phosphating film formation if nitrate or nitroguanidine promoter is added. The treatment temperature is usually 80 to 100 ° C, the treatment time is 10 to 20 minutes, and the membrane weight is 7.5 g/m 2 or more.

Zinc phosphating is also widely used as a kind of rust-proof phosphating. Usually, nitrate is used as a promoter. The treatment temperature is 80-90 ° C, the treatment time is 10-15 min, the phosphating film is more than 7.5 g/m 2 , and the phosphating film is microscopic. The structure is generally a compact pile of needles.

General process of rust-proof phosphating:

Degreasing and descaling - water cleaning - surface conditioning activation - phosphating - water cleaning - chromate treatment - drying - grease or dye treatment

The workpiece treated by the strong alkali strong acid will cause the phosphating film to coarsen, and the surface adjustment activation can refine the crystal grains. Zinc phosphating can be adjusted by oxalic acid and colloidal titanium. Manganese phosphating can be activated by insoluble manganese phosphate suspension. Iron phosphating generally does not require adjustment of the activation process. The phosphating workpiece can be greatly improved in rust resistance by chromate sealing. For example, after varnishing or dyeing treatment, the rust resistance can be improved by several or even several times, as shown in Table 1.

Table 1 Effect of Phosphating Film and Oil Coating on Corrosion Resistance

Material rust time (h) (salt mist ASTM B117-64)

Bare steel 0.5

Steel + oil 15.0

Steel +16g/m2 zinc phosphating 4.0

Steel + zinc phosphating + oiling 550.0

The phosphating treatment refers to a chemical treatment method in which a metal surface is brought into contact with an acidic solution containing dihydrogen phosphate to form a chemically reactive surface to form a stable insoluble inorganic compound film layer on the metal surface. The film formed is referred to as a phosphate film. Its film formation mechanism is: (taking zinc as an example)

a) metal dissolution process

When the metal is immersed in the phosphating solution, it first reacts with phosphoric acid in the phosphating solution to form a generation of iron phosphate, and a large amount of hydrogen is precipitated. Its chemical reaction is;

Fe+2H3PO4=Fe (H2PO4)2+H2↑ (1)

The above formula shows that at the beginning of phosphating, only the metal dissolves, and no film is formed.

b) acceleration of the accelerator

The hydrogen released by the previous reaction is adsorbed on the surface of the metal workpiece, thereby preventing the formation of the phosphate film. Therefore, an oxidizing accelerator is added to remove hydrogen. Its chemical reaction formula is:

3Zn(H2PO4)2+Fe+2NaNO2=Zn3(PO4)2+2FePO4+N2↑+2NaH2PO4+4H2O (2)

The above formula is the mechanism of action of sodium nitrite as a promoter.

c) hydrolysis reaction and tertiary dissociation of phosphoric acid

The basic component in the phosphating bath is an acid phosphate of one or more heavy metals, and its molecular formula Me(H2PO4)2, which is soluble in water, undergoes a hydrolyzed overtone method at a certain concentration and pH value. Free phosphoric acid:

Me(H2PO4)2=MeHPO4+H3PO4 ( 3 )

3MeHPO4=Me3(PO4)2+H3PO4 ( 4 )

H3PO3=H2PO4-+H+=HPO42-+2H+=PO43-+3H+ ( 5 )

As the hydrogen ion concentration on the surface of the metal workpiece drops sharply, the dissociation equilibrium of the phosphate groups shifts to the right and eventually becomes phosphate.

d) Formation of phosphate film

When the trivalent phosphate dissociated from the metal surface and the metal ions (such as zinc ions, calcium ions, manganese ions, and ferrous ions) in the phosphating bath (such as zinc ions, calcium ions, manganese ions, and ferrous ions) are saturated, the crystal is deposited on the metal workpiece. On the surface, the grains continue to grow until a continuous, water-insoluble, firm, phosphating film is formed on the surface of the metal workpiece.

