High temperature reactivity of two chromium-containing alloys in impure helium

Kinetics and mechanism of reaction between water vapor, carbon monoxide and a chromia-forming nickel base alloy

“…Even if these impurities are highly diluted in helium -only between a few tens to hundreds of µbar -they can react at high temperature, typically between 850°C and 1000°C, with structural metallic materials. Gas-metal interactions involved in HTR heat exchangers have been discussed in numerous papers published in the past three decades [1][2][3][4][5][6] and more recently in France [7][8][9][10][11][12][13][14] and in the USA [15][16][17]. These works have demonstrated that depending on the composition of the chromia-former alloy, on the working temperature (850-1000°C) and on the partial pressures of the various gas species in helium, two main behaviours could be distinguished: either a passive corrosion behaviour based on the formation of a stable, slow-growing, Cr-rich oxide layer on the surface or an active corrosion with the destruction of the surface oxide layer and the subsequent decarburization or carburization of the alloy [9].…”

Investigation on Material Degradation of Alloy 617 in High Temperature Impure Helium Coolant

“…KEYWORDS : Impure Helium, VHTR, Corrosion, Thermodynamics, Surface Reaction H2 20Pa, H2O 0.5Pa, CH4 2Pa, and CO 5Pa was used, based on previous reports [1][2][3][4][5].…”

NGNP Reactor Coolant Chemistry Control Study

“…The critical temperature can change with the concentration of various impurities in the helium coolant-specifically, the amount of CO. Quadakkers obtained a curve that shows the relationship between the critical temperature and the concentration or partial pressure of CO. 26,27 The simple relationship between the CO content and the critical temperature is that increasing the CO content will increase the critical temperature to a point. This is an important consideration when deciding on a chemistry composition for a system that operates around the critical temperature of 900-970°C.…”

“…This is an important consideration when deciding on a chemistry composition for a system that operates around the critical temperature of 900-970°C. 27 When the carbon concentration of the coolant enters a region of carburization or decarburization, the protective oxide layer can be destroyed. At the critical temperature, there is a CO partial pressure where too little or too much will result in rapid carburization or decarburization.…”

“…Another system change that may influence helium chemistry is the higher reactor outlet temperatures, which could result in lower concentrations of CO 2 but potentially higher CO concentrations in some nuclear power plants. 26,27,28 The key to maintaining strong materials in the harsh high-temperature environment is to maintain the protective oxide layer, which can be formed and destroyed over and over again. This type of cycling can be detrimental to the life of the material because there is a finite amount of chromium in the base metal.…”