JP7524916B2 - Method for measuring the amount of corrosion on test specimens - Google Patents

Description

The present invention relates to a method for measuring various test specimens, and in particular to a method for measuring the amount of corrosion of test specimens made of metallic materials.

In areas where snow melting salt is spread on roads in winter, measures against perforation corrosion of automobiles are an important issue. Perforation corrosion occurs inside the joints (overlaps) of plates formed when plate-shaped metal materials such as steel plates and aluminum alloy plates are joined together by spot welding or crimping. In joints, the inside (inner surface) is not fully treated with chemical conversion coating or electrocoating, and moisture and salt tend to accumulate, so the corrosion rate can be faster than that of general surfaces other than the inside of the joint. Furthermore, unlike corrosion on general surfaces, the degree of corrosion inside the joint cannot be determined from the appearance, making it difficult to manage. Therefore, the life until perforation corrosion occurs in the joint is an important factor in setting the plate thickness of automobile structural materials.

The rate of perforation corrosion of the target metal material is of fundamental importance in understanding the actual lifespan until perforation corrosion occurs in the operating environment. The corrosion rate is the differential value of the amount of corrosion, and is calculated by measuring the change in the amount of corrosion over time and using a regression equation from the corrosion amount curve. In other words, to measure the corrosion rate at a certain point in time, a series of past data is required, in which many test pieces (test specimens) are first exposed to the air, and then the test specimens are sequentially collected at regular intervals to determine the amount of corrosion from the initial state of the test specimens.

Generally, the amount of corrosion of a test specimen is quantitatively evaluated by removing the corrosion products that have formed on the test specimen due to corrosion, and then measuring the amount of plate thickness reduction using a micrometer or by measuring by gravimetric method. Because the test specimen is destroyed during the evaluation, it is impossible to track the amount of corrosion over time for the same test specimen. Therefore, when collecting data on the change in the amount of corrosion over time, it is necessary to prepare a large number of test specimens in order to measure the corrosion rate. Preparing a large number of test specimens and collecting, managing, and analyzing that data requires a great deal of effort.

Regarding corrosion on the general surface of weathering steel materials used without painting, various methods have been proposed to judge the state of rust, which is thought to correlate with the corrosion rate, and to easily estimate the corrosion rate. For example, the five-level evaluation criteria shown in Non-Patent Document 1 are used as criteria for judging the appearance of rust. This criteria is used to assign a score to the roughness of the rust appearance and the presence or absence of peeling rust. In other words, if fine, adherent rust is formed on the surface, it is judged that the corrosion rate has been sufficiently reduced and there are no problems in terms of maintenance.

"Joint Research Report on the Application of Weathering Steel to Bridges (XV)", Public Works Research Institute, Ministry of Construction, Kozai Club, Japan Bridge Construction Association

However, the above five-point evaluation criteria for determining the appearance of rust cannot be applied to corrosion that occurs inside joints of test specimens that cannot be seen with the naked eye, for example.

The present invention has been made in consideration of the above circumstances, and aims to provide a method for measuring the amount of corrosion of a test specimen that can measure the corrosion of the test specimen nondestructively and can measure the amount of corrosion over time using the same test specimen.

In order to achieve the above object, the inventors have conducted intensive research and discovered that the amount of corrosion of a test specimen that has been exposed to the environment and has developed corrosion can be measured non-destructively using computed tomography (CT).

The present invention has the following configuration.
[1] A method for measuring the amount of corrosion of a test specimen, comprising:
Selecting a corroded region in each of a plurality of parallel cross-sectional images generated by a reconstruction process using a computed tomography method for the test specimen to be measured;
a method for measuring the amount of corrosion of a test specimen, the method comprising: measuring the amount of corrosion of the test specimen by integrating the selected corroded region for the plurality of parallel cross-sectional images.
[2] The method for measuring the amount of corrosion of a test specimen according to [1], wherein the computed tomography is a computed tomography using any one of X-rays, neutron rays, and synchrotron radiation.
[3] The method for measuring the amount of corrosion of a test specimen according to [1] or [2], wherein the computed tomography is an X-ray computed tomography using X-rays with a tube voltage of 50 kV or more and 300 kV or less.
[4] The method for measuring the amount of corrosion of a test specimen according to any one of [1] to [3], wherein the test specimen is a laminated test specimen in which two or more sheets of metal material are joined together within the plate surface.
[5] The method for measuring the amount of corrosion of a test specimen according to any one of [1] to [4], wherein the amount of corrosion is a depth of a corrosion pit or a corrosion volume.
[6] A device for measuring the amount of corrosion of a test specimen used in the method for measuring the amount of corrosion according to any one of [1] to [5],
An apparatus for measuring the amount of corrosion of a test piece, comprising: a rotating base on which a test piece to be measured is placed; a radiation generating unit that irradiates the test piece with radiation consisting of X-rays, neutron beams, or synchrotron radiation; a detection unit that detects the radiation that has passed through the test piece and generates a radiation transmission signal; and an analysis unit that takes in the radiation transmission signal to generate a radiation transmission image and rotates the rotating base to accumulate the radiation transmission images obtained while rotating the test piece placed on the rotating base, thereby generating a three-dimensional reconstructed image and performing image analysis.

