A chromium-free passivation solution for zinc surface and application thereof

Abstract

The present application relates to a kind of zinc surface chromium-free passivation solution and its application, the pH value of the zinc surface chromium-free passivation solution is 8±0.5, it at least dissolves the following substances: (i) transition metal acid or transition metal salt, (ii) hydrogen fluoride or fluoridated salt, (iii) hydrogen peroxide or metal peroxide.The transition metal acid or transition metal salt used in the zinc surface chromium-free passivation solution is reduced into dense transition metal oxide on the surface of zinc oxide itself, zinc surface is oxidized due to the double oxidation of transition metal acid radical and peroxide, forms zinc oxide, and part of divalent zinc is combined into zinc fluoride with fluoride ion, finally forms the passivation film of three substances coexisting of transition metal oxide, zinc oxide, zinc fluoride, and the salt spray corrosion resistance of zinc surface after passivation exceeds 168 hours.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal surface treatment, in particular to a zinc surface chromium-free passivation solution and application thereof. BACKGROUND

[0002] 08Al cold rolled steel sheet (CRS) is a commonly used commercial body panel, and the metal consumption is large, but the corrosion resistance is poor. It usually needs several surface treatment steps, such as shot blasting (or pickling), degreasing, phosphating and cathodic electrophoresis (or powder spraying), which has problems of complex process, high cost, large energy consumption, environmental pollution and the like.

[0003] Galvanizing is the most basic and widely used corrosion protection method for steel surface, which is widely used in various fields of national economy, such as metallurgy, building materials, electric power, transportation and agriculture. Galvanizing includes cold zinc spraying and hot galvanizing, and hot galvanizing method is commonly used for steel surface corrosion protection. It is to make zinc atoms and iron atoms form a eutectic phase structure on the surface of the metal matrix by physical and chemical changes under certain temperature, pressure and other external conditions, so as to make the material have special properties of electric conductivity, thermal conductivity, super hardness, wear resistance, corrosion resistance and other properties. Due to the limitation of external conditions such as temperature and pressure, the composition and structure of the coating will be affected to a certain extent. Although hot galvanizing can make the coating thickness reach several hundred microns, the surface roughness of the coating is high, and the zinc tumor and clumping phenomenon are common, which reduces the corrosion resistance. In addition, the hot galvanizing process has large energy consumption, long process and serious pollution, resulting in high cost in all aspects. In order to further improve the corrosion resistance of the galvanized layer, post-treatment is generally needed, such as chromate passivation treatment. The chromate passivation is uniform, dense and has strong corrosion resistance, and can realize the oxidation corrosion of zinc. However, the chromium-free passivation solution still contains carcinogenic hexavalent chromium, which is not an ideal chromium-free passivation solution for galvanizing post-treatment. Although the toxicity of trivalent chromium passivation film is only 1% of that of hexavalent chromium, and the excellent corrosion resistance of the passivation film is retained, a small amount of hexavalent chromium is often mixed in the trivalent chromium passivation film, and there is a possibility that trivalent chromium will be converted into hexavalent chromium during long-term use.

[0004] Chinese patent document CN113445039A discloses a zinc surface chromium-free passivation solution and a passivation method. The chromium-free passivation solution contains soluble silicate and soluble cerium (IV) salt. The soluble silicate is a film former, and the soluble cerium (IV) salt is an oxidizing film former. The pH value of the chromium-free passivation solution is 2.0-3.0. The chromium-free passivation solution is acidic and has a certain corrosion effect on the zinc surface, and the passivation film can only resist corrosion for 24 hours in a neutral salt spray. SUMMARY

[0005] In order to further improve the salt spray corrosion resistance of the zinc surface, the present application provides a zinc surface chromium-free passivation solution and application thereof.

[0006] The technical solution provided by the present application is as follows.

[0007] In the first aspect, the present application provides a zinc surface chromium-free passivation solution, whose pH value is 8±0.5, and at least the following substances are dissolved in the solution: (i) transition metal acid or transition metal acid salt, (ii) hydrogen fluoride or fluoride salt, and (iii) hydrogen peroxide or metal peroxide.

[0008] In the zinc surface chromium-free passivation solution provided by the present application, the transition metal acid or transition metal acid salt is both an oxidizing agent and a film forming agent. In the chromium-free passivation solution, the main role of the oxidizing agent is oxidation, which oxidizes the zinc to zinc oxide, and the role of the film forming agent is film forming. The transition metal acid or transition metal acid salt is oxidized to transition metal oxide film after oxidizing the 0-valent zinc to 2-valent zinc. The hydrogen fluoride or fluoride salt plays the role of a film forming agent in the chromium-free passivation solution, and F combines with the 2-valent zinc on the zinc surface to form a zinc fluoride film. The hydrogen peroxide or metal peroxide plays the role of an oxidizing agent in the chromium-free passivation solution, and can oxidize the 0-valent zinc on the zinc surface to form a zinc oxide film. Therefore, the zinc surface chromium-free passivation solution provided by the present application can form three kinds of films on the zinc surface, which are independent of each other and uniformly distributed on the surface of the zinc plating layer.

