Hydrogen Charging of Aluminum using Friction in Water

Summary

In order to introduce high amounts of hydrogen in aluminum and aluminum alloys, a new method of hydrogen charging was developed, called the friction in water procedure.

Abstract

A new method of hydrogen charging of aluminum was developed by means of a friction in water (FW) procedure. This procedure can easily introduce high amounts of hydrogen into aluminum based on the chemical reaction between water and non-oxide coated aluminum.

Introduction

In general, aluminum base alloys have higher resistance to environmental hydrogen embrittlement than steel. The high resistance to hydrogen embrittlement of aluminum alloys is due to oxide films on the alloy surface blocking hydrogen entry. To evaluate and compare the high embrittlement sensitivity between aluminum alloys, hydrogen charging is usually performed prior to mechanical testing^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17}. However, it is known that hydrogen charging aluminum is not easy, even when utilizing hydrogen charging methods such as cathodic charging¹⁵, slow strain rate deformation under humid air¹⁶, or hydrogen plasma gas charging¹⁷. The difficulty of hydrogen charging aluminum alloys is also due to the oxide films on the aluminum alloy surface. We postulated that higher amounts of hydrogen could be introduced into aluminum alloys if we could remove the oxide film continuously in water. Thermodynamically¹⁸, pure aluminum without oxide film reacts easily with water and generates hydrogen. Based on this, we have developed a new method of hydrogen charging of aluminum alloys based on the chemical reaction between water and non-oxide aluminum. This method is able to add high amounts of hydrogen into aluminum alloys in a simple way.

Protocol

1. Material preparation

1. Use 1 mm thick plates made of an aluminum-magnesium-silicon alloy containing 1 mass% Mg and 0.8 mass% Si (Al-Mg-Si).
2. Make test pieces from the Al-Mg-Si alloy plates having a gauge length of 10 mm and width of 5 mm.
3. Anneal the test pieces at 520 ˚C for 1 h using an air furnace. Quench in water as a solution heat treatment.
4. Anneal the test pieces at 175 ˚C for 18 h as a peak aging heat treatment (T6-temper).
5. Polish the surface of the test pieces using silicon carbide emery paper (#2000) without water.
6. Measure the weight of the polished specimens to a precision of 0.0001 g using an electric balance
7. Measure the thickness and width of the gauge part of the specimens to a precision of 0.001 mm using an optical comparator.

2. FW procedure (Figure 1)

1. Attach two Al-Mg-Si alloy specimens using glue to a triangular, prism-shaped stirrer made by a fluorocarbon polymer.
2. Prepare a cylinder glass container with an empty top as a reaction vessel.
3. Attach a round polishing paper made by silicon carbides, #2000 with a diameter of 10 mm, using double sided tape in the bottom inside of the container.
4. Place the triangular, prism-shaped stirrer with two specimens on the polishing paper at the bottom surface of the glass container.
5. Pour 100 mL of distilled water into the glass container from the top.
6. Cover the glass container with a round rubber piece with three holes (for a gas inlet, for a gas outlet, and for a pH probe at the top of the glass container).
7. Fill the glass container with high purity (99.999%) argon at a constant flow rate of 20 mL/min after closing the rubber cover.
8. Connect the gas outlet to a gas chromatograph (GC) with a semiconductor hydrogen sensor (detection limit: 5 ppb).
9. Wait until the gas in the container is replaced by argon.
10. Rotate the triangular, prism-shaped stirrer with two specimens on a magnetic stirrer with a constant rotating speed at room temperature.
11. Measure the hydrogen generation during the stirrer rotation using the GC, taking one measurement every 2 min.
12. Measure the pH of the water in the container during the stirrer rotation.
13. Remove the two specimens from the triangular, prism-shaped stirrer by immersion in acetone with an ultrasonic vibration for 5 min after the FW procedure.
14. Measure the weight and thickness of the specimens again after the FW procedure using the electric balance and an optical comparator, respectively.

3. Hydrogen absorption by the FW procedure

1. After the FW procedure, cut a specimen to a rectangular shape of 1 x 5 x 10 mm.
2. Place the specimen inside a quartz tube with a diameter of 10 mm connected to a GC with a semiconductor hydrogen sensor.
3. Flow high purity (99.999%) argon gas in a quartz tube with a constant flow rate of 20 mL/min.
4. Heat the quartz tube with the specimen using a tubular furnace at a constant heating rate, 200 ˚C/h.
5. Measure the thermal hydrogen desorption of the specimen after the FW procedure using the GC.

4. Material evaluation after the FW procedure

1. Carry out tensile tests (at least 3x, to ensure repeatability) in laboratory air with a crosshead speed of 2 mm/min using a specimen that has been treated by the FW procedure.
2. Measure the tensile properties (e.g., tensile strength, fracture strain) obtained from the stress-strain curve in the tensile test.
3. Observe the fracture behavior with a secondary electron microscope (SEM) after the tensile test.

Results

Hydrogen generation/absorption by the FW procedure
Figure 2 shows the hydrogen generation behavior during the FW procedure of Al-Mg-Si alloys containing different amounts of iron from 0.1 mass % to 0.7 mass %. The specimen continuously emitted a high amount of hydrogen when the stirrer started to rotate. This suggests that hydrogen was generated by a chemical reaction caused by the friction between the alloy surface and water. In addition, the pH value of the water dur...

Discussion

One important aspect of the FW procedure is the attachment of the two specimens to the magnetic stirrer. Because the center of the stirrer bar becomes the non-friction zone, it is best to avoid the attachment of the specimens at the center of the stirrer bar.

Control of the rotation speed of the stirrer bar is also important. When the speed is more than 240 rpm, it becomes difficult to maintain the reaction vessel on the stage of the magnetic stirrer. When the FW procedure is carried out at hi...

Materials

Name	Company	Catalog Number	Comments
Air furnace	GC	QC-1
Aluminum alloy plates	Kobe Steel		Al/1.0 mass% Mg/0.8 mass% Si
Electric balance	A&D	HR-200
Glass container			Custom made
Magnetic stirrer	CORNING	PC-410D
Optical Comparator	NIKON	V-12B
pH meter	Sato Tech	PH-230SDJ
Quartz tube			Custom made
Rotary polishing machine	IMT	IM-P2
Secondary electrom microscope	JOEL	JSM-5310LV
Sensor gas chromatograph	FIS Inc.	SGHA
Silicon carbide emery paper	IMT	531SR
Tensile testing machine	Toshin Kogyo	SERT-5000-C
Tubular furnace	Honma Riken		Custom made