Engineering
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Applicability Analysis of Assessment Methods for Morphological Parameters of Corroded Steel Bars
Summary November 1st, 2018
This paper measures the geometry and the amount of corrosion of a steel bar using different methods: mass loss, calipers, drainage measurements, 3D scanning, and X-ray micro-computed tomography (XCT).
Transcript
This video aims to measure the surface morphology of intact and corroded steel bars. We will demonstrate and evaluate five different measures, including mass loss, vernier caliper measurements, drainage measurements, 3D scanning, and XCT of the steel bar. 3D scanning is the best method for measurement of the spacial variability of corrosion penetration in the surface of a corroded bar.
The corrosion measurements obtained with the 3D-scanning method would enable engineering to evaluate the safety and service life of existing engineering structures in our society much more precisely and reliably. Demonstrating the procedure will be Mr.Huilai Han, a technician from a laboratory, and Mr.Shenglin Cui, a technician from Shenzhen Hong Rong's three-dimensional technology. To begin, mark a polished 500-millimeter-long, 14-millimeter-diameter steel bar in 10-millimeter increments.
Then, use zeroed vernier calipers to measure the diameter of the bar at the first mark with the jaws gently touching the bar. Perform three more measurements, rotating the calipers by 45 degrees each time, for a total of four measurements at 45-degree intervals. Repeat this process for each mark on the bar.
Average the diameters at each mark and calculate the cross sections. Then, cut 30 millimeters from each end of the bar to obtain two 30-millimeter-long bar specimens and one 440-millimeter-long bar specimen. Weigh each specimen on a digital electronic scale and record the readings.
Mark the 30-millimeter-long bar specimens at 10-millimeter intervals, starting five millimeters from the left side. Determine the average diameter at each of these marks as previously described. Next, set up a static-hydraulic, electromechanical universal testing machine and place a glass displacement cylinder under the machine head.
Fill the cylinder with tap water to just reach the outlet. Then, place an electronic scale below the outlet of the container. Place a 200-milliliter beaker on the scale in-line with the outlet.
After that, clamp a bar specimen vertically in the head of the EUT machine. Move the EUT head down vertically until the bar just touches the surface of the water. Record the initial reading on the digital scale.
Then, set the EUT machine to move the bar downward at 10 millimeters per minute. Run the machine to displace the bar to the first 10-millimeter mark. Then, record the final reading on the electronic scale.
Repeat this process for each 10-millimeter segment of the bar specimen until the entire bar is submerged. Let the bar air-dry for one hour afterwards. Calculate the uniform cross section of each 10-millimeter segment of the bar from the displaced water masses.
Test each of the three specimens in this way. Next, spray each dry bar specimen with white flaw-detection developer, and allow the specimens to dry in air. Then, place a coated specimen on the platform of the 3D scanner.
Calibrate the bar specimen position using a label marked with a random array of small, white dots. Then, scan the specimen along its length, develop a spacial model, and generate morphological data from the model. Repeat this for each specimen.
Next, place one 30-millimeter-long specimen on the rotatable platform of an x-ray micro-computed tomography system. Close the XCT instrument. Open the instrument software and move the specimen to the correct position for scanning.
Fill in the desired pixel size and magnification factor. Then, scan the specimen, and generate the geometric parameters. Repeat the process for the other 30-millimeter-long specimen.
After acquiring the measurements of the non-corroded steel bar, strip 50 to 60 millimeters of insulation from a two-meter length of multi-stranded electrical wire. Strip a shorter segment from the other end. Fix the longer, exposed end of the wire to one end of the 440-millimeter-long specimen with insulation tape firmly wrapped around 70 millimeters of the end of the bar.
Then, firmly wrap 70 millimeters of the other end of the bar with insulation tape. Next, mix epoxy adhesive with hardener in a one-to-one ratio. Apply the epoxy uniformly to each insulated end of the bar to protect the ends from corrosion.
Once the epoxy is dried, fill a plastic tank equipped with a copper plate with an aqueous 3.5%by weight solution of sodium chloride in tap water. Place the bar specimen in the tank. Then, connect the wire attached to the bar to the positive terminal of a DC power supply.
Connect the copper plate to the negative terminal of the power supply. Set the power supply to produce a corrosion current density of 2.5 micro-amperes-per-square-centimeter throughout the bar. Apply the current for the duration needed to attain the desired level of corrosion per Faraday's law.
Then, turn off the current, disconnect the bar specimen, and soak it in a 12%by volume aqueous solution of hydrochloric acid for 30 minutes to remove corrosion products. After that, transfer the bar specimen to a saturated lime-water solution to neutralize the acid residue. Then, rinse the bar with tap water and allow it to dry in air.
Next, mark the corroded surface at 10-millimeter intervals. Weigh the corroded bar horizontally on a digital electronic scale, and calculate the average area of the corroded bar. Determine the average diameter at each 10-millimeter mark using vernier calipers and calculate the cross sections.
Then, calculate the cross section of each 10-millimeter segment of the corroded bar using the displaced water drainage method. After that, spray the specimen with white developer and take a 3D scan. Lastly, cut a 30-millimeter segment from the corroded bar and scan it with XCT.
The measured diameters of the in-tact steel bar did not vary significantly along its length, but consistent variation was observed between the 45-degree measurement and the 135-degree measurement, indicating that the bar was elliptical. The mass loss, caliper measurement, and 3D scanning techniques produced similar values with low variation. The drainage method measurements of the 440-millimeter-long specimen suffered from multiple sources of uncertainty, including the dryness of the bar and the surface tension of the water.
The 30-millimeter specimens were analyzed with XCT, which produced values consistent with other techniques. Overall, calipers, XCT, and 3D scanning produced similar values with minimal variation. Thus, caliper measurements were the simplest way to obtain accurate measurements of non-corroded bars.
Corrosion resulted in substantial variation and cross-sectional shape throughout the bar, which could not be captured with the mass loss method. While calipers were more sensitive to the shape variation, they could not account for pitting in the surface of the specimen. XCT and 3D scanning produced similar values, but XCT is limited by the need for small samples with flat ends.
Thus, 3D measurement were favored for analyzing the morphology of corroded steel bars. A vernier caliper is the best tool for measuring a non-corroded steel bar's surface morphology. And, it's easy to operate.
And, it's quite economical. The drainage method measurement may be affected by some uncertainty, so further improvements to the measurement device are needed. Although the XCT measurement can accurately measure the residual cross-sectional area of a corroded steel bar, it is limited by the length of the specimen.
The 3D scanning method is the most optimal method for the measurements of a corroded steel bar because it is precise, economical, and efficient. It can also generate additional useful information about the corroded bar.
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