2Zn2++Fe2++2PO43-+4H2O→Zn2Fe (PO4)2.4H2O↓ ( 6 )

3Zn2++2PO42-+4H2O=Zn3 (PO4)2.4H2O ↓ ( 7 )

A part of the ferrous ions dissolved in the metal workpiece is consumed as a component of the phosphating film, and the ferrous ions remaining in the phosphating bath are oxidized to ferric ions, and the chemistry of the formula (2) occurs. In response, the formed phosphating sediment is mainly composed of ferrous phosphate and a small amount of Me3(PO4)2.

There are several classification methods for phosphating:

1 According to the phosphate classification of the phosphating solution

There are zinc phosphate, manganese phosphate, and iron phosphate. In addition, there is a zinc-calcium system in which calcium is added to zinc phosphate, and a "ternary system" phosphating of nickel and manganese is added to zinc phosphate.

2 Classification according to the temperature of phosphating

There are high temperature (above 80 degrees) phosphating, medium temperature (50 to 70 degrees) phosphating and low temperature phosphating (below 40 degrees).

3 Classification by phosphating construction method

There are spray phosphating, impregnating phosphating, spray immersion combined phosphating, and brushing phosphating.

4 Classification by quality of phosphating film

There are weight type (7.5g/m2 or more), medium type (4.3~7.5g/m2), lightweight type (1.1~4.3g/m2) and extra-lightweight type (0.3~1.1g/m2).

The iron salt phosphating film is the thinnest, and its film weight is (0.3~1.1) g/m2, which is lightweight. Zinc salt phosphating depends on the formula and can be classified into light, medium or heavy phosphating films. The membrane weight ranges from (1.0 to 5.0) g/m2. The principle of phosphating film formation can be explained by the theory of supersaturation. That is, when the ion product constituting the phosphate film reaches the solubility product of the insoluble phosphate, a phosphate film is deposited on the surface of the metal. The main component of the phosphating material is acid phosphate, which has a molecular formula of Me(H2PO4)2. The metal ion Me is usually zinc, manganese, iron or the like. These acid phosphates are all soluble in water. In an acidic phosphating solution containing an oxidizing agent and various additives, the dihydrogen phosphate salt is dissociated to produce a metal ion Me and a phosphate ion, but at this time, the ion product does not reach the solubility product of the insoluble phosphate, and no film is produced. Deposition:

Me(H2PO4)2→Me2++H2PO4-→HPO4-+H+→PO4-+H+

The metal dissolution reaction occurs when the phosphating solution is brought into contact with the surface of the metal to be treated at an appropriate temperature.

Fe+2H+→Fe2++H2↑

Due to the above reaction, H+ is continuously consumed at the interface between iron and phosphating solution, causing the pH to rise, which in turn promotes the three-step dissociation reaction. Then, the concentration of Me2+ and PO43- at the interface is continuously increased until [Me2+][PO43-]>Lme3(PO4)2, and deposition of Me3(PO4)2 insoluble phosphate is formed to cover the metal surface to form a phosphate film.

However, the hydrogen generated by the above formula adsorbs on the surface of the metal, causing so-called cathodic polarization, so that the phosphating reaction is understood to be hindered. Therefore, a certain amount of oxidant is added as a cathode depolarizer to ensure that the phosphating reaction is completed within a prescribed time. Hydrogen is oxidized to water by an oxidant to remove it. The Fe2+ generation part is involved in film formation to form Zn2Fe(PO4)2.4H2O, and the remaining part is oxidized to Fe3+. Fe3+ and PO43- combine to form FePO4 with a small concentration, which is precipitated and excluded from the system.

2 Phosphating film quality assessment project and method

1 Appearance visual method

The appearance of a good phosphating film is uniform and complete, no metal highlights, no white ash. The zinc-based phosphating film is a gray film, and the iron-based phosphating is an iridescent film. Aluminum and aluminum alloys are colorless or colored aluminum films.

