Phosphating of conversion coating on the hottest m

2022-08-21
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Conversion film on metal surface phosphating

after oil and rust removal, the metal surface is usually treated chemically to prevent re rusting, so that a protective film is formed on the metal surface, which is usually only a few microns, mainly to enhance the adhesion between the coating and the substrate, and a thicker film can also enhance the rust prevention performance. The commonly used surface chemical conversion methods include oxidation, phosphating and passivation. Among them, phosphating is the central link of chemical treatment. It is a simple, reliable, low-cost and simple process method to greatly improve the corrosion resistance of metal workpieces. It is widely used in industry

1. Standards related to phosphating process

metals (mainly steel) are treated with solutions containing phosphates such as zinc (Zn), manganese (MN), chromium (CR) and iron (FE), forming an insoluble phosphate film on the surface of the base metal. This process is called phosphating. Phosphating makes the metal surface form a protective film with good adhesion. Taking zinc phosphate as an example, in the presence of oxidant, the generated phosphating film is the crystal of zn3 (PO4) 2 · 4h20 and zn2fe (PO4) 2 · 4h20. The phosphating film is bright, gray and porous (the porosity is 0.5%~1.5% of the surface area), and the film thickness is usually 0.1-50 μ m。

as for phosphating process, there are corresponding standard systems in China and internationally, It can be implemented by reference:

GB/t11376-1997 phosphate conversion coating of metals

GB/t6807-2001 technical conditions for phosphating treatment of steel workpieces before painting

GB/t12612-1990 general technical conditions for multifunctional steel surface treatment fluid

ISO 9717-1990 (E) phosphate conversion coating of metals - Method for determining requirements

iso10546-1993 (E) Chemical conversion coatings - rinsing and non rinsing chromate conversion coatings on aluminum and aluminum alloys

principle, abbreviated symbols and inspection methods of phosphating treatment of metals in DIN 50942-1973

ANSI/ASTM/AMS 2480c painting substrate phosphating treatment

2 can continuously narrow the gap with the world's advanced level and the role of phosphating

phosphate conversion coatings can be applied to iron, aluminum, zinc, cadmium and their alloys, which can be used as the final finishing layer, It can also be used as the middle layer of other overburden layers, and its functions mainly include the following aspects

2.1 improve corrosion resistance

although the phosphating film is thin, because it is a non-metallic non-conductive isolation layer, it can turn the good conductor on the surface of the metal workpiece into a bad conductor, inhibit the formation of micro electricity on the surface of the metal workpiece, and then effectively prevent the corrosion of the film. Table 1 lists the effect of phosphating film on the corrosion resistance of metals. Table 1 corrosion resistance of samples of different coating protective steel soaked in salt water

the time when the protective film first appears corrosion in 3%nacl solution/h no coating

phosphating film

nickel plating

chromium plating

phosphating film plus paraffin

two layers of baking paint

phosphating film plus one layer of baking paint

long-term anticorrosive coating

phosphating film plus long-term anticorrosive coating 0.1

1

10 ~ 13

23 ~ 24

60

70

500h test without corrosion Corrosion

1000h test no corrosion

>2000h test no corrosion

2.2 improve the adhesion between the substrate and the coating or other organic finishing layers

the phosphating film and the metal workpiece are a closely integrated structure. There is no obvious boundary between them. The porous nature of phosphating film enables sealers and coatings to penetrate into these pores and closely combine with phosphating film, so as to improve the adhesion

2.3 provide a clean surface

phosphating film can only grow on the surface of metal workpiece without oil stain and rust layer. Therefore, metal workpiece after phosphating treatment can provide a clean, uniform, grease free and rust free surface

2.4 improve the cold working properties of materials, such as wire drawing, pipe drawing, extrusion, etc

2.5 improve the surface friction performance to promote its sliding

3. Classification of phosphating

there are many classification methods of phosphating treatment, and the following are commonly used in industry

