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Analysis of phosphating technology of normal temperature phosphating solution
In this paper, the status quo and development trend of phosphating phosphating skills at room temperature are summarized from the aspects of phosphating membrane formation process, composition of normal temperature phosphating solution and selection of propellant. The normal temperature phosphating solution was the most active and fastest-developing phosphating product at that time. It defeated the high energy consumption of high and medium temperature phosphating, high cost and low power, and had low energy consumption, low cost and low pollution. The characteristics such as rapid speed have been widely recognized, and many related reports have been reported in recent years.
1 Phosphating mechanism The phosphating reaction is a typical and somewhat opposite, and is essentially an electrochemical response. When the metal is immersed in the phosphating solution, many micro-corrosive cells are formed on the outside, fine etching occurs, and the acid concentration decreases at the solution metal interface.
At normal temperature, the oxidation of the metal is weak, and a small amount of oxidant must be added as a phosphating reaction propellant, so that the mixed electric appearance of the metal surface is located at the electrode potential of the oxidizing propellant.
2 Phosphating process and composition of phosphating film The phosphating process was studied by two methods: corrosion voltage and current. It was pointed out that phosphating can be divided into seven stages: corrosion from the beginning of the substrate, film formation and over-corrosion. Scanning electron microscopy carefully investigated the progress of phosphating film formation at room temperature: when the steel sample was immersed in the phosphating solution, the test piece formed a rainbow color film (which can be a mixture of Fe2O3 and FeO), and then some active sites formed a crystal nucleus. With the extension of the soaking time and the growth of the nucleus, this growing position seems to have little contact with the original arrangement of the steel. Phosphating crystallization of normal temperature phosphating solution at normal temperature is slow. EDX analysis found that the elements in the microcrystal grains are Zn, Fe, and P, and the elements in the film that are far from the grain are Fe and P. The crystal form, film composition and content of the normal temperature phosphating solution were analyzed by X-ray and electron probe, and the imagination of improving the skill was proposed. The above work explored the phosphating process in a different way, reminding the connection between the composition and function of the phosphate film.
3 composition of normal temperature phosphating solution
3.1 Fundamental film-forming substances
The film-forming substance contains metal ions (such as Zn2+, Fe2+, etc.), phosphate ions, and a certain concentration of free phosphoric acid. Usually Zn2+ is 1.5 to 10 g/L (normally, iron ions are not directly added to the phosphating solution, and the immersion concentration is high.
The pH is 2.5 to 3.5, and the acid ratio (total acid/acid) is 20 to 50.
Phosphating method total acidity / point temperature / °C
Free acid / point to acid ratio (total acid / acid)
pH value
Impregnated phosphating
25~30
<20
0.5
>50
>3.5
25~30
0.5 to 1
30~40
3.1 to 3.4
>30
1 to 1.5
20~30
2.8~3.0
Spray phosphating
15~20
30~40
0.1 to 0.5
50～80
3.5 to 3.9
3.2 Phosphating propellant
3.2.1 Oxidation propellant
(1) NO-3 is also a commonly used oxidant, and can be directly involved in the normal temperature phosphating solution, the content is about 0.8 ~ 10g / L. The higher the ratio of NO-3/PO3-4, the faster the phosphating film of normal temperature phosphating solution is formed, but too high will cause the film to yellow, and the single use of NO-3 will make the phosphating film of normal temperature phosphating solution coarse.
(2) NO-2 is the most commonly used propellant and is often used in conjunction with NO3. However, NO-2 is not stable and easy to differentiate. The phosphating solution using NO-2 as propellant is double-packed, quantitatively mixed during use, and added regularly. The NO-2 content should be strictly controlled at 0.1 to 1.0 g/L. The content is small and the propulsion is weak.
(3) ClO-3 is usually only used for zinc phosphating. ClO-3 is relatively stable and does not need to be added frequently. The concentration is 0.5% to 1.0%. However, the corrosive nature of Cl-Y causes the appearance of "white spots" on the phosphate film, so it is not used alone.
3.2.2 Metal ion propellant
Adding a metal salt (usually a nitrate) to the phosphating solution, such as a metal salt with a positive potential such as Cu, Ni, Mn, Ca, Co, is favorable for crystal nucleation and grain refinement, and is advantageous for accelerating normal temperature phosphating. The process of liquid phosphating.
(1) Cu2+ A very small amount of copper salt will greatly improve the phosphating speed. When the working solution contains Cu2+ from 0.002% to 0.004%, the phosphating speed is improved by more than 6 times, but the increase of copper must be moderate. Otherwise, the copper film will replace the phosphate film and its function will decrease.
(2) Ni2+ is the most useful and most commonly used phosphating propellant. It not only accelerates phosphating, refines crystallization, but also improves the corrosion resistance of the film.
The Ni2+ content should not be too low, otherwise the film layer is thin; unlike the Cu salt, many of the Ni salts have no adverse effects, but will increase the cost. The Ni2+ content is usually controlled to be 0.7 to 5.0 g/L.
(3) Ca2+ grows according to the growth of the phosphating crystal, and it is made into a short rod shape and a granular shape, and the crystal grains are refined. However, it requires a higher temperature and is generally not suitable for normal temperature phosphating solutions.
3.3 Phosphating Auxiliaries
Such substances are mainly used to stabilize phosphating solution, refine crystallization, and improve membrane function. NaF can be used as a buffer to stabilize the pH of the phosphating solution over a certain period of time (eg 3 ± 0.5). A small number of external active agents, such as OP emulsifier and sodium dodecyl sulfonate, can improve the crystallization rate of the phosphating solution to the workpiece. Octadecylamine (ODA) can make the phosphate film thinner, with enhanced adhesion and improved corrosion resistance.
4 normal temperature phosphating solution phosphating skills
(1) Pre-treatment before phosphating Before dephosphorization, degreasing and derusting must be carried out. Phosphating membrane function is related to pre-treatment quality. Many common faults in phosphating are caused by poor pre-treatment. 
(2) Appearance adjustment. The principle and function of appearance adjustment are analyzed in detail. The surface conditioners were classified more specifically. The surface modifier suitable for phosphating of normal temperature phosphating solution is colloidal titanium phosphate, and its manufacture has been reported more specifically.
5 Trends in phosphating skills of normal temperature phosphating solution
From the coarse crystal thick film to the microcrystalline film, a phosphating film with excellent and uniform corrosion resistance is obtained. In place of the NO2 system, in order to overcome the double package of the NO-2 system, the development of a new propellant that can replace the NO-2 and can be mixed with the concentrate has made everyone's hobby. There are many researches in this area, such as the use of hydroxylamine sulfate (HAS).
[Article source: Hefei Xianghe Phosphate Coating Co., Ltd. http:// 】
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