CN110361313B - Electrochemical test method for quantitatively evaluating porosity of phosphating film - Google Patents
Electrochemical test method for quantitatively evaluating porosity of phosphating film Download PDFInfo
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Abstract
The invention relates to an electrochemical test method for quantitatively evaluating the porosity of a phosphating film, which comprises the following specific steps: (1) placing a phosphating film sample in electrolyte for an electrochemical alternating current impedance test to obtain a Nyquist diagram and a Bode diagram; (2) fitting the low-carbon mild steel sample of the base metal in the electrolyte solution by using the Nyquist diagram obtained in the step (1)
And the interfacial differential capacitance C of the phosphating film sample in the electrolytedl(ii) a (3) Fitting an experimental frequency point number N' corresponding to the phosphating film from high frequency to penetration phase or penetration frequency in the alternating current impedance test frequency range through the Bode diagram obtained in the step (1); (4) and (4) calculating the porosity according to the parameters obtained in the steps (2) and (3). Compared with the prior art, the method for quantitatively evaluating the porosity of the phosphating film is easy to implement, quick, simple and convenient and high in accuracy.
Description
Technical Field
The invention relates to a method for measuring the porosity of a phosphate film, in particular to an electrochemical test method for quantitatively evaluating the porosity of the phosphate film.
Background
The phosphorization of carbon steel, aluminum alloy, zinc alloy, cadmium and other metal materials is one of the methods widely adopted in surface treatment, the purpose of the metal surface phosphorization is mainly shown in two aspects, one is applied to the pretreatment process of coating to increase the adhesive force of a coating, the other is to obtain a corrosion-resistant, lubricating and electric-insulating functional phosphorization film, in addition, the phosphorization film has the decorative effect, and the post-treatment of the phosphorized metal parts such as binding oil or paraffin wax can provide temporary protection effect[1]. Among the characteristics of the phosphating film, the corrosion resistance is an important use performance index of the phosphating film, the conventional test method and the electrochemical test method are mainly used for testing and evaluating the corrosion resistance of the phosphating film at present, the conventional test method mainly comprises a copper sulfate pitting test and a brine test, the two test methods can directly evaluate the corrosion resistance of the phosphating film, but only can give a qualitative result, the test endpoint needs to be judged by depending on experience when the film is thick, larger artificial errors are easily introduced, and the detection result has larger subjectivity. The electrochemical test method for the corrosion resistance of the phosphating film mainly comprises a potentiodynamic scanning method and an electrochemical alternating current impedance method, wherein the potentiodynamic scanning method and the electrochemical alternating current impedance method are used for testing the sample coated with the phosphating film in the electrolyteCorrosion potential, corrosion current and polarization resistance of corrosion, thereby quantitatively evaluating the corrosion resistance of the phosphating film[2,3]。
The electrochemical characterization method for the corrosion resistance of the phosphating film has the advantage of high accuracy. In fact, besides the corrosion potential, the corrosion current and the polarization resistance of corrosion, the porosity of the phosphating film and the corrosion resistance of the film layer have good correspondence, and the method is an important characteristic of the phosphating film, measures the porosity of the phosphating film layer and can quantitatively evaluate the corrosion resistance of the phosphating film.
The quantitative evaluation of the porosity of the phosphate film can be obtained by calculating through image analysis software[4]The principle is that the color of the defect positions such as pores, cracks and the like can be seen to be darker from the SEM appearance of a phosphating film sample, a proper threshold value is set by the color level of the pore positions through Photoshop software, the area higher than the threshold value is white, the area lower than the threshold value is black, so that a gray-scale picture with clear black and white is generated, then the percentage ratio of the black area in the gray-scale picture is calculated through image analysis software, and the porosity of the phosphating film is calculated. Furthermore, Notter et al[5]Method for electrochemical experiment by adopting potentiodynamic scanning, Weng, Lendvay and the like[1,6]The porosity of the phosphating film is quantitatively evaluated by adopting an electrochemical alternating-current impedance experiment method. In fact, the mathematical model for evaluating the porosity of the phosphate film by electrochemical methods of Notter, Weng and the like is derived from A.T.A.Jenkins, R.D.Armstrong[7]Reported mathematical models for coating porosity (as shown in figures 1-2). The model in fig. 1 considers that in the electrochemical ac impedance test of porosity, the membrane is insulating, and after the electrolyte penetrates into the metal surface through the membrane pores, the area of corrosion of the base metal is equal to the area of the pores, and the calculation formula of porosity can be expressed by the following equation:
wherein A isdPorosity, p-electrolyte resistivity in pores (Ω. cm), d-coating thickness (cm), RpoIn electrolysis of the base metalPolarization resistance in liquid (Ω · cm)2)。
The mathematical model in fig. 2 considers that in the electrochemical ac impedance test, the membrane is insulated, after the electrolyte permeates into the metal surface through the membrane pores, the corroded area of the base metal is not equal to the area of the membrane surface pores, the corroded area is larger than the area of the membrane surface pores, and the calculation formula of the porosity can be represented by the following equation:
wherein A isd-a porosity of the porous material,
differential capacitance (μ F) of the base metal in the electrolyte, CdlDifferential capacitance of the film in the electrolyte (μ F).
