Design method of high corrosion resistant conversion coating on steel based on nucleation control

Abstract

The present invention discloses a method for designing a highly corrosion-resistant steel conversion film based on nucleation control, and relates to the technical field of steel conversion films. The method comprises: establishing a data set of film-forming substances for steel conversion film design; screening the substances most easily deposited within the pH range of steel phosphating to determine the main salt type and film-forming substances for phosphating; screening an acid ratio that keeps the conversion film solution stable and close to the critical supersaturation within the film-forming temperature range; determining the temperature value and acid ratio of the conversion film solution with the highest supersaturation, with the constraint that the higher the supersaturation, the higher the nucleation rate; and adding an oxidant and a chelating agent to further increase the nucleation kinetics, ultimately obtaining a highly corrosion-resistant steel conversion film solution based on nucleation control. The present invention improves the corrosion resistance of the steel conversion film by controlling nucleation, fully combining thermodynamic and kinetic factors, and providing a new approach for the design of steel phosphating processes. The method is reliable.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical conversion films for steel, and in particular to a design method for a highly corrosion-resistant conversion film for steel based on nucleation control. Background Art

[0002] Chemical conversion coatings are an effective means of improving the corrosion resistance of steel. However, current corrosion resistance tests for steel conversion coatings, such as those conducted in salt spray tests, typically measure around 72 hours, with some rarely exceeding 120 hours. Therefore, further post-processing is required to improve the corrosion resistance of steel, which increases costs. Furthermore, conventional chemical conversion coating solutions for steel often utilize orthogonal methods, resulting in a massive amount of experimental work and a lack of systematic theoretical guidance. This requires significant manpower, time, and material investment.

[0003] Q345 steel is a low-alloy, high-strength structural steel. Due to its excellent mechanical properties and high cost-effectiveness, it is widely used in engineering fields such as construction, bridges, ships, and pressure vessels. However, in complex corrosive environments such as marine atmosphere and industrial pollution, its insufficient corrosion resistance has gradually become a key bottleneck restricting its long-term service. The corrosion mechanism of traditional carbon steel is mainly electrochemical corrosion. Although trace alloying elements (such as Mn and Si) in Q345 steel can improve strength, they may aggravate local micro-battery effects and accelerate failure behaviors such as pitting and stress corrosion cracking. Especially in high humidity, high salt fog or acidic medium environments, a loose rust layer is easily formed on the surface of the substrate, resulting in a rapid decline in protective performance, which seriously threatens the safety and economy of the structure. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art or related art.

[0005] To this end, the present invention provides a design method for a steel high corrosion resistant conversion film based on nucleation control, comprising the following steps:

[0006] Determine the film-forming substance dataset, obtain the metal ions used for film formation in the steel phosphating process and their corresponding film-forming substances, and establish the film-forming substance dataset for steel conversion film design;

[0007] Screen the main salt system and film-forming substances, establish a thermodynamic deposition phase diagram of the metal ion concentration and pH value of the film-forming substance based on the solubility product principle, screen the substances that are most easily deposited within the pH range of steel phosphating, and determine the main salt type and film-forming substance for phosphating;

[0008] The acid ratio parameters were optimized, the film-forming temperature range was selected, and conversion film solutions with different acid ratios were designed. After two thermal cycles of heating to the target temperature, cooling and collecting the supernatant, and then heating to the target temperature again, the acid ratio that kept the conversion film solution stable and close to the critical supersaturation within the film-forming temperature range was screened.

[0009] The nucleation conditions were regulated, with the higher the supersaturation, the higher the nucleation rate. The supersaturation of the conversion film solution at different temperatures and acid ratios within the film-forming temperature range was calculated, and the temperature and acid ratio of the conversion film solution with the highest supersaturation were determined.

[0010] Optimizing the additive ratio, adding oxidants and complexing agents to further enhance nucleation, and determining the type and amount of oxidants and complexing agents in the conversion coating solution with the highest supersaturation through experimental testing and in combination with corrosion resistance and cost, ultimately obtaining a steel high corrosion-resistant conversion coating solution based on nucleation control;

[0011] The high corrosion resistant conversion film for steel is prepared by using the high corrosion resistant conversion film solution based on nucleation control.

