Electrochemical Investigation of Stellite 6 Alloy in Morpholine Corrosive Environment

Abstract

The corrosion resistance of Stellite 6 alloy in morpholine solution with pH 9.5 is investigated using the electrochemical test method, simulating the amine environment of the boiler feed water service condition in coal power plants. Polarization test is performed on Stellite 6 alloy under the low potential varying from-0.4 VSCE to 1.2 VSCE and is also conducted under a constant high potential (4 VSCE) in order to fail the sample surface. 17-4PH stainless steel, which is also a common material for the application of the boiler feed water in coal power plants, is tested simultaneously under the same conditions for comparison. It is shown that the polarization curve 17-4PH steel from the low potential test has an apparent passivation region indicating a protective oxide film formed on the sample surface, but Stellite 6 only exhibits a tendency to passivate. Both samples after the failure tests under the high potential (4 VSC) are analyzed using SEM/EDX. The surface morphologies indicate that the former is severely corroded in the solution while the latter is less corroded. The corrosion mechanisms of Stellite 6 alloy and 174PH stainless steel in morpholine solution are discussed with assistance of the Pourbaix diagrams.