Notter et al used a potentiodynamic scanning electrochemical test method to test the porosity of the phosphating film and Weng et al [1] reported mathematical models for quantitatively evaluating the porosity of the phosphating film by an electrochemical alternating current impedance test method, all consider the phosphating film to be insulating, and the phosphating film does not participate in electrochemical dissolution in the electrochemical test process.
Although the above test methods for evaluating the porosity of the phosphating film have a certain theoretical basis, the test methods only consider the microscopic surface morphology of the phosphating film, do not touch the internal structure condition of the crystallization of the phosphating film, and do not consider the influence of the thickness of the phosphating film on the porosity test, and the porosity data calculated by the methods inevitably deviate from the real condition of the porosity of the phosphating film.
Reference documents:
[1]Weng D,Jokiel P,Uebleis A,et a1.Corrosion and characteristics of zinc and manganese phosphate coatings[J].Surface and Coatings Technology,1996,88:147-156.
[2] hederan, Wang Minghao, Rui Zhengdan, etc., research on the zinc-manganese phosphating solution with high corrosion resistance and electrochemical analysis of phosphating film [ J ]. proceedings of Hunan university, 2009,36(4):65-69.
[3] Linebelan, Lu-Wentang, class of pore, et al.
[4] Shao hong, Chen Ting, Qichangyang, etc. the influence of sealing treatment on the phosphating film performance under the 316L stainless steel ultrasonic field [ J ] China surface engineering, 2017,30(1):63-69.
[5]Notter I M,Gabe D R.Polarisation resistance methods for measurement of the porosity of thin metal coatings[J].Corrosion Science,1993,34(5):851.
[6]Lendvay-gyorik G,Meszaros G,Lengyel B.A simple testing method for quality control of phosphate coatings based on impedance measurements[J].Journal of Applied Electrochemistry,2002,32(8):891.
[7]A.T.A.Jenkins,R.D.Armstrong.Comments on the article“Use of electrochemical impedance spectroscopy for the study of corrosion protection by polymer coatings”by F.Mansfeld[J].Journal of Applied Electrochemistry,1995,25:1143-1144.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide an electrochemical test method for quantitatively evaluating the porosity of a phosphate film.
The purpose of the invention can be realized by the following technical scheme:
an electrochemical test method for quantitatively evaluating the porosity of a phosphating film comprises the following specific steps:
(1) placing a phosphating film sample in electrolyte for an electrochemical alternating current impedance test to obtain a Nyquist diagram and a Bode diagram;
(2) fitting the low-carbon mild steel sample of the base metal in the electrolyte solution by using the Nyquist diagram obtained in the step (1)
And the interfacial differential capacitance C of the phosphating film sample in the electrolytedl；
(3) Fitting an experimental frequency point number N' corresponding to the phosphating film from high frequency (10kHz) to a penetration phase or penetration frequency in the alternating current impedance test frequency range through the Bode diagram obtained in the step (1);
(4) and (3) calculating the porosity according to the parameters obtained in the steps (2) and (3), wherein the calculation formula for evaluating the porosity is as follows:
wherein A isd-a porosity of the porous material,
differential capacitance (μ F) of the base metal in the electrolyte, CdlDifferential capacitance (μ F), K-proportionality coefficient of the phosphating film in the electrolyte
Wherein N' is the number of experimental frequency points corresponding to the AC impedance test frequency range from high frequency to penetration phase or penetration frequency of the phosphating film, NoIs the total experimental frequency point number in the AC impedance test frequency range of the phosphating film.
Preferably, in step (1): the electrochemical alternating-current impedance experiment adopts a three-electrode system, a phosphating film sample is placed in a three-electrode electrolytic cell, and an electrochemical workstation is adopted for testing.
More preferably, in the three-electrode system, the working electrode is a base metal low-carbon steel sample and a phosphated low-carbon steel phosphating film sample respectively, the auxiliary electrode is a platinum black electrode, and the reference electrode is an Ag/AgCl/KCl aqueous solution electrode.
More preferably, the area of the working electrode is 1cm2。
More preferably, the concentration of the KCl aqueous solution in the reference electrode is 3 mol/L.
Preferably, the amplitude of the alternating current signal in the electrochemical workstation is +/-10 mV.
Preferably, the electrolyte is a 3.5 wt.% NaCl solution.
Preferably, the electrolysis temperature is 20 ℃.
Preferably, the scanning frequency range in the electrochemical workstation is 10KHz-0.01 Hz.