The present invention provides a method for measuring a test specimen that can measure corrosion of the test specimen nondestructively and can measure the amount of corrosion over time using the same test specimen.

According to the present invention, it is possible to quantitatively evaluate corrosion occurring on the surface or inside of a test specimen nondestructively, and to track and evaluate the amount of corrosion in the same test specimen. According to the present invention, it is possible to reduce the number of test specimens to be evaluated, and further to reduce the effects of measurement variability between test specimens.

FIG. 1 is a diagram showing an example of a test specimen. FIG. 1 shows conditions for SAE J2334 cyclic corrosion test. A test specimen (bonded test piece) was measured using X-ray CT, and corroded areas were selected from the reconstructed cross-sectional images of the plate surface at each position within the plate thickness. FIG. 1 is a diagram showing the state of corrosion of the mating surfaces of the same test specimen (jointed test piece) when a complex cyclic corrosion test is performed using the same test specimen and the test specimen is measured by X-ray CT at every predetermined cycle. FIG. 2 is a graph showing the change in corrosion amount of the same test specimen depending on the number of corrosion test cycles measured in the examples.

The following describes an embodiment of the present invention. Note that the following description shows one preferred embodiment of the present invention, and the present invention is not limited in any way by the following description.

In the present invention, for a test specimen to be measured, a corroded area is selected in each of a number of parallel cross-sectional images generated by reconstruction processing using computed tomography, and the amount of corrosion of the test specimen is measured by integrating the selected corroded areas for the multiple parallel cross-sectional images.

First, we will explain the test specimen to be measured.

(Test specimen)
The material of the test object to be measured is preferably a metal material. Examples of the metal material include any metal material, including ordinary steel (iron alloy), stainless steel, aluminum alloy, magnesium alloy, and titanium alloy. In addition, in the present invention, a laminated test object having a joint in which the same or different materials are joined can be measured. The present invention is particularly useful when a laminated test object having a joint (overlap) formed by joining two or more plate materials in the plate plane (stacking and joining in the plate thickness direction) is measured.

To evaluate corrosion (perforation corrosion) inside the joints of test specimens, homogeneous laminated test specimens made of the same metal material or heterogeneous laminated test specimens made of different metal materials can be used. Furthermore, heterogeneous laminated test specimens made of plastic materials such as carbon fiber reinforced plastics or composite materials and metal materials can also be used. By using such various laminated test specimens as the measurement subject, it is also possible to measure the corrosion (perforation corrosion state) of metal materials in contact with these materials. Furthermore, it is also possible to use test specimens in which these test specimens have been painted as the measurement subject.

The shape, area, clearance, etc. of the joint of the test specimen are not limited, but it is desirable to simulate the shape of the joint that may be formed in the actual product. The method of forming the joint is not limited, but it is preferable to use the same method as the actual product. As the method, in addition to fusion joining such as resistance spot welding, arc welding, and brazing, various mechanical joining methods such as mechanical clinching, friction stir welding (FSW), and bolt fastening can be used.

