Accelerated Corrosion of Candidate Materials for High Temperature Power Plants

Abstract

Introduction The decreasing supply of fossil fuel sources, coupled with the increasing concentration of green house gases has made it necessary to maximize the efficiency of power generation (1). Increasing the outlet temperature of these power plants will result in an increase in efficiency (2). To increase their efficiency, coal fired power plants currently use both supercritical water (SCW) and ultrasupercritical water (USCW) with the most advanced designs allowing for a decrease of 58000 kg/hr in the coal fed to produce the same power as a conventional coal power plant (1, 3). By employing high temperature fluid, including supercritical water, the plant efficiency can be increased to 45%, compared to traditional reactors which currently operate at ~33% (4, 5). Experimental Experiments were conducted in the recently established supercritical water loop (SCWL) facility at UNR. This unique facility allows accelerated corrosion testing and mechanical behavior studies to be conducted simultaneously. The primary loop environment that was used for these experiments contained deionized water held at a pressure of 27 MPa and temperatures ranging from room temperature up to 600°C. A secondary high pressure pump was used to supply an electrolyte containing 1 g/L of boric acid at a flow rate of 1.5mL/min with a chlorinated silver wire used as a reference electrode. Materials studied include; austenitic steels (Nitronic 50 stainless steels 304 and 316), nickel based alloys (Inconel 625 and 718) and a high carbon steel (A285 class C). Although A285 will not come into contact with the working fluid it will be studied due to the low amount of alloying elements when compared to the austenitic and nickel based alloys. A Gamry PCI4 potentiostat was used for polarization of these samples. The oxide layer that was formed was characterized to determine the corrosion characteristics in this environment. Surface chemistry of the oxide layer was studied using X-ray photoelectron, Raman, and infrared spectroscopies. The corrosion products in the electrolyte were analyzed using inductively coupled plasma atomic emission. Results The corrosion behavior of the high carbon steel A285 class C samples were studied using potentiodynamic polarization at temperatures ranging from 20°C to 600°C. The results indicate that the corrosion potential becomes more noble with increasing temperature while the corrosion current exhibited no such trend. Tests conducted at 600oC indicate the possibility of the formation of protective oxide layer. Surface chemistry of the films formed on the samples was found to vary based on the test conditions. Acknowledgements: This study was supported by the Department of Energy, NEUP under Contract DE-NE0000454 and Nuclear Regulatory Commission (NRC) under award NRC-HQ-11-G-38-0039. D.R. is supported under NRC Fellowship award NRC-38-10-949.