The matrix metal sample is low-carbon mild steel with the size of 30mm multiplied by 50mm multiplied by 0.5 mm. And (3) dipping the base metal sample material into a medium-temperature phosphating solution for phosphating for 10min to prepare a phosphating film sample.
The medium-temperature zinc phosphating solution comprises the following components and operating parameters: zinc dihydrogen phosphate (Zn (H)2PO4)2)33g/L of zinc nitrate (Zn (NO)3)2·6H2O)95g/L, nickel nitrate (Ni (NO)3)2·6H2O1 g/L, sodium fluoride (NaF)1.2g/L, cerium nitrate (Ce (NO))3·6H2O)0-100mg/L, the temperature is 65 ℃, and pure water is used for preparing the phosphating solution and cleaning the sample.
In the Bode diagram, the phase and the frequency corresponding to the intersection point of the tangent line of the obviously-rising part of the capacitive reactance arc phase at the high frequency and the tangent line of the basically-unchanged part of the phase at the high frequency are obtainedτAnd logfτTwo parameters respectively represent the penetrating phase and the corresponding penetrating frequency of the electrolyte penetrating the phosphating film from the pores on the surface of the phosphating film to the surface of the base metal in the testing process of an electrochemical alternating-current impedance experiment from high frequency to low frequency, and the two parameters are closely related to the compactness of the phosphating film and the thickness of the phosphating film. The AC impedance experiment starts from high frequency to phiτOr logfτThe total number of the experimental frequencies corresponding to the time is N'.
Compared with the prior art, the invention has the following beneficial effects:
1. the structure conditions of the interior of the phosphorization film crystal, such as crystal grain size, crystal shape, superposition between crystal grains, compactness, holes, cracks and the like, can be better reflected, and the influence of the thickness of the phosphorization film on the porosity is considered.
2. Can well reflect the real situation of the porosity of the phosphating film and can correspond to the corrosion resistance of the phosphating film.
3. The method for quantitatively evaluating the porosity of the phosphating film by adopting an electrochemical alternating-current impedance experimental method is easy to implement, quick, simple and convenient and high in accuracy.
Drawings
Figure 1 is a schematic diagram of a mathematical model reported by a.t.a.jenkins, r.d.armstrong for coating porosity;
fig. 2 is a schematic diagram of a mathematical model reported by a.t.a.jenkins, r.d.armstrong for coating porosity;
FIG. 3 is a schematic view of the porosity of the phosphating film of the invention;
FIG. 4 is a Nyquist plot of the AC impedance of the phosphating film sample of example 1;
FIG. 5 is a graph of the AC impedance Bode of a sample phosphating film of example 1;
FIG. 6 is a Nyquist plot of the AC impedance of the phosphating film sample of example 2;
FIG. 7 is a graph showing the AC impedance Bode of a phosphating film sample in example 2;
FIG. 8 is a Nyquist plot of the AC impedance of the phosphating film sample of example 3;
FIG. 9 is a graph of the AC impedance Bode of a sample phosphating film of example 3;
FIG. 10 is a Nyquist plot of the AC impedance of the phosphating film sample of example 4;
FIG. 11 is a graph of the AC impedance Bode of a sample phosphating film of example 4;
FIG. 12 is a Nyquist plot of the AC impedance of the phosphating film sample of example 5;
FIG. 13 is a graph showing the AC impedance Bode of a phosphating film sample in example 5;
FIG. 14 is a Nyquist plot of the AC impedance of the phosphating film sample of example 6;
FIG. 15 is a graph showing the AC impedance Bode of a sample of a phosphating film of example 6.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments.
The test equipment used in the following examples:
low carbon mild steel (30mm x 50mm x 0.5mm), 250mL three-electrode system electrolytic cell, 260 type platinum black electrode, silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L) ], AutolabPGST302/FRA electrochemical workstation, QCC-A type magnetic thickness gauge. And (3) measuring the thickness of the phosphating layer by using a QCC-A type magnetic thickness gauge, measuring 3 points on the surface of the sample after the gauge is corrected, and averaging to obtain the thickness of the phosphating layer.
Example 1
The medium-temperature zinc phosphating solution comprises the following components and operating parameters: zinc dihydrogen phosphate (Zn (H)2PO4)2)33g/L of zinc nitrate (Zn (NO)3)2·6H2O)95g/L, nickel nitrate (Ni (NO)3)2·6H2O1 g/L, sodium fluoride (NaF)1.2g/L, cerium nitrate (Ce (NO))3·6H2O)0mg/L, temperature 65 ℃. 33g of zinc dihydrogen phosphate, 95g of zinc nitrate, 1g of nickel nitrate and 1.2g of sodium fluoride are respectively weighed, the weighed chemical reagents are dissolved into 500mL of purified water under stirring, the solution is diluted to 1000mL after being completely dissolved, the solution is uniformly stirred and heated and is kept at 65 ℃, and the medium-temperature zinc-series phosphating solution is obtained.
