Molten (Na, K)₂SO₄-NaCl salt corrosion of Ni₃Al alloy

Abstract
Two Ni₃Al alloys namely Ni₃AlB (Ni-18 3Al-0.2B (in at.%)) and Ni₃AlBCrZr (Ni-18.0Al- 0.2 B-6.0Cr-0.5Zr (in at.%)) were tested. Specimens of 15 x 5 x 3 mm were cut from the extruded bars. A commercial nickel base superalloy IN 738 was used as a reference material. The corrosion tests were conducted at 750, 800 and 850 °C in (0.9 Na, 0.1 K)₂SO₄-25 NaCl (in mole %) melt in air. The measurements include free corrosion potential (E_corr) and linear polarization resistance (R_p) . The test results show that during a test at 800 °C the E_corr of IN 738 shifted gradually to a more noble direction and then stayed around -200 mV. With the same tendency, after 30 h its R_p reached about 100 ohm cm² and become stable. The results proved that a protective oxide scale formed gradually on IN 738. In contrast, the corrosion behavior of Ni₃Al-0.2 B was somewhat different. During test at 800 °C, its E_corr dropped rapidly to -970 mV soon after immersion and R_p was only about 5.6 ohm cm², e.g. one order of magnitude lower than that of IN 738. Obviously, the corrosion rate of Ni₃Al-0.2 B was much higher, which can also be proved by the corrosion morphology that IN 738 was kept intact with a thin oxide scale on the surface after 100 h, whilst the whole specimen of Ni₃Al-0.2 B was entirely corroded after only 6 h immersion.