Method for evaluating phosphorylated film

Abstract

To provide a method for evaluating the coating state of a phosphorylated film formed on the surface of an iron substrate (steel plate or the like).SOLUTION: The present invention is a method for evaluating a coating state of a phosphorylated film formed on an iron substrate, the method including a measurement step of applying a current to at least a pair of electrodes in close contact with the phosphorylated film to obtain a DC characteristic or an AC characteristic regarding the phosphorylated film, and a determination step of comparing the DC characteristic or the AC characteristic with a predetermined characteristic to determine the coating state. The electrode may be made of, for example, conductive rubber or conductive resin. The AC characteristic is, for example, an impedance at a frequency within a predetermined range, or a relaxation time when a relaxation time distribution function becomes local maximum or maximum. According to the present invention, for example, a coating state of a phosphorylated film serving as a base layer for electrodeposition coating can be inspected at a site such as a production line.SELECTED DRAWING: Figure 3B

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the coating condition of a phosphoric acid film. [Background technology]

[0002] Members and structures (e.g., car bodies made of steel plates) exposed to the external environment (sunlight, wind, rain, corrosive gases, etc.) may be coated on their exterior surfaces to not only improve their design but also to prevent corrosion and deterioration. Phosphate treatment is often used as a base treatment for this purpose. The phosphate film (base layer) formed by this treatment is corrosion-resistant and roughens the painted surface, improving the adhesion of the paint film. In other words, the quality of the base treatment and phosphate film formation affects the deterioration and peeling of the paint film, and ultimately the corrosion resistance of the object to be painted.

[0003] For this reason, it is important to understand (by inspection, etc.) the state of the phosphate oxide film (covering condition). Visual inspections performed on production lines depend on the skill of the person in charge, so a method for easily and quantitatively evaluating the covering condition of the phosphate oxide film has been desired. Proposals related to this have been made, for example, in the following patent documents. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2006-53130 [Patent Document 2] Patent Publication No. 2004-18869 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 proposes an inspection method in which light from a red light-emitting diode is irradiated onto a phosphate conversion coating formed on an automobile body (steel plate) before electrodeposition coating, and the amount of the conversion coating deposited is estimated from the reflectance.

[0006] Patent Document 2 estimates the amount of aluminum conversion coating from the amount of energy of fluorescent X-rays. Although the inspection object and the type of irradiated light are different, Patent Document 2 and Patent Document 2 have in common the point that the amount of coating is estimated from reflected light.

[0007] The mass (amount of coating) of the conversion coating estimated using such light does not adequately reflect the coating condition (coverage rate, etc.). Furthermore, measurements using reflected light are easily affected by the measurement environment (light leakage, etc.), making them difficult to adopt on production lines.

[0008] The present invention has been made in view of the above circumstances, and has as its object to provide a method for evaluating a phosphoric acid film formation process that is different from conventional methods. [Means for solving the problem]

[0009] As a result of extensive research, the inventors have discovered that the coating condition of a phosphate oxide film can be determined based on the electrical characteristics (DC characteristics or AC characteristics) obtained by passing a current between electrodes attached to the surface of the phosphate oxide film. By expanding on this finding, the inventors have completed the present invention, which will be described below.

[0010] <Evaluation method for phosphorus oxide film formation> The present invention is a method for evaluating the coating condition of a phosphate film formed on an iron substrate, comprising: a measurement step of passing a current through at least a pair of electrodes placed in close contact with the phosphate film to determine DC or AC characteristics of the phosphate film; and a determination step of comparing the DC or AC characteristics with predetermined characteristics to determine the coating condition.

[0011] According to the method for evaluating a phosphate film of the present invention (simply referred to as the "evaluation method"), it is possible to efficiently and simply perform an appropriate evaluation of the coating state of a phosphate film formed on the surface of an iron substrate.

[0012] For example, it is possible to quickly grasp (estimate) on-site whether the surface of an iron substrate is sufficiently coated with a phosphate oxide film and whether the coating rate is above a predetermined value for a product extracted from a process line.

