JP2022110360A - Method for measuring defect rate of corrosion-resistant film - Google Patents

Abstract

To provide a method for measuring a defect rate of a corrosion-resistant film, capable of obtaining a value sufficiently close to an actual defect rate of a corrosion-resistant film in a short time.SOLUTION: A method for measuring a defect rate of a corrosion-resistant film in a member including a metallic first base material and a corrosion-resistant film formed on a front surface of the first base material comprises: obtaining a critical passivation current density of the member by sweeping potential of the member at a sweep rate within the range of 250-360 mV/min in an electrolytic solution; obtaining a critical passivation current density of a second base material by sweeping potential of the second base material made of metal being the same as that of the first base material at the sweep rate in the electrolytic solution; and calculating a defect rate of the corrosion-resistant film based on a formula (1): R=(Ia/Ib)×100 (where R is a defect rate (%) of the corrosion-resistant film, Ia is the critical passivation current density of the member, and Ib is the critical passivation current density of the second base material).SELECTED DRAWING: Figure 2

Description

The present invention relates to a method for measuring the defect rate of corrosion resistant films.

Non-Patent Document 1 describes a critical passivation current density method (CPCD method) as a method for electrochemically evaluating defects in corrosion-resistant dry coating films. The CPCD method utilizes the fact that the critical passivation current of metals exhibiting active-passive transition properties is proportional to the area of the metal to assess defects in corrosion resistant coatings. Specifically, when a metal exhibiting active-passive transition properties with a corrosion resistant coating is brought to an active potential, the metal exposed at the bottom of defects such as pinholes penetrating the coating dissolves. Therefore, the critical passivation current is proportional to the exposed metal area, ie, the defect area of the corrosion resistant coating. Therefore, from the critical passivation current, the defect area fraction of the corrosion resistant coating can be calculated. In Non-Patent Document 1, the potential of the metal is swept from −0.45 V to 0.40 V at a speed of 23 mV/min to obtain a polarization curve and determine the critical passivation current.

Katsuhisa Sugimoto, "Current Status of Defect Evaluation of Corrosion-Resistant Dry Coating Films", Zairyo-to-Kankyo, 44, 308-313 (1995)

Since the defect rate of the corrosion-resistant film calculated from the critical passivation current by the method of Non-Patent Document 1 is much larger than the actual defect rate, it is necessary to correct the calculated value. In the method of Non-Patent Document 1, it takes about 45 minutes to measure the critical passivation current, but it is desirable to be able to calculate the defect rate in a shorter time.

Therefore, a method for measuring the defect rate of a corrosion-resistant film is provided, which can obtain a value sufficiently close to the actual defect rate of the corrosion-resistant film in a shorter time.

According to one aspect of the present invention, a method for measuring the defect rate of a corrosion resistant film of a member provided with a metal first substrate and a corrosion resistant film formed on the surface of the first substrate, comprising:
determining the critical passivation current density of the member by sweeping the potential of the member in the electrolyte at a sweep rate in the range of 250-360 mV/min;
Sweeping the potential of a second substrate made of the same metal as the first substrate in the electrolytic solution at the sweep speed to determine the critical passivation current density of the second substrate;
Formula (1) below:
R = (Ia/Ib) x 100 (1)
(Wherein, R is the defect rate (%) of the corrosion resistant film, Ia is the critical passivation current density of the member, and Ib is the critical passivation current density of the second substrate)
calculating the defect rate of the corrosion resistant film according to
A method is provided comprising:

The method of measuring the defect rate of a corrosion resistant coating of the present disclosure can obtain a value sufficiently close to the actual defect rate of the corrosion resistant coating in a shorter time.

1 is a polarization curve obtained in Comparative Example 1. FIG. FIG. 2 is a graph plotting the calculated defect rate against the actually measured defect rate in Examples 1 and 2 and Comparative Examples 1 and 2. In FIG. 3 is a cross-sectional SEM image of the specimen of Comparative Example 1. FIG. 4 is a cross-sectional SEM image of the specimen of Example 2. FIG.

