Impure Helium Research Articles

Corrosion behaviors of Inconel 617 and Incoloy 800H in impure helium with different CO contents at high temperatures

The helium coolant of the very high-temperature gas-cooled reactor (VHTR) is expected to contain trace impurities, which will cause corrosion to superalloys at high temperatures. The high-temperature corrosion behaviors of Inconel 617 and Incoloy 800H were investigated in the impure helium with various CO levels. For Inconel 617, the microclimate reaction occurred at high temperatures, releasing large amounts of CO and destroying the oxide layer. The corrosion model was established according to the chemical thermodynamics theory, showing that the reaction temperature is related to the CO content. However, this corrosion behavior was not observed for Incoloy 800H. The corrosion difference between these alloys was analyzed in this study, indicating that the silicon oxide interlayer of Incoloy 800H may inhibit the microclimate reaction. Although the interlayer could attenuate the chemical destruction of the oxide layer, the adhesion may be reduced leading to thermal damage, which was proved by the experiment in this study. This research clarified the different corrosion mechanisms of these two alloys and provided guidance for the control scheme of CO content in the primary circuit of HTGR.

Carburization and tensile behavior of Alloy 617 in impure helium containing a part-per-million level of CH4 at 950 °C

The carburization and tensile behavior of Alloy 617 corroded at 950 °C for 100 h in the impure helium containing methane were investigated. In helium containing 20 ppm CH4, Alloy 617 exhibited slight carburization and held its plasticity. However, in helium containing 200 ppm CH4, Alloy 617 was carburized to a depth of over 20 % and lost plasticity. In addition, the stress-strain curves in the plastic phase at room temperature exhibited a serrated flow phenomenon in both two environments, primarily caused by dynamic interactions between carbide-induced crack pinning and stress-concentration-induced crack unpinning.

Effect of impurity ratios on the high-temperature corrosion of Inconel 617 and Incoloy 800H in impure helium

Helium is utilized to be the coolant of the high-temperature gas-cooled reactor (HTGR), which contains trace impurities in the primary circuit. The impurities may cause various corrosion behaviors of superalloys at elevated temperatures. In this study, the oxidation and carburization of Inconel 617 and Incoloy 800H at 950 °C in impure helium were investigated. According to thermodynamics and kinetics, the different ratios PH2/PH2O and PCH4/PH2O were set in this research, which leads to different oxidation and carburization behaviors of alloys at 950 °C. When PH2O is at a constant concentration, high PH2 would reduce the oxidation capacity of the atmosphere, and high PCH4 will cause severe carburization. Due to the presence of the silicon oxide interlayer, the oxide layer of Incoloy 800H has poor adhesion and it is easier to be destroyed at high temperatures. It may be the reason that Incoloy 800H has weaker resistance to carburization.

High-temperature reaction kinetics of Inconel 617 in impure helium

High-temperature reaction kinetics of Inconel 617 in impure helium

Study on corrosion of T-22 alloy under high temperature condition of VHTR

ABSTRACT T-22 alloy is applied to the steam generator of the high-Temperature Reactor–Pebblebed Modules (HTR-PM). In this study, a corrosion experiment of T-22 alloy was carried out for 50 h under the environment of air, vacuum, and impure helium at ultra-high temperature (950°C). After the experiment, the corrosion behaviors of T-22 alloy were characterized and analyzed by electronic balance, scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), carbon sulfur analyzer, and gas chromatography. The results show that the T-22 alloy was seriously corroded and destroyed under the air ingress accident; in the vacuum environment, the alloy appeared mass loss and slight decarburization; in the impure helium environment, the microclimate reaction would occur on the surface of T-22 alloy, and the oxide on the surface of the alloy would be destroyed and the alloy decarburized obviously; CO2 in the ppm range will accelerate the oxidation and decarburization of the alloy.

High temperature corrosion of two superalloys in the impure helium environment

ABSTRACT The helium coolant of a very high temperature gas-cooled reactor contains traces of impurities, which have an adverse effect on the structural metallic materials at elevated temperatures. The chromium-containing superalloys Inconel 617 and Incoloy 800 H have been the representative materials with outstanding performance, and, in this study, their corrosion behaviors were investigated under the impure helium, which was the typical HTGR operating environment. A continuous chromia-based surface scale can be developed in the impure helium, but after being above a critical temperature this oxide layer may be destroyed. This destructive phenomenon may be general for the high-temperature alloys such as Inconel 617 and Haynes 230, however, Incoloy 800 H can spontaneously resist this damage by the silica layer. This study was focused on the critical temperature calculations and measurements for Alloy 617 and 800 H, and the different corrosion behavior has been discussed.

