Corrosion of High Entropy Alloys in Molten Salts

ABSTRACT

High Entropy Alloys (HEAs) are a new class of multicomponent alloys that contain five or more elements in equal or near-equal amounts in atomic percent, resulting in extraordinary mechanical properties. Turbine blades are one of many potential applications for HEAs. Gas turbine blades exposed to temperatures in the 650-900°C range in marine atmospheres, can be subject to accelerated corrosion beneath a thin molten eutectic salt film consisting of sodium chloride and sodium sulfate. This type of corrosion (Type II hot corrosion) can eventually lead to engineering failure. Understanding the corrosion behavior of HEAs in the presence of a thin fused film of these salts would therefore be relevant for this application.

In this study, coupons of a multicomponent AlCoCrFeNi alloy and selected nickel-based alloys were exposed to a molten NaCl-Na₂SO₄ eutectic salt mixture at 700°C in the presence of a platinum-catalyzed SO2/air mixture. In situ electrochemical techniques were utilized to characterize the corrosion behavior of these alloys. The morphology of the attack was studied using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The results are discussed and comparisons between the different alloy systems are made.

INTRODUCTION

High Entropy Alloys (HEAs) are a new class of alloys with the potential for use in advanced engineering applications. They exhibit an attractive combination of wear resistance, hardness and high temperature strength coupled with relatively low densities.^1,2 Unlike traditional alloys, HEAs contain five or more elements with equal or near-equal atomic percent.^1,2 The result of this unconventional alloying gives high entropy alloys extraordinary properties. Turbine blades are one of many potential applications that are being explored for HEAs.³ In gas turbine environments, salts can deposit on the turbine blades and form a thin molten film. The interaction between the metal, a thin layer of molten salt and the environment is called hot corrosion.⁴ Literature is sparse in regards to the hot corrosion behavior of these new materials. Therefore, it is important to evaluate the hot corrosion resistance of these alloys to determine their suitability for service under these conditions.