[0013] Evaluation Device The present invention can also be understood as an evaluation device for a phosphate film. For example, the present invention may be an evaluation device for a phosphate film, comprising: two or more electrodes disposed in close contact with a phosphate film formed on an iron substrate; measuring means (analysis means) for applying current between at least one pair of the electrodes to determine DC or AC characteristics of the phosphate film; and determining means for comparing the DC or AC characteristics with predetermined characteristics to determine the coating condition of the phosphate film.

[0014] "others" (1) In this specification, the terms "means" and "steps" can be interpreted interchangeably, and the elements of a "product (apparatus)" and those of a "method" can be interchangeable.

[0015] (2) Unless otherwise specified, "x to y" in this specification includes a lower limit of x and an upper limit of y. Any numerical value included in the various numerical values ​​or ranges described in this specification may be used as a new lower limit or upper limit to create a new range, such as "a to b." [Brief explanation of the drawings]

[0016] [Figure 1] 1 is an overview of a measurement system (example). [Figure 2] 1 shows the relationship between the coverage rate of the phosphoric acid oxide film and the DC resistance value. [Figure 3A] 10 shows the frequency characteristics of impedance for phosphoric acid film formation with different coverage rates. [Figure 3B] 10 shows the relaxation time distribution function spectra for phosphoric acid films formed with different coverage rates. [Figure 4] 1 shows the relationship between the coverage rate of the phosphoric acid film and impedance. [Figure 5] 1 shows an example of an observed image of a phosphate film and an example of the correlation between the coverage rate and the amount of film. DETAILED DESCRIPTION OF THE INVENTION

[0017] One or more components arbitrarily selected from this specification may be added to the above-described components of the present invention. The contents described in this specification may be applied not only to "products" but also to "methods" as appropriate. Which embodiment is best depends on the target, required performance, etc.

[0018] Subject The object of evaluation was the phosphoric acid formed on the iron substrate.

[0019] The iron substrate may be made of pure iron, iron alloy, intermetallic compound containing iron, or composite material. The form of the iron substrate is not important. The iron substrate may be in its raw form after being cast or rolled, or may be machined or surface-treated. A typical example is steel sheet, more specifically, galvanized steel sheet. The galvanization may be either hot-dip galvanization or electrogalvanization (including chromate treatment). Elements present near the surface (treated surface) of the iron substrate may be reflected in the components of the phosphate film.

[0020] A phosphate oxide film is typically formed by phosphate treatment on the surface (surface to be treated) of an iron substrate. This phosphate treatment film may be formed (primed) using a treatment solution containing metal ions such as zinc ions, calcium ions, and iron ions in addition to phosphate ions. Typical phosphate treatments include zinc phosphate treatment, zinc calcium phosphate treatment, and iron phosphate treatment. The source of the metals (zinc, iron, etc.) contained in the phosphate treatment film may be the treatment solution or the iron substrate.

[0021] "electrode" The electrode can be of any shape (shape, size) or material as long as it can adhere to the phosphate film. Electrodes that are thin, foil-like, or flexible (deformable) will adhere easily to the phosphate film, which has a finely uneven or wavy surface.

[0022] The substrate of the electrode may be a metal such as platinum, or may be rubber or resin, which are less rigid and more flexible than metals. A good example of an electrode is a rubber electrode (including a resin electrode), which has a matrix (substrate) of rubber or resin in which a filler made of a conductive material (metal, graphite, etc.) is dispersed, resulting in overall conductivity.

[0023] It is preferable to have at least one pair of electrodes. The iron substrate itself can be considered as an electrode, but using two or more electrodes makes it easier to attach and detach, measure, etc.

[0024] The electrodes may be arranged in a plane, with multiple electrodes on the same phosphate film, or in a face-to-face arrangement, with one electrode on the phosphate film and the other on the iron substrate. When evaluating a thin phosphate film with uneven or wavy surfaces, an in-plane arrangement is easier to measure and evaluate than a face-to-face arrangement.

[0025] The electrodes should be spaced apart enough to avoid short-circuiting. The spacing (the distance between adjacent electrode edges) is, for example, about 1 to 30 mm, 3 to 15 mm, or 5 to 10 mm. Three or more electrodes (electrode group) may be placed on the phosphoric acid film, and electricity may be passed or measurements may be performed between a selected pair of electrodes. The electrodes are provided with terminals or leads (e.g., lead wires) for connection to an external circuit. The form and wiring method of these are not important.