High-Temperature Corrosion Behavior of Incoloy 800H Alloy in the Impure Helium Environment

The helium coolant in the primary circuit of the high-temperature gas-cooled reactor (HTGR) contains traces of impurities, which can induce the corrosion of superalloys when exposed to elevated temperatures. The superalloy damage caused by the corrosion could threaten the safe operation of the reactor. In this work, the corrosion behavior of a representative superalloy (chromium-rich iron base alloy Incoloy 800H) was investigated under the impure helium at different typical temperatures of HTGR. An experimental setup developed for studying the high-temperature corrosion of superalloys was used to investigate the chemical reactions and corrosion behaviors of Incoloy 800H. It was found that CO2 is an important oxygen source in the reaction with chromium, and CO is released as the product. In addition, the observation and computation of the critical temperature (TC) of the reaction between CO2 and carbon in the alloy show that TC is much lower than that (TA) of the microclimate reaction, which indicates that CO2 can protect the scale from destruction. Furthermore, the slight decarbonization of the alloy was found above TC. Also, a model developed by the thermodynamic analysis was proposed to explain the mechanism of slight decarbonization and predict the critical temperature when the CO2-C reaction occurs. This work presents a guideline for protecting the oxide scale of superalloys used in HTGR.

The high temperature corrosion of Incoloy 800H alloy at three different atmospheres

ABSTRACT Incoloy 800 H alloy is one of the candidate materials for high-temperature gas-cooled reactor steam generator heat transfer tubes. Previous studies have shown that corrosion phenomena such as oxidation and carburization occurred in high-temperature impure helium gas. On this basis, the corrosion behavior of Incoloy 800 H alloy in air, impure helium, and vacuum was investigated. After the experiment, electronic balance, scanning electron microscope, X-ray energy spectrum analysis, X-ray diffraction analysis, and gas chromatograph were used to characterize the corrosion behavior of the alloy. The results showed that CO is the main oxidation and carburization gas of Incoloy 800 H alloy in impure helium. Contents in ppm CO2 can supplement the reduction of CO and prevent the destruction of oxidation layer and carburization layer; The mass change of the alloy after corrosion in air at 950°C met the oxidation parabola law, the corrosion layer on the alloy surface fell off, and Si migration and accumulation in different degrees occurred in air and impure helium experiments; Carbon transfer and decarburization occurred in the alloy in a vacuum.

Design and analysis of the helium purification system for the NSRRC cryogenic system

Helium is an expensive consumable in cryogenic facilities and is used widely in space, medical and energy research. At NSRRC, liquid helium is used as a coolant for superconducting magnets and SRF cavities . Minor contaminants such as nitrogen, oxygen, moisture and oil will be picked up when liquid helium circulates in large scale cryogenic systems and such contaminants can crystalize and cause damage to the cold box turbo expanders resulting in system damage and failure . Therefore, a helium purification system is designed as an integral part of the cryogenic system to conserve helium by providing 99.9995% pure helium to the liquefier after eliminating contaminants. The NSRRC helium purification process is based on two principles, the first one being a cryo-sorption device using activated charcoal and a molecular sieve and the other being a cryo-condensation unit using a tubular heat exchanger. The purifier has been designed to purify impure helium with overestimated contaminants of as much as 2.5% nitrogen and 2.5% oxygen with a mass flow rate of 475 nm3/hr and delivering a pressure of 17 bar(a) of impure helium to the purifier, the actual impurity will be much lower than the actual design contaminants. In this paper, calculation and design of the helium purification system and components composed of one double pipe counter flow heat exchanger, one vessel and tube heat exchanger, one pre-cooler and one charcoal vessel will be discussed together with charcoal mass requirement calculations and design of other components. In NSRRC, helium is liquefied and is used as a coolant for the SRF system and for cryogenic undulators. During a cryogenic cycle in the cryogenic system, helium may pick up contaminants such as moisture, oxygen, oil, and nitrogen, which have a higher freezing point than liquid helium and will crystalize. These frozen impurities will then affect plant capacity and operation such as alternating flow characteristics, damaging moving parts like turbines in the cold box causing the overall cooling efficiency to drop. Eliminating the contaminants is therefore very important for the cryogenic system.

