CN102269729B - Method for detecting quality of phosphating film - Google Patents

Abstract

The invention discloses a method for detecting the quality of a phosphating film. Firstly, a phosphating sample is processed into a test piece with a surface area not less than 1 cm ² , and a three-electrode electrochemical testing system is adopted, wherein the working electrode is a test piece, a reference The electrode is a calomel electrode, the auxiliary electrode is a platinum wire mesh, and the solute concentration of the electrolyte is 0.45% to 0.55% potassium dihydrogen phosphate test solution at room temperature. After the inflection point appears on the curve, record the time when the inflection point appears. When the inflection point occurs at t≤300s, the phosphating film quality is poor; when the inflection point occurs at 300s<t≤700s, the phosphating film quality is average; when the inflection point occurs at 700s<t≤1000s, the phosphating film quality is good; When the inflection point appears time t>1000s, the phosphating film quality is very good.

Description

A kind of detection method of phosphating film quality

technical field

The invention provides a method for detecting the quality of a phosphating film, in particular an electrochemical test method for detecting the crystallization state of the phosphating film.

Background technique

As an important surface pretreatment technology, phosphating has been widely developed and applied in corrosion resistance, coating, lubrication and finishing. The crystallization state of the phosphating film includes the crystal size of the phosphating film grains, the uniformity of the phosphating film and the compactness of the phosphating film. The comprehensive effect of these three aspects will directly reflect the quality of the phosphating film and directly affect its quality. Therefore, it is particularly important to detect the crystallization state of the phosphating film. At present, the methods for detecting phosphating film include: film weight method, copper sulfate pitting test, salt water test, electrochemical impedance method and SEM observation, etc. The film weight method is mainly to measure the weight of the phosphating film. This method cannot detect the crystallization state of the phosphating film; the copper sulfate pitting test can only detect the compactness of the phosphating film, and cannot detect the crystallization state of the phosphating film. Comprehensive detection; salt water test is the main method to detect the corrosion resistance of phosphating film. At the same time, due to the harsh test conditions and long test period, it is easy to introduce large human errors; electrochemical impedance method can accurately and objectively detect phosphating film. A digital test technology for film quality, but the data analysis of this method is too complicated, and because the commonly used electrolyte is 3.5% NaCl solution, the Cl ^- in this solution is highly corrosive, and the corrosion resistance of the phosphating film is poor , when the phosphating film is immersed in the electrolyte, it may change the properties of the phosphating film, increase or decrease the porosity of the phosphating film, or change the interface structure of the substrate/phosphating film, etc., which will affect the test results ; SEM observation is currently the most intuitive and convincing method to detect the quality of phosphating film, but this method requires special equipment, complex operation, and professional operators, so it cannot be widely used.

In summary, the current methods for detecting phosphating film have the following shortcomings:

1. The detection is not comprehensive: the crystallization state of the phosphating film cannot be comprehensively detected, and only the local characteristics of the crystallization state of the phosphating film can be detected;

2. Poor detection accuracy: the test conditions are harsh, the test time is long, and the human error is large;

3. Data analysis is complex;

4. The operation is complex and requires special test equipment;

5. The electrolyte is highly corrosive, which can easily cause test errors in the quality of the phosphating film.

Contents of the invention

Aiming at the problems existing in the existing methods for detecting the quality of the phosphating film, the purpose of the present invention is to provide a comprehensive, accurate and convenient method for detecting the quality of the phosphating film.

In order to achieve the above-mentioned purpose of the invention, the solution of the present invention is: utilize constant current/potentiostatic instrument, test the relational curve of open circuit potential and time of phosphating film in phosphate solution, detect phosphating according to the time that this curve appears inflection point film quality.

The open circuit potential refers to the electrode potential of the metal immersed in the electrolyte when no external current passes through it. It often changes with the type of electrode, the composition of the electrolyte and other conditions. The inflection point of the relationship between the open circuit potential and time of different phosphating plates is due to the fact that when the phosphating plate is immersed in the electrolyte, the relationship between the open circuit potential and time of the phosphating film is first tested. The corrosion resistance of the film is poor, so the phosphating film will be "broken down" after soaking in the electrolyte for a certain period of time to expose the substrate metal. There is an inflection point in the relationship curve, and the time when the inflection point appears represents the time when the phosphating film is "broken down". The length of this time represents the quality of the phosphating film. . The quality of the phosphating film of the same series mainly depends on the size, density and uniformity of the phosphating film grains. The phosphating film with uniform particle size is of good quality.