[0009] On the basis of the above technical solution, the transition metal acid is molybdic acid, and the transition metal acid salt is molybdate. Preferably, the molybdate is ammonium molybdate, and the pH value is 8.

[0010] On the basis of the above technical solution, in the zinc surface chromium-free passivation solution, the concentration of the transition metal acid or transition metal acid salt is 0.01-10 g / L, the concentration of the fluoride salt is 0.01-5 g / L, and the concentration of the hydrogen peroxide or metal peroxide is 0.01-10 g / L. Preferably, in the zinc surface chromium-free passivation solution, the concentration of the transition metal acid or transition metal acid salt is 4-6 g / L, the concentration of the fluoride salt is 1-3 g / L, and the concentration of the hydrogen peroxide or metal peroxide is 4-6 g / L.

[0011] In the second aspect, the present application provides a composition for preparing the above-mentioned zinc surface chromium-free passivation solution, which comprises: (i) transition metal acid or transition metal acid salt, (ii) hydrogen fluoride or fluoride salt, (iii) hydrogen peroxide or metal peroxide, and (iv) alkaline pH adjuster.

[0012] In the third aspect, the present application provides a zinc surface chromium-free passivation method, which comprises immersing the zinc surface into the above-mentioned zinc surface chromium-free passivation solution, keeping the temperature at 25-42℃, and reacting for 3-20 minutes. The zinc surface is a pure zinc surface or a zinc alloy surface.

[0013] On the basis of the above technical solution, the temperature is 35-40℃, and the reaction time is 5-10 minutes.

[0014] On the basis of the above technical solution, the zinc alloy surface is a GA galvanized surface or a GI galvanized surface.

[0015] In a fourth aspect, the present application provides a steel product surface treatment method, comprising:

[0016] galvanizing the steel product surface;

[0017] immersing the zinc surface into the above-mentioned zinc surface chromium-free passivation solution, keeping the temperature at 25-42℃, and reacting for 3-20 minutes.

[0018] On the basis of the above technical solution, the galvanizing the steel product surface comprises:

[0019] providing a galvanizing solution: the galvanizing solution comprises zinc salt, complexing agent, surfactant, pH adjuster;

[0020] galvanizing treatment: taking the steel product as the cathode and hot galvanized iron alloy plate as the anode, and performing constant voltage and constant current galvanizing under room temperature stirring condition.

[0021] On the basis of the above technical solution, the concentration of the zinc salt in the galvanizing solution is 3-10 g / L.

[0022] Compared with the prior art, the present application has at least the following beneficial effects:

[0023] The zinc surface chromium-free passivation solution provided by the present application does not need to be performed under acidic condition, avoiding corrosion of the zinc surface and the surface of the steel product not galvanized in the acidic environment, and forming a passivation film coexisting with transition metal oxide, zinc oxide and zinc fluoride on the zinc surface, so that the salt spray corrosion resistance of the treated steel product can reach more than 168 hours. DETAILED DESCRIPTION

[0024] To make the objectives, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0025] The zinc surface chromium-free passivation solution provided by the present application has a pH value of 8±0.5, and at least dissolves the following substances: (i) transition metal acid or transition metal acid salt, (ii) hydrogen fluoride or fluoride salt, and (iii) hydrogen peroxide or metal peroxide.

[0026] The present application utilizes the unstable state of transition metal (e.g. vanadium, chromium, manganese, cobalt, iron) in high valence in transition metal acid or transition metal acid salt, the property of transition metal acid or transition metal acid salt oxidizing 0-valence zinc while transforming itself into dense transition metal oxide film, and the specific transition metal is molybdenum, the transition metal acid salt is molybdate, and the water-soluble molybdate is used, such as ammonium, alkali metal, magnesium molybdate. The process of zinc surface passivation with chromium-free passivation solution is a solid-liquid heterogeneous reaction process, and the reaction only occurs at the interface of solid and liquid. The oxidant molecules and film-forming agent molecules in the chromium-free passivation solution need to migrate to the interface of the chromium-free passivation solution and the zinc plating layer to form a film. In the traditional combination of oxidant and film-forming agent, the oxidant molecules and film-forming agent molecules will form various complex components during the solid-liquid interface reaction process, hindering the oxidation of the oxidant and the film formation of the film-forming agent, thus easily leading to uneven film formation. The transition metal acid or transition metal acid salt as the oxidation and film-forming agent is reduced to low-valence transition metal oxide after oxidizing zinc at the interface of the chromium-free passivation solution and the zinc plating layer during molecular motion, and the transition metal oxide is in-situ film-formed at the interface of the chromium-free passivation solution and the zinc plating layer, which not only accelerates the passivation and film formation rate, but also effectively prevents the influence of other molecular motion of the chromium-free passivation solution in the oxidation and film formation series process, improving the completeness, uniformity and density of the passivation and film formation.

