Abstract

Aircraft engines, turbines, and industrial machinery operating at high temperatures near marine environments are prone to accelerated degradation of their components due to corrosion under a thin film of fused salt (“hot corrosion”). Typical salts formed under these conditions are sodium sulfate from fuels and sodium chloride from the marine environment leading to low temperature hot corrosion (Type II). In general, the addition of chromium to alloys is known to increase their corrosion resistance, especially in mitigating attacks from molten sulfates. However, systematic studies on the corrosion resistance of binary nickel-chromium alloys to molten salts as a function of their chromium content are sparse. In this study, the corrosion kinetics of Type II hot corrosion at 700°C were determined for UNS N02200, Ni-Cr alloys with 2.5, 5, 7.5 and 10 wt% Cr as well as the UNS N06600 alloy in a Na2SO4 - 30.8 wt% NaCl eutectic salt using DC electrochemical techniques in a stagnant air closed system and a flowing 1vol% SO2 – air blend (g). Metallography was used to validate the reliability of high temperature electrochemistry as an accelerated test for hot corrosion. Although the electrochemical test could not discriminate the influence of chromium in the long term performance of Ni-Cr model alloys, metallography confirmed that increasing Cr-contents in the alloy resulted in the formation of a continuous protective chromia layer at Cr-contents greater than and equal to 7.5 wt%.

Introduction

Turbine blades in aircraft engines operating in marine environments and hardware in coal and biomass-fired power generation as well as in waste incineration plants may be subjected to accelerated degradation caused by thin molten salt films1 These salt films are generally formed due to reactions between sulfur (impurity in the fuel), oxygen (from the atmosphere) and alkali-based salts (from the operating environment). The salt formed as a consequence of the reactions is subsequently deposited on to the surface of metallic alloys, as a thin film. This molten salt film leads to accelerated corrosion attack that is referred to as Type I or Type II Hot Corrosion. Type I Hot Corrosion or High Temperature Hot Corrosion (HTHC) is defined as hot corrosion that occurs at or above the melting point of a relatively pure salt.2,3 Type II Hot Corrosion or Low Temperature Hot Corrosion (LTHC) occurs at a lower temperature in the presence of multiple salts and/or contaminants. During hot corrosion the metal and metal oxides are degraded by the molten film salt leading to the loss of engineering function. The reaction mechanism for this phenomena is electrochemical in nature because of the transfer of electrons 4-6. The purpose of this research was to develop a hot corrosion exposure test protocol along with an electrochemical test protocol that would allow for the investigation of Type II hot corrosion of Ni-Cr binary alloys using a Na2SO4 + NaCl eutectic melt at 700 and 800°C. While it is well known that increasing the percentage of chromium in alloys is beneficial for hot corrosion resistance, the critical amount of chromium to achieve this protection has not been systematically studied, e.g., in binary Ni-Cr alloys.

This content is only available via PDF.
You can access this article if you purchase or spend a download.