Using 1 for a base metal mild low carbon steel sample (30mm multiplied by 50mm multiplied by 0.5mm)#～5#And sequentially polishing the metallographic abrasive paper, cleaning with purified water, and soaking into a medium-temperature phosphating solution for phosphating for 10min to prepare a phosphating film sample. Measuring the thickness of the phosphating layer of a phosphating film sample by using a QCC-A type magnetic thickness gauge, and then performing electrochemical alternating-current impedance spectroscopy measurement, wherein the specific operation process is as follows:
200mL of the prepared 3.5 wt% NaCl solution is measured by a measuring cylinder, poured into a 250mL three-electrode electrolytic cell and then put into a water bath kettle at a constant temperature of 20 ℃. The geometric area is 1cm2The phosphating film sample is taken as a working electrode and placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is taken as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L)]The test instrument is an autolab PGST302/FRA electrochemical workstation, the amplitude of an alternating voltage signal is +/-10 mV, the frequency range of the alternating voltage signal is 10KHz-0.01Hz, an impedance spectrum experiment is carried out under the conditions of open circuit potential and dissolved oxygen-containing atmosphere, and the total frequency spectrum number N of the experimentoIs 61. The electrochemical AC impedance Nyquist diagram (FIG. 4) and Bode diagram (FIG. 5) of the phosphating film sample were obtained. Fitting a Nyquist diagram by electrochemical analysis software to obtain the differential capacitance C of the phosphate film sample in the electrolytedlAnd obtaining the experimental frequency point number N' corresponding to the phosphating film sample from high frequency to penetration phase or penetration frequency in the alternating current impedance test frequency range according to the electrochemical impedance spectrum Bode diagram.
The geometric area is 1cm2The matrix metal mild steel sample is used as a working electrode and is placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is used as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L) ]]Performing electrochemical alternating current impedance test on the reference electrode by using the same test instrument and test conditions to obtain a Nyquist diagram of the base metal sample, and fitting the Nyquist diagram by using electrochemical analysis software to obtain the differential capacitance of the base metal low-carbon mild steel sample in the electrolyte
Further by the formula
And calculating the porosity of the phosphating film. The results of calculation of each parameter and porosity are shown in table 1.
TABLE 1
Example 2
The medium-temperature zinc phosphating solution comprises the following components and operating parameters: zinc dihydrogen phosphate (Zn (H)2PO4)2)33g/L of zinc nitrate (Zn (NO)3)2·6H2O)95g/L, nickel nitrate (Ni (NO)3)2·6H2O1 g/L, sodium fluoride (NaF)1.2g/L, cerium nitrate (Ce (NO))3·6H2O)20mg/L, temperature 65 ℃. 33g of zinc dihydrogen phosphate, 95g of zinc nitrate, 1g of nickel nitrate, 1.2g of sodium fluoride and 20mg of cerium nitrate are respectively weighed, the weighed chemical reagents are dissolved into 500mL of purified water under stirring, the solution is diluted to 1000mL after being completely dissolved, and the solution is uniformly stirred, heated and kept at 65 ℃ to obtain the medium-temperature zinc-series phosphating solution.
Using 1 for a base metal mild low carbon steel sample (30mm multiplied by 50mm multiplied by 0.5mm)#～5#And sequentially polishing the metallographic abrasive paper, cleaning with purified water, and soaking into a medium-temperature phosphating solution for phosphating for 10min to prepare a phosphating film sample. Use of QCC-A type magnetism for phosphating film samplesThe thickness meter measures the thickness of the phosphating layer, and then electrochemical alternating-current impedance spectroscopy measurement is carried out, and the specific operation process is as follows:
200mL of the prepared 3.5 wt% NaCl solution is measured by a measuring cylinder, poured into a 250mL three-electrode electrolytic cell and then put into a water bath kettle at a constant temperature of 20 ℃. The geometric area is 1cm2The phosphating film sample is taken as a working electrode and placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is taken as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L)]The test instrument is an autolab PGST302/FRA electrochemical workstation, the amplitude of an alternating voltage signal is +/-10 mV, the frequency range of the alternating voltage signal is 10KHz-0.01Hz, an impedance spectrum experiment is carried out under the conditions of open circuit potential and dissolved oxygen-containing atmosphere, and the total frequency spectrum number N of the experimentoIs 61. The electrochemical AC impedance Nyquist plot (FIG. 6) and Bode plot (FIG. 7) of the phosphate film sample were obtained. Fitting a Nyquist diagram by electrochemical analysis software to obtain the differential capacitance C of the phosphate film sample in the electrolytedlAnd obtaining the experimental frequency point number N' corresponding to the phosphating film sample from high frequency to penetration phase or penetration frequency in the alternating current impedance test frequency range according to the electrochemical impedance spectrum Bode diagram.