3.1 according to the type of phosphating film

phosphating can be divided into six categories: zinc series, zinc calcium series, zinc manganese series, manganese series, iron series and amorphous iron series

see Table 2 for the characteristics of various phosphating films. Table-2 classification and characteristics of phosphating film

phosphating film category basic components of phosphating film iron matrix unit area film (g/m2) crystal type zinc series zn2fe (PO4) · 4h2ozn3 (PO4) 2 · 4h2o1 ~ 40 stereotyped crystal structure, dendritic, acicular, with many voids, zinc calcium series zn2ca (PO4) 2 · 4h2ozn2fe (PO4) 2 · 4h2ozn3 (PO4) 2 · 4h2o1 ~ 15 dense particles, sometimes with large acicular particles, Zinc manganese system with less voids zn2fe (PO4) 2 · 4h2ozn3 (PO4) 2 · 4H2O (Mn, Fe) 5h2 (PO4) 4 · 4h2o1 ~ 40 particle dendritic needle mixed crystal form, manganese system with less voids (Mn, Fe) 5h2 (PO4) 4 · 4h2omn3 (PO4) 2 · 3H2O acid iron phosphate manganese 1 ~ 40 dense particles, iron system with less voids fe5h2 (PO4) 4 · 4h2o5 ~ 20 particles, and amorphous iron system fe3h2 (PO4) 2 · 8h2ofe2o3fepo42 with more voids.5 ~ 1.5 thin film, The structure is amorphous phase plane distribution

3.2 according to the classification of phosphating film quality

in practical application, it can be divided into heavy weight, sub heavy weight, light weight and sub light weight according to the film quality per unit area (g/m2). See Table 4 for its functions. Generally, the thin film has good adhesion and good corrosion resistance. The film required for coating pretreatment is 0.5~7.5g/m2. Generally, the zinc phosphating film is controlled at 1~4.5g/m2, the iron phosphating film is controlled at 0.2~1g/m2, and the phosphating film is controlled at 1~3g/m2 when matched with cathodic electrophoresis or powder coating

table-3 relationship between phosphating film quality and use

Quality Classification film quality/(g/m2) main components of film use sub lightweight

lightweight

sub heavyweight

heavyweight 0.2 ~ 1

1.1 ~ 4.5

4.6 ~ 7.5

> 7.5 iron phosphate, calcium phosphate, etc.

zinc phosphate, etc.

zinc phosphate, etc.

zinc phosphate Manganese phosphate is used as the bottom layer for workpieces with large deformation

as the general bottom layer

for workpieces with basically no deformation

as the bottom layer

for rust prevention, not as the bottom layer

3.3 classification according to phosphating treatment temperature

(1) high temperature phosphating treatment temperature is 80 ~ 90 ℃ 5 Spring amplitude: 0 (5) 0mm;. The advantages are that the formula is simple, the phosphating speed is fast, and the corrosion resistance, hardness and heat resistance of the phosphating film are high. The disadvantages are high bath temperature, high energy consumption, large evaporation, large amount of sediment and high cost. The phosphating film formed is thick and rough. Generally, phosphating before coating is not used

(2) medium temperature phosphating phosphating treatment temperature is 60 ~ 75 ℃. The advantages are that the phosphating speed is fast, the phosphating crystal is fine, and the corrosion resistance is good, but the phosphating film is still thick, and the gloss of the coating film after coating is not good. It is generally applicable to the corrosion resistance protective layer and the bottom layer of spraying and painting, but it is not applicable to the bottom layer of electrophoresis and electrostatic powder spraying

(3) low temperature phosphating phosphating treatment temperature is 35 ~ 55 ℃. The power of low-temperature phosphating film formation mainly depends on the accelerator and other substances in the formula. The formed phosphating film is thin and dense, flat and smooth, stable bath solution, less sediment, low energy consumption, simple maintenance and low comprehensive cost. It is the main technology of coating primer treatment at home and abroad at present

(4) phosphating at room temperature phosphating process without heating at room temperature. The power of phosphating film formation completely depends on the accelerator composition in the formula. Saving energy and reducing equipment investment are new development trends, but phosphating speed is relatively invasive, which is not suitable for large quantities of products. Phosphating formula is complex, tank solution maintenance and adjustment is difficult, tank solution concentration is high, but the comprehensive cost is low, which is the development direction

