A zinc metal chromium-free passivating agent and its preparation and application

Abstract

The present invention belongs to the field of metallic zinc passivation, and specifically relates to a chromium-free passivator for zinc metal, which is an alkaline aqueous solution containing an aluminate source, an ammonia source, and an enhancer; the enhancer includes an ethylenediaminetetraacetic acid source; and the pH of the alkaline aqueous solution is greater than 13. In addition, the present invention also includes the preparation of the passivator and the application of metallic zinc passivation. The passivation system described in the present invention can achieve synergy, and can realize controllable etching of the zinc surface at the pH and in-situ conversion into a self-protective layer. This can not only effectively circumvent the problem of strong corrosion of zinc in a strong alkaline system, but can also unexpectedly achieve in-situ passivation and self-protection of the zinc surface in a synergistic manner, which is conducive to obtaining a better passivation effect.

Description

Technical Field

[0001] The present invention relates to the field of metal passivation, in particular to the field of metal zinc passivation. Background Art

[0002] Steel is the most widely used metal, but the annual losses from corrosion are enormous. Galvanizing is a common method used to protect steel from corrosion. While zinc coatings offer a high degree of protection for steel, they corrode rapidly in corrosive media, producing white corrosion products that coat the surface. These white corrosion products are hydroxides or zinc salts. Therefore, if the zinc coating is not passivated after galvanizing, the zinc will come into contact with substances such as oxygen and water in the air, and corrosion will occur. The surface of the zinc coating will gradually darken due to corrosion, and once the zinc coating is damaged, the steel itself will corrode. Considering this, the surface of the deposited coating must be passivated during the galvanizing process.

[0003] Currently, the most widely used metal surface passivation method is still the chromate passivation process using a chromium compound solution. This process is also the primary method used for the passivation of zinc in domestic industry. Hexavalent chromium compounds are present in both chromate solutions and chromate coatings. However, due to their high toxicity and carcinogenicity, many countries have banned the use of hexavalent chromium for passivation. Given this, the development of chromate-free passivation processes is gaining increasing attention.

[0004] For example, patent CN110158095A discloses a method for preparing LDH on the surface of galvanized steel. The main ingredients are zinc nitrate, ammonium nitrate, sodium metaaluminate, and sodium nitrate. The advantage of this passivation method is that it can form a passive film on the galvanized steel surface at room temperature without heating, thereby improving the corrosion resistance of the galvanized steel. However, the passivation time of this passivation solution is as long as 12 to 16 hours. This long passivation period may not only cause uneven thickness of the passivation film on different parts of the galvanized steel, but also is not conducive to its industrialization. Summary of the Invention

[0005] In order to solve the problem that the existing zinc metal passivation has an unsatisfactory passivation effect, the purpose of the present invention is to provide a zinc metal chromium-free passivator, aiming to provide a controllable corrosion-self-protection passivation effect.

[0006] The second object of the present invention is to provide a method for passivating metallic zinc using the zinc metal chromium-free passivator.

[0007] Metallic zinc is an amphoteric metal that is susceptible to severe corrosion under highly alkaline conditions. This hinders the close bonding of the passivation layer and the passivation of the metallic zinc. The main solution in the industry to this problem is to use a relatively low pH and perform ex situ deposition passivation with an external metal source, such as zinc. However, this method has unsatisfactory passivation effects. To address the problems of metallic zinc passivation, the present invention provides the following solutions:

[0008] A chromium-free passivating agent for zinc metal, which is an alkaline aqueous solution containing a metaaluminate source, an ammonia source, and an enhancer;

[0009] The enhancer comprises an EDTA source;

[0010] The pH of the alkaline aqueous solution is greater than 13.

[0011] The present invention breaks away from the inherent idea that zinc metal is strongly corroded under strong alkaline conditions and is not conducive to passivation, and innovatively provides an idea for passivation in a system with a pH greater than 13. It is further discovered that, at the said pH, further combined control of the said aluminate source, ammonia source, and ethylenediaminetetraacetic acid source can unexpectedly achieve synergy, and the zinc surface can be controllably etched at the said pH and converted in situ into a self-protective layer. This not only effectively avoids the problem of strong corrosion of zinc in a strong alkaline system, but can also unexpectedly synergistically achieve in-situ passivation and self-protection of the zinc surface, which is conducive to obtaining a better passivation effect.

[0012] In the present invention, the combination of pH and aluminate source, ammonia source, and enhancer containing ethylenediaminetetraacetic acid source is the key to synergistically solving zinc corrosion under high alkaline conditions and synergistically achieving in-situ self-protection passivation.

[0013] In the present invention, the aluminate source in the passivating agent exists in the form of aluminate, which can be provided by a water-soluble aluminate or an aluminum source that can be converted into aluminate under the system.

