Role of Superficially Applied SnO₂ Inhibitor in Controlling High Temperature Corrosion of Some Fe-, Co- and Ni-Base Superalloys

Abstract

Inhibitors and fuel additives have been used with varying success to combat high temperature corrosion of metals and alloys. In this work, the role of a superficially applied SnO₂ coating to combat high temperature corrosion of some superalloys viz Superfer 800H (Alloy A), Superco 605 (Alloy B) and Superni 75 (Alloy C) has been investigated. Accelerated corrosion testing of the coated as well as bare superalloys was done in a molten salt environment (Na₂SO₄–60 %V₂O₅₋) at 900 °C for 50 cycles. Each cycle consisted of 1 h heating in a silicon carbide tube furnace followed by 20 min cooling in ambient air. Weight change measurements after each cycle were taken by an electronic balance having an accuracy of 0.01 mg. XRD, SEM and EPMA analyses of the exposed specimens were carried out to characterize the oxide scales. The bare superalloys showed more overall weight gains, in general, in comparison with their SnO₂ coated counterparts. The corrosion rate for the bare Co-base alloy was found to be highest, whereas that for the Fe-base Superfer 800H a lowest. The percentage decrease in the weight gain in alloys A, B and C with superficially applied SnO₂ was found to be 17.2, 66 and 50 % respectively after 30 cycles. The effectiveness of the SnO₂ was perhaps due to its non-reactive nature with the corroding species and high melting point (1630 °C).