Effect of Impurity Ratios on the High Temperature Corrosion of Inconel 617 and Incoloy 800h in Impure Helium

Effect of Impurity Ratios on the High Temperature Corrosion of Inconel 617 and Incoloy 800h in Impure Helium

Effects of High Temperature Corrosion in Vacuum and Impure Helium Environment on Mechanical Properties of Vhtr Superalloys

Effects of High Temperature Corrosion in Vacuum and Impure Helium Environment on Mechanical Properties of Vhtr Superalloys

Tribochemical changes of alloy 800HT under sliding contact at elevated temperature in impure helium environment

This study investigates the chemical changes of the surface of Alloy 800HT during sliding contact under helium atmosphere in the presence of impurities at 750 °C, which is of interest for the nuclear industry. The alloy samples are exposed to helium (He) atmosphere at high temperature for long duration of 500 h to identify the chemical and topographical changes of the alloy surface. The oxidation rate constant is reported based on the weight change data. For the tribo-experimental samples, the surfaces inside and outside the wear track are studied thoroughly using techniques such as secondary electron microscopy, energy dispersive spectroscopy, x-ray diffraction, Raman spectroscopy and secondary ion mass spectroscopy. The formation and evolution of surface oxides are discussed in He atmosphere, compared with air. The presence of Fe-rich thin layer of oxide on top of a thicker Cr-rich oxide and a mixed spinel-based inner layer on the bulk is found. The persistent presence of thicker Fe3O4in the wear track in high temperature air proved to be the reason of lower friction property. Pre-oxidation of the samples in air or He does not improve but worsen the tribological behavior in He atmosphere.

Oxidation of SiCf-SiC CMC cladding tubes for GFR application in impure helium atmosphere and materials interactions with tantalum liner at high temperatures up to 1600°C

The oxidation kinetics of SiCf-SiC cladding tube segments and sandwich tubes with an inner tantalum layer in impure helium atmosphere prototypical for GFRs was investigated at ambient pressure in the temperature range between 900 °C and 1600 °C using a thermogravimetric device. The transition from passive oxidation (formation of a protective silica scale) to active oxidation (volatilization of silica due to the formation of SiO and other volatile species) occurred between 1200 °C and 1300 °C. Hence, during target operational temperatures of 900–1000 °C, the CMC cladding material should withstand long-term operation with respect to oxidation and corrosion. At higher temperatures beyond 1300 °C, i.e. under accident conditions, volatilization of composite constituents would lead to serious degradation of the SiC-based cladding tubes. Severe interactions between the tantalum layer and silicon carbide with the formation of Ta silicides and carbides were found to become significant at temperatures higher than 1300 °C.

Investigation of Oxidation Behavior of Alloy 617 under Air/Helium Environments at 950℃, 니켈기 합금 Alloy 617의 950℃ 대기/헬륨 분위기에서 산화거동 고찰

Alloy 617 is a candidate Ni-based superalloy for intermediate heat exchanger (IHX) of a high-temperature gas reactor (VHTR), because of its good creep strength and corrosion resistance at high temperature. Small amount of impurities such as H2O, H2, CO and CH4 are introduced inevitably in helium, as a coolant during operation of a VHTR. Reactions of material and impurities are accelerated with increase of temperature to 950℃ of operating temperature of a VHTR, leading to material corrosion aggravation. In this circumstance, high-temperature corrosion tests were performed at 950℃ in air and impure helium environments, up to 250 hours in this study. Oxidation rate of 950℃ in an air environment was higher than that of impure helium, explained by difference in outer oxide morphology and microstructure as a function of oxygen partial pressure. An equiaxed Cr-rich surface oxide layer was formed in an air environment, and a columnar Cr-rich oxide was formed in an impure helium environment.

Experimental investigation on the cooling helium circulation of the internal purifier pilot plant

The internal purifier is utilized to remove impurities from impure helium, keep the helium free of any gaseous contaminants. However, there is few report on the performance improvement of the internal purifier. In this paper, we present a novel circuit of the experimental platform with GM cryocooler providing a separated cooling helium circulation as the refrigeration power supply for investigating the performance the internal purifier. Namely, that circuit is easily constructed by extracting a small stream of helium from the high-pressure side of the GM cryocooler serving as the cooling helium circulation. The experiment is then carried out to test the mass flow and the temperature of the cooling helium circulation. The impact of the modification on the cold head is also estimated. It is finally demonstrated that the circuit is sufficient to guarantee the cooling supply of the whole experimental system, which means that the modification is suitable and efficient.

