An Investigation on the Chemical Variation in TLP Joint Line and Its Effect on High-temperature Corrosion Behavior of the Bonded IN738-Lc

Abstract

Turbine blade face with crack generation during high-temperature application. Due to its high cost of manufacturing, repair of the damaged section is usually more desirable than its replacement. transient liquid phase (TLP) bonding is a common joining procedure used for maintenance of a gas turbine damaged vein and blade. This paper describes how the TLP joint area resist hot corrosion at high working temperatures. Microstructure and metal loss were studied from scanning electron microscope (SEM) images, and corrosion products were identified by XRD and EDS analyses. SEM observations revealed that the TLP area is composed of three distinguished regions: a thermally solidified zone with about 15 µm thickness, the isothermally solidified zone with about 32 µm thickness, and diffusion affected zone with about 54 µm thickness. Exposing test samples to Na2SO4/NaCl (3:1 wt%) salt mixture at 750 °C, creates a 40 µm of porous oxide scales containing the outer layer of chromium oxide and the inner layer of chromium–nickel oxides. Moreover, 160 µm of internal attacks were observed and were identified as sulfide attack. The corrosion attacks in the diffusion affected area were observed in the form of micro-cracks and internal sulfide island. The morphological together with cross-sectional studies on corroded samples showed that the thickness of corrosion product in TLP joint was more than that of area far from TLP centerline. Surfaces on the side of TLP line acted as the short-circuit path to deliver chromium and nickel to the corrosion exposure. Moreover, cracks which were formed during hot corrosion in TLP area, acted as short-circuit inward diffusion of sulfur, resulting in internal sulfidation. It is also suggested that the formation of boride and silicide intermetallics in the TLP area also weakens the corrosion resistance.