Evaluating the Corrosion Resistance of AA7075 Containing Mn-Rich Phases Fabricated By Spark Plasma Sintering

Abstract

Aluminum alloy 7075 (AA7075) are known to have light-weight and high strength. AA7075 mainly contains alloying elements such as Zn, Mg, Cu, and Fe to increase strength by intermetallic particles (IMPs). However, Cu-containing and Fe-containing IMPs show higher potential relative to the Al-matrix and act as initiation sites of localized corrosion such as pitting in chloride solutions1-2). In particular, Cu-containing IMPs in AA7075 become cathode and strongly affect the initiation and growth of localized corrosion2). As for AA7075, it was reported that breakdown of Al-oxide films occurs around Cu-containing IMPs due to an increase in pH by the oxygen reduction reaction, followed by pitting. Therefore, it would appear that the inhibition of the breakdown of passive films around the IMPs AA7075. Spark plasma sintering (SPS) is a sintering technique which has been applied to fabricate various materials. When stainless steel powders and pure Mo powders are mixed and sintered by SPS, stainless steel containing Mo-rich phases was fabricated3).In this study, AA7075 containing Mn-rich phases was fabricated by SPS. Gas-atomized AA7075 powders and pure Mn powders were mixed and sintered at 773 K for 15 min. After sintering, hot-forging was conducted at 773 K. After hot-forging, heat-treatment was conducted at 688 K for 2 h, followed by furnace-cooling. After that, the specimens were polished with a diamond paste down to 2.5 µm. The surface of specimens was observed using an optical microscope and a field emission scanning electron microscope (FE-SEM) equipped with energy dispersive X-ray spectroscopy system (EDS). The open circuit potential (OCP) of the specimen was measured for 72 h in 0.1 M NaCl at pH 6.0. With the exception of the electrode area, the surfaces of specimens were coated with an epoxy resin. The size of the exposed electrode area was ca. 5 mm × 5 mm. A dip-dry corrosion test was conducted to evaluate the corrosion resistance of specimens. The specimens were dipped in 0.1 M NaCl (pH 6.0) for 20 min and dried at 298 K and 50%RH for 60 min.Mn-rich phases were detected in Mn-added AA7075. Mn-rich phases consisted of pure Mn and Al-Mn intermetallic phases. The size of Mn-rich phases was ca. 20 - 150 µm. In the OCP measurements, OCP of Mn-added AA7075 decreased immediately after the immersion in 0.1 M NaCl, and the minimum value of the OCP of the Mn-added AA7075 was found to be lower than that of Mn-free AA7075. It was observed that the OCP of 5.0 mass% Mn-added AA7075 was lower than that of Mn-free AA7075. After the OCP measurements, the corrosion of Mn-rich phases was confirmed, and the color of these phases changed to dark. In the cyclic dip-dry corrosion test, mass-loss decreased with Mn content. After 40 cycles, mass-loss of 5.0 mass% Mn-added AA7075 was twice as lower as that of Mn-free AA7075. After the cyclic dip-dry corrosion test, pitting corrosion of the matrix was observed and Mn phases were dissolved.