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Promoting Protective Scale Formation at Lower Temperatures via Surface Finishing: Effects on the Establishment, Structure, and Chemistry in Haynes 214 High-Temperature Oxidation-Resistant Nickel Alloy

Haynes® 214® is a Ni-Cr-Al-Fe alloy widely employed for its excellent resistance to oxidation and corrosion in high-temperature oxidizing environments, such as direct flame impingement, burner assemblies, and catalytic converter internals of automobiles. The resistance is attributable to the formation of a protective and adherent aluminum oxide scale. The oxidation behavior of 214, however, is temperature-dependent. Alumina-scale formation on 214 is kinetically favorable at exposure temperatures above 955 °C; below this temperature a less-protective scale containing nickel oxide is apt to form. This could be problematic for lower-temperature applications, particularly those involving multiple oxidants. Surface finishing of metallic parts and structures is often done for aesthetic or practical reasons. For example, it has been shown that the surface treatment can impact the corrosion resistance of metals.[1,2] More recently in a study of Ni-Al and Ni-Al-Cr alloys at 1100 °C, surface abrasion was used to directly promote Al 2 O 3 -scale establishment.[3] Inspired by this, we investigated whether surface modification via abrasion could be used to facilitate Al 2 O 3 -scale formation on 214 alloys at lower temperatures. This presentation discusses our study of the impact of typical surface finishes on the alloy structure and the associated response to short-term oxidation exposures. In the current study, three surface treatments were investigated: polishing with graded SiC sandpapers finishing at 500- or 1200-grit, and vapor blasting (VB) with 300 mesh silica beads. The surface-finished samples were then oxidized in air at 800 °C for 2 hours. Surface and topographical characterization was performed using atomic force microscopy (AFM) and scanning electron microscopy (SEM), while focused ion beam (FIB-SEM) cross-sectional lift-outs were examined using scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDXS), and ASTAR nanoscale orientation mapping to characterize the subsurface. This examination revealed that surface finishing does indeed impact the oxide scale formation on 214 down to the nano-scale, and provide insight into the relative importance of various structural aspects.