When measuring the amount of corrosion inside the joint of a laminated test piece by X-ray computed tomography (X-ray CT) using X-rays, it is preferable to use X-rays with an X-ray tube voltage of 50 kV or more in order to obtain a clear contrast between the corroded and uncorroded parts (sound parts) in the radiographic images and three-dimensional reconstruction images described below and to accurately measure the amount of corrosion. The X-ray tube voltage is more preferably 70 kV or more, and even more preferably 100 kV or more. There is no particular limit to the upper limit of the X-ray tube voltage, but as an example, the X-ray tube voltage is preferably 300 kV or less. In addition, in order to obtain a clear image, it is preferable to use X-rays with an X-ray tube current of 50 μA or more and 10 mA or less. As the test piece, a laminated test piece in which plate materials made of iron alloys are joined within the plate surface is used, and the plate thickness of the laminated test piece (the total plate thickness of the plate materials) is preferably 10 mm or less. The plate thickness of the laminated test piece is more preferably 4 mm or less, and even more preferably 2 mm or less. In addition, the short side of the joint when the laminated test specimen is viewed in plan is preferably 70 mm or less, and more preferably 40 mm or less. By using such a laminated test specimen, when the test specimen is rotated (spinned) while being photographed as described below, X-rays can be sufficiently transmitted through the joint of the rotating test specimen, making it easier to accurately measure corrosion (hole corrosion) inside the joint.

(Computed Tomography)
In the present invention, the corrosion of a test specimen is measured non-destructively by computer tomography (CT). Any type of computer tomography may be used as long as it uses radiation that can penetrate the test specimen to be measured. For example, computer tomography may use X-rays, neutron rays, or synchrotron radiation. Considering the maintenance and management of the device and the measurement costs, X-ray CT using X-rays is preferable, and it is most preferable to use a general-purpose industrial X-ray CT.

A computed tomography apparatus (CT apparatus) used as a measuring device for measuring the amount of corrosion of a test specimen includes a base on which the test specimen to be measured is placed, a radiation generating unit that irradiates radiation toward the test specimen, a detection unit that detects the radiation that has passed through the test specimen and generates a radioactive transmission signal, and an analysis unit that takes in the radioactive transmission signal detected by the detection unit and performs image analysis. The base is a rotatable base (rotating base), and when the base rotates, the test specimen placed on the base rotates (rotates) around the axis of rotation of the base. The radiation generating unit is disposed so that the optical axis of the radiation irradiated from the radiation generating unit and the axis of rotation of the base are perpendicular. The analysis unit is composed of a computer, and takes in the radioactive transmission signal detected by the detection unit to generate a radioactive transmission image. Furthermore, the analysis unit generates a three-dimensional reconstruction image by accumulating the radioactive transmission images obtained by photographing the base (test specimen) while rotating, and then generates a cross-sectional image based on this three-dimensional reconstruction image (reconstruction processing). The analysis unit can generate cross-sectional images in any direction as cross-sectional images by using a computer program or software included in the analysis unit, and can generate multiple cross-sectional images at any pitch (the interval between the cross-sectional images to be generated). For example, if the test object is a plate material, multiple parallel cross-sectional images perpendicular to the plate thickness direction can be generated, and multiple parallel cross-sectional images perpendicular to the plate length direction or width direction can be generated.

For example, in the case of X-ray computed tomography (X-ray CT), the absorption coefficient of X-rays increases in proportion to the atomic number. In other words, the heavier the element, the more X-rays it absorbs. Also, if there are regions with different electron densities in the test specimen, the higher the electron density, the more X-rays it absorbs. Therefore, when corrosion occurs in the test specimen and corrosion products such as iron oxide or voids are generated in the test specimen, the region becomes sparser than the uncorroded part (sound part), and therefore the absorption of X-rays is lower than in the uncorroded part. For this reason, when a cross-sectional image is generated based on a three-dimensional reconstruction image generated by reconstruction processing using X-ray CT for a corroded test specimen, the cross-sectional image provides a contrast between the uncorroded part (sound part) and the corroded part of the test specimen. In the present invention, the region with a different contrast from the uncorroded part (sound part) in the cross-sectional image generated by reconstruction processing using CT is selected as the corroded part (corroded area) for the test specimen.

In the present invention, in each of the multiple parallel cross-sectional images generated by the reconstruction process, an area that has a contrast different from that of the uncorroded area (healthy area) is selected as the corroded area. This selection may be performed, for example, visually or by using image processing software. The amount of corrosion of the test specimen is determined by integrating the selected corroded area for the multiple parallel cross-sectional images (integrating the corroded area in the vertical direction of the multiple parallel cross-sectional images).

The amount of corrosion can be measured (calculated) as, for example, the depth of corrosion (corrosion pits) or the corrosion volume. In addition, the amount of corrosion can be calculated as the remaining thickness of the test specimen (remaining plate thickness) by subtracting the measured depth of the corrosion pits from the thickness of the test specimen (plate thickness) before the corrosion test.