[0012] Furthermore, the design method is used for designing a high corrosion resistant conversion coating for Q345C steel, comprising the following steps:

[0013] Determine the film-forming substance dataset, obtain the metal ions used for film formation in the phosphating process of Q345C steel and their corresponding film-forming substances, and establish the film-forming substance dataset for the Q345C steel conversion film design;

[0014] Screen the main salt system and film-forming substances, establish a thermodynamic deposition phase diagram of the metal ion concentration and pH value of the film-forming substances based on the solubility product principle, screen the substances that are most easily deposited within the pH range of Q345C steel phosphating, and determine the main salt type and film-forming substances for Q345C steel phosphating;

[0015] The acid ratio parameters were optimized, and the film-forming temperature range was selected as 60°C-90°C. Q345C steel conversion coating solutions with different acid ratios were designed. After two thermal cycles of heating to the target temperature, cooling and collecting the supernatant, and then reheating to the target temperature, the acid ratio that kept the Q345C steel conversion coating solution stable and close to the critical supersaturation within the range of 60°C-90°C was screened.

[0016] The nucleation conditions were regulated, with the higher the supersaturation, the higher the nucleation rate. The supersaturation of the Q345C steel conversion coating solution at different target temperatures and acid ratios in the range of 60°C-90°C was calculated, and the temperature and acid ratio for the Q345C steel conversion coating solution with the highest supersaturation were determined.

[0017] The additive ratio was optimized, and oxidants and complexing agents were added to further enhance nucleation. Through experimental testing and in combination with corrosion resistance and cost, the types and addition amounts of oxidants and complexing agents in the Q345C steel conversion coating solution with the highest supersaturation were determined, ultimately resulting in a Q345C steel high corrosion-resistant conversion coating solution based on nucleation control.

[0018] The high corrosion resistant conversion film of Q345C steel is prepared by using the high corrosion resistant conversion film solution of Q345C steel based on nucleation control.

[0019] Furthermore, the film-forming material data set in the Q345C steel conversion film design includes: manganese phosphide, zinc phosphide, calcium phosphide and iron phosphide.

[0020] Furthermore, the screening of the main salt system and film-forming substances includes: within the pH value of 2-4 for phosphating Q345C steel, screening the most easily deposited substance as MnHPO4, and determining that the main salt type for phosphating Q345C steel is manganese-based and the film-forming substance is MnHPO4.

[0021] Furthermore, in the optimized acid ratio parameters, the preparation method of the Q345C steel conversion coating solution with different acid ratios is to add different amounts of sulfuric acid to the reference solution; wherein the manganese-phosphorus ratio of the reference solution is 2:3, the manganese ion concentration is 0.2 mol / L, and the dihydrogen phosphate concentration is 0.3 mol / L.

[0022] Furthermore, the nucleation conditions are controlled, including: calculating the MnHPO4 and H2PO4 at different target temperatures. - Solubility product K sp , the reaction quotient Jsp is calculated based on the acid ratio at different target temperatures, thereby calculating the supersaturation values ​​of the Q345C steel conversion film solution at different target temperatures and different acid ratios, and then determining the temperature value and acid ratio of the Q345C steel conversion film solution with the highest supersaturation.

[0023] Furthermore, it was calculated that the temperature of the Q345C steel conversion coating solution with the highest supersaturation was 90° C., and the acid ratio was 14.68.

[0024] Furthermore, the optimized additive ratio includes: using sodium nitrite as an oxidant and experimentally optimizing the amount of sodium nitrite added within a sodium nitrite concentration range of 0.5 g / L-2 g / L.

[0025] Furthermore, the optimization of the additive ratio includes: using EDTA as a complexing agent and experimentally optimizing the amount of EDTA added within the EDTA concentration range of 3 g / L-22.8 g / L.

[0026] Furthermore, the formula of the Q345C steel high corrosion-resistant conversion film solution based on nucleation control is: manganese sulfate concentration is 0.2 mol / L, sodium dihydrogen phosphate concentration is 0.3 mol / L, acid ratio is 14.68, sodium nitrite addition amount is 1.5 g / L, EDTA addition amount is 3 g / L, and film forming temperature is 90°C.

[0027] Compared with the prior art, the present invention has at least the following beneficial effects:

[0028] The present invention provides a design method for a high-corrosion-resistant steel conversion film based on nucleation control. The method combines film formation kinetics and thermodynamics, designs the conversion film solution based on the idea of ​​"acid ratio" theory, screens out the main salt and determines the content based on the thermodynamic phase diagram, and quickly screens out the acid ratio of the conversion solution that is in a critical state and can still remain stable at each temperature by heating to the target temperature, cooling, and reheating the supernatant to observe whether precipitation occurs. With supersaturation as a constraint, the acid ratio and temperature are changed to calculate the supersaturation of the solution, thereby determining the optimal acid ratio and temperature conditions, thereby determining the basic parameters of steel phosphating (main salt, temperature, acid ratio), and then accelerating metal dissolution and thus accelerating nucleation by adding an oxidant and a chelating agent. The optimal content is determined through experiments to optimize the formula.