[0027] The hydrogen fluoride or fluoride salt in the zinc surface chromium-free passivation solution of the present application forms ZnF2 with 2-valence zinc, which is slightly soluble in water, insoluble in ethanol, and soluble in hot acid, hydrochloric acid, nitric acid and ammonium hydroxide, and has good salt spray resistance. The fluoride salt in the zinc surface chromium-free passivation solution of the present application uses water-soluble fluoride salt, such as ammonium or alkali metal fluoride salt.

[0028] The hydrogen peroxide or metal peroxide in the zinc surface chromium-free passivation solution of the present application can oxidize 0-valence zinc into a dense zinc oxide film. The metal peroxide can use sodium peroxide.

[0029] The pH value of the chromium-free passivation solution is controlled to be 8±0.5 in the present application; preferably, the pH value of the chromium-free passivation solution is controlled to be 8. The traditional chromium-free passivation solution utilizes oxidation and film-forming agent to oxidize and passivate the zinc surface, which needs to react in an acidic environment. However, experiments show that the effect is best when the pH value of the chromium-free passivation solution is 8. If the pH value of the chromium-free passivation solution is too low, i.e. pH≤7, the passivation film is rough, the adhesion is poor, and the corrosion resistance of the passivation film is reduced. If the pH value of the chromium-free passivation solution is too high, i.e. pH≥9, the stability of the chromium-free passivation solution is poor, some complex by-products are produced, and the uniformity and density of the passivation film are difficult to guarantee. Therefore, in order to obtain a dense passivation film, it is appropriate to control the pH value of the chromium-free passivation solution to be 8, so as to obtain a passivation film with appropriate thickness and density in the passivation process, and to meet the requirements of corrosion resistance.

[0030] In some possible embodiments, the concentration of the transition metal acid or transition metal acid salt in the zinc surface chromium-free passivation solution is 0.01-10 g / L, the concentration of the fluorinated salt is 0.01-5 g / L, and the concentration of the hydrogen peroxide or metal peroxide is 0.01-10 g / L. In order to improve the corrosion resistance of the chromium-free passivation film, the concentration of the transition metal acid or transition metal acid salt in the zinc surface chromium-free passivation solution is controlled to be ≥4 g / L. Preferably, the concentration of the transition metal acid or transition metal acid salt is 4-6 g / L, the concentration of the fluorinated salt is 1-3 g / L, and the concentration of the hydrogen peroxide or metal peroxide is 4-6 g / L. More preferably, the concentration of the transition metal acid or transition metal acid salt is 5-6 g / L, the concentration of the fluorinated salt is 2 g / L, the concentration of the hydrogen peroxide or metal peroxide is 5 g / L, and the concentration of the hydroxyl ion is 1-2 g / L.

[0031] In some possible embodiments, the pH value of the zinc surface chromium-free passivation solution is 8, and the solution contains ammonium molybdate, sodium fluoride and hydrogen peroxide. The concentration of the ammonium molybdate is 5-6 g / L, the concentration of the sodium fluoride is 2 g / L, and the concentration of the hydrogen peroxide is 5 g / L.

[0032] Based on the above zinc surface chromium-free passivation solution, the present application provides a composition for preparing the zinc surface chromium-free passivation solution, which comprises: (i) a transition metal acid or transition metal acid salt, (ii) a hydrogen fluoride or fluorinated salt, (iii) hydrogen peroxide or a metal peroxide, and (iv) an alkaline pH adjuster. The four components do not react with each other, the alkaline pH adjuster is soluble in water, and does not contain cations that are prone to form precipitates with fluorine ions and transition metal acid radicals. Preferably, the pH adjuster is sodium hydroxide.

[0033] In some possible embodiments, the transition metal acid or transition metal acid salt, the fluorinated salt, the hydrogen peroxide or the metal peroxide are independently packaged or mixed packaged after being weighed according to the proportion.

