A comparative study of oxide scales grown on stainless steel and nickel-based superalloys in ultra-high temperature supercritical water at 800 °C

Abstract

This study investigates the oxidation behavior of several stainless steels and nickel-based superalloys exposed to supercritical water at 800 °C for 12 h. Characterization of the resulting oxide layers were conducted using weight change measurements, X-ray diffraction, scanning/transmission electron microscopy, and energy dispersive spectroscopy. Although the exposure time is only 12 h, the thickness of the oxide layers formed was as high as 1 μm, comprising different spinel structures. The influence of alloying elements such as Al, Nb, Mo, Mn and Ti on the corrosion behavior is investigated and possible corrosion mechanisms for each candidate alloy are discussed.

Introduction

The Canadian supercritical water-cooled reactor (SCWR) concept being developed under Canada’s Generation IV National Program [1], [2], [3], [4] requires the selection of candidate fuel cladding alloys that can withstand the peak cladding temperature, which could reach 800 °C. The operating conditions in an SCWR core include high temperature and pressure, and radiation flux. Candidate materials need to have acceptable dimensional stability, be resistant to oxidation and irradiation creep under constant stress and high temperature oxidation, possess acceptable ductility, toughness, creep rupture strength, and resistance to stress corrosion cracking (SCC) [3], [5], [6]. Many commercial alloys, for example, austenitic steels (such as SS 304, SS 316) and nickel-based alloys, suffer from high corrosion rates or SCC in supercritical water (SCW) [3], [6] and are therefore unsuitable for application in an SCWR core.

There have been numerous investigations of the oxidation of alloys in SCW; note that temperatures well above the critical temperature there is no distinction between SCW and water at a temperature above the critical temperature but at a pressure below the critical pressure (typically called superheated steam in the power industry) [6], [7], [8], [9], [10], [11], [12]. Materials research for reactors operating at temperatures above the thermodynamic critical point of water started in the early 1950s [7]. Some of the highest temperatures reported for SCW corrosion tests were those reported by Boyd in 1956 [7], who studied the corrosion behavior of Ni–Cr–Fe alloys such as 410, 302, 347, 309, 310 stainless steels, and nickel-base alloys such as 625, 617, 718 as a function of temperatures. It is known that alloying elements and impurities can change the oxidation and corrosion susceptibility of the base metal at high temperature. In Fe–Ni–Cr alloys, outward diffusion of iron and nickel and reaction with oxygen leads to the formation of oxides such as Fe₃O₄, Fe₂O₃, FeO, and NiO, depending on the oxygen content, temperature and exposure time to SCW. Chromium plays an important role in the formation of protective oxide layers on the alloy surface in the form of chromium oxide (Cr₂O₃) or Cr-based oxides with the spinel structure [12], [13], [14], [15], [16]. More importantly, alloying elements such as molybdenum, titanium, manganese, aluminum and niobium influence oxide layer formation by formation of different phases in the oxide scale [8], [9], [10], [11], [12], [17], [18]. Additionally, as there are differences in the potentials for different oxide compositions, localized corrosion can be accelerated at high temperature leading to pitting. Finally, diffusion rate differences at high temperature may result in the depletion or enrichment of alloying elements leading to localized corrosion. It is worth nothing that at the high operating pressure of the SCWR it is possible for alloying elements and/or oxides to dissolve into the SCW, resulting in loss of the oxide layer [4], [13], [14], [19]. Several mechanisms for oxidation of alloys in SCW, such as solid-state growth and metal dissolution/oxide precipitation, have been proposed [20], [21], [22], [23], [24].

As there have been few studies of the oxidation of candidate alloys in SCW at 800 °C, in this work, several candidate alloys were tested for their corrosion susceptibility in SCW at 800 °C and 25 MPa for 12 h. The oxide scales and pitting formed during exposure to SCW were characterized using weight gain/loss measurements, X-ray diffraction (XRD), and scanning/transmission electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). Oxidation and pitting mechanisms are discussed.