[0029] The design method of the present invention has a rich theoretical basis, takes the control of nucleation as the starting point to improve the corrosion resistance of the steel conversion film, fully combines thermodynamics and kinetic factors, obtains various basic parameters required for phosphating through thermodynamic and kinetic calculations, and has a good effect after phosphating, providing ideas for the design of steel phosphating process. In addition, this method changes the previous situation in which the phosphating parameters of the steel conversion film are obtained through a large number of orthogonal experiments and relying on work experience during the design process, greatly reduces the trial and error of experiments and the scale of experiments, reduces time and cost, and obtains a conversion film with good performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a graph showing the relationship between the ion concentration of phosphoric acid and pH provided in Example 1 of the present invention.

[0031] Figure 2 Thermodynamic deposition phase diagram for different metal ion concentrations and pH values ​​provided in Example 1 of the present invention.

[0032] Figure 3 This is a relationship diagram between the acid ratio and the supersaturation of the conversion coating solution at different temperatures provided in Example 1 of the present invention.

[0033] Figure 4 XRD patterns of chemical conversion films prepared in conversion film solutions at different phosphating temperatures provided in Example 1 of the present invention.

[0034] Figure 5 The macroscopic morphology of the chemical conversion film prepared in the conversion film solution with different phosphating temperatures provided in Example 1 of the present invention after salt spray test for different times.

[0035] Figure 6This is a graph showing the copper sulfate corrosion resistance time of the chemical conversion film prepared in conversion coating solutions at different phosphating temperatures provided in Example 1 of the present invention.

[0036] Figure 7 Polarization curves of chemical conversion films prepared in conversion film solutions at different phosphating temperatures provided in Example 1 of the present invention.

[0037] Figure 8 Polarization resistance diagram of the chemical conversion film prepared in the conversion film solution at different phosphating temperatures provided in Example 1 of the present invention.

[0038] Figure 9 This is a salt spray test diagram of different time for the chemical conversion film prepared in the conversion film solution with different NaNO2 addition amounts provided in Example 1 of the present invention.

[0039] Figure 10 This is a graph showing the copper sulfate corrosion resistance time of the chemical conversion film prepared in the conversion film solution with different NaNO2 addition amounts provided in Example 1 of the present invention.

[0040] Figure 11 Polarization curves of chemical conversion films prepared in conversion film solutions with different NaNO2 addition amounts provided in Example 1 of the present invention.

[0041] Figure 12 Polarization resistance diagram of the chemical conversion film prepared in the conversion film solution with different EDTA addition amounts provided in Example 1 of the present invention.

[0042] Figure 13 This is a diagram of salt spray experiments at different times on chemical conversion films prepared in conversion film solutions with different EDTA addition amounts provided in Example 1 of the present invention.

[0043] Figure 14 The hydrogen evolution acceleration diagram (a), weight loss acceleration diagram (b), and hydrogen evolution weight loss current density diagram (c) of the chemical conversion film prepared in the conversion film solution with different EDTA addition amounts provided in Example 1 of the present invention.

[0044] Figure 15 The figures are comparative diagrams of the salt spray test of the Q345C steel with conversion coating prepared in Example 1 of the present invention and the bare Q345C steel in Comparative Example 1; wherein, (a) is a 24h salt spray test diagram of the bare Q345C steel substrate provided in Comparative Example 1; and (b) is a 504h salt spray test diagram of the Q345C steel with conversion coating prepared by the formula under the optimal conditions of Example 1. DETAILED DESCRIPTION

[0045] In order to better understand the above technical solution, the technical solution of the present invention is described in detail below through specific embodiments.

[0046] The embodiments of the present invention provide a method for designing a high-corrosion-resistant conversion coating for steel by theoretically guiding the design of the high-corrosion-resistant conversion coating for steel, thereby solving the technical problems of long experimental cycles and wasted costs. The method provides a method for designing a high-corrosion-resistant conversion coating for steel that is theoretically sound, low-cost, and has low trial-and-error costs, including the following steps:

[0047] Step 1: Determine the film-forming substance dataset, obtain the metal ions used for film formation in the steel phosphating process and their corresponding film-forming substances, and establish the film-forming substance dataset for steel conversion film design;

[0048] Step 2: Screen the main salt system and film-forming substances. Based on the solubility product principle, establish a thermodynamic deposition phase diagram of the metal ion concentration and pH value of the film-forming substances. Within the pH range of steel phosphating, screen the substances that are most easily deposited to determine the main salt type and film-forming substances for phosphating.