The geometric area is 1cm2The matrix metal mild steel sample is used as a working electrode and is placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is used as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L) ]]Performing electrochemical alternating current impedance test on the reference electrode by using the same test instrument and test conditions to obtain a Nyquist diagram of the base metal sample, and fitting the Nyquist diagram by using electrochemical analysis software to obtain the differential capacitance of the base metal low-carbon mild steel sample in the electrolyte
Further by the formula
And calculating the porosity of the phosphating film. The results of calculation of each parameter and porosity are shown in table 2.
TABLE 2
Example 3
The medium-temperature zinc phosphating solution comprises the following components and operating parameters: zinc dihydrogen phosphate (Zn (H)2PO4)2)33g/L of zinc nitrate (Zn (NO)3)2·6H2O)95g/L, nickel nitrate (Ni (NO)3)2·6H2O1 g/L, sodium fluoride (NaF)1.2g/L, cerium nitrate (Ce (NO))3·6H2O)40mg/L, temperature 65 ℃. 33g of zinc dihydrogen phosphate, 95g of zinc nitrate, 1g of nickel nitrate, 1.2g of sodium fluoride and 40mg of cerium nitrate are respectively weighed, the weighed chemical reagents are dissolved into 500mL of purified water under stirring, the solution is diluted to 1000mL after being completely dissolved, and the solution is uniformly stirred, heated and kept at 65 ℃ to obtain the medium-temperature zinc-series phosphating solution.
Using 1 for a base metal mild low carbon steel sample (30mm multiplied by 50mm multiplied by 0.5mm)#～5#And sequentially polishing the metallographic abrasive paper, cleaning with purified water, and soaking into a medium-temperature phosphating solution for phosphating for 10min to prepare a phosphating film sample. Measuring the thickness of the phosphating layer of a phosphating film sample by using a QCC-A type magnetic thickness gauge, and then performing electrochemical alternating-current impedance spectroscopy measurement, wherein the specific operation process is as follows:
200mL of the prepared 3.5 wt% NaCl solution is measured by a measuring cylinder, poured into a 250mL three-electrode electrolytic cell and then put into a water bath kettle at a constant temperature of 20 ℃. The geometric area is 1cm2The phosphating film sample is taken as a working electrode and placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is taken as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L)]The test instrument is an autolab PGST302/FRA electrochemical workstation, the amplitude of an alternating voltage signal is +/-10 mV, the frequency range of the alternating voltage signal is 10KHz-0.01Hz, an impedance spectrum experiment is carried out under the conditions of open circuit potential and dissolved oxygen-containing atmosphere, and the total frequency spectrum number N of the experimentoIs 61. The electrochemical AC impedance Nyquist diagram (FIG. 8) and Bode diagram (FIG. 9) of the phosphate film sample were obtained. Fitting a Nyquist diagram by electrochemical analysis software to obtain the differential capacitance C of the phosphate film sample in the electrolytedlFrom the Bode diagram of the electrochemical impedance spectrumThe number of experimental frequency points N' corresponding to the penetration phase or the penetration frequency from high frequency to the alternating current impedance test frequency range of the phosphating film sample.
The geometric area is 1cm2The matrix metal mild steel sample is used as a working electrode and is placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is used as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L) ]]Performing electrochemical alternating current impedance test on the reference electrode by using the same test instrument and test conditions to obtain a Nyquist diagram of the base metal sample, and fitting the Nyquist diagram by using electrochemical analysis software to obtain the differential capacitance of the base metal low-carbon mild steel sample in the electrolyte
Further by the formula
And calculating the porosity of the phosphating film. The results of calculation of each parameter and porosity are shown in table 3.
TABLE 3
Example 4
The medium-temperature zinc phosphating solution comprises the following components and operating parameters: zinc dihydrogen phosphate (Zn (H)2PO4)2)33g/L of zinc nitrate (Zn (NO)3)2·6H2O)95g/L, nickel nitrate (Ni (NO)3)2·6H2O1 g/L, sodium fluoride (NaF)1.2g/L, cerium nitrate (Ce (NO))3·6H2O)60mg/L, temperature 65 ℃. 33g of zinc dihydrogen phosphate, 95g of zinc nitrate, 1g of nickel nitrate, 1.2g of sodium fluoride and 60mg of cerium nitrate are respectively weighed, the weighed chemical reagents are dissolved into 500mL of purified water under stirring, the solution is diluted to 1000mL after being completely dissolved, and the solution is uniformly stirred, heated and kept at 65 ℃ to obtain the medium-temperature zinc-series phosphating solution.