The number of parallel cross-sectional images can be set appropriately depending on the accuracy of the desired measurement value, the load of analysis, etc. For example, although the load of analysis will increase, by reducing the pitch and integrating the corrosion area for a larger number of cross-sectional images to measure the amount of corrosion, it is possible to get closer to the actual measured amount of corrosion.

In addition, in this embodiment, the amount of corrosion was measured by selecting a corroded area in the cross-sectional image generated by the reconstruction process, but conversely, it is also possible to select an area of uncorroded (healthy) parts in the cross-sectional image, measure the area of the uncorroded parts by integrating this for multiple parallel cross-sectional images, and subtract this from the entire test piece before the corrosion test to obtain the amount of corrosion.

In addition, when using X-ray CT, it is desirable to adjust the tube voltage and tube current of the X-ray CT device appropriately according to the object to be measured, and to adjust the contrast between corroded and uncorroded areas (healthy areas) on the radiographic images and three-dimensional reconstructed images.

For example, in a laminated test specimen in which two steel plates with a thickness of 0.2 mm to 5 mm are overlapped in the plate plane, and the length of the short side of the joint when the test specimen is viewed in a plane is 70 mm or less, a three-dimensional reconstruction image with clear contrast can be obtained by setting the tube voltage of the X-ray CT to 100 kV to 300 kV. If the plate thickness is 0.2 mm or more, it becomes easier to distinguish between the corrosion of the joint surface and the surface (outer surface) of the steel plate. If the plate thickness is 5 mm or less and/or the length of the short side of the joint is 70 mm or less, X-rays are more likely to penetrate and a clear image is more likely to be obtained. In this case, the tube current is preferably 50 μA or more, and more preferably 3 mA or more. In addition, since the size of the area of the imaging field of view affects the resolution of the three-dimensional reconstruction image, it is preferable to set it according to the required measurement accuracy. Although it depends on the model of the device, the diameter of the imaging field of view is set to be approximately 1000 times the required resolution, making it possible to measure with the target accuracy. There are no limitations on the calculation method used for the reconstruction process, but for example, the commonly used filtered back projection method can be used.

The above-described method for measuring the amount of corrosion of a test specimen of the present invention allows the amount of corrosion of the test specimen to be measured non-destructively, and allows the amount of corrosion to be measured over time using the same test specimen.

Conventionally, in order to measure the amount of corrosion of a test specimen, it was necessary to remove the corrosion products generated by corrosion from the test specimen (rust removal), and then measure the thickness loss of the test specimen with a micrometer and measure the weight of the removed corrosion products. Furthermore, when the test specimen was a laminated test specimen, in order to measure the amount of corrosion inside the joint (perforation corrosion), it was necessary to disassemble the laminated test specimen and perform the above operations. Therefore, the measurement of the amount of corrosion using the same test specimen was limited to one time, and it was impossible to track the amount of corrosion over time using the same test specimen. Furthermore, when evaluating the change in the amount of corrosion over time, it was necessary to prepare a large number of test specimens, and using a large number of test specimens for evaluation also caused measurement variations between test specimens. Furthermore, depending on the type of material of the test specimen, it was difficult to predict when corrosion would occur, and especially when the test specimen was a laminated test specimen, it was necessary to disassemble the test specimen and check each time to confirm whether corrosion had occurred inside the joint, and it was necessary to prepare more test specimens.

According to the present invention, the amount of corrosion of a test specimen can be measured nondestructively, and the amount of corrosion can be measured over time using the same test specimen, so there is no need to prepare a large number of test specimens as described above. According to the present invention, the corrosion occurring on the surface or inside of the same test specimen can be quantitatively evaluated nondestructively, making it possible to track and evaluate the amount of corrosion in the same test specimen. According to the present invention, the number of test specimens to be evaluated can be reduced, and the effects of measurement variations between test specimens can be reduced. In addition, the degree of corrosion, the timing at which corrosion occurs, and how corrosion progresses (corrosion rate and direction of corrosion progress) can also be evaluated nondestructively. In particular, when the present invention is applied to the measurement of a combined test specimen, the process of disassembling the combined test specimen to measure the amount of corrosion is not required, so the effects of the present invention can be enjoyed even more.

Below, examples of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following examples.

