JPH0634552A - Method and apparatus for automatic measurement of depth of local corrosion - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide an automatic measuring device for the depth of localized corrosion that occurs in large numbers on the surface of corrosion resistant materials such as stainless steel. [Structure] When measuring the depth of localized corrosion using a microscope, move the test piece while focusing on the surface where corrosion has not occurred, and the presence or absence of local corrosion depending on the change in the amount of light reflected from the sample at that time. Then, the movement of the test piece is stopped together with the presence of the corrosion hole of the desired size, and the depth of the corrosion hole is measured from the relationship between the movement amount of the objective lens and the reflected light amount. As the microscope, a non-confocal type and a confocal type are used, but when the confocal type is used, the accuracy of corrosion hole detection and depth measurement becomes extremely high. Further, the depth of each portion of the corrosion hole can be measured by scanning the inside of the visual field by using a laser beam having a minute spot size as a light source and dividing the visual field into a large number of pixels. [Effect] Compared with the conventional method, it is possible to measure the depth of the corrosion hole with high accuracy in a short time. Many corrosion holes can be automatically measured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for automatically measuring the depth of localized corrosion damage such as pitting corrosion generated on the surface of a corrosion resistant material such as stainless steel, and an apparatus therefor.

[0002]

2. Description of the Related Art Even with corrosion resistant materials such as stainless steel and aluminum alloys, pit-like localized corrosion represented by pitting corrosion occurs in an environment containing corrosive substances such as seawater and sea salt particles. To know the corrosion resistance of a material accurately,
It is important to measure the depth distribution of these pitting corrosions, and in the case of actual structures, it is indispensable to estimate the time variation of the maximum pitting depth for estimating the life of the material.

Conventionally, a typical method for measuring the depth of pitting corrosion is to search for pitting corrosion with the naked eye using an optical microscope and focus on the bottom of pitting corrosion and the free surface without pitting corrosion, respectively. The depth is calculated from the amount of movement of the objective lens at that time. This is because humans are involved in all operations such as detection of pitting corrosion and depth measurement by focusing, and there is a drawback that the measurement accuracy and measurement speed depend on the observer, and the measurement takes an extremely long time. did.

In Japanese Unexamined Patent Publication (Kokai) No. 3-199399, an optical non-contact displacement gauge is attached to an XYZ electric stage, and the surface of the test piece is scanned by this to determine the pitting depth of the test piece. There is disclosed a device for measuring the displacement of the. However, since the laser displacement meter is used to measure the pitting depth, the size and shape of the pitting that can be measured depend on the diameter of the laser beam and the angle between the incident optical axis and the light receiving axis of the laser. An ordinary laser displacement meter has a laser beam diameter of 50.
Since the angle between the incident light and the light receiving axis is about 30 μm, the pitting depth on the surface of corrosion-resistant material such as sharply shaped stainless steel with an opening diameter of about 50 μm and a depth of 50 μm or more is accurately measured. Difficult to do. As described above, up to now, no technique has been developed to rapidly measure the pitting depth formed on the surface of stainless steel or the like.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for automatically measuring the pitting depth of the surface of a corrosion resistant material such as stainless steel, and an apparatus therefor. Was made as.

[0006]

As a result of studying a system for automatically measuring the depth of pitting corrosion, the present inventors have devised a method of detecting the presence or absence of pitting corrosion and a method of using a microscope, whereby the diameter is 50 μm. We have succeeded in inventing a device that can measure pitting corrosion to a sufficient degree automatically.

The gist of the present invention is to measure the depth of localized corrosion such as pitting corrosion that occurs on the surface of corrosion resistant materials such as stainless steel and aluminum alloys, using a microscope. While moving the test piece while focusing, at the same time monitoring the change in the intensity of the reflected light from the test piece, the presence or absence of pitting corrosion of a target size or more is determined in the observation field of view, and the target pit Equipped with a means for stopping the scanning of the test piece at the same time as confirming the corrosion and shifting to the depth measurement operation, the depth of pitting corrosion is measured from the relationship between the amount of movement of the objective lens and the amount of reflected light. And an automatic pitting depth measuring method and device.