[0026] The method for adhering the electrode to the phosphate film is not critical. For example, the electrode can be held on the phosphate film by adhering it with tape or adhesive (paste), by mechanical pressure, or by magnetic pressure. If the iron substrate is ferromagnetic, fixing the electrode with a magnet allows the electrode to be held to the iron substrate with a substantially constant pressure (adhesion force) and can also be easily detached. This allows for simple and efficient measurement and evaluation of the phosphate film. The magnetic source can be a permanent magnet placed on the electrode or magnetic particles embedded in a rubber electrode. A conductive bonded magnet can also be used as the rubber electrode if it does not affect the measurement.

[0027] Evaluation Method (1) Measurement Electric current is passed between electrodes attached to the phosphate film to measure DC or AC characteristics of the phosphate film. DC characteristics include, for example, DC resistance. AC characteristics include, for example, impedance spectrum, relaxation time spectrum, dielectric constant (dielectric spectrum), dielectric loss tangent (tan δ), conductance, capacitance, and apparent capacitance. AC characteristics are measured using, for example, an LCR meter or an impedance analyzer. The impedance spectrum (frequency characteristics of impedance) is subjected to relaxation time distribution analysis (DRT) to obtain a relaxation time distribution function spectrum, from which the relaxation time spectrum corresponding to the coverage (coverage) of the phosphate film can be determined. The relaxation time is primarily the delay time of the motion of permanent dipoles and is thought to be due to dielectric relaxation phenomena caused by polarization (electronic polarization, ionic polarization, or orientation polarization).

[0028] The DC characteristics are measured by applying, for example, a voltage of 25 to 100 V or 40 to 75 V and a current of 0.05 to 0.5 A or 0.1 to 0.3 A. The AC characteristics are measured by applying, for example, a voltage of 0.01 to 0.5 V or 0.05 to 0.3 V. The frequency range is, for example, 10 -3 ~10 3 Hz or 10 -2 ~10 2 Hz.

[0029] (2) Judgment The coverage of the phosphate film is determined by comparing the measured DC or AC characteristics with predetermined characteristics (predetermined values). The coverage may be determined simply by determining whether the surface of the iron substrate to be treated is substantially entirely covered, or by quantitatively estimating the proportion of the surface of the iron substrate to be treated that is covered by the phosphate film (coverage rate). Instead of the coverage rate, the mass of the phosphate film per unit area (film amount) may be evaluated (estimated).

[0030] Specifically, for example, the DC resistance value or the impedance at a specific frequency may be compared with a predetermined value (threshold value), or the impedance in a predetermined frequency range or the relaxation time when the relaxation time distribution function is at its maximum or longest may be compared with a reference value set for each coverage rate.

[0031] Because a phosphate film is made of an inorganic compound, its DC resistance and impedance are much greater than those of an iron substrate (a conductor). Although this depends on the type and form of the phosphate film, when measured using a rubber electrode, for example, if the DC resistance is 15 kΩ or more or 20 kΩ or more, the impedance (at 1 Hz) is 15 kΩ or more or 20 kΩ or more, or the relaxation time at which the relaxation time distribution function peaks is 0.01 s or more, 0.1 s or more, or even 1 s or more, it can be estimated that substantially the entire treated surface of the iron substrate (in terms of coverage, 90% or more, or even 95% or more) is covered with the phosphate film.

[0032] 《Application》 The phosphate oxide film to be evaluated is formed on the surface of, for example, a mobile body (car body, ship body, aircraft body), a building, a bridge, infrastructure piping, etc. Such a phosphate oxide film may be the outermost layer or a base layer for a paint (resin paint film). An example of a paint applied to the phosphate oxide film is electrodeposition painting. The evaluation method of the present invention may be used for sampling inspection or 100% inspection on-site or on a production line. [Example]

[0033] The coating state was evaluated based on the characteristics obtained by passing an electric current through the phosphoric acid oxide film.The present invention will be described in more detail based on these specific examples.

[0034] Subject As shown in Figure 1, a hot-dip galvanized (plating thickness: 0.2 mm) steel sheet (strip shape: 70 mm × 150 mm × t0.7 mm / iron substrate) was phosphate-treated to form a phosphate oxide film on the steel sheet surface.