[0034] Based on the above zinc surface chromium-free passivation solution, the present application provides a zinc surface chromium-free passivation method, which comprises immersing the zinc surface into the above zinc surface chromium-free passivation solution, and keeping the temperature at 25-42°C for 3-20 minutes.

[0035] In some possible embodiments, the zinc alloy surface is a GA galvanized surface or a GI galvanized surface.

[0036] Based on the above zinc surface chromium-free passivation solution, the present application provides a steel product surface treatment method, which comprises:

[0037] Galvanizing the surface of the steel product;

[0038] Immersing the zinc surface into the above zinc surface chromium-free passivation solution, and keeping the temperature at 25-42°C for 3-20 minutes.

[0039] In some possible embodiments, the galvanizing of the surface of the steel product comprises:

[0040] Providing a galvanizing solution: the galvanizing solution comprises a zinc salt, a complexing agent, a surfactant, a pH adjuster;

[0041] Electro-galvanizing treatment: taking the steel product as the cathode and the GA plate as the anode, the constant voltage and constant current electro-galvanizing is carried out under the stirring condition at room temperature.

[0042] In some possible embodiments, the concentration of the zinc salt in the galvanizing solution is 3-10 g / L.

[0043] In some possible embodiments, the steel product is a vehicle steel plate, and the surface treatment method of the steel product comprises two parts of the electro-galvanizing treatment of the surface of the steel plate and the chromium-free passivation treatment of the galvanized surface, and the two parts are in a sequential relationship. First, the electro-galvanizing treatment is carried out on the surface of the steel plate, and then the chromium-free passivation treatment is carried out on the surface of the galvanized layer after the electro-galvanizing treatment is completed. Finally, the combined electro-galvanized layer of the intermediate layer of the galvanized layer and the outermost layer of the chromium-free passivation film is obtained on the surface of the steel plate. It should be emphasized that the second step of the chromium-free passivation treatment after the electro-galvanizing treatment of the steel product can also be used as an independent surface treatment process for the surface corrosion prevention treatment of the pure zinc workpiece, the GA galvanized workpiece or the GI galvanized workpiece. Unlike the existing chromium-free passivation solution, the chromium-free passivation solution of the present application does not need to add chromate as an oxidizing agent.

[0044] In some possible embodiments, the surface treatment method of the steel product provided by the present application comprises the following steps: the construction of the electro-galvanizing device, the preparation of the galvanizing solution, the oil removal, the electro-galvanizing and the chromium-free passivation, and the intermediate water washing step is omitted.

[0045] In some possible embodiments, in the galvanizing solution, the concentration of the zinc salt is 0.01-20 g / L, the concentration of the complexing agent is 0.01-5 g / L, the concentration of the surfactant is 0.01-0.5 g / L, the concentration of the pH adjuster is 0.01-30 g / L, and the conductivity of the deionized water is less than 5 μs / cm. Preferably, in the galvanizing solution, the concentration of the zinc salt is 3-10 g / L, the concentration of the complexing agent is 2-3 g / L, the concentration of the surfactant is 0.2-0.4 g / L, and the concentration of the pH adjuster is 10-20 g / L. More preferably, in the galvanizing solution, the concentration of the zinc salt is 5 g / L, the concentration of the complexing agent is 3 g / L, the concentration of the surfactant is 0.3 g / L, and the concentration of the pH adjuster is 15 g / L.

[0046] In some possible embodiments, the zinc salt comprises one or more of zinc nitrate, zinc sulfate, zinc chloride, zinc oxide, zinc acetate, zinc formate, the complexing agent comprises one or more of tartaric acid, citric acid, oxalic acid, formic acid, acetic acid, ethylenediaminetetraacetic acid, sodium tartrate, sodium citrate, sodium ethylenediaminetetraacetate, the surfactant comprises one or more of sodium dodecyl benzene sulfonate, sodium dodecyl sulfonate, OP-10, cetyl ammonium bromide, sorbitol, and the pH regulator comprises one or more of ammonia, sodium carbonate, sodium bicarbonate, sodium hydroxide, hydrochloric acid, nitric acid, sulfuric acid, formic acid, acetic acid, citric acid, tartaric acid, oxalic acid, and lactic acid.

[0047] Preferably, the pH regulator comprises an organic acid, and the surfactant is an anionic surfactant. More preferably, in the zinc plating solution, the zinc salt is zinc oxide, the organic complexing agent is tartaric acid, the surfactant is sodium dodecyl benzene sulfonate, and the organic acid is formic acid.