Hereinafter, embodiments will be described with reference to the drawings as appropriate. The present invention is not limited to the following embodiments, and various design changes can be made without departing from the spirit of the invention described in the claims. In the drawings referred to in the following description, the same members or members having similar functions are denoted by the same reference numerals, and repeated description may be omitted. Also, the dimensional ratios in the drawings may differ from the actual ratios for convenience of explanation, and some members may be omitted from the drawings. In addition, in the present application, a numerical range represented using the symbol "~" includes the numerical values described before and after the symbol "~" as the lower and upper limits, respectively.

In the method according to the embodiment, the defect rate of the corrosion resistant film of the member provided with the metal first base material and the corrosion resistant film formed on the surface thereof is measured.

(1) Measurement of Critical Passivation Density of Member The metal constituting the first substrate is not particularly limited as long as it exhibits active state-passive state transition characteristics. Metals exhibiting active-passive transition properties include, for example, chromium, nickel, aluminum, carbon steel, and stainless steel (eg, SUS304, SUS316L).

Examples of the corrosion resistant film include Ti, TiN, SiO ₂ and resin films. The corrosion resistant film may be a single layer film or a multilayer film. The corrosion resistant film can be formed by any film forming method such as sputtering, ion plating, plating, chemical vapor deposition (CVD), and the like.

Using the member as a working electrode and using a reference electrode and a counter electrode, the density of the flowing current is measured while sweeping the potential of the member in the electrolytic solution. If there is a portion where the corrosion resistant film is defective and the first substrate is exposed, the current density increases as the potential is swept. If the sweep of the potential is continued, a passivation film is formed on the exposed surface of the first base material, so that no current flows. The maximum current density during this period is called the critical passivation current density.

For example, an Ag/AgCl electrode can be used as the reference electrode. For example, a Pt electrode can be used as the counter electrode. As the electrolytic solution, for example, a solution of sulfuric acid and potassium thiocyanate can be used.

The range over which the potential of the member is swept may be appropriately determined according to the electrolytic solution, the first base material, and the corrosion resistant film. The sweep rate should be in the range of 250-360 mV/min. The sweep speed may be constant.

(2) Measurement of Critical Passivation Density of Second Substrate The second substrate is composed of the same metal as that of the first substrate. No corrosion resistant film is formed on the surface of the second base material. The critical passivation density of the second substrate is measured under the same measurement conditions as the critical passivation density of the member described above.

(3) Calculation of Defect Rate of Corrosion-Resistant Film Based on the measured critical passivation density of the member and the critical passivation density of the second substrate, the defect rate of the corrosion-resistant film of the member is determined. Specifically, the following formula (1):
R = (Ia/Ib) x 100 (1)
(Wherein, R is the defect rate (%) of the corrosion resistant film, Ia is the critical passivation current density of the member, and Ib is the critical passivation current density of the second substrate)
Calculate the defect rate of the corrosion resistant film according to

In the method according to the embodiment, by measuring the critical passivation density by sweeping the potential at a rate within the range of 250 to 360 mV / min, as shown in the examples described later, the calculated defect rate is The value is sufficiently close to the actual defect rate. Moreover, in the method according to the embodiment, the potential is swept at a higher speed than the conventional technique described in Non-Patent Document 1, so the measurement time can be shortened.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the present invention is not limited to these Examples.

Comparative example 1
A Ti film having a thickness of 1 μm was formed as a corrosion-resistant film on the surface of a plate-shaped first base material made of SUS304 by a sputtering method. Next, the Ti film was scribed so that the area ratio of defects in the Ti film (defect rate (actual value)) was 1.1%, and a test piece was produced.

A solution of sulfuric acid and potassium thiocyanate was prepared as an electrolyte. The specimen was held by a holder and immersed in the electrolytic solution. At this time, the holder insulated the area other than the measurement area (area of 1 cm ² ) of the specimen from the electrolytic solution. Immediately after the specimen was immersed in the electrolytic solution, the potential of the specimen was swept from -0.45 V to 0.4 V at a constant rate of 20 mV/min to obtain a polarization curve. An Ag/AgCl electrode was used as a reference electrode, and a Pt electrode was used as a counter electrode. The obtained polarization curve is indicated by a solid line in FIG. The critical passivation current density Ia of the specimen was obtained from the maximum value of the current density in the polarization curve.

Similarly, the polarization curve of the second substrate made of SUS304 without the Ti film was obtained. The obtained polarization curve is indicated by a dashed line in FIG. The critical passivation current density Ib of the second substrate was obtained from the maximum current density in the polarization curve.