[0049] Step 3: Optimizing the acid ratio parameters, selecting the film-forming temperature range, and designing conversion film solutions with different acid ratios. The conversion film solutions are subjected to two thermal cycles of heating to the target temperature, cooling to remove the supernatant, and then reheating to the target temperature. The acid ratio that keeps the conversion film solution stable and close to the critical supersaturation within the film-forming temperature range is selected.

[0050] Step 4: Controlling the nucleation conditions, with the constraint that the higher the supersaturation, the higher the nucleation rate, the supersaturation of the conversion film solution at different temperatures and different acid ratios within the film forming temperature range is calculated, and the temperature and acid ratio of the conversion film solution with the highest supersaturation are determined;

[0051] Step 5: Optimizing the additive ratio, adding an oxidant and a complexing agent to further enhance nucleation, and determining the type and amount of the oxidant and complexing agent in the conversion coating solution with the highest supersaturation through experimental testing and in combination with corrosion resistance and cost, ultimately obtaining a steel high corrosion-resistant conversion coating solution based on nucleation control;

[0052] Step 6: preparing a steel high corrosion resistant conversion film using the steel high corrosion resistant conversion film solution based on nucleation control.

[0053] In the embodiment of the present invention, the solubility product constant of the corresponding phosphide of the element is taken into consideration. K sp , the corresponding critical deposition line can be calculated. The lower the critical deposition line, the earlier the deposition occurs. Therefore, the solubility product constant of the phosphide corresponding to the candidate set of metal ions is obtained by querying and calculating K sp , and use this to calculate the corresponding critical deposition line to determine the deposition order of alloying elements.

[0054] The embodiments of the present invention take into account that the pH at the metal solution interface increases instantaneously due to the dissolution of iron into ferrous iron, which loses hydrogen ions to generate hydrogen gas, thereby rapidly increasing the concentration of monohydrogen phosphate. The critical deposition line of the phosphating film-forming substance can reflect the deposition order of the film-forming substance. Therefore, the metal ions and film-forming substances can be screened according to the deposition order of the corresponding film-forming products in the metal ion candidate set, with the deposition order being the front as the constraint condition, so that the main salt obtained by screening can quickly form a dense conversion film during phosphating, thereby improving the corrosion resistance of the steel.

[0055] In an embodiment of the present invention, metal ions that are easily film-forming are screened through a thermodynamic phase diagram to complete the screening of the main salt. The ratio of the film-forming metal ion to the phosphate is determined based on the structure of the material deposited in the solution within the phosphating pH range according to the thermodynamic phase diagram. The temperature range of the phosphating series is roughly defined by the type of film-forming metal ion selected, and the final phosphating temperature is determined by calculating the supersaturation of the solution at the temperature points within this range. It should be noted that different temperatures of the screened main salt have different trends in affecting supersaturation. Therefore, according to this method, the optimal temperature parameters under different series of phosphating conditions can be obtained.

[0056] It can be understood that the design method of the high corrosion-resistant conversion film for steel provided in the embodiment of the present invention is carried out on the basis of thermodynamics. In steel phosphating, thermodynamics is the basis, and the corrosion resistance of the conversion film depends on kinetic factors. Therefore, on the basis of ensuring the feasibility of thermodynamics, the nucleation kinetics is added. Since the nucleation rate and the supersaturation have an exponential growth relationship, the nucleation rate can be greatly improved by increasing the supersaturation, and the temperature and acid ratio are the key factors affecting the supersaturation of the solution. Therefore, the parameters of the conversion film solution are changed with the acid ratio and temperature as variables, and the process parameters that make the conversion film corrosion resistant are selected by calculating the supersaturation of the conversion solution under different conditions.

[0057] Since the phosphating process is based on the three steps of "dissolution", "ionization" and "deposition", the adjustment of the supersaturation of the solution is based on the two steps of "ionization" and "deposition". Dissolution is the first step of phosphating, so it is necessary to increase the dissolution rate. The improvement of the last two steps is meaningful. Therefore, the dissolution process of the metal is promoted by adding oxidants and chelating agents, so the content of oxidants and chelating agents must be screened.

[0058] The experimental method of the present invention is introduced below, which is as follows:

[0059] The samples used for hydrogen evolution, weight loss, and salt spray tests measured 25 mm × 25 mm × 3 mm, while the electrochemical samples measured 10 mm × 10 mm × 10 mm. The samples were polished to a 1000# grit with SiC sandpaper, rinsed with alcohol, air-dried, and placed in a desiccating dish for later use. The chemicals were weighed into a beaker, thoroughly stirred with deionized water, and dissolved until fully dissolved. The mixture was then brought to a volume of 1000 ml in a volumetric flask. The acid ratio was adjusted by adding sulfuric acid. The temperature in the water bath was set to the target temperature. During heating, the sample was covered with plastic wrap to prevent concentration changes caused by evaporation. A thermometer was used to monitor the temperature of the conversion coating solution. Once the temperature reached the target temperature, the sample was suspended with fishing line and placed in the conversion coating solution for 10 minutes. The sample was then removed, rinsed with deionized water, and air-dried with a hair dryer before being tested for corrosion resistance.