[0048] In some possible embodiments, the parameters of the constant-voltage and constant-current electroplating satisfy that the electric work is 0.4-12 J, preferably, the electric work is 2-10 J, and more preferably, the electric work is 4-5 J.

[0049] In some possible embodiments, the electric work is maintained at 4-5 J during the zinc plating on the surface of the steel product, and in the zinc plating solution, the zinc salt is zinc oxide with a concentration of 5 g / L, and the organic acid is formic acid with a concentration of 15 g / L.

[0050] In some possible embodiments, the steel product is an 08Al cold-rolled steel plate.

[0051] In some possible embodiments, the steel product and the anode are subjected to degreasing and surface conditioning before use, washed with water after degreasing, then subjected to surface conditioning, washed in the surface conditioning solution for 10-30 s, rinsed after being taken out, and finally subjected to electroplating. The degreasing solution comprises 0.01-20 g / L of a strong base, 0.01-40 g / L of a weak base, and 0.01-5 g / L of a surfactant, and the surfactant comprises one or more of sodium dodecyl benzene sulfonate, sodium dodecyl sulfonate, OP-10, cetyl ammonium bromide, and sorbitol, and the active component has a concentration of 0.5-5 g / L.

[0052] In some possible embodiments, the zinc electroplating process comprises the following steps: maintaining the liquid level of the zinc plating solution to be higher than the height of the surface to be treated in the zinc plating solution environment, magnetically stirring at room temperature with a rotation speed of 1-1000 rpm, setting the voltage and current parameters of the direct-current power supply, maintaining a constant voltage of 0.1-15 V and a constant current of 0.1-3 A, starting the electroplating reaction, and electroplating for 30-40 min.

[0053] The technical solutions of the present application are described in detail below through examples:

[0054] Embodiment

[0055] The present application takes 08Al cold-rolled steel plate as the treatment object, and provides a steel product surface treatment method, comprising the following steps:

[0056] (1) Construction of electroplating device:

[0057] 08Al cold-rolled steel plate with a size of 11 cm*7 cm*0.08 cm is used as the cathode, and the metal clamp (cathode end) connected by the direct current power supply wire is fixed on one side of a 1L glass beaker. The same size GA plate is used as the anode, and the metal clamp (anode end) connected by the direct current power supply wire is fixed. The above device is placed on a magnetic stirrer with heating function.

[0058] (2) Preparation of zinc plating solution:

[0059] The above glass beaker is added with 1g / L, 3g / L, 5g / L, 8g / L, and 10g / L of main salt zinc oxide and deionized water (conductivity less than 5us / cm), 3g / L of tartaric acid as a complexing agent, 0.3g / L of sodium dodecyl benzene sulfonate as a surfactant, and 15g / L of formic acid as a pH adjuster. The solution is magnetically stirred at room temperature until it is clear and transparent at a speed of 500 revolutions / min.

[0060] (3) Oil removal:

[0061] The surface of the 08Al cold-rolled steel plate (15 cm*7 cm*0.8 mm) that has been polished with 400 mesh sandpaper to remove rust is wiped clean with clean degreasing cotton and then placed in a degreasing solution. The degreasing solution comprises 10g / L of strong alkali, 20g / L of weak alkali, and 3g / L of surfactant. The degreasing solution is placed in a 3L beaker, and the sample is stirred at 50-60°C for 10 min. Then, the sample is rinsed with tap water for 30s and dried with a hair dryer. The oil-removed and dried sample is weighed on an electronic balance, and the mass is m0. The same operation is performed on the GA plate, and the oil-removed and dried GA plate is weighed, and the mass is m2.

[0062] (4) Electroplating:

[0063] The 08Al cold-rolled steel plate and GA plate after oil removal in step (3) are placed in a 1L cylindrical glass plating tank, kept relative and parallel with a distance of 85cm, the 08Al cold-rolled steel plate is fixed to the inner wall of the plating tank by a clamp connected to the cathode of a direct current power supply, and the GA plate is fixed to the inner wall of the plating tank by a clamp connected to the anode of the direct current power supply. The initial maximum voltage is set to 10V, the current value is set to any fixed value in the range of 0.01-5A in the constant current mode, the plating time is 30min, the power supply is turned off, the anode and cathode plates are washed with tap water for 30s, and then dried by hot air blowing, and the mass of the dried cathode plate is m1 and the mass of the dried anode plate is m3. The effective area of the electrode is S, unit: dm 2 , Mcp is the mass of the cathode coating per unit area, and Map is the thinning amount of the anode per unit area. The following formulas are used to express them:

[0064] Mcp=(m1-m0) / S (unit: g / dm 2 ) I

[0065] Map=(m2-m3) / S (unit: g / dm 2 ) II

[0066] W=U×I×t (unit: J) III

[0067] Among them, the effect of electric work on the mass of the coating per unit area under several main salt concentrations is studied, and the research results are as follows:

[0068] The main salt concentration is 1g / L: the electric work is 0.44J, the mass of the coating per unit area is 11.69g / m 2 ; the electric work is 1.75J, the mass of the coating per unit area is 22.08g / m 2 ; the electric work is 4.38J, the mass of the coating per unit area is 33.77g / m 2 ;

[0069] The main salt concentration is 3g / L: the electric work is 4.38J, the mass of the coating per unit area is 54.55g / m 2 ; the electric work is 6.72J, the mass of the coating per unit area is 77.92g / m 2 ; the electric work is 12.6J, the mass of the coating per unit area is 107.79g / m 2 ;

[0070] The main salt concentration is 5g / L: the electric work is 2.97J, the mass of the coating per unit area is 45.45g / m 2 ; the electric work is 4.19J, the mass of the coating per unit area is 74.03g / m 2 ; the electric work is 4.73J, the mass of the coating per unit area is 83.12g / m 2; the electric work is 5.28J, the coating mass per unit area is 98.70;

[0071] The main salt concentration is 8g / L: the electric work is 4.88J, the coating mass per unit area is 96.10g / m 2 ; the electric work is 5.18J, the coating mass per unit area is 102.60g / m 2 ; the electric work is 5.86J, the coating mass per unit area is 103.90g / m 2 ; the electric work is 8.75J, the coating mass per unit area is 128.57g / m 2 ;

[0072] The main salt concentration is 10g / L: the electric work is 2.54J, the coating mass per unit area is 74.03g / m 2 ; the electric work is 2.80J, the coating mass per unit area is 83.12g / m 2 ; the electric work is 4.48J, the coating mass per unit area is 107.79g / m2; the electric work is 9.92J, the coating mass per unit area is 155.84g / m 2 ; the electric work is 11.67J, the coating mass per unit area is 206.49g / m 2 .

[0073] Further, the embodiment makes a relevant research on the influence of the main salt concentration on the coating mass per unit area under the same electric work (4-5J), and the research results are as follows:

[0074] The main salt concentration is 1g / L, the coating mass per unit area is 30-35g / m 2 ;

[0075] The main salt concentration is 3g / L, the coating mass per unit area is 50-60g / m 2 ;

[0076] The main salt concentration is 5g / L, the coating mass per unit area is 70-90g / m 2 ;

[0077] The main salt concentration is 8g / L, the coating mass per unit area is 90-100g / m 2 ;

[0078] The main salt concentration is 10g / L, the coating mass per unit area is 100-120g / m 2 .

[0079] Further, the embodiment makes a relevant research on the influence of the main salt concentration, the organic acid formic acid concentration and the current density on the coating mass per unit area under the same electric work, and the research results are as follows

[0080] Main salt concentration is 5g / L, organic acid concentration 15g / L, current density 1.3A / dm2 2 Under the condition, electroplating 20min, thinning per unit area 0.55g / dm2 2

[0081] Main salt concentration is 5g / L, organic acid concentration 15g / L, current density 1.3A / dm2 2 Under the condition, electroplating 30min, thinning per unit area 0.79g / m2 2

[0082] Main salt concentration is 5g / L, organic acid concentration 15g / L, current density 1.3A / dm2 2 Under the condition, electroplating 35min, thinning per unit area 0.95g / m2 2

[0083] Main salt concentration is 5g / L, organic acid concentration 15g / L, current density 1.3A / dm2 2 Under the condition, electroplating 40min, thinning per unit area 1.06g / m2.

[0084] The above results show that there is a linear relationship between current density and the mass of the coating per unit area, the mass per unit area of the cathode increases with the increase of the current density, and the mass per unit area of the anode decreases with the increase of the current density; the use of organic acid instead of inorganic acid effectively reduces the accelerated corrosion of strong acid on the anode plate; the addition of complexing agent balances the metal cations in the plating solution, so that the concentration of metal ions is maintained at a stable level, ensuring the uniformity of the coating thickness; the addition of anionic surfactant keeps the surface of the cathode plated part wet and clean, and the active anion groups adsorbed on the surface also play a role in promoting the reduction of zinc ions to zinc metal, and at the same time, the adsorption of active groups on the surface of the plated part can also improve the adhesion between the coating and the substrate, ensuring the uniformity, integrity and smoothness of the coating surface.