3.4 according to the classification of phosphating treatment process

phosphating process mainly includes immersion method, spraying method and brushing method, and its functions and characteristics are shown in Table 4. Table-4 characteristics of various phosphating methods

characteristics

impregnation method

spraying method

coating thickness

purpose

adaptability

production scale

phosphating temperature

film layers of various thicknesses can be obtained

various uses

small and medium-sized workpieces of various shapes

small batch

various temperatures

medium and thin film can be obtained

coating bottom layer or corrosion prevention between processes

large workpiece

Large batch

medium and low temperature

can obtain medium and thin film

coating primer or inter process corrosion prevention

small and medium-sized workpieces

large batch

low temperature

(1) dip phosphating is suitable for dealing with workpieces with complex shapes, with less sediment and easy equipment maintenance. The disadvantages are long phosphating time, high treatment concentration and rough crystallization of the film

(2) spray phosphating is applicable to the treatment of plates with relatively simple geometry. Due to the combination of the impact force during spraying and the chemical action during phosphating, the phosphating speed of spraying is increased, the concentration is low, and the crystallization of the film is fine and uniform. The disadvantages are that the internal part of the workpiece is not easy to phosphatize, and it is also prone to corrosion. There is a lot of sediment sprayed, which leads to large equipment investment and difficult maintenance

(3) brushing phosphating is suitable for phosphating or local phosphating of large steel components. It can obtain medium and thin phosphating film, with less equipment investment and convenient phosphating. The disadvantage is that the phosphating film is not uniform enough and is greatly affected by human factors

other classification methods can also be divided into nitrate type, nitrite type, chlorate type, organic nitride type, molybdate type, etc. according to the type of phosphating accelerator; According to the classification of whether to wash after phosphating, it is divided into water washing phosphating solution and non water washing phosphating solution; According to the number of sediment in phosphating tank, it can be divided into multi slag phosphating and low slag phosphating; According to the classification of whether the accelerator is supplemented separately, it can be divided into the phosphating of containing promoting agent and the phosphating of promoting agent supplemented separately; Classify according to whether the phosphating solution contains nitrite and nickel salt, etc

4. Symbols of phosphating film

4.1 the sequence of symbols of phosphating film is composed of

① phosphating film type, which is represented by the abbreviations shown in Table 3

② indication symbol of phosphating film application

g - friction reduction; Under the changes of paste assembly and time flow

i - electrical insulation

r - corrosion resistance and enhanced adhesion of organic coatings or adhesives

z - cold deformation processing lubrication

③ the film mass per unit area, in g/m2, with an allowable error of 30%

④ symbol for post-treatment method of phosphating film

a - varnish, coating or other polymer materials

d - inorganic or organic salt sealing

e - staining

f - dip and apply oil and grease

s - saponification

w - apply wax

2. Examples of symbols of phosphating film

znph · R · 4 · a

znph - zinc phosphating film

r - used for corrosion resistance and enhancing the adhesion of organic coatings or adhesives

4 - the film mass per unit area is (4 soil 1.2 to avoid export risk) g/m2

a - varnish paint or other organic paint used for post-treatment

in practical application, the four parts of the symbol are not necessarily written out. For example, for the workpiece that does not need post-treatment, the symbol can only be represented by the first three parts. If the demander does not give the film quality per unit area, table 3 or other corresponding values can be selected

5. Development of phosphating technology

the research of phosphating technology in China started relatively late. After the mid-1980s, with the rapid development of automobile, refrigerator, washing machine and other household appliance industries and the acceleration of technology introduction, phosphating technology has gradually been valued, promoted and applied. Since the mid-1980s, Japan, Germany, the United States and other industrial developed countries have begun to set up professional companies of phosphating technology in China, which has been further expanded in recent years. Domestic research institutes and enterprises engaged in phosphating technology include:

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