The specific method is: the phosphating sample is processed into a test piece with a surface area of not less than ^1cm2 , and a three-electrode electrochemical test system is used, wherein the working electrode (i.e. the research electrode) is the test piece, and the reference electrode is the calomel electrode ( SCE), the auxiliary electrode is a platinum wire mesh, and the solute concentration of the electrolyte is 0.45% to 0.55% potassium dihydrogen phosphate test solution according to the mass percentage. The test of the relationship between the open circuit potential and time is carried out at room temperature. When the curve appears an inflection point Finally, record the time when the inflection point appears, and check the quality of the phosphating film according to the time when the inflection point appears.

When the inflection point occurs at t≤300s, the phosphating film quality is poor; when the inflection point occurs at 300s<t≤700s, the phosphating film quality is average; when the inflection point occurs at 700s<t≤1000s, the phosphating film quality is good; When the inflection point appears time t>1000s, the phosphating film quality is very good.

The present invention has the following beneficial effects:

(1) The method is not disturbed by human factors, and the test solution has little damage to the phosphating film, and can reflect the quality difference of different phosphating films with quantitative values, and the detection result is accurate.

(2) The method comprehensively reflects the quality of the phosphating film by combining various characteristics of the crystalline state of the phosphating film, and can fully reflect the quality of the phosphating film.

(3) The test time of this method is about 1000s, and the test time is short.

(4) In this method, the quality of the phosphating film is detected by the time when the open circuit potential curve appears an inflection point, and the analysis of the test results is simple.

(5) The method uses simple equipment and is easy to operate.

Description of drawings

The open circuit potential curve diagram of the plate in Fig. 1a;

The SEM observation picture of the plate in Fig. 2a;

The open circuit potential curve of the plate in Figure 3b;

The SEM observation picture of the plate in Figure 4b;

The open circuit potential curve diagram of Fig. 5c plate;

Figure 6c SEM observation image of the plate.

The open circuit potential curve diagram of Figure 7d plate;

SEM observation image of the plate in Fig. 8d.

Detailed ways

The best embodiment of the present invention will be described below by detecting the quality of the phosphating film of four different phosphating films.

Step 1. Take four samples a, b, c, and d with different phosphating coatings.

Step 2, process the above sample into a test piece of 100mm×100mm.

Step 3, using the M273 constant current/potentiostatic instrument, using a three-electrode electrochemical test system, in a 0.5% potassium dihydrogen phosphate solution by mass percentage, respectively have different phosphating films for a, b, c, and d Test the open circuit potential curve of the test piece. When the open circuit potential curve has an inflection point, the test ends, and the time when the inflection point appears is recorded. The test polarization curves are shown in Figure 1, Figure 3, Figure 5, and Figure 7.

Step 4. According to the time of occurrence of the inflection point on the polarization curve measured in step 3, the quality of the phosphating film with different phosphating film test pieces is detected according to Table 1, and the results are shown in Table 2. Table 1 is the phosphating film quality inspection table, and Table 2 is the test results of the phosphating film quality of four different phosphating films a, b, c, and d.

Table 1 Phosphate film quality inspection form

Phosphating coating quality grade very good good generally Difference The time when the inflection point appears t, s ＞1000 700＜t≤1000 300＜t≤700 ≤300

Table 2a, b, c, d four kinds of phosphating film quality inspection results

Template Inflection point appearance time t, s Phosphating film quality inspection results a 240 Poor, the grains of the phosphating film are coarse and uneven, and many areas cannot be phosphated b 630 Generally, the phosphating film is relatively thick and uniform, and the phosphating film cannot be applied in some areas c 780 Well, the phosphating film grains are relatively small, uniform and dense d 1050 Very good, the phosphating film grains are fine, uniform and dense

SEM observations were carried out on the above four different phosphating films, and the observation results are shown in Figure 2, Figure 4, Figure 6, and Figure 8. After comparing the test results in Table 2 with the observation results in Fig. 2, Fig. 4, Fig. 6 and Fig. 8, it can be found that the results of the present invention detecting the quality of phosphating film are in good agreement with the results of SEM observation .

Claims (1)

1. the detection method of a phosphorization film quality, it is characterized in that: the test open circuit potential of phosphating coat in phosphate solution and the relation curve of time, occur that according to this curve the time of flex point detects the quality of phosphating coat, be first not less than 1cm by being processed into surface area through the model of phosphatization ²test piece, adopt three-electrode electro Chemical test macro, wherein working electrode is that test piece to be measured, contrast electrode are that mercurous chloride electrode, auxiliary electrode are platinum gauze, the potassium dihydrogen phosphate test solution that electrolytic solution solute concentration is 0.45%～0.55% according to mass percent, carry out at ambient temperature the test of the relation curve of open circuit potential and time, when curve occurs after flex point, there is the time of flex point in record.