2 Microstructural microscopy

The phosphating film is magnified to 100 to 1000 times by a metallographic microscope or an electron microscope, and the crystal shape, size, and arrangement are observed. The crystal shape is preferably a columnar crystal. The crystal size is preferably small, generally controlled to be several tens of micrometers or less, and the more uniform the row portion, the smaller the porosity is, the better.

3 thickness (or gravimetric method) determination

The phosphating film method for the steel plate is to immerse the phosphating plate at 75 degrees, the mass fraction is removed in the chromic acid solution specified by the national standard (10~15) min to remove the phosphating film, and then the film with the difference in weight before and after the film layer is removed. weight.

3 Corrosion performance measurement method

The most commonly used is the copper sulfate drip test. Now, after the next process, the salt spray test, the heat resistance test or the cycle test are carried out according to the user's requirements.

4 impact test

It is often measured together after painting. When the coated phosphating plate is subjected to an impact test with 49 N/cm, the phosphating film can be determined when no radioactive crack occurs at the impact point of the back surface of the impacted sample. The quality is better.

5 secondary adhesion determination

The adhesion measured after the phosphating film was applied was a single adhesion. The adhesion measured after the temperature-resistant water test under certain conditions is called secondary adhesion. Generally, the adhesion method is determined by the cross-hatch method on the sample plate after the water resistance test, and the peeling degree of the coating film is observed after the tape is peeled off, and generally the parallel comparison experiment is performed.

6 Determination of porosity of phosphating film

A solution of 14% NaCL and 3% potassium ferricyanide solution, 0.1% by mass of a surfactant in distilled water, was stored in a brown bottle for 24 hours and filtered through a filter paper. When using, cut the filter paper into paper pieces with a width of 2.5 cm. Dip the paper into the above solution with plastic tweezers, and then drop the excess test solution and cover it with the surface of the phosphating film. After 1 minute), the test paper was removed, and the surface of the film layer was observed, and a blue spot indicates a void portion.

7 alkali resistance of phosphating film

Comparing the phosphating film in a immersion lye solution of 0.1 mol/L sodium hydroxide at 25 degrees for 5 minutes before and after, the amount of phosphating film dissolved in the lye can be obtained.

8 Phosphate film acid resistance

The acid resistance of the phosphate film was evaluated by comparing the amount of dissolution of the phosphate film in an acid solution having a pH of 2.

9 phosphating film P ratio

The P ratio is originally defined as P/(P+H), where P is dizinc-iron phosphate and H is zinc phosphate, so the P ratio indicates the ratio of di-zinc-iron phosphate in the phosphate film. The phosphating film with a higher P ratio is less likely to lose water and is less likely to rehydrate, and its corrosion resistance is better than that of a phosphating film having a lower P ratio.

3 Factors affecting phosphation

There are many factors affecting phosphating. When the quality of the phosphating film is problematic, it can be considered from the aspects of phosphating process parameters, accelerators, phosphating processes (including equipment) and the surface of the steel to be treated.

Effect of a phosphating process parameter

1 Total acidity: Total acidity is too low, phosphating must be affected, because total acidity is an indicator reflecting the concentration of phosphating solution. The significance of controlling the total acidity is to keep the filming ion concentration in the phosphating solution within the necessary range.

2 Free acidity: If the free acidity is too high or too low, it will have adverse effects. If it is too high, it will not form a film, and it will be prone to yellow rust; the stability of too low phosphating solution will be threatened, resulting in additional residue. The free acidity reflects the content of free H+ in the phosphating solution. The significance of controlling the free acidity is to control the degree of dissociation of dihydrogen phosphate in the phosphating solution, and to control the concentration of the filming ions in a necessary range. During the use of phosphating solution, the free acidity will increase slowly. At this time, the alkali should be used for neutralization and adjustment. Pay attention to slowly adding and stirring thoroughly. Otherwise, the excessive concentration of the alkali solution will produce unnecessary residue. Alkali, the higher the free acidity. It is of no practical significance to look at free acidity and total acidity, and must be considered together.