[0085] (5) Chromium-free passivation

[0086] The 08Al cold-rolled steel plate and GA plate after electroplating zinc in step (4) are respectively placed in a chromium-free passivation solution containing 0.01-10g / L ammonium molybdate, 0.01-5g / L sodium fluoride, 0.01-10g / L hydrogen peroxide, 0.01-2g / L alkaline pH adjuster, the reaction temperature is 15-45℃, the stirring speed is 200-400r / min, the reaction time is 5-10min, and then the plated part is washed with tap water for 30s and dried.

[0087] (5.1) To examine the effect of reaction temperature on the salt spray corrosion resistance of the chromium-free passivation film on the surface of the GA plate:

[0088] ​​The reaction temperature was changed, and the reaction temperature included 15℃, 25℃, 30℃, 35℃, 40℃ and 45℃. The research results are shown in Table 1.

[0089] Table 1 Influence of reaction temperature on salt fog corrosion resistance of GA plate surface chromium-free passivation film

[0090]

[0091] The above results show that the reaction temperature of the zinc surface chromium-free passivation method based on the above chromium-free passivation solution can obtain a chromium-free passivation film with good salt fog corrosion resistance when the reaction temperature is 25-42℃, further, the reaction temperature is preferably 30-40℃, and further, the reaction temperature is preferably 35-40℃.

[0092] (5.2) Influence of reaction time on salt fog corrosion resistance of GA plate surface chromium-free passivation film:

[0093] The reaction temperature was changed, and the reaction temperature included 15℃, 25℃, 30℃, 35℃, 40℃ and 45℃. The research results are shown in Table 1.

[0094] Table 2 Influence of reaction time on salt fog corrosion resistance of GA plate surface chromium-free passivation film

[0095] Reaction time Salt spray corrosion resistance 3 Red rust appeared at 120h 5 Red rust appeared at 168h 10 Red rust appeared at 168h 15 Red rust appeared at 120h 20 Red rust appeared at 120h

[0096] The above results show that the reaction time of the zinc surface chromium-free passivation method based on the above chromium-free passivation solution can obtain a chromium-free passivation film with good salt fog corrosion resistance when the reaction time is 3-20min, further, the reaction time is preferably 5-10min.

[0097] (5.3) Influence of ammonium molybdate concentration and chromium-free passivation solution aging time on salt fog corrosion resistance of GA plate surface chromium-free passivation film:

[0098] The aging time was 5 days, 6 days and 1 day. The research results are shown in Table 3.

[0099] Table 3 Influence of ammonium molybdate concentration on salt fog corrosion resistance of GA plate surface chromium-free passivation film

[0100]

[0101]

[0102] As shown in Table 3, the concentration of ammonium molybdate in the newly prepared chromium-free passivation solution is 5 g / L, and the salt spray corrosion resistance of the chromium-free passivation film obtained by passivating the GA plate immediately is 168 h; the concentration of ammonium molybdate in the newly prepared chromium-free passivation solution is reduced to 4 g / L after being placed for 5 days, and the salt spray corrosion resistance of the chromium-free passivation film obtained by passivating the GA plate at this time is 120 h; the concentration of ammonium molybdate in the chromium-free passivation solution placed for 5 days is increased to 6 g / L by adding ammonium molybdate at 2 g / L, and the salt spray corrosion resistance of the chromium-free passivation film obtained by passivating the GA plate at this time is 192 h; the chromium-free passivation solution placed for 6 days at room temperature continues to be used, and the concentration of ammonium molybdate ions is reduced to 4-5 g / L, and the salt spray corrosion resistance of the chromium-free passivation film obtained by passivating the GA plate at this time is 144 h; the concentration of ammonium molybdate in the chromium-free passivation solution placed for 5 days is increased to 5 g / L by adding ammonium molybdate at 1 g / L, and the salt spray corrosion resistance of the chromium-free passivation film obtained by passivating the GA plate at this time is 168 h; the concentration of ammonium molybdate in the chromium-free passivation solution placed for 5 days is increased to 6 g / L by adding ammonium molybdate again at 1 g / L, and the salt spray corrosion resistance of the chromium-free passivation film obtained by passivating the GA plate at this time is 192 h.

[0103] The above results show that the salt spray corrosion resistance time of the chromium-free passivation film obtained by passivating the GA plate with the newly prepared chromium-free passivation solution is higher than that of the aged chromium-free passivation solution; as the concentration of ammonium molybdate increases, the aged chromium-free passivation solution recovers its activity and becomes a fresh chromium-free passivation solution, and the corrosion resistance of the newly prepared chromium-free passivation film layer is significantly improved compared to that of the chromium-free passivation film layer prepared by the chromium-free passivation solution placed for several days, and the salt spray corrosion resistance of the chromium-free passivation film on the surface of the GA plate is enhanced as the concentration of ammonium molybdate in the newly prepared chromium-free passivation solution increases. Therefore, using the newly prepared chromium-free passivation solution can ensure appropriate cost and good salt spray corrosion resistance. When the chromium-free passivation solution is aged for several days, the concentration of ammonium molybdate decreases, the content of effective components of the chromium-free passivation solution decreases, and the activity decreases, so a certain amount of ammonium molybdate needs to be added to adjust the chromium-free passivation solution to the ideal state.