Further, in the above, by using an apparatus having a confocal optical system as the microscope, the depth of focus is made extremely shallow, and the accuracy of the pitting depth for determining and measuring the presence of pitting corrosion is improved. It becomes possible. Also,
By scanning the light source of the microscope with laser light and scanning the measurement field of the microscope with minute spots, the resolution in the plane can be improved and the depth of each spot can be measured.

[0009]

The present invention will be described in detail below. The first feature of the apparatus of the present invention is to detect the presence or absence of pitting in the visual field by monitoring the change in the reflected light intensity in the visual field of the microscope while scanning the test piece while focusing on the surface where pitting does not occur. Is to judge.

The relationship between the focal position of the microscope and the reflected light intensity is as shown in FIG. The intensity of reflected light becomes brightest when the object to be observed is focused by the optical lens system of the microscope. Therefore, when the test piece is scanned by focusing on the surface where pitting does not exist, the part where pitting does not come into focus, so when there is pitting in the visual field, the reflected light intensity is weak. Become. By converting the reflected light intensity of the field of view of the microscope into an electric signal from the monitor and electrically setting a certain threshold value, it is possible to detect the presence of pitting corrosion and shift to the next measurement operation. Since the mouth of pitting corrosion is almost circular, it is possible to set a certain threshold for the diameter of pitting corrosion that is the target of automatic measurement, if the relationship between the reflected light intensity and the diameter or area is verified. It is possible.

Further, by using a confocal optical system for the microscope used, the sensitivity of pitting corrosion detection is improved. The confocal optical system is the one schematically shown in FIG. 3, in which slits or pinholes 4a and 4b are provided in front of the light source 5 and the light receiver 6. In this case, the defocused light cannot pass through the pinhole or the slit and is removed by the slit, so that the strongly reflected light is detected only when the light is in focus. As a result, if the observation field of view is focused on a surface without pitting corrosion, the reflected light is almost completely cut off because the portion where pitting corrosion occurs is out of focus. Will be improved.

Further, the diameter of the pitting corrosion in the scanning direction of the laser light is measured in real time by scanning the measurement field of view with laser light having a small spot diameter as the light source and monitoring the reflected light at that time. It is also possible. In this case,
It is also possible to use the diameter of pitting corrosion as a threshold for detecting pitting corrosion.

As a method of focusing on a surface free from pitting corrosion, a laser displacement meter having a beam diameter sufficiently larger than that of pitting corrosion is used to control the distance between the objective lens and the test piece constant, and the objective There is a system that keeps the distance between the objective lens and the test piece constant by controlling the ejection pressure of the air nozzle attached near the lens to be constant. Even when using a normal laser displacement meter, if the surface shape changes drastically like pitting corrosion, the light from inside the pitting is scattered and does not return to the light receiving element, resulting in no pitting corrosion. The displacement of the surface will be measured.

The second feature of the apparatus of the present invention is that the depth of pitting corrosion is measured from the relationship between the amount of movement of the objective lens and the amount of reflected light. Generally, the image observed with a microscope is
When the objective lens is moved in the direction normal to the sample surface, including the case of the confocal optical system, the brightness becomes maximum at the position where the focus is achieved, and the brightness becomes darker as the focus shifts. Therefore, when pitting exists in the visual field, the amount of movement of the objective lens and the brightness of the visual field are as shown in FIG. Point A in the figure indicates the position where the reflected light intensity is the maximum, and is the mouth of pitting (the surface of the test piece).
Corresponds to the position of. Also, point B in the figure indicates the position of the bottom of pitting corrosion. The amount of movement of the objective lens between AB in the figure is the target pitting depth. Since the microscope has a depth of focus depending on the lens characteristics and the optical system, the point where the brightness is zero cannot be directly set as the point B (bottom of pitting). The extent to which the point B deviates from the position C where the brightness becomes completely zero depends on the type of optical system and light source of the microscope used, and therefore it is necessary to perform the test using a standard sample. If necessary, a mathematical process such as differentiating the lens movement-luminance curve once to obtain a point B at a position having a certain inclination or extrapolating to the low-luminance side after the smoothing process is required. When a confocal optical system is used, BC is generally used.
The distance between them is about several times the depth of focus.

In the case of using a system in which the light source of the microscope is laser light and scanning is performed within the observation field of view of the microscope, the spot size of the laser beam corresponds to the in-plane resolution. It is possible to handle it separately. When the change in the reflected light intensity of each pixel is measured by moving the objective lens, each pixel has the maximum brightness at the in-focus position. Therefore, for all the pixels, a curve as shown in FIG. 5 can be obtained by measuring the relationship between the amount of movement when moving the objective lens and the number of pixels showing the maximum brightness at that time.