[0035] Palbond L45 (PB-L45) from Nihon Parkerizing Co., Ltd. was used for the phosphate treatment. The outline of the process is as follows: First, the surface of the steel sheet was cleaned with a solution using an alkaline degreasing agent, and then rinsed with water to degrease it. After degreasing, the steel sheet was immersed in a treatment solution (bath), rinsed with water, and then dried with hot air. The treatment solution contained phosphate ions and zinc ions. The phosphate treatment was performed on both sides of the hot-dip galvanized steel sheet.

[0036] The coverage of the steel sheet by the phosphate oxide film was adjusted by adjusting the concentration of the treatment solution, immersion time, etc. The coverage was calculated by observing the treated surface with a scanning electron microscope or a digital microscope and determining the exposed rate of the metal surface.

[0037] Observation images of the phosphate oxide film (coverage: 80%, 100%) are shown in Figure 5. The mass of each phosphate oxide film with different coverage was measured to determine the mass per unit area (referred to as "film mass"). The results are also shown in Figure 5. As can be seen from the table in Figure 5, there was a positive correlation between the film mass and the coverage. The coverage can be roughly estimated from the film mass, but the two were not strictly proportional.

[0038] "electrode" (1) Placement As shown in Figure 1, a pair of electrodes was placed in an in-plane arrangement or in an opposing arrangement. In the in-plane arrangement, each electrode was placed on the upper surface of the phosphate oxide film, approximately 8 mm apart (distance between the edges). In the opposing arrangement, one electrode was placed on the upper surface of the phosphate oxide film, and the other electrode was placed on the lower surface of the phosphate oxide film. Note that the steel plate itself (the part not covered with the phosphate oxide film) may also serve as the other electrode.

[0039] (2) Type A rubber electrode and a platinum electrode were prepared, both measuring 7 mm x 7 mm.

[0040] A conductive silicon rubber sheet (0.5 mm thick / As One Corporation: 3-1945-01) was used for the rubber electrode, and platinum foil (0.2 mm thick) was used for the platinum electrode.

[0041] (3) Retention A permanent magnet (fixed magnet) was placed on each electrode to magnetically attach the electrode to the steel plate. Note that each electrode was previously provided with a lead (wiring).

[0042] "measurement" The leads of each electrode were connected to a measuring device, which was either an insulation resistance meter (IR4051 manufactured by HIOKI) or an electrochemical analyzer (ModuLab manufactured by Solartron Analytical).

[0043] The DC resistance value (R: DC characteristic value) was measured using an insulation resistance meter when a constant voltage or a constant current was applied (measurement step).

[0044] The impedance spectrum (frequency characteristics of impedance / AC characteristics) obtained by passing AC current of various frequencies was measured using an electrochemical analyzer (measurement step). This impedance spectrum was subjected to relaxation time distribution analysis to obtain the relaxation time distribution function spectrum (AC characteristics) showing the relationship between the relaxation time (τ) and the relaxation time distribution γ. At this time, the AC frequency range: 10 -1 ~10 6 The frequency was 10 Hz, applied voltage (amplitude): 0.1 V, and measurement interval: 10 points / dec. For the analysis of relaxation time distribution (DRT), open source software (DRT tools.m / MIT) was used.

[0045] 《DC characteristics》 (1) Figure 2 shows the relationship between the coverage of the phosphate oxide film and the DC resistance (applied voltage: 50 V) obtained from the electrodes placed in-plane on each phosphate oxide film.

[0046] As can be seen from Figure 2, whether rubber electrodes or platinum electrodes were used, the DC resistance (R) was clearly higher (e.g., R≧15 kΩ) when the coverage was 100% than when the coverage was 80% or less. This tendency was more pronounced when platinum electrodes were used.

[0047] (2) On the other hand, the DC resistance values ​​obtained by arranging the electrodes opposite each other are shown in Table 1.