[0104] According to the test results, the salt spray resistance time of the chromium-free passivation film obtained by using the chromium-free passivation solution aged for 5 days is reduced to 120 h compared to the newly prepared chromium-free passivation solution, and the salt spray corrosion resistance is reduced by 28.6%; if 40% ammonium molybdate is added, the salt spray corrosion time is increased to 192 h, and the corrosion resistance is improved by 60% compared to before the addition of ammonium molybdate; if 20% ammonium molybdate is added to the chromium-free passivation solution aged for 1 day, the corrosion resistance can be improved by 14% compared to before the addition of ammonium molybdate.

[0105] (5.4) The influence of the pH value of the chromium-free passivation solution on the color and adhesion of the chromium-free passivation film was investigated.

[0106] The reaction temperature is 35℃, the reaction time is 5 minutes, the concentration of ammonium molybdate is 5g / L, the concentration of sodium fluoride is 2g / L, the concentration of hydrogen peroxide is 5g / L, and the pH value is changed, including 6, 8 and 9. The research results are shown in Table 4.

[0107] Table 4 Influence of pH value on the color and adhesion of chromium-free passivation film

[0108]

[0109] The above results show that when the pH value of the chromium-free passivation solution is 8, the color and adhesion of the chromium-free passivation film are the best.

[0110] Through the above examples of the treatment conditions of the chromium-free passivation film and the salt spray corrosion resistance of the galvanized layer, the zinc surface chromium-free passivation method provided by the present application optimizes the temperature to 35℃, the passivation time to 5 minutes, the pH value to 8, the concentration of ammonium molybdate to 5-6g / L, the concentration of sodium fluoride to 2g / L, the concentration of hydrogen peroxide to 5g / L, and the concentration of sodium hydroxide to 1-2g / L. The salt spray corrosion resistance of the chromium-free passivation film obtained by passivating the zinc surface with the newly prepared chromium-free passivation solution is 168-192 hours (the thickness of the galvanized layer is 11μm), while the salt spray corrosion resistance of the galvanized layer without the chromium-free passivation film is not more than 120 hours. The chromium-free passivation method can be used for the sealing treatment of the galvanized layer, and significantly improves the corrosion resistance of the galvanized layer.

[0111] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples. Any changes, modifications, substitutions, combinations and simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement methods, and all shall be included in the protection scope of the present application.

Claims

1. A chromium-free passivation solution for zinc surfaces, characterized in that: The pH is adjusted to 8 ± 0.5 by sodium hydroxide, and the following substances are dissolved in the solution: (i) molybdic acid or molybdate, (ii) hydrogen fluoride or fluoride salt, (iii) hydrogen peroxide or metal peroxide; wherein: In the chromium-free passivation solution for zinc surfaces, the concentration of molybdic acid or molybdate is 4–10 g / L, the concentration of hydrogen fluoride or fluoride salt is 0.01–5 g / L, and the concentration of hydrogen peroxide or metal peroxide is 0.01–10 g / L.

2. A chromium-free passivation method for zinc surfaces, characterized in that: The zinc surface is immersed in the chromium-free passivation solution of zinc surface as described in claim 1, and the temperature is maintained at 35-40°C for 5-10 minutes to complete the reaction; the zinc surface is a pure zinc surface or a zinc alloy surface.

3. A surface treatment method for steel products, characterized in that, include: Zinc plating is performed on the surface of steel products; the zinc surface is immersed in the chromium-free passivation solution of zinc surface as described in claim 1, the temperature is maintained at 35-40°C, and the reaction is carried out for 5-10 minutes.

4. The surface treatment method for steel products according to claim 3, characterized in that, The zinc plating on the surface of steel products includes: Zinc plating solution is provided: The zinc plating solution includes zinc salts, complexing agents, surfactants, and pH adjusters; Electroplating zinc treatment: using steel products as cathodes and GA plates as anodes, constant voltage and constant current electroplating is carried out under room temperature stirring conditions.

5. The surface treatment method for steel products according to claim 4, characterized in that: The concentration of zinc salt in the zinc plating solution is 3–10 g / L.

Images