Aluminum alloy 7075 (AA7075) is known as a light-weight and high strength material. AA7075 mainly contains alloying elements such as Zn, Mg, Cu, and Fe to increase strength by intermetallic particles (IMPs). Cu-containing and Fe-containing IMPs in aluminum alloys act as initiation sites of localized corrosion such as pitting in chloride solutions^1-3). These IMPs in aluminum alloys show higher potential relative to the Al-matrix and act as cathodic reaction sites⁴⁾. As for AA7075, it was reported that an increase in pH by the oxygen reduction reaction around Cu-containing IMPs contributes to the dissolution of Al-matrix, followed by pitting³⁾. Therefore, it would appear that the inhibition of the oxygen reduction reaction on the IMPs prevents the pitting corrosion of 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 fabricated⁵⁾.

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 0.25 µm. AA7075 was fabricated same procedure as AA7075 containing Mn-rich phases. 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). Electrochemical measurements were performed in 0.1 M NaCl at pH 6.0 adjusted with NaOH. 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 or 100 µm × 100 µm. Surface treatment of AA7075 was performed in 0.1 M MnSO₄. The specimens were polarized in the solution at -0.9 V vs. Ag/AgCl (3.33 M KCl) for 5 hours. A dip-dry corrosion test was conducted to evaluate the corrosion resistance of the 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.

Al- and Mn-containing films were formed on Cu-containing IMPs by surface treatment in 0.1 M MnSO₄. In the 20 cycles of dip-dry corrosion test, MnSO₄-treated specimen showed less discoloration than non-treated specimen. It was found that the Al- and Mn-containing films inhibited the oxygen reduction reaction by cathodic polarization measurements. Mn-rich phases were detected in Mn-added AA7075. Mn-rich phases consisted of Mn, Al-Mn, and Al-Mn-Cu intermetallic phases. The size of Mn-rich phases was ca. 20 - 150 µm. In the 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 dip-dry corrosion test, pitting corrosion of the matrix was observed and Mn phases were dissolved. Al- and Mn-containing films were formed on Cu-containing IMPs around the Mn-rich phases after the corrosion test. It was found that the Al- and Mn-containing films inhibited the oxygen reduction reaction by cathodic polarization measurements.

References:

1. H. Kakinuma, I. Muto, Y. Oya, Y. Kyo, Y. Sugawara and N. Hara, J. Electrochem. Soc., 166, C19-C32 (2019).

2. H. Kakinuma, I. Muto, Y. Oya, T. Momii, Y. Sugawara and N. Hara, J. Electrochem. Soc., 168, 021504 (2021).

3. N. Birbilis, M.K. Cavanaugh and R.G. Buchheit, Corros. Sci., 48, 4202-4215 (2006).

4. Y. Zhu, K. Sun and G. S. Frankel, J. Electrochem. Soc., 165, C807-C820 (2018).

5. H. Saito, I. Muto and Y. Sugawara, Mater. Trans., 61, 2248-2251 (2020).