When charging aluminum alloys with hydrogen, the presence of oxide film on the surface is a challenge. To solve this problem, we have developed a method that can introduce high amount of hydrogen into aluminum alloys using friction in water. The method is easiest to understand when the rotation of the stir bar and the specimen on the polishing paper can be visualized.
Success depends on stabilizing the rotation of the stir bar. Demonstrating the procedure will Ms.Michiko Arayama, an undergraduate student from my laboratory. Fabricate the aluminum alloy test pieces as described in the text.
Perform a solution heat treatment by heating the test pieces in an air furnace at 520 degrees Celsius for one hour and then quenching the test pieces in water. For the next step, the peak-aging heat treatment, anneal the test pieces at 175 degrees Celsius for 18 hours. The final step in preparing the test pieces is polishing the surface using silicon carbide emery paper.
Before proceeding to the friction in water procedure, weigh and measure the test pieces. Use an electric balance to weigh the test pieces to a precision of 0.0001 grams. Use an optical comparator to measure the thickness and width of the test pieces to a precision of 0.0001 millimeters.
The friction in water procedure is carried out in a magnetically stirred custom-made reaction vessel. To begin, use glue to attach two test pieces to a fluorocarbon polymer triangular stir bar. Next, prepare the reaction vessel, a custom-made glass container.
Use double-sided tape to attach polishing paper to the inside bottom of the reaction vessel. Place the reaction vessel on the magnetic stirrer. Then place the test pieces and stir bar on top of the polishing paper and the reaction vessel, and add 100 milliliters of distilled water.
Place the rubber cover on the reaction vessel. Connect the gas inlet to high-purity argon, and turn on the gas. Connect the gas outlet to a gas chromatograph.
Inset the pH probe into the vessel through the rubber cover. Turn on the argon. Once the gas in the reaction vessel as been completely replaced by the argon, the apparatus is ready for charging the aluminum alloy test pieces.
Turn on the magnetic stirrer. To stabilize movement of the stir bar in the water, controlling the rotation speed is important. A speed ranging from 60 rpm to 240 rpm is best.
Every two minutes, measure the hydrogen concentration using the gas chromatograph, and measure the pH. After one hour, turn off the magnetic stirrer, and remove the test pieces and stir bar from the reaction vessel. To detach the test pieces from the stir bar, immerse them in acetone, and apply ultrasonic vibration for five minutes.
Before proceeding to the next step, measure the weight and thickness of the test pieces. Use a tensile test machine to measure the material properties of the test pieces. Set the crosshead speed of the machine to two millimeters per minute.
Then measure the stress-strain relationship for the test pieces. To calculate the amount of hydrogen absorbed during the friction in water procedure, first measure the hydrogen released by the test pieces when heated. Cut the test piece to a rectangular shape of one by five by 10 millimeters.
Place the test piece inside a quartz tube with a diameter of 10 millimeters, and place the quartz tube in a tubular furnace. Connect the tube to the gas chromatograph and to the argon gas supply. Turn on the flow of argon gas.
Heat the quartz tube containing the test piece, increasing the temperature at a constant rate of 200 degrees Celsius per hour until it reaches a temperature of 625 degrees Celsius. While the quartz tube and test piece are being heated, use the gas chromatograph to measure the hydrogen released every two minutes. Aluminum-magnesium-silicon alloys with three different iron concentrations were subjected to the friction in water procedure, 0.1%iron, 0.2%iron, and 0.7%iron.
Regardless of the iron concentration, the test pieces emitted large amounts of hydrogen during the procedure. Thermal desorption analysis was performed on both uncharged and hydrogen-charged samples. Regardless of the iron concentration of the alloy, the total hydrogen concentration increased as a result of the friction in water procedure.
Compared to the method of pre-stained in humid air, friction in water is an effective method of hydrogen charging an aluminum alloy. Thermal desorption analysis showed a higher hydrogen release rate for alloy charged using the friction in water procedure, and the calculated hydrogen concentration was substantially higher. Compared to uncharged alloy samples, hydrogen-charged alloy samples showed lower ductility.
This indicates that the friction in water procedure resulted in hydrogen embrittlement. Secondary electron microscopy was used to examine the fracture morphology of aluminum-magnesium-silicon alloy containing 0.1%iron. After the friction in water procedure, the morphology changed to a grain boundary fracture.
This indicates that hydrogen atoms introduced by the friction in water procedure enhanced the decohesion of grain boundaries, leading to hydrogen embrittlement. It is possible to hydrogen-charge aluminum alloys through exposure and humid air with brittle deformation with a slow stirring rate. However, the current method result in a greater amount of hydrogen charging.
This method makes it easy for researchers to evaluate the hydrogen embrittlement sensitivity of aluminum alloys that have a variety of different chemical compositions. It may be applied to the development of hydrogen storage materials.