[0014] Preferably, the water-soluble metaaluminate is at least one of sodium metaaluminate, potassium metaaluminate, and ammonium metaaluminate;

[0015] Preferably, the aluminum source material is at least one of aluminum hydroxide, aluminum chloride, aluminum sulfate, aluminum nitrate, aluminum silicate, aluminum sulfide, and potassium aluminum sulfate.

[0016] In the present invention, the ammonia source of the system is at least one of free ammonia, ammonia water, and ammonium salt.

[0017] In the present invention, the EDTA source in the passivation system is in an ionic state, which can be provided by a water-soluble compound capable of ionizing EDTA, preferably EDTA and its water-soluble salts.

[0018] In the present invention, EDTA is used as an enhancer, and the pH and process are controlled in combination, thereby synergistically avoiding corrosion of metallic zinc under high alkalinity and achieving in-situ self-protection without the addition of an external metal, thereby further improving the passivation effect.

[0019] In the present invention, the enhancer further comprises at least one of acetate (such as sodium salt), glucose, polyacrylate (such as sodium salt), thiourea, ethylenediamine, polyethylene glycol, and ethylene glycol ethyl ether.

[0020] Preferably, the enhancer is a complex of ethylenediaminetetraacetic acid (EDTA) and thiourea. Studies have shown that, under the above process, the combination of the enhancer can achieve a better synergistic effect and bring about better in-situ self-protection performance of metallic zinc.

[0021] In the present invention, when a composite reinforcing agent is used, the content of EDTA radical is greater than or equal to 15 mol%, preferably 25 to 75 mol%.

[0022] In the zinc metal chromium-free passivator of the present invention, the concentration of the aluminate source in the alkaline aqueous solution is less than or equal to 5.0M, preferably 0.2 to 1M; more preferably 0.4 to 0.8M; and even more preferably 0.6 to 0.8M.

[0023] Preferably, the concentration of the enhancer is less than or equal to 2.0 M, preferably 0.1 to 0.5 M; more preferably 0.2 to 0.3 M;

[0024] Preferably, the concentration of the ammonia source is less than or equal to 4.0M; more preferably 0.3 to 2.0M; and even more preferably 1 to 2M;

[0025] Preferably, the pH of the chromium-free passivating agent is 13.2 to 15, more preferably 13.5 to 15. The pH is supplemented and regulated by a strong base, such as an alkali metal hydroxide (such as at least one of sodium hydroxide and potassium hydroxide).

[0026] The present invention also provides a method for preparing the passivating agent, which comprises dissolving a metaaluminate source and a reinforcing agent in water, and then adjusting the pH of the system to the required condition by adding an ammonia source and a strong base.

[0027] The present invention also provides a zinc metal passivation method, which comprises immersing the zinc metal in the zinc metal chromium-free passivating agent for passivation.

[0028] In the present invention, the zinc metal is zinc alone or a zinc-containing alloy.

[0029] The present invention can control the temperature and other conditions of the passivation process as needed. For example, the passivation temperature can be room temperature (such as 15 to 40° C.).

[0030] In the present invention, the passivation time can be adjusted as needed. For example, the passivation time is 10 seconds to 30 minutes, preferably 2 to 5 minutes.

[0031] Beneficial effects

[0032] In the present invention, under the passivation components and pH, controllable etching of the metallic zinc surface can be achieved, and it can be simultaneously converted in situ into a dense self-protective layer. This not only avoids strong corrosion under highly alkaline conditions, but also unexpectedly utilizes its own zinc to be converted in situ into a dense self-protective layer, which can show a better passivation protection effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The Tafel curves are those of unpassivated metallic zinc and those of metallic zinc passivated with the passivating agent described in Example 5.

[0034] Figure 2 The AC impedance curves of unpassivated metal zinc and the metal zinc passivated with the passivating agent described in Example 5 are shown. DETAILED DESCRIPTION

[0035] To facilitate understanding of the present invention, the present invention will be described more comprehensively and meticulously below with reference to the accompanying drawings and embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0036] In the present invention, the metal zinc substrate can be a metal zinc element or an alloy. The following case is an example of a research object using electrolytic zinc prepared by electrolysis, which is prepared by the following steps:

[0037] Stainless steel was used as the cathode plate and DSA anode was used as the anode plate. Electrolysis conditions: the electrolyte was 100 g / L zinc sulfate 7 hydrate, 20 g / L boric acid, 1 g / L thiourea, sulfuric acid was added to adjust the pH of the electrolyte to 1-2, and the current density was 3 A / dm 2 , electrolysis temperature 25 ° C, electrolysis time 30 min, to prepare an electrolytic zinc product, the prepared electrolytic zinc product is used for subsequent passivation experiments and passivation layer performance evaluation.