Formula (1) below:
R = (Ia/Ib) x 100 (1)
(Wherein, R is the defect rate (%) of the specimen, Ia is the critical passivation current density of the specimen, and Ib is the critical passivation current density of the second substrate)
The defect rate of the Ti film was calculated according to

Similarly, test specimens with Ti film defect rates (measured values) of 0%, 2.1%, and 3.2% were prepared, and the critical passivation current density Ia of each specimen was measured. 1), the defect rate of the Ti film in each specimen was calculated.

FIG. 2 shows a graph in which the defect rate (calculated value) is plotted against the defect rate (measured value).

After calculating the defect rate, the cross-section of the defective portion of the specimen was observed with a scanning electron microscope (SEM). A SEM image is shown in FIG.

Example 1
The use of test specimens with Ti film defect rates (measured values) of 0%, 0.05%, 1%, and 1.7%, and the sweep speed of the potential when obtaining the critical passivation current density The defect rate (calculated value) of each specimen was determined in the same manner as in Comparative Example 1, except that the voltage was 250 mV/min. The results are shown in FIG.

Example 2
Using specimens with Ti film defect rates (measured values) of 0%, 1.1%, 2.1%, and 4.4%, and sweeping the potential when determining the critical passivation current density The defect rate (calculated value) of each specimen was obtained in the same manner as in Comparative Example 1, except that the speed was 360 mV/min. The results are shown in FIG. Also. After calculating the defect rate, the cross section of the defective portion of the specimen was observed by SEM. A SEM image is shown in FIG.

Comparative example 2
The use of test specimens with Ti film defect rates (measured values) of 0%, 0.05%, 1%, and 1.7%, and the sweep speed of the potential when obtaining the critical passivation current density The defect rate (calculated value) of each specimen was obtained in the same manner as in Comparative Example 1, except that the voltage was 400 mV/min. The results are shown in FIG.

The dashed line in FIG. 2 indicates the case where the measured defect rate and the calculated defect rate completely match. As can be seen from FIG. 2, in Examples 1 and 2 in which the critical passivation current density was obtained by sweeping the potential at a sweep speed within the range of 250 to 360 mV / min, the calculated value of the defect rate was different from the measured value. The values were close enough. Specifically, the deviation of the calculated value from the measured value was within ±25%. On the other hand, in Comparative Examples 1 and 2 in which the potential was swept at a sweep rate of less than 250 mV/min or more than 360 mV/min to determine the critical passivation current density, the calculated defect rate was significantly different from the measured value.

As shown in FIG. 3, in the specimen of Comparative Example 1, it was confirmed that there was a portion where SUS304, which was the first substrate, did not exist under the Ti film (left side in FIG. 3). This suggests that the corrosion of the first substrate progressed excessively around the defective portion of the Ti film during the measurement of the polarization curve, and the first substrate was corroded to the portion covered with the Ti film. is doing. As a result, the area of the surface of the first substrate where the passivation film is formed in contact with the electrolyte becomes larger than the actual defect area of the Ti film, and as a result, the defect calculated from the critical passivation current density It is believed that the defect rate was much higher than the actual defect rate.

On the other hand, as shown in FIG. 4, in the test piece of Example 2, the first base material corroded only at the defective portion of the Ti film, and excessive corrosion of the first base material was not observed. Therefore, it is considered that the calculated value of the defect rate is sufficiently close to the measured value of the defect rate.

Claims (1)

A method for measuring the defect rate of a corrosion resistant film of a member comprising a metal first substrate and a corrosion resistant film formed on the surface of the first substrate, comprising:
determining the critical passivation current density of the member by sweeping the potential of the member in the electrolyte at a sweep rate in the range of 250-360 mV/min;
Sweeping the potential of a second substrate made of the same metal as the first substrate in the electrolytic solution at the sweep speed to determine the critical passivation current density of the second substrate;
Formula (1) below:
R = (Ia/Ib) x 100 (1)
(Wherein, R is the defect rate (%) of the corrosion resistant film, Ia is the critical passivation current density of the member, and Ib is the critical passivation current density of the second substrate)
calculating the defect rate of the corrosion resistant film according to
method including.

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