[0060] 1) Copper sulfate spot test

[0061] The composition of the copper sulfate drop solution is: 41g / L CuSO4·5H2O, 35g / L NaCl, 13ml / L HCl. At room temperature, place a drop of the test solution on the surface of the phosphating sample and observe how long it takes for the drop to change from sky blue to deep yellow or light red.

[0062] 2) Electrochemical testing

[0063] The electrochemical tests in this application were performed using a Princeton P4000 Series A electrochemical workstation from AMETEK, USA. A three-electrode system was used, with a Q345C steel specimen as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode. The potentiodynamic polarization curve was scanned from -0.3 V vs OCP to 1.6 V vs Ref at a scan rate of 0.333 mV / s. The test frequency range for the electrochemical impedance spectroscopy was: 10 5 ~ 10 -2 Hz, and the sinusoidal perturbation was 10mV. The impedance data were fitted using Zview software. The test frequency range of the electrochemical impedance spectroscopy was 10 5 to 10 -2 Hz, with a perturbation amplitude of 10 mV. Polarization curves and electrochemical impedance spectroscopy measurements were performed at a constant temperature of 30 ± 1°C. To ensure experimental reproducibility, three replicates were tested for each condition.

[0064] 3) Salt spray test

[0065] Samples were subjected to salt spray testing according to the salt spray test standard (ASTM B117-03) at a temperature of 35 ± 1°C. The original macroscopic morphology of the samples was recorded and, after the start of the test, removed and photographed every 24 hours. The changes in the sample's morphology before and after the test were used to determine the extent of corrosion and compare corrosion resistance. Five replicates of each sample were tested to ensure the accuracy of the results.

[0066] Example 1

[0067] Taking Q345C steel as an example, Example 1 provides a design method for a high corrosion-resistant conversion film on Q345C steel based on nucleation control, comprising the following steps:

[0068] Step 1: Obtain the common or reported metal ions used for film formation in steel phosphating processes and their corresponding film-forming substances, and establish a dataset of possible film-forming substances for the design of conversion coatings on Q345C steel;

[0069] Step 2: Based on the solubility product constant, a thermodynamic deposition phase diagram of the metal ion concentration and pH of the film-forming substance is drawn. Within the pH range of Q345C steel phosphating, the height of the deposition line is used as a constraint condition to screen out the most easily deposited substance, determine the main salt type and film-forming substance for Q345C steel phosphating; and determine the concentration of the main salt of the film-forming solution based on the screened film-forming substance.

[0070] Step 3: Selecting a film-forming temperature range, designing multiple groups of conversion film solutions with different acid ratios, heating to the target temperature, cooling, and collecting the supernatant, and reheating to the target temperature to screen out an acid ratio that keeps the Q345C steel conversion film solution stable and close to the critical supersaturation;

[0071] Step 4: Based on the idea of ​​"acid ratio" theory, with the constraint that higher supersaturation means higher nucleation rate, the supersaturation of Q345C steel conversion coating solution at different acid ratios and temperatures was calculated, and the temperature condition and acid ratio with the highest supersaturation were screened out;

[0072] Step 5: adding an oxidant and a complexing agent to further increase nucleation. By experimental testing and combining corrosion resistance and cost, the type and addition amount of the oxidant and complexing agent in the Q345C steel conversion coating solution with the highest supersaturation are determined, and finally a Q345C steel high corrosion resistance conversion coating solution based on nucleation control is obtained.

[0073] Step 6: Prepare a high corrosion resistant conversion film for Q345C steel using a high corrosion resistant conversion film solution for Q345C steel based on nucleation control.

[0074] In step 1, the data set of possible film-forming substances in the conversion coating design of Q345C steel includes manganese-based, zinc-based, calcium-based, and iron-based phosphating.

[0075] In step 2, the solubility products of manganese, zinc, calcium and iron phosphides are consulted or calculated. Based on the solubility product constant, the total phosphorus content is tentatively set at 0.3 mol / L. According to the relationship between the ion concentration of phosphoric acid and pH, as shown in the figure Figure 1 As shown, the thermodynamic deposition phase diagrams for different metal ion concentrations and pH values ​​are drawn as shown in Figure 2 As shown in the figure, within the pH range of steel phosphating, i.e. pH 2-4, with the height of the deposition line as the constraint condition, the most easily deposited substance was screened out as MnHPO4, and the main salt type of phosphating was determined to be manganese series and the film-forming substance was determined to be MnHPO4.