Using 1 for a base metal mild low carbon steel sample (30mm multiplied by 50mm multiplied by 0.5mm)#～5#Metallographic abrasive paper is beaten in proper orderGrinding, washing with purified water, and soaking in medium-temperature phosphating solution for phosphating for 10min to prepare a phosphating film sample. Measuring the thickness of the phosphating layer of a phosphating film sample by using a QCC-A type magnetic thickness gauge, and then performing electrochemical alternating-current impedance spectroscopy measurement, wherein the specific operation process is as follows:
200mL of the prepared 3.5 wt% NaCl solution is measured by a measuring cylinder, poured into a 250mL three-electrode electrolytic cell and then put into a water bath kettle at a constant temperature of 20 ℃. The geometric area is 1cm2The phosphating film sample is taken as a working electrode and placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is taken as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L)]The test instrument is an autolab PGST302/FRA electrochemical workstation, the amplitude of an alternating voltage signal is +/-10 mV, the frequency range of the alternating voltage signal is 10KHz-0.01Hz, an impedance spectrum experiment is carried out under the conditions of open circuit potential and dissolved oxygen-containing atmosphere, and the total frequency spectrum number N of the experimentoIs 61. The electrochemical AC impedance Nyquist plot (FIG. 10) and Bode plot (FIG. 11) of the phosphate film sample were obtained. Fitting a Nyquist diagram by electrochemical analysis software to obtain the differential capacitance C of the phosphate film sample in the electrolytedlAnd obtaining the experimental frequency point number N' corresponding to the phosphating film sample from high frequency to penetration phase or penetration frequency in the alternating current impedance test frequency range according to the electrochemical impedance spectrum Bode diagram.
The geometric area is 1cm2The matrix metal mild steel sample is used as a working electrode and is placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is used as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L) ]]Performing electrochemical alternating current impedance test on the reference electrode by using the same test instrument and test conditions to obtain a Nyquist diagram of the base metal sample, and fitting the Nyquist diagram by using electrochemical analysis software to obtain the differential capacitance of the base metal low-carbon mild steel sample in the electrolyte
Further by the formula
And calculating the porosity of the phosphating film. Calculation results of parameters and porosityAs shown in table 4.
TABLE 4
Example 5
The medium-temperature zinc phosphating solution comprises the following components and operating parameters: zinc dihydrogen phosphate (Zn (H)2PO4)2)33g/L of zinc nitrate (Zn (NO)3)2·6H2O)95g/L, nickel nitrate (Ni (NO)3)2·6H2O1 g/L, sodium fluoride (NaF)1.2g/L, cerium nitrate (Ce (NO))3·6H2O)80mg/L at a temperature of 65 ℃. 33g of zinc dihydrogen phosphate, 95g of zinc nitrate, 1g of nickel nitrate, 1.2g of sodium fluoride and 80mg of cerium nitrate are respectively weighed, the weighed chemical reagents are dissolved into 500mL of purified water under stirring, the solution is diluted to 1000mL after being completely dissolved, and the solution is uniformly stirred, heated and kept at 65 ℃ to obtain the medium-temperature zinc-series phosphating solution.
Using 1 for a base metal mild low carbon steel sample (30mm multiplied by 50mm multiplied by 0.5mm)#～5#And sequentially polishing the metallographic abrasive paper, cleaning with purified water, and soaking into a medium-temperature phosphating solution for phosphating for 10min to prepare a phosphating film sample. Measuring the thickness of the phosphating layer of a phosphating film sample by using a QCC-A type magnetic thickness gauge, and then performing electrochemical alternating-current impedance spectroscopy measurement, wherein the specific operation process is as follows:
200mL of the prepared 3.5 wt% NaCl solution is measured by a measuring cylinder, poured into a 250mL three-electrode electrolytic cell and then put into a water bath kettle at a constant temperature of 20 ℃. The geometric area is 1cm2The phosphating film sample is taken as a working electrode and placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is taken as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L)]The test instrument is an autolab PGST302/FRA electrochemical workstation, the amplitude of an alternating voltage signal is +/-10 mV, the frequency range of the alternating voltage signal is 10KHz-0.01Hz, an impedance spectrum experiment is carried out under the conditions of open circuit potential and dissolved oxygen-containing atmosphere, and the total frequency spectrum number N of the experimentoIs 61. The electrochemical AC impedance Nyquist plot (FIG. 12) and Bode plot (FIG. 13) of the phosphate film sample were obtained. By electrochemical divisionThe analysis software fits the Nyquist diagram to obtain the differential capacitance C of the phosphating film sample in the electrolytedlAnd obtaining the experimental frequency point number N' corresponding to the phosphating film sample from high frequency to penetration phase or penetration frequency in the alternating current impedance test frequency range according to the electrochemical impedance spectrum Bode diagram.
The geometric area is 1cm2The matrix metal mild steel sample is used as a working electrode and is placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is used as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L) ]]Performing electrochemical alternating current impedance test on the reference electrode by using the same test instrument and test conditions to obtain a Nyquist diagram of the base metal sample, and fitting the Nyquist diagram by using electrochemical analysis software to obtain the differential capacitance of the base metal low-carbon mild steel sample in the electrolyte
Further by the formula
And calculating the porosity of the phosphating film. The results of calculation of each parameter and porosity are shown in table 5.
