A286 alloy (AISI 660) is an austenitic precipitation hardness stainless steel that is widely used in the gas turbine industry, due to its high-temperature strength and oxidation resistance [1]. A286 alloy contains higher Ni (24.23 wt.%) and lower Cr (14.5 wt.%) than AISI 316 and AISI 304. The main precipitation strengthening phase of this alloy is gamma phase γ' [Ni₃(Al, Ti)], and second phase particles, which form during solution treatment and aging, include eta phase η (Ni₃Ti) and metal carbides and nitrides (TiC, TiN).

Unlike the relation between precipitates (γ', η) and pitting corrosion [2], the influence of TiC and TiN on the pitting corrosion is rarely studied. To investigate the influence of the various second phase particles on the corrosion behavior of A286 alloys, three samples were employed, including the hot-rolled (HR), HR + solution treatment at 980 °C for 1 h, and HR + solution treatment + aging at 720 °C for 16 h.

The corrosion resistance of the various A286 alloy samples were examined by potentiodynamic tests in 3.5 wt.% NaCl solution. The microstructure was investigated via scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS).

The second phase particles of HR A286 were mainly TiC and a few TiN, which were several micrometers in size and distributed intragranularly in the austenite matrix (Fig. 1a, 1d, and 1e). After the solution treatment, many TiC particles with sizes of a couple of micrometers precipitated along the austenite grain boundaries (Fig. 1b). Further aging resulted in the formation of γ' and η particles (Fig.1c). The potentiodynamic curves (Fig.1f) showed that the HR A286 had the lowest pitting potential (E_pit) and smallest passivation range, suggesting the formation of TiC particles did not deteriorate the corrosion resistance. Meanwhile, the γ' and η particles resulted in a slight decrease in the E_pit and passivation range.

Reference:

[1] H. De Cicco, M.I. Luppo, L.M. Gribaudo, J. Ovejero-García, Microstructural development and creep behavior in A286 superalloy, Materials Characterization 52 (2004) 85-92.

[2] P. De Tiedra, O. Martín, M. San-Juan, Potentiodynamic study of the influence of gamma prime and eta phases on pitting corrosion of A286 superalloy, Journal of Alloys and Compounds 673 (2016) 231-236.