Corrosion Performance of Valve Seat Hardfacing Materials in Amine Environment of Boiler Feed Water Service

Abstract

As the valve seat hardfacing materials in boiler feed water service of coal power plants, the corrosion behavior of Stellite 6 alloy and 17-4PH stainless steel in amine solutions is studied using electrochemical test methods. Morpholine, cyclohexylamine, and sodium hydroxide (NaOH) solution with pH 9.5 are selected to be the corrosive media. The first solution is a typical amine medium, the second one is selected to examine if different amine media with the same pH can result in any change in the corrosion behavior of these materials. Sodium hydroxide is chosen to offset the effect of pH change. Polarization test is performed on Stellite 6 alloy and 17-4PH stainless steel 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 surfaces. The polarization curves 17-4PH stainless steel from the low potential tests show an apparent passivation region indicating a protective oxide film formed on the sample surface, but Stellite 6 alloy only exhibits a tendency to passivate. The Stellite 6 alloy and 17-4PH stainless steel samples after the failure tests under the high potential (4 VSC) are analyzed using SEM/EDX. It is shown that the former is severely corroded in all the solutions while the latter is only corroded in morpholine solution and it is very corrosion-resistant in sodium hydroxide solution. The corrosion mechanisms of Stellite 6 alloy and 17-4PH stainless steel in amine solutions are discussed with assistance of the Pourbaix diagrams.