TABLE 5
Example 6
The medium-temperature zinc phosphating solution comprises the following components and operating parameters: zinc dihydrogen phosphate (Zn (H)2PO4)2)33g/L of zinc nitrate (Zn (NO)3)2·6H2O)95g/L, nickel nitrate (Ni (NO)3)2·6H2O1 g/L, sodium fluoride (NaF)1.2g/L, cerium nitrate (Ce (NO))3·6H2O)100mg/L, temperature 65 ℃. 33g of zinc dihydrogen phosphate, 95g of zinc nitrate, 1g of nickel nitrate, 1.2g of sodium fluoride and 100mg of cerium nitrate are respectively weighed, the weighed chemical reagents are dissolved into 500mL of purified water under stirring, the solution is diluted to 1000mL after being completely dissolved, and the solution is uniformly stirred, heated and kept at 65 ℃ to obtain the medium-temperature zinc-series phosphating solution.
Using 1 for a base metal mild low carbon steel sample (30mm multiplied by 50mm multiplied by 0.5mm)#～5#And sequentially polishing the metallographic abrasive paper, cleaning with purified water, and soaking into a medium-temperature phosphating solution for phosphating for 10min to prepare a phosphating film sample. Measuring the thickness of the phosphating layer of a phosphating film sample by using a QCC-A type magnetic thickness gauge, and then performing electrochemical alternating-current impedance spectroscopy measurement, wherein the specific operation process is as follows:
200mL of the prepared 3.5 wt% NaCl solution is measured by a measuring cylinder, poured into a 250mL three-electrode electrolytic cell and then put into a water bath kettle at a constant temperature of 20 ℃. The geometric area is 1cm2The phosphating film sample is taken as a working electrode and placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is taken as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L)]The test instrument is an autolab PGST302/FRA electrochemical workstation, the amplitude of an alternating voltage signal is +/-10 mV, the frequency range of the alternating voltage signal is 10KHz-0.01Hz, an impedance spectrum experiment is carried out under the conditions of open circuit potential and dissolved oxygen-containing atmosphere, and the total frequency spectrum number N of the experimentoIs 61. The electrochemical AC impedance Nyquist plot (FIG. 14) and Bode plot (FIG. 15) of the phosphate film sample were obtained. Fitting a Nyquist diagram by electrochemical analysis software to obtain the differential capacitance C of the phosphate film sample in the electrolytedlAnd obtaining the experimental frequency point number N' corresponding to the phosphating film sample from high frequency to penetration phase or penetration frequency in the alternating current impedance test frequency range according to the electrochemical impedance spectrum Bode diagram.
The geometric area is 1cm2The matrix metal mild steel sample is used as a working electrode and is placed in a three-electrode electrolytic cell, a 260 type platinum black electrode is used as an auxiliary electrode, and a silver-silver chloride electrode [ Ag/AgCl, KCl (3.0mol/L) ]]Performing electrochemical alternating current impedance test on the reference electrode by using the same test instrument and test conditions to obtain a Nyquist diagram of the base metal sample, and fitting the Nyquist diagram by using electrochemical analysis software to obtain the differential capacitance of the base metal low-carbon mild steel sample in the electrolyte
Further by the formula
And calculating the porosity of the phosphating film. The results of calculation of each parameter and porosity are shown in table 6.
TABLE 6
From the results of the film thickness parameters and the porosity of the example 1 to the example 6, it can be seen that the film thickness of the phosphating films has an influence on the porosity, and the porosity of the films has a significant reduction trend along with the increase of the film thickness, such as the example 2, the example 3, the example 4 and the example 6, which is mainly because the superposition effect among crystal grains is significant when the film thickness is increased, and the porosity of the phosphating films is reduced, such as porosity, holes and cracks. However, the porosity of the film layer is related to the stacking effect among crystal grains, and also related to the internal structure condition of the crystal of the phosphating film, such as crystal grain size and crystal shape, so that the increase of the film layer thickness has no necessary correlation with the reduction of the porosity, and the parameters such as example 2 and example 1, example 4, example 5 and example 6 show that the porosity is not necessarily reduced when the film layer thickness is increased.
In the embodiment, the different concentrations of the cerium nitrate are mainly used for controlling the thickness of the membrane layer, the thickness of the membrane layer can be increased when the concentration of the cerium nitrate is less than 40mg/L, and the thickness of the membrane is slightly fluctuated when the concentration of the cerium nitrate is more than 40mg/L and basically can be regarded as constant.
