Effect of Superficially Applied Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating on High-Temperature Corrosion Behavior of Ni-Base Superalloys

Inhibitors and oxide additives have been investigated with varying success to control high-temperature corrosion. Effect of Y₂O₃ on high-temperature corrosion of Superni 718 and Superni 601 superalloys was investigated in the Na₂SO₄-60 pct V₂O₅ environment at 1173 K (900 °C) for 50 cycles. Y₂O₃ was applied as a coating on the surfaces of the specimens. Superni 601 was found to have better corrosion resistance in comparison with Superni 718 in the Na₂SO₄-60 pct V₂O₅ environment. The Y₂O₃ superficial coating was successful in decreasing the reaction rate for both the superalloys. In the oxide scale of the alloy Superni 601, Y and V were observed to coexist, thereby indicating the formation of a protective YVO₄ phase. There was a distinct presence of a protective Cr₂O₃-rich layer just above the substrate/scale interface in the alloy. Whereas Cr₂O₃ was present with Fe and Ni in the scale of Superni 718. Y₂O₃ seemed to be contributing to better adhesion of the scale, as comparatively lesser spalling was noticed in the presence of Y₂O₃.     

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

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 %V₂O_5?) 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).     

Chromate passivation of hot dipped Zn25Al alloy coatings

Abstract

A low concentration chromate passivation treatment has been successfully applied to a new type of hot dipped Zn2 5Al alloy coating, and the corrosion resistance of the chromate passive film has been assessed using the copper accelerated acetic salt spray (CASS) test, electrochemical measurements, and sea water immersion testing. The results showed that the corrosion resistance of the Zn2 5Al alloy coating was significantly better after the chromate passivation treatment. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses showed that the composition (at.-%) of the low concentration chromate passive film was: 5·5S–3·4Na–11·8C–7·9Ti–41·6O–1 3·7Cr–16·0Zn. Aluminium was not found in the film, which is attributed to the dissolution behaviour of the Zn2 5Al alloy coating in acidic chromate solution.