High temperature corrosion of structural materials under gas‐cooled reactor helium

Abstract

The Generation IV International Forum has selected six promising nuclear power systems for further collaborative investigations and development. Among these six concepts, two candidates are Gas Cooled Reactors (GCR), namely the Very High Temperature Reactor (VHTR) and the Gas-cooled Fast Reactor (GFR). The CEA has launched a R&D program on the metallic materials for application in an innovative GCR. Structural GCR alloys have been extensively studied in the past three decades. Some critical aspects for the steels and nickel base alloys resistance under the service conditions are microstructural stability, creep strength and compatibility with the cooling gas. The coolant, namely helium, proved to contain impurities mainly H 2 , CO, CH 4 , N 2 and steam in the microbar range that interact with metals at high temperature. Surface scale formation, bulk carburisation and/or decarburisation can occur, depending on the atmosphere characteristics, primarily the effective oxygen partial pressure and carbon activity, on the temperature and on the alloys chemical composition. These structural transformations can notably influence the mechanical properties: carburisation may induce a loss in toughness and ductility whereas decarburisation impedes the creep strength.There is a valuable theoretical as well as practical knowledge on the corrosion of high temperature alloys in the primary circuit of a GCR but this past experience is not sufficient to qualify every component in a future reactor. On the one hand, the material environment could be significantly different from the former GCR's, especially regarding the higher temperature. On the other hand, the materials of interest are partly different. Ni-Cr-W alloys, for instance, may offer significant improvement in the maximum operating temperature as far as the mechanical properties are concerned. However, their corrosion resistance toward the GCR atmosphere is still unknown.We describe here our first corrosion tests of Haynes 230, a high strength Ni base alloy containing tungsten, under low oxidising helium at 750 C or 950 C. The experiments were carried out in a purposely-designed device that allows controlling low impurity partial pressure. The test duration was up to 1000 h. The corrosion behaviour was assessed through gravimetry and microscopy. Some specimens were investigated using EDS and DRX. The results under low oxidizing potential are compared to the oxidation kinetics in air. Tentative interpretations are proposed based on published models.

Chromia-Assisted Decarburization of W-Rich Ni-Based Alloys in Impure Helium at 1273 K (1000 °C)

“…Similar decarburization also was observed in other alloys such as Inconel 617 and Haynes 230. [32][33][34]36,[46][47][48] The measurements also indicate that A2 alloy was decarburized rapidly within the first 10 to 15 hours of exposure with the sample at 50 hours exposure showing complete carbon loss. However, the rate of decarburization was slower in the carbide-containing A8 alloy.…”

“…The fact that Cr 2 O 3 is not seen on the surface is indicative of the kinetics of the chromia-assisted decarburization reaction Cr 2 O 3 (s) + 3C alloy (ss) fi 2Cr (ss) + 3CO(g). [34,[36][37][38] Since it has been shown earlier that, in the presence of a pre-grown Cr 2 O 3 , all of the carbon in the alloy (0.08 wt pct) is removed within 50 hours, the formation of Cr 2 O 3 must be the rate-limiting step since the alloy carbon content in the as-fabricated alloys was still 0.045 wt pct after decarburizing for 25 hours. In other words, the fast diffusing carbon is readily available at the alloy surface whereas the growth of chromium oxide occurs at a slower pace.…”

“…[2,4,[16][17][18][19][20][21][22][23][24][25][26] The prospective alloys generally considered are based on Ni-Cr, Ni-Cr-Mo, Fe-Ni-Cr, or Ni-Cr-W-(Mo) systems and include Inconel 617, [17,[27][28][29][30][31][32][33][34][35][36] Incoloy 800, [17,[28][29][30] Alloy 800H, [17,29,31,33] Hastelloy X/XR, [17,18,28,30,31,33,34] Nimonic PE16, [28] Nimonic 80A, [17] Nimonic 86, [29,31] Alloy 713LC, [17,33] IN-100, [33] and Haynes 230. [34,[36][37][38] Of particular mention are high-W content Ni- …”

Corrosion of high temperature metallic materials in VHTR

“…This paper reviews published data [4][5][6][7][8][9][10][11] that deal with the corrosion resistance of candidate high temperature metallic alloys, Alloys 617 and 230, in the specific environment of the reactor primary circuit. It is shown that the reactive impurities H 2 O, CO, H 2 , and CH 4 in the cooling gas interact with Ni alloys at high temperature causing corrosion effects that can significantly influence the in-depth microstructure, and consequently the mechanical properties.…”

High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO2 as Impurities

“…The helium inevitably contains parts per million (ppm) level of CO, CO 2 , H 2 , H 2 O, and CH 4 as impurities, which arise mainly from reactions between the hot graphite core and in-leakage of O 2 , N 2 , and water vapor from seals, welds, and degassing of reactor materials such as fuel, thermal insulation, and in-core structural materials. [4,5] Typical concentrations of helium impurities in various gascooled reactors are shown in Table I.…”