[0038] In the following case, the passivation of the electrolytic zinc product: the configured chromium-free passivator is placed in a constant temperature water tank, the surface of the cathode plate after electrolysis is rinsed with deionized water, and then the cathode plate is placed in the chromium-free passivator so that the chromium-free passivator immerses the surface of the cathode plate. Passivation is carried out at 30°C for 3 minutes. After passivation is completed, it is taken out and rinsed with deionized water, and then dried to obtain the passivated metal zinc product. The electrochemical performance of the passivated metal zinc product is tested.

[0039] In the present invention, in the following cases, the passivating agent, on the basis of the alkalinity constructed by sodium metaaluminate and ammonia water, is further regulated to a desired pH value by sodium hydroxide.

[0040] The molar concentration of the ammonia solution is calculated as NH3.

[0041] Example 1:

[0042] Sodium metaaluminate and a reinforcing agent (disodium ethylenediaminetetraacetic acid) were dissolved in water, and ammonia was added to obtain an alkaline aqueous solution. In the alkaline aqueous solution, the concentration of sodium metaaluminate was 0.6M, the concentration of the reinforcing agent was 0.2M, the concentration of ammonia was 1M, and the pH was controlled at 13.5. The self-corrosion current was measured to be 1.826×10 -6 A / cm 2 .

[0043] Example 2:

[0044] Compared with Example 1, the only difference is that, while maintaining the amounts of sodium metaaluminate, enhancer, and ammonia source unchanged, the pH of the solution system is regulated by sodium hydroxide, respectively: (A): pH 14; (B): pH 14.5; (C): pH 15;

[0045] The results are: (A): 1.472×10 -6 A / cm 2 ;(B):7.485×10 -7 A / cm 2 ; (C): 8.743×10 - 7 A / cm 2 .

[0046] Example 3:

[0047] Compared with Example 1, the only difference is that the concentrations of sodium metaaluminate and disodium edetate are changed:

[0048] A: The concentration of sodium metaaluminate is 0.4M, and the concentration of disodium EDTA is 0.1M;

[0049] B: The concentration of sodium metaaluminate is 0.8M, and the concentration of disodium EDTA is 0.3M;

[0050] C: The concentration of sodium aluminate is 0.8M, and the concentration of disodium EDTA is 0.1M.

[0051] The results are: (A): 1.952×10 -6 A / cm 2 ;(B):9.374×10 -7 A / cm2 ;

[0052] (C): 4.735×10 -6 A / cm 2 .

[0053] Example 4:

[0054] Compared with Example 1, the only difference is that the type of enhancer is changed: A: the enhancer is disodium edetate and sodium acetate in a molar ratio of 1:3; B: the enhancer is disodium edetate and glucose in a molar ratio of 1:2; C: the enhancer is disodium edetate and sodium polyacrylate in a molar ratio of 3:1; D: the enhancer is disodium edetate and thiourea in a molar ratio of 1:1; E: the enhancer is disodium edetate and ethylenediamine in a molar ratio of 1:2; F: the enhancer is disodium edetate and polyethylene glycol in a molar ratio of 4:1; G: the enhancer is disodium edetate and ethylene glycol ether in a molar ratio of 1:1.

[0055] The results are: (A): 6.547×10 -7 A / cm 2

[0056] (B):4.155×10 -7 A / cm 2

[0057] (C): 5.935×10 -7 A / cm 2

[0058] (D): 3.684×10 -7 A / cm 2

[0059] (E): 6.276×10 -7 A / cm 2

[0060] (F): 6.562×10 -7 A / cm 2

[0061] (G): 8.160×10 -7 A / cm 2

[0062] Example 5:

[0063] Sodium metaaluminate and an enhancer (the enhancer is disodium ethylenediaminetetraacetic acid and thiourea in a 1:1 molar ratio) are dissolved in water, and the pH of the solution is controlled with aqueous ammonia to obtain an alkaline aqueous solution. In the alkaline aqueous solution, the concentration of sodium metaaluminate is 0.8 M, the concentration of disodium ethylenediaminetetraacetic acid is 0.3 M, the molar concentration of aqueous ammonia is 2.0 M, and the pH is 14.0.

[0064] The self-corrosion current was measured to be 1.894×10 -7 A / cm 2 .

[0065] Comparative Example 1

[0066] Compared with implementation 1, the only difference is the lack of enhancer.

[0067] The self-corrosion current was measured to be 5.736×10 -6 A / cm 2 .

[0068] Comparative Example 2

[0069] Compared with embodiment 1, the only difference is that EDTA is replaced by an equimolar amount of sodium acetate.