(Test specimen)
In this example, in order to observe the change over time in the corrosion state of the gap of the steel material formed by welding, a joint test piece shown in Fig. 1 was used as a joint test piece. As shown in Fig. 1, a steel plate (plate 1) of 60 mm x 80 mm x 1 mm size and a steel plate (plate 2) of 40 mm x 60 mm x 1 mm size were overlapped in the plate thickness direction and resistance spot welded at two places to prepare a joint test piece. This was subjected to zinc phosphate conversion treatment and electrodeposition coating to prepare a joint test piece in which the surface was painted and the inside of the joint (gap) was not painted, i.e., the inside of the joint (gap) was not painted, and this joint test piece was used as the measurement subject.

(Corrosion Test)
The bonded test pieces were subjected to a wet-dry repeated combined cycle corrosion test, which was specified in SAE J2334 (5 days) and consisted of manual salt water immersion and wet-dry repeated cycles (FIG. 2). X-ray CT measurements were performed before the start of the corrosion test and every 15 cycles during the corrosion test.

(X-ray CT scan)
The X-ray CT device used was an industrial X-ray CT device manufactured by RF Corporation. This X-ray CT device has the same configuration as the above-mentioned CT device, and has an X-ray tube as a radiation generating unit. In addition, the test specimen is placed on a pedestal, and the test specimen placement surface of the pedestal is parallel to the horizontal plane.

Using the X-ray CT device, the above-mentioned bonded test piece was supported on a base using a resin jig so that the steel plate surface of the test piece (bonded test piece) was perpendicular to the optical axis of the X-rays (so that the long sides of plates 1 and 2 shown in Figure 1 were vertical). Images were then taken under conditions of tube voltage: 80 kV, tube current: 3 mA. In the analysis section, a three-dimensional image was reconstructed using the filtered back projection method, and cross-sectional images of the plate surface were generated at 100 μm pitch in the plate thickness direction and output to a monitor.

Examples of the above-mentioned cross-sectional images are shown in Figures 3 and 4. Figure 3 is a cross-sectional image of a bonded test piece at the end of 60 cycles during a corrosion test. Figure 3 shows cross-sectional images generated at positions 200 μm, 400 μm, 600 μm, and 800 μm in the thickness direction of plate 2 of the bonded test piece, parallel to the mating surface, with the mating surface of the bonded test piece at the 0 μm position. Figure 4 shows cross-sectional images of the mating surface (0 μm position) of the bonded test piece generated for the same test body (bonded test piece) every 15 cycles of the corrosion test.

In Figure 3, from the cross-sectional images of the plate surface at each position within the plate thickness, areas with a different contrast from the uncorroded areas (sound areas) were visually selected as corroded areas, and the corroded area within the surface at each position was calculated. Furthermore, the corroded area was integrated in the plate thickness direction, and this was determined as the amount of corrosion (corrosion volume) of the bonded test piece.

As above, for the same test specimen (joint test piece), the relationship between the amount of corrosion (corrosion volume) measured every 15 cycles of the corrosion test and the number of corrosion test cycles is shown in Figure 5.

In this way, the present invention makes it possible to measure the corrosion of a test specimen nondestructively, and to measure the amount of corrosion over time using the same test specimen.

Note that the embodiments and examples disclosed herein are illustrative in all respects, and the present invention is not limited to these examples.

The method and device for measuring the amount of corrosion of a test specimen of the present invention can quantitatively evaluate the corrosion of the surface of a test specimen and the corrosion of holes inside the test specimen in a non-destructive manner, and can track and evaluate the amount of corrosion in the same test specimen. According to the present invention, it is possible to reduce the number of test specimens to be evaluated and to reduce the effects of measurement variability between test specimens, which is useful not only for the development of various materials but also for evaluating the severity of corrosive environments.

Claims (1)

A method for measuring the amount of corrosion of a test specimen, comprising the steps of:
Selecting a corroded region in each of a plurality of parallel cross-sectional images generated by a reconstruction process using a computed tomography method for the test specimen to be measured;
The amount of corrosion of the test specimen is measured by integrating the selected corroded area for the plurality of parallel cross-sectional images , and the measurement of the amount of corrosion of the test specimen is performed over time using the same test specimen;
The computed tomography is an X-ray computed tomography using X-rays with a tube voltage of 100 kV or more and 300 kV or less and a tube current of 3 mA or more and 10 mA or less,
The test specimen is a laminated test specimen having a joint in which two steel plates having a plate thickness of 0.2 mm or more and 5 mm or less are overlapped in the plate plane, and the length of the short side of the joint when the laminated test specimen is viewed in a plane is 70 mm or less,
A method for measuring the amount of corrosion of a test specimen, wherein the amount of corrosion is a depth of a corrosion pit or a corrosion volume .

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