[0076] Based on the 1:1 manganese-phosphorus ratio of the selected film-forming substance MnHPO4, the main salt of the film-forming solution was determined to have a manganese-phosphorus ratio of 2:3 and a concentration of 0.2 mol / L manganese ion and 0.3 mol / L dihydrogen phosphate. It is understandable that additional phosphoric acid is required to control the pH and maintain a stable pH during the reaction. The concentrations of 0.2 mol / L and 0.3 mol / L are designed to keep the solution in a supersaturated state, thereby enabling rapid nucleation during the phosphating process.

[0077] In step 3, the film-forming temperature range is selected to be 60°C-90°C. By adding sulfuric acid to the conversion solution to change the pH of the solution, the acid ratio of the solution is measured to obtain multiple groups of conversion film solutions with different temperatures and different acid ratios. The solutions with different acid ratios are heated to four target temperatures (60°C, 70°C, 80°C, and 90°C), cooled, and the supernatant is taken. The solution is then heated to the four target temperatures (60°C, 70°C, 80°C, and 90°C) for a second time to screen out the conversion film solution that keeps the solution stable, i.e., does not precipitate again.

[0078] In step 4, the reaction enthalpy of MnHPO4 at different temperatures is calculated using the Van't Hoff equation. K sp and H2PO4 - of K sp According to the acid ratio, the acid ratio at the target temperature is measured to calculate the reaction quotient Jsp, and then the supersaturation under different temperatures and different acid ratios is calculated for comparison. Figure 3 As shown, it was found that the supersaturation was highest when the temperature was 90℃ and the acid ratio was 14.68, so the temperature and acid ratio were determined accordingly.

[0079] See also Figure 5-Figure 8It can be seen that the time for the sample of the conversion film prepared at a temperature of 90℃ to experience large-area corrosion in the neutral salt spray test is longer than that at the other three temperature conditions, and the corrosion resistance of the conversion film improves as the temperature increases, with large-area corrosion only occurring at 672h. The copper sulfate drip test shows that the color change time at a temperature of 90℃ is the longest, and the longer the color change time as the temperature increases, the better the corrosion resistance. The polarization curve shows that the corrosion current density decreases with increasing temperature, indicating that the corrosion resistance improves with increasing temperature. The polarization resistance change diagram shows that the polarization resistance at 90℃ is higher than that at other temperatures, indicating that the corrosion resistance of the conversion film is better than that under other conditions. According to the above characterization results, the experimental results are consistent with the calculated results to a great extent, indicating that the calculation method is reliable.

[0080] In step 5, the nucleation kinetics were further increased by adding an oxidant. Due to the strong oxidizing properties and low cost of sodium nitrite, sodium nitrite was used as a strong oxidant, and the concentration range was locked in between 0.5 g / L and 2 g / L. Four experimental points were taken: 0.5 g / L, 1 g / L, 1.5 g / L, and 2 g / L. The main salts, manganese sulfate, were controlled at 0.2 mol / L, sodium dihydrogen phosphate at 0.3 mol / L, the acid ratio was adjusted to 14.68, the temperature was 90°C, and the EDTA concentration was 3 g / L. The experiment screened out the optimal amount of sodium nitrite at 1.5 g / L.

[0081] See also Figures 9-12 It can be seen that the same conclusion can be drawn through salt spray test, copper sulfate drip test, polarization and impedance test: the corrosion resistance is best when the sodium nitrite concentration is 1.5g / L, so the sodium nitrite concentration is selected as 1.5g / L.

[0082] In step 5, the nucleation kinetics were further enhanced by adding a complexing agent. Since EDTA greatly promotes electrochemical cathode and anode reactions, four experimental points were selected: 3g / L, 8g / L, 15g / L, and 22.8g / L. The main salts, manganese sulfate, were controlled at 0.2mol / L, sodium dihydrogen phosphate at 0.3mol / L, the acid ratio was adjusted to 14.68, the temperature was 90°C, and the sodium nitrite concentration was 1.5g / L. The experiment was conducted. Taking into account corrosion resistance and cost factors, the EDTA addition amount was selected to be 3g / L.