[0070] The self-corrosion current was measured to be 4.558×10 -6 A / cm 2 .

[0071] Comparative Example 3

[0072] Compared with Example 1, the only difference is that no ammonia water is added, that is, the chromium-free passivating agent is a solution of sodium metaaluminate and a reinforcing agent (disodium ethylenediaminetetraacetic acid), and the concentrations of the components are the same as in Example 1.

[0073] The self-corrosion current was measured to be 7.218×10 -6 A / cm 2 .

[0074] Comparative Example 4

[0075] Compared with Example 1, the only difference is that the pH is regulated by sulfuric acid, that is, the pH of the system is 12.5. The other concentrations of sodium aluminate, enhancer, and ammonia water are the same as those in Example 1.

[0076] The self-corrosion current was measured to be 5.673×10 -5 A / cm 2 .

[0077] The electrochemical properties of unpassivated and passivated metal zinc were tested, and the Tafel polarization curve results are as follows: Figure 1 As shown in Table 1, the AC impedance curve is as follows Figure 2 shown.

[0078] Table 1 Performance comparison of electrolytic zinc products before and after passivation

[0079] sample Self-corrosion potential (V) <![CDATA[Self-corrosion current (A / cm 2 )]]> Unpassivated zinc -1.195 1.093e-5 Zinc after passivation -1.014 1.894e-7

[0080] Depend on Figure 1 From the electrochemical test results in Table 1, it can be seen that after passivation, the self-corrosion potential of the metal zinc product shifted significantly to the positive side, with the self-corrosion potential shifting positively by 0.18 V, and the corresponding self-corrosion current decreased by 2 orders of magnitude. The corrosion resistance of the electrolytic metal zinc product has been significantly improved.

[0081] In the AC impedance curve, the size of the capacitive arc reflects the corrosion resistance of the passivation film. Generally speaking, the larger the diameter of the capacitive arc, the better the corrosion resistance of the passivation film. Figure 2 It can be seen that the AC impedance curve after passivation consists of a high-frequency capacitive reactance arc and a low-frequency capacitive reactance arc. Compared with the AC impedance curve of the unpassivated sample, it can be seen that the high-frequency capacitive reactance arc is significantly increased after passivation, which shows that the passivation film can effectively hinder the transfer of charges and ions and inhibit the occurrence of corrosion reactions.

Claims

1. A chromium-free passivating agent for zinc metal, characterized in that: It is an alkaline aqueous solution containing aluminate source, ammonia source and enhancer; The enhancer comprises an ethylenediaminetetraacetic acid source; and further comprises at least one of sodium acetate, glucose, sodium polyacrylate, thiourea, ethylenediamine, polyethylene glycol, and ethylene glycol ether; The ammonia source is at least one of free ammonia and ammonia water; In the enhancer, the content of EDTA radical is greater than or equal to 15 mol.%; The pH of the alkaline aqueous solution is greater than 13; In alkaline aqueous solution, the concentration of the aluminate source is 0.2~1.0M; the concentration of the enhancer is 0.1~0.5M; The concentration of the ammonia source is 0.3~2.0M.

2. The zinc metal chromium-free passivator according to claim 1, wherein The aluminate source is a water-soluble aluminate or an aluminum source that can be converted into aluminate in an alkaline aqueous solution system.

3. The zinc metal chromium-free passivator according to claim 2, wherein The water-soluble metaaluminate is at least one of sodium metaaluminate, potassium metaaluminate, and ammonium metaaluminate; The aluminum source material is at least one of aluminum hydroxide, aluminum chloride, aluminum sulfate, aluminum nitrate, aluminum silicate, aluminum sulfide, and potassium aluminum sulfate.

4. The zinc metal chromium-free passivator according to claim 1, wherein The EDTA source is a water-soluble compound that can ionize EDTA.

5. The zinc metal chromium-free passivator according to claim 4, wherein The EDTA source is EDTA and its water-soluble salt.

6. The zinc metal chromium-free passivator according to claim 1, wherein In the enhancer, the content of EDTA radical is 25-75 mol%.

7. The zinc metal chromium-free passivator according to any one of claims 1 to 6, wherein In alkaline aqueous solution, the concentration of aluminate source is 0.4~0.8M; The concentration of the enhancer is 0.2~0.3M; The concentration of the ammonia source is 0.3~2.0M; The pH is 13.2~15.

8. A zinc metal passivation method, characterized in that, The zinc metal is immersed in the zinc metal chromium-free passivator according to any one of claims 1 to 7 for passivation.

9. The zinc metal passivation method according to claim 8, wherein The zinc metal is zinc element or zinc alloy.

10. The zinc metal passivation method according to claim 8, wherein The passivation time is 10 seconds to 30 minutes.

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