3 acid ratio: acid ratio refers to the ratio of total acidity to free acidity. Generally speaking, the acid ratio is in the range of 5 to 30. The formula with relatively small acid has high free acidity, slow film formation speed, long phosphating time and high temperature. The acid is relatively large in formulation, the film formation speed is fast, the phosphating time is short, and the required temperature is low. Therefore, the acid ratio must be controlled.

4 Temperature: The phosphating temperature is the same as the acid ratio and is also a key factor in film formation. Different formulations have different temperature ranges, and in fact, he controls the concentration of filming ions in the phosphating solution. The temperature is high, the dissociation degree of dihydrogen phosphate is large, and the concentration of film-forming ions is correspondingly higher. Therefore, the relationship can be improved by lowering the temperature while increasing the acid ratio, and the film formation can also be achieved. The relationship is as follows:

70°C 60°C 50°C 40°C 30°C 20°C

1/5 1/7 1/10 1/15 1/20 1/25

After the production unit has determined a certain formula, the temperature should be strictly controlled. If the temperature is too high, a large amount of sediment will be produced, and the phosphating solution will lose its original balance. When the temperature is too low, the concentration of the filming ions does not reach the concentration product, and a complete phosphating film cannot be formed. When the temperature is too high, the dissociation degree of soluble phosphate in the phosphating solution is increased, the ion concentration of the film is greatly increased, unnecessary sediment is generated, the active component in the phosphating solution is wasted, and the original balance is forced to be damaged. A new temperature equilibrium is formed. For example, when the temperature of the low temperature phosphating solution rises out of control, the dissociation reaction of H2PO4→H++PO43- proceeds to the right, thereby increasing the phosphate concentration and producing zinc phosphate precipitation. The acid ratio of the phosphating solution is automatically increased. When the phosphating solution returns to its original temperature, the original balance cannot be recovered. Therefore, in practice, when the phosphating solution exceeds a certain temperature and then lowers to the original temperature, if it is not adjusted, it may not be phosphatized. From the point of reducing sediment, stabilizing the bath, and ensuring the quality, the temperature change of the phosphating solution is as small as possible.

5 Time: Each recipe has a defined process time. The time is too short, the film formation amount is insufficient, and a dense phosphate film layer cannot be formed. The time is too long, and as the crystallization continues to grow on the formed film, a thick film having a loose surface may be produced.

Second, the impact of accelerators

Accelerators are an essential ingredient, and without them, phosphating will lose its meaning. Promoters in phosphating solutions, mainly referred to as certain oxidizing agents. The oxidant is a chemically reactive accelerator used as a cathodic depolarizer in a phosphating formulation. His main role is to accelerate the discharge rate of hydrogen ions at the cathode, and to accelerate the acid etching rate in the first stage of phosphating, so it can be called a catalyst for metal corrosion. When the metal surface is exposed to the phosphating solution, the following reaction occurs first:

Fe+2H+→Fe2++H2↑

This reaction can consume a large amount of hydrogen ions, promote the pH of the solid-liquid interface, and then promote the three-stage dissociation equilibrium of the dihydrogen phosphate in the phosphating solution to the right, so that the zinc ion concentration and the phosphate concentration are dissolved at the interface. The film is formed into a film. If no effective substance is added, the retention of hydrogen evolved from the cathode causes the cathode to be polarized, so that the reaction cannot proceed, and the deposition of the phosphate film cannot continue. Therefore, any substance that accelerates this reaction will accelerate phosphating. The oxidant acts as a cathodic depolarization to accelerate the reaction.

Commonly used oxidants are nitrates, nitrites, hydrogen peroxide, bromates, iodates, molybdates, organic nitro compounds, organic peroxides, and the like. The most commonly used is mainly nitrate


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