Effect of grain boundary characteristics on intergranular corrosion resistance of 6061 aluminum alloy extrusion
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I. INTRODUCTION
THE extrusions of the 6061 aluminum alloy are widely used for structural components because of their good extrudability and relatively high strength. However, it is well known that copper addition to Al-Mg-Si alloys reduces their resistance to intergranular corrosion[1,2] limiting their use in corrosive environments such as at sea. By the way, the application of Al-Mg-Si alloys as automobile body sheets is increasing, and there are some reports about improvement in intergranular corrosion resistance of Al-Mg-Si-Cu alloys by controlling the artificial aging condition or lowering the corrosion potential by zinc addition.[1] For use as body sheets, solution heat treatments are carried out after cold rolling. On the other hand, most of the Al-Mg-Si alloy extrusions are produced by press quenching, which is a method of combining solution heat treatment and extrusion.[3] It is important to determine the behavior of Al-Mg-Si-Cu alloy extrusions with regard to intergranular corrosion because their microstructures are different from sheet materials. In this study, the behavior of the 6061-T6 extrusions with regard to intergranular corrosion was investigated and the effect of the microstructures and grain boundary characteristics on the intergranular corrosion was discussed. II. EXPERIMENTAL PROCEDURE The 6061 aluminum alloy billet shown in Table I was prepared by direct chill casting with a diameter of 250 mm and then homogenized at 550 ⬚C. After homogenization, the billet was heated to 520 ⬚C for about 300 seconds using an induction heater and was extruded into the shape shown in Figure 1 at a speed of 133 mm/s. The extrusion was then press quenched with water. Artificial aging was carried out at 175 ⬚C for 8 h.
T. MINODA, Research Engineer, and H. YOSHIDA, General Manager, are with the Research and Development Center, Sumitomo Light Metal Industries, Ltd., Nagoya 455-8670, Japan. Contact e-mail: tadashi [email protected] Manuscript submitted July 9, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
The 50 ⫻ 60-mm specimens for the intergranular corrosion tests were cut from the upper part of the extrusions. The intergranular corrosion tests were carried out in accordance with ISO/DIS 11846 (method B). The specimens were degreased with acetone and treated with mixed acid for surface preparation prior to the corrosion tests. After pretreatment, the specimens were immersed at room temperature in an aqueous solution containing 3 mass pct NaCl and 1 vol pct HCl. The corrosion tests were then carried out for 24 hours as the standard condition and 96 and 240 hours as the extended test periods. For the 96- and 240-hour tests, the solutions were renewed every 24 hours. To clarify the differences in the intergranular corrosion behavior between the surface of the extrusions and the center thickness, the specimen was ground to half-thickness and polished with a #1200 abrasive paper, and then the intergranular corrosion test was carried out for 24 hours. After the corrosion tests, the specimens were washe
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