Comparative example
The corrosion resistance of the phosphating film is an important characteristic, and the conventional experimental method for evaluating the corrosion resistance of the film mainly comprises a copper sulfate drop test method, wherein the copper sulfate drop test method is used for detecting the corrosion resistance of the film and comprises the following components in percentage by weight: copper sulfate (CuSO)4·5H2O)41 g/L; 35g/L of sodium chloride (NaCl); 13mL/L of 0.1mol/L hydrochloric acid (HCl), and the balance of distilled water. At 15-23 deg.C, a drop of test solution was applied to the phosphated surface while a stopwatch was pressed. When the drop was on the surface of the phosphating film, changing from sky blue to pale yellow or reddish, the time was recorded.The method can directly evaluate the corrosion resistance of the phosphating film. The porosity of the phosphating film has a better corresponding relation with the corrosion resistance of the film, the larger the porosity is, the poorer the corrosion resistance of the film is, so the corrosion resistance of the phosphating film can be determined by measuring the porosity, and the results of the conventional copper sulfate spot experiment method and the electrochemical test method for quantitatively evaluating the porosity of the phosphating film are shown in the following table.
TABLE 7
The longer the test time of the conventional copper sulfate drop is, the better the corrosion resistance of the phosphating film is, but the method can only give qualitative results, and when the film layer is thicker, the test endpoint needs to be judged by experience, so that larger human errors are easily introduced. As can be seen from the data in the table, the electrochemical test method for quantitatively evaluating the porosity of the phosphating film has better corrosion resistance corresponding to the film and better accuracy compared with the conventional experiment result.
The embodiments described above are described to facilitate an understanding and use of the invention by those skilled in the art. It will be readily apparent to those skilled in the art that various modifications to these embodiments may be made, and the generic principles described herein may be applied to other embodiments without the use of the inventive faculty. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications within the scope of the present invention based on the disclosure of the present invention.
Claims (7)
1. An electrochemical test method for quantitatively evaluating the porosity of a phosphating film is characterized by comprising the following specific steps:
(1) placing a phosphating film sample in electrolyte for an electrochemical alternating current impedance test to obtain a Nyquist diagram and a Bode diagram;
(2) fitting the low-carbon mild steel sample of the base metal in the electrolyte solution by using the Nyquist diagram obtained in the step (1)
And the interfacial differential capacitance C of the phosphating film sample in the electrolytedl；
(3) Fitting an experimental frequency point number N' corresponding to the phosphating film from high frequency to penetration phase or penetration frequency in the alternating current impedance test frequency range through the Bode diagram obtained in the step (1);
(4) and (3) calculating the porosity according to the parameters obtained in the steps (2) and (3), wherein the calculation formula for evaluating the porosity is as follows:
wherein A isd-a porosity of the porous material,
differential capacitance (μ F) of the base metal in the electrolyte, CdlDifferential capacitance (μ F), K-proportionality coefficient of the phosphating film in the electrolyte
Wherein N' is the number of experimental frequency points corresponding to the AC impedance test frequency range from high frequency to penetration phase or penetration frequency of the phosphating film, NoIs the total experimental frequency point number in the test frequency range of the alternating current impedance of the phosphating film;
wherein, the phase and frequency corresponding to the intersection point of the tangent line of the capacitive reactance arc phase ascending part at high frequency and the tangent line of the phase unchanged part at high frequency in the Bode diagram are obtained to obtain phiτAnd logfτTwo parameters respectively represent the penetration phase and the corresponding penetration frequency phi of the electrolyte penetrating the phosphating film from the pores on the surface of the phosphating film to the surface of the base metal in the testing process of the electrochemical alternating-current impedance experiment from high frequency to low frequencyτAnd logfτThe two parameters are closely related to the compactness of the phosphating film and the thickness of the phosphating film;
in the step (1): the electrochemical alternating-current impedance experiment adopts a three-electrode system, a phosphating film sample is placed in a three-electrode electrolytic cell, and an electrochemical workstation is adopted for testing;
in the three-electrode system, the working electrode is a base metal low-carbon steel sample and a phosphated low-carbon steel phosphating film sample respectively, the auxiliary electrode is a platinum black electrode, and the reference electrode is an Ag/AgCl/KCl aqueous solution electrode.
2. The electrochemical test method for quantitatively evaluating the porosity of phosphide film as claimed in claim 1, wherein the area of working electrode is 1cm2。
3. The electrochemical test method for quantitatively evaluating the porosity of the phosphate film according to claim 1, wherein the concentration of the KCl aqueous solution in the reference electrode is 3 mol/L.
4. The electrochemical test method for quantitatively evaluating the porosity of the phosphate film according to claim 1, wherein the amplitude of the alternating current signal in the electrochemical workstation is ± 10 mV.
5. The electrochemical test method for quantitatively evaluating the porosity of the phosphate film according to claim 1, wherein the electrolyte is 3.5 wt.% NaCl solution.
6. The electrochemical test method for quantitatively evaluating the porosity of a phosphate film according to claim 1, wherein the electrolysis temperature is 20 ℃.
7. The electrochemical test method for quantitatively evaluating the porosity of the phosphate film according to claim 1, wherein the scanning frequency range in the electrochemical workstation is 10KHz to 0.01 Hz.
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