Hot corrosion of TBC‐coated components upon combustion of low‐sulfur fuels

Abstract

Gas turbine reliability is a crucial requirement for passenger safety in aviation and a secure energy supply. Hence, corrosive degradation of combustor parts, vanes, and blades in gas turbines must be prevented. One of the most severe forms of corrosion is alkali‐sulfate‐induced hot corrosion, which is associated with internal sulfidation of components and is usually anticipated to fade in importance in the absence of sulfur. However, the literature suggests that hot corrosion might still occur in low‐sulfur combustion gases. In this study, established thermodynamic modeling methods are used to analyze the low‐sulfur hot corrosion regime. Liquid sodium chromate is found to be stable in these conditions. A comparison of calculation results and engine findings suggests that high alkali levels can negatively impact thermal barrier coating life even if sulfur is absent in the fuel. Laboratory tests are carried out to validate the chromate formation on MCrAlY‐coated specimens. It is shown that molten sodium chromate can alter the oxidation behavior of MCrAlY, promoting the formation of voluminous spinel. This represents a new and different form of hot corrosion compared to type I hot corrosion.

A Comprehensive Understanding of Thermal Barrier Coatings (TBCs): Applications, Materials, Coating Design and Failure Mechanisms

“…Oxygen will penetrate through the pores of the TC and may diffuse deeply into the BC, potentially leading to the destruction of the protective oxide layer [229]. Hot corrosion occurs in stages, beginning with initiation, when salts reach the surface of the component and break the protective oxide layer, creating conditions for the propagation of the attack, thus distinguishing between type I hot corrosion at high temperature, above the melting point of salts (800-950 • C), and type II hot corrosion at a lower temperature, below the melting point (600-800 • C) [207,230]. These can be identified by the presence of the main corrosive salts Na 2 SO 4 (MP 884 • C), NaCl (MP 801 • C) and V 2 O 5 (MP 690 • C), which can form mixtures among themselves and with other elements [231][232][233][234].…”

Hot Corrosion Behavior of Single-Layered Gd2Zr2O7, Sm2Zr2O7, and Nd2Zr2O7 Thermal Barrier Coatings Exposed to Na2SO4 + MgSO4 Environment

“…The "type II" regime primarily occurs at lower temperatures, typically between 600 and 800 • C. Within this temperature range, the corrosion process exhibits specific characteristics and mechanisms different from those observed in the higher temperature regime. On the other hand, the "type I" regime dominates at higher temperatures, typically around 800 to 950 • C. The corrosion behaviour in this regime is notably distinct from the type II regime and showcases its unique features and mechanisms [24,25]. The initial stage of the attack involves dissolving the metal's protective oxide using the available molten sulfate deposit.…”