Generally, since the area of the pitted hole is smaller than the observation field, the maximum value of the curve indicates that the free surface of the test piece without pitting is in focus. The position where the number of pixels with the maximum brightness becomes zero when the objective lens is brought close to the test piece corresponds to the bottom of pitting corrosion. Strictly speaking,
The position where the maximum number of pixels is zero does not completely coincide with the bottom of the pit. This is because, when measuring the maximum brightness position for each pixel, errors due to the depth of focus of the microscope and scattered light are mixed in. Therefore, the bottom of pitting corrosion is as shown in FIG. It is necessary to test what position it corresponds to using a standard test piece. If necessary, a mathematical process such as differentiating the lens movement amount-luminance curve once to obtain a point B at a position having a certain inclination or extrapolating to the low luminance side after smoothing processing is required. According to the principle described above,
By detecting the pitting to be measured and measuring the depth, it is possible to automatically measure the pitting depth.

[0017]

EXAMPLE FIG. 1 shows a block diagram of a prototype pitting depth measuring device. Table 1 shows the combination of the type of light source and the optical system used. That is, as the light source 12 of the microscope,
50W halogen lamp and 1.5mW He-Ne output
Laser (The spot diameter when passing through the objective lens is about 1μ
m) was used, and as the light receiving element 11, a Si photodiode was used when the light source was a halogen lamp, and a Photomal and a CCD camera were used when the light source was laser light. With respect to the combination of the respective light receiving elements, an experiment was performed with and without a slit having a width of about 50 μm in front of the light receiving elements. A confocal optical system has a slit.

In any case, the objective lens 17 is 50
A product with a numerical aperture of 0.95 was used. The stage 8 on which the test piece 7 is placed is an electric stage driven by a stepping motor, and the microscope stage is vertically driven by a stage 9 with a servomotor to provide a reference surface (mirror-polished horizontal surface) provided on the sample stage. ) Was measured by the laser displacement meter 13, and the amount of movement of the objective lens, that is, the sample stage in the vertical direction was measured and used for measuring the pitting depth. In addition, we also examined a method of attaching a servomotor to the focus adjustment handle attached to the microscope and measuring the rotation angle with a rotary encoder to measure the distance between the test piece and the objective lens. In order to focus on the surface of the test piece which is free from pitting corrosion, the air ejection nozzle 10 is attached to the objective lens so that the ejection pressure is constant, that is, the distance between the lens and the test piece is constant. It was controlled by moving the sample stage up and down.

The driving of the electric stage and the vertical movement of the stage were controlled by the computer 16 through the motor drivers 14 and 15. The amount of light received from the test piece was measured by the computer 16 by converting it into a voltage. When the test piece is moved while automatically focusing with the air nozzle, and the presence of pitting corrosion is found when the reflected light satisfies the preset conditions, the stage is stopped and the pitting depth measurement operation starts. I decided. When a halogen lamp was used as the light source, a threshold value was set for the reflected light intensity.
When scanning the inside of the field of view using laser light, the change in the intensity of the scanning line of the reflected light is monitored, and pitting corrosion is detected when the range of focus deviation in the horizontal direction of the field of view exceeds a certain value. I set it as what I did. In the pitting depth measuring operation, the objective lens is separated from the autofocus position by 15 μm, and then the objective lens is brought close to the test piece within the working distance of the lens, and the amount of light received at that time is measured.

When a halogen lamp is used as the light source, the change in the amount of light in the field of view and the amount of movement of the objective lens are measured and recorded, the position of maximum brightness and the position where the brightness becomes zero are calculated, and the error depending on the depth of focus is calculated. Is corrected by software, and the value of the pitting depth is converted into a data file, and then the operation for moving the stage for recognizing the pitting corrosion is performed again.

When the light source is laser light, in the pitting depth measuring operation, the position of the objective lens showing the highest brightness is measured for each pixel in the visual field, and the objective lens movement amount and the maximum brightness pixel number are measured. After calculating the relationship, calculate the position with the largest number of brightest pixels on the surface of the test piece, calculate the position of the objective lens where the number of brightest pixels is zero, and correct the error depending on the depth of focus with software. The value of the pitting depth is converted into a data file, and then the operation for moving the stage for recognizing the pitting corrosion is performed again.