High temperature alloys stability testing in impure helium

Abstract Within the national and international research program of materials for advanced nuclear reactors Czech organizations contributed with several tests of metallic alloys. The specimens of the alloys were first exposed in the long term (up to 1500 hours) in simulated advanced gas cooled reactor coolant environment at 750-900 °C. After exposure the degradation of tested materials was explored, especially changes in material microstructure, corrosion damage and corrosion layer composition and in some cases also changes in mechanical properties were observed. In this paper selected results of exposure tests in high temperature helium of alloy 800 H, austenitic steel 316L and high-temperature nickel alloys are presented.

Creep Crack Growth Behavior of Alloys 617 and 800H in Air and Impure Helium Environments at High Temperatures

The environmental degradation of intermediate heat exchanger (IHX) materials in impure helium has been identified as an area with major ramifications on the design of very high-temperature reactors (VHTR). It has been reported that in some helium environments, non-ductile failure is a significant failure mode for Alloy 617 with long-term elevated-temperature service. Non-ductile failure of intermediate exchangers can result in catastrophic consequences; unfortunately, the knowledge of creep crack initiation and creep crack growth (CCG) in candidate alloys is limited. Current codes and code cases for the candidate alloys do not provide specific guidelines for effects of impure helium on the high-temperature behavior. The work reported here explores creep crack growth characterization of Alloy 617 and Alloy 800H at elevated temperatures in air and in impure helium environments, providing information on the reliability of these alloys in VHTR for long-term service. Alloy 617 was found to exhibit superior CCG resistance compared to Alloy 800H. For Alloy 617 tested at 973 K (700 °C), a notable increase in the resistance to crack growth was measured in air compared to that measured in the helium environment; CCG results for Alloy 800H suggest that air and helium environments produce similar behavior. Testing of grain boundary-engineered (GBE) Alloy 617 samples revealed that, although the technique produces superior mechanical properties in many respects, the GBE samples exhibited inferior resistance to creep crack growth compared to the other Alloy 617 samples tested under similar conditions. Grain size is noted as a confounding factor in creep crack growth resistance.

Effect of thermal exposure in helium on mechanical properties and microstructure of 316L and P91

In this paper, the effects of high temperature exposure in air as well as in impure He on mechanical properties of 316L and P91 steels were investigated. The experimental programme was part of material design of new experimental facility – high temperature helium loop. Some of the specimens were exposed in air at 750 °C for up to 1000 h. Another set of specimens were exposed in impure helium containing 1 ppmv CO2, 2 ppmv O2, 35 ppmv CH4, 250 ppmv CO and 400 ppmv H2 at 750 °C for up to 1000 h. Metalographical analysis, tensile tests, fracture toughness and hardness tests of exposed and non-exposed specimens were carried out. After the exposure both in air and He, the ultimate tensile strength of P91 decreased significantly more than that of 316L. After the exposure in He, the fracture toughness of 316L was reduced to 60% while fracture toughness of P91 showed no significant changes. The hardness of P91 decreased with exposure time in air. The measurement of the hardness of 316L was very scattered the most probably due to the heterogeneities in microstructure, the trend was not possible to evaluate.

Testing of degradation of alloy 800 H in impure helium at 760 °C

The base metal, weld metal and heat affected zone specimen of alloy 800 H were exposed to impure helium at 760°C for up to 1500h. Helium impurities included 100vppm of H2, 500vppm of CO and 100vppm CH4. The weight gain of alloy 800 H specimens were found to be higher than those of ferritic and austenitic stainless steel tested in similar environment. On the surface, corrosion product layers contained Cr and also Ti or Mn and other oxides. In some cases spalling of these layers was observed. Under corrosive layers the C, O, Ti, Cr and Al rich formations were also found. The exposure had no significant effect to hardness and micro hardness of tested alloy.

Development and performance evaluation of external helium purifier

Helium purification system is vital for removing gaseous and oil aerosol/oil vapour impurities in helium which can freeze and damage ultra high-speed turbo-expanders and choke fine passages of cryogenic heat exchangers or fine filters of helium refrigerators/liquefiers. An external helium purifier is designed and fabricated for purification of 10 Nm3/hr flow of impure helium with maximum N2 impurity of 1%. It consists of an evacuated super-insulated cold box, a helical coil heat exchanger with tube in tube type configuration, an LN2 bath and a charcoal adsorber maintained at LN2 temperatures. The main features of the purifier are its compactness, low LN2 consumption, ease and simplicity of charcoal regeneration. Performance evaluation of the purifier is carried out by observing the impurities (N2, moisture and hydrocarbons) in the purified helium gas at its exit. Total LN2 consumption and effectiveness of heat exchanger used for recovery of cold of outgoing helium are also evaluated.