[0083] See also Figure 13-14It can be seen that the addition of EDTA greatly promotes the hydrogen evolution and weight loss of metal dissolution, indicating that the addition of EDTA accelerates the dissolution of the metal, greatly increases the concentration of monohydrogen phosphate ions in the next step of the deposition reaction, and promotes the deposition reaction. However, with the increase of EDTA concentration, it is found that the acceleration of hydrogen evolution and weight loss decreases, that is, the increase in EDTA concentration has a relatively small effect on the improvement of metal dissolution. The salt spray test results show that the corrosion resistance of 3g / L is much better than that of no EDTA, and the corrosion resistance is best under the condition of 8g / L. Taking into account the industrial cost issues and corrosion resistance, 3g / L is selected as the best condition.

[0084] Finally, the optimal formula of the high corrosion-resistant conversion coating solution for Q345C steel based on nucleation control was obtained: the main salt manganese sulfate was 0.2 mol / L, sodium dihydrogen phosphate was 0.3 mol / L, the acid ratio was adjusted to 14.68, the temperature was 90℃, the sodium nitrite concentration was 1.5 g / L, and the EDTA addition amount was 3 g / L.

[0085] In addition, Example 1 investigated the treatment of Q345C steel with a conversion coating solution containing 0.2 mol / L manganese sulfate as the main salt, 0.3 mol / L sodium dihydrogen phosphate, 14.68 acid ratio, 1.5 g / L sodium nitrite concentration, and 3 g / L EDTA at phosphating temperatures of 60°C, 70°C, 80°C, and 90°C. The XRD patterns of the prepared conversion coatings are shown in FIG. Figure 4 shown. Figure 4 The diffraction results show that although Q345C steel is treated with different phosphating temperatures, the substance on the surface of the conversion film does not change, and the main substance is MnHPO4. This further verifies that the substance of the chemical conversion film preliminarily determined by the thermodynamic phase diagram is MnHPO4. At the same time, with the increase of temperature, the diffraction peak intensity of the MnHPO4 phase increases accordingly, which indicates that with the increase of temperature, the content of MnHPO4 on the coating surface increases.

[0086] Comparative Example 1

[0087] Comparative Example 1 is bare Q345C steel without conversion coating.

[0088] Results and Discussion

[0089] Figure 15 The salt spray test diagram of bare Q345C steel provided in Comparative Example 1 and the salt spray test diagram of Q345C steel phosphating treated with the optimal formula of the conversion coating solution in Example 1 are shown. Figure 15 It can be seen that the steel that has not been converted has large-scale corrosion in 24 hours under salt spray conditions, while the steel that has been converted under optimal parameters has smaller-area corrosion in 504 hours, and its corrosion resistance has been greatly improved.

[0090] In summary, the corrosion resistance of the manganese phosphate film prepared in Example 1 is greatly improved compared with the previous conversion film without any pre-treatment and post-treatment.

[0091] It is easy for those skilled in the art to understand that, under the premise of no conflict, the above-mentioned advantageous methods can be freely combined and superimposed. The above is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application. The above is only a preferred embodiment of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and variations can be made without departing from the technical principles of the present application. These improvements and variations should also be regarded as the scope of protection of the present application.

Claims

1. A design method for a steel high corrosion resistant conversion film based on nucleation control, characterized in that: The steps include: Determine the film-forming substance dataset, obtain the metal ions used for film formation in the steel phosphating process and their corresponding film-forming substances, and establish the film-forming substance dataset for steel conversion film design; Screen the main salt system and film-forming substances, establish a thermodynamic deposition phase diagram of the metal ion concentration and pH value of the film-forming substance based on the solubility product principle, screen the substances that are most easily deposited within the pH range of steel phosphating, and determine the main salt type and film-forming substance for phosphating; The acid ratio parameters were optimized, the film-forming temperature range was selected, and conversion film solutions with different acid ratios were designed. After two thermal cycles of heating to the target temperature, cooling and collecting the supernatant, and then heating to the target temperature again, the acid ratio that kept the conversion film solution stable and close to the critical supersaturation within the film-forming temperature range was screened. The nucleation conditions were regulated, with the higher the supersaturation, the higher the nucleation rate. The supersaturation of the conversion film solution at different temperatures and acid ratios within the film-forming temperature range was calculated, and the temperature and acid ratio of the conversion film solution with the highest supersaturation were determined. Optimizing the additive ratio, adding oxidants and complexing agents to further enhance nucleation, and determining the type and amount of oxidants and complexing agents in the conversion coating solution with the highest supersaturation through experimental testing and in combination with corrosion resistance and cost, ultimately obtaining a steel high corrosion-resistant conversion coating solution based on nucleation control; The high corrosion resistant conversion film for steel is prepared by using the high corrosion resistant conversion film solution based on nucleation control.