With the above-described apparatus and measurement procedure, a diamond needle was applied to the stainless steel to give a depth of 50 μm.
m and 30 μm indentations (hemispheres) were measured. The measurement results are shown in Table 1. Laser spot diameter 30μ for comparison
The measurement result by the laser displacement meter of m is also shown. As is clear from the results, it is possible to measure a large number of pit depths extremely accurately and quickly by using the device of the present invention.

[0023]

[Table 1]

[0024]

By using the method and apparatus of the present invention described above, it is possible to accurately and quickly measure the depth of localized corrosion such as pitting corrosion that occurs on the surface of a corrosion resistant material such as stainless steel. .

[Brief description of drawings]

FIG. 1 is a block diagram of a prototype pitting depth automatic measuring device.

FIG. 2 is a diagram showing a focus position with respect to a relationship between a moving amount of an objective lens of a microscope and reflected light intensity.

FIG. 3 is a schematic diagram of a confocal optical system.

FIG. 4 is a diagram showing the positions of the surface of a test piece and the bottom of pitting corrosion with respect to the relationship between the intensity of reflected light and the amount of movement of an objective lens in a non-scanning microscope.

FIG. 5 is a diagram showing the positions of the surface of the test piece and the bottom of pitting corrosion with respect to the relationship between the position where each pixel shows the highest brightness in the scanning microscope and the amount of movement of the objective lens at that time.

[Explanation of symbols]

 1,7 Test piece 2,17 Lens 3 Mirror 4a, 4b Pinhole 5,12 Light source 6,11 Light receiving element 8 XY axis electric stage 9 Z axis electric stage 10 Autofocus air nozzle 13 Laser displacement difference 14,15 Stage driver 16 computers

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shoji Murota 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Corporate Technology Development Division

Claims (6)

[Claims] 1. A method for automatically measuring the depth of localized corrosion such as pitting corrosion that occurs on the surface of a material by a microscope, wherein the light from the light source is focused on the surface of the object (test piece) where local corrosion has not occurred. By moving the subject in the plane of the subject while adjusting the above and monitoring the change in the intensity of the reflected light from the subject, local corrosion with a size larger than the expected size (threshold value) in the observation field of view. Is present, and when there is local corrosion having a size equal to or greater than the threshold value, the movement of the subject is stopped, and the process proceeds to a process for measuring the depth of the local corrosion, In the process, a local corrosion depth automatic measuring method is characterized in that the depth of local corrosion is detected from the relationship between the amount of movement (displacement) of the objective lens and the amount of reflected light. 2. The microscope according to claim 1, wherein a microscope having a confocal optical system is used as the microscope, and the depth of focus is made extremely shallow so that the presence of local corrosion and the accuracy of measuring the depth of local corrosion can be improved. Automatic local corrosion depth measurement method described. 3. The local corrosion according to claim 1, wherein the light source is a laser beam, and the microscopic spot is scanned in the measurement field of view of the microscope to improve the resolution on a plane and the depth of each spot can be measured. Automatic depth measurement method. 4. A device for automatically measuring the depth of localized corrosion such as pitting corrosion generated on the surface of a material by a microscope, wherein the light source (12) and an autofocus for focusing on a surface where local corrosion does not occur. The means (10), the displacement gauge (13), the stage driver (15) and the Z-axis moving stage (9), and the subject (test piece) (7) are set in the X-axis direction and Y direction.
There is an XY moving stage (8) and a stage driver (14) that can be displaced in the axial direction, an objective lens (17), a light receiving element (11), and local corrosion having a size equal to or larger than a threshold value in an observation visual field. In addition to determining whether or not there is local corrosion having a size equal to or greater than the threshold value, the movement of the objective lens is stopped to measure the depth of local corrosion by stopping the movement of the object and the reflection and displacement. A computer that calculates and calculates the depth of local corrosion from the relationship with the amount of light (1
6) An automatic local corrosion depth measuring device comprising: 5. The local corrosion depth automatic measuring device according to claim 4, wherein the microscope has a confocal optical system. 6. The local corrosion depth automatic measuring device according to claim 4, wherein the light source (12) has a laser light oscillation device.