[0048] As can be seen from Table 1, when a constant voltage (50 V) was applied using rubber electrodes, the DC resistance value increased significantly (e.g., R≧10 kΩ) when the coverage was 100% compared to when the coverage was 80% or less. When a constant current (200 mA) was applied, a non-conductive state (insulating state) was observed except when current was applied from the platinum electrode to a phosphate oxide film with a coverage of 20% or less. Note that "insulating" in Table 1 refers to a non-conductive state where the DC resistance is infinite (∞), and "short-circuited" refers to a conductive state where the DC resistance is approximately zero.

[0049] (3) Figure 2 and Table 1 show that the coverage of the phosphate oxide film (coverage rate) can be evaluated by appropriately combining the type and arrangement of electrodes and the current application method (constant voltage or constant current) (evaluation step).

[0050] For example, it was found that when rubber electrodes are used, whether they are placed in-plane on the phosphate oxide film or placed opposite to it, it is possible to determine whether the coverage of the phosphate oxide film is near 100% (for example, whether the coverage is 80% or less) based on the DC resistance value.

[0051] 《AC characteristics》 (1) The frequency characteristics (impedance spectrum) of impedance |Z| obtained by applying AC current (applied voltage: AC 100 mA) to rubber electrodes placed in-plane on each phosphoric acid film are summarized in Figure 3A. As is clear from Figure 3A, the impedance spectrum at a coverage rate of 100% is clearly different from the impedance spectrum at a coverage rate of 80% or less. This tendency is evident in the low frequency range (10 2 Hz or less), and its impedance is, for example, |Z| ≥ 10 7 It became Omega.

[0052] The relaxation time distribution function spectrum obtained from the impedance spectrum is shown in Figure 3B. In Figure 3B, the vertical axis is normalized by the maximum value of each peak (γ(τ)max). As is clear from Figure 3B, peaks in the relaxation time distribution function spectrum were observed at different relaxation times (τ) corresponding to each coverage. In particular, when the coverage was 100%, the relaxation time (τ: AC characteristics) at which peaks appeared was significantly longer (e.g., τ≧1) than when the coverage was 80% or less.

[0053] Incidentally, when AC current was applied to platinum electrodes arranged in-plane, a similar tendency to that observed when rubber electrodes were used was observed.

[0054] (2) On the other hand, when AC current was applied to the electrode placed opposite the phosphoric acid film, no clear correlation was observed between the coverage of the phosphoric acid film and its impedance spectrum. For reference, the typical |Z| 1Hz The relationship between the impedance at 1 Hz and the coverage is shown in Figure 4.

[0055] (3) From Figures 3A and 3B (collectively referred to as "Figure 3"), it was found that by focusing on the AC characteristics (impedance in a specific frequency range and the relaxation time at the peak of the relaxation time distribution function spectrum) obtained using electrodes (especially rubber electrodes) arranged in-plane, it is possible to determine whether almost the entire surface to be treated is covered with a phosphate oxide film (whether the coverage rate is around 100% or whether the coverage rate is 80% or less), and even to estimate the specific coverage rate itself.

[0056] Thus, it was confirmed that the present invention makes it possible to evaluate the coating condition of the phosphate film.

[0057] [Table 1]

Claims

1. A method for evaluating the coating condition of a phosphate film formed on an iron substrate, comprising: a measuring step of applying a current to at least a pair of electrodes attached to the phosphorylation film to determine DC or AC characteristics of the phosphorylation film; and a determining step of comparing the DC characteristic or the AC characteristic with a predetermined characteristic to determine the coating condition.

2. 2. The method for evaluating a phosphate film according to claim 1, wherein the electrodes are made of conductive rubber or conductive resin.

3. 3. The method for evaluating a phosphate film according to claim 1, wherein the electrode is held on the phosphate film by magnetic force.

4. 2. The method for evaluating a phosphate film according to claim 1, wherein the AC characteristic is an impedance in a predetermined frequency range or a relaxation time when a relaxation time distribution function reaches a local maximum or a maximum value.

5. 2. The method for evaluating a phosphate film according to claim 1, wherein the phosphate film is a phosphate-treated film containing zinc.

6. 2. The method for evaluating a phosphate film according to claim 1, wherein the phosphate film is a base layer to be painted.

7. The method for evaluating a phosphate oxide film according to claim 1 or 6, wherein the iron substrate is a zinc-plated steel sheet.

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