2. The design method of a steel high corrosion resistant conversion film based on nucleation control according to claim 1, characterized in that: The design method is used for designing a high corrosion resistant conversion coating for Q345C steel, and includes the following steps: Determine the film-forming substance dataset, obtain the metal ions used for film formation in the phosphating process of Q345C steel and their corresponding film-forming substances, and establish the film-forming substance dataset for the Q345C steel conversion film design; Screen the main salt system and film-forming substances, establish a thermodynamic deposition phase diagram of the metal ion concentration and pH value of the film-forming substances based on the solubility product principle, screen the substances that are most easily deposited within the pH range of Q345C steel phosphating, and determine the main salt type and film-forming substances for Q345C steel phosphating; The acid ratio parameters were optimized, and the film-forming temperature range was selected as 60°C-90°C. Q345C steel conversion coating solutions with different acid ratios were designed. After two thermal cycles of heating to the target temperature, cooling and collecting the supernatant, and then reheating to the target temperature, the acid ratio that kept the Q345C steel conversion coating solution stable and close to the critical supersaturation within the range of 60°C-90°C was screened. The nucleation conditions were regulated, with the higher the supersaturation, the higher the nucleation rate. The supersaturation of the Q345C steel conversion coating solution at different target temperatures and acid ratios in the range of 60°C-90°C was calculated, and the temperature and acid ratio for the Q345C steel conversion coating solution with the highest supersaturation were determined. The additive ratio was optimized, and oxidants and complexing agents were added to further enhance nucleation. Through experimental testing and in combination with corrosion resistance and cost, the types and addition amounts of oxidants and complexing agents in the Q345C steel conversion coating solution with the highest supersaturation were determined, ultimately resulting in a Q345C steel high corrosion-resistant conversion coating solution based on nucleation control. The high corrosion resistant conversion film of Q345C steel is prepared by using the high corrosion resistant conversion film solution of Q345C steel based on nucleation control.

3. The design method of steel high corrosion resistant conversion film based on nucleation control according to claim 2, characterized in that: The film-forming material data set used in the Q345C steel conversion film design includes: manganese phosphide, zinc phosphide, calcium phosphide and iron phosphide.

4. The design method of a steel high corrosion resistant conversion film based on nucleation control according to claim 3, characterized in that: The screening of the main salt system and the film-forming substance includes: screening the most easily deposited substance as MnHPO4 within the pH value of 2-4 for phosphating Q345C steel, and determining that the main salt type for phosphating Q345C steel is manganese series and the film-forming substance is MnHPO4.

5. The design method of steel high corrosion resistant conversion film based on nucleation control according to claim 2, characterized in that: In the optimized acid ratio parameters, the preparation method of the Q345C steel conversion coating solution with different acid ratios is to add different amounts of sulfuric acid to a reference solution; wherein the manganese-phosphorus ratio of the reference solution is 2:3, the manganese ion concentration is 0.2 mol / L, and the dihydrogen phosphate concentration is 0.3 mol / L.

6. The design method of a steel high corrosion resistant conversion film based on nucleation control according to claim 5, characterized in that: The nucleation conditions are controlled, including: calculating the MnHPO4 and H2PO4 at different target temperatures. - Solubility product K sp , the reaction quotient Jsp is calculated based on the acid ratio at different target temperatures, thereby calculating the supersaturation values ​​of the Q345C steel conversion film solution at different target temperatures and different acid ratios, and then determining the temperature value and acid ratio of the Q345C steel conversion film solution with the highest supersaturation.

7. The design method of a steel high corrosion resistant conversion film based on nucleation control according to claim 6, characterized in that: According to calculation, the temperature of the Q345C steel conversion coating solution with the highest supersaturation is 90° C., and the acid ratio is 14.

68.

8. The method for designing a steel high corrosion resistant conversion film based on nucleation control according to claim 2, characterized in that: The optimized additive ratio includes: using sodium nitrite as an oxidant, and optimizing the amount of sodium nitrite added in an experiment within the range of sodium nitrite concentration of 0.5 g / L-2 g / L.

9. The design method of a steel high corrosion resistant conversion film based on nucleation control according to claim 2, characterized in that: The optimized additive ratio includes: using EDTA as a complexing agent, and optimizing the amount of EDTA added within the EDTA concentration range of 3 g / L-22.8 g / L.

10. The design method of steel high corrosion resistant conversion film based on nucleation control according to claim 2, characterized in that: The formula of the Q345C steel high corrosion-resistant conversion film solution based on nucleation control is as follows: manganese sulfate concentration is 0.2 mol / L, sodium dihydrogen phosphate concentration is 0.3 mol / L, acid ratio is 14.68, sodium nitrite addition amount is 1.5 g / L, EDTA addition amount is 3 g / L, and film forming temperature is 90°C.

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