Protective coatings for very high temperature reactor applications

The future very high temperature reactors (VHTR) are nuclear systems that shall operate at a maximum temperature of about 950 °C. Primary circuit materials thus require good creep and corrosion resistance on very long time. Use of high‐strength alloys with protective coatings could significantly improve the service life of high temperature reactor components. However, coating systems are mainly designed for shorter term purposes, often under extremely aggressive atmospheres, that cannot be extrapolated to the VHTR environment. We present our first investigations on the environmental resistance of Alloy 800H coated with two different protective systems under VHTR representative conditions: NiAl(Pt)/EBPVD ZrO₂(Y) and NiCrAl(Y)/CVD ZrO₂(Y). Isothermal exposures were carried out up to 1000 h at 950 °C in impure helium. This specific atmosphere was shown to induce formation of a surface oxide scale together with carburisation of the bare Alloy 800H. After high temperature exposure to impure helium, the microstructure of the coated specimens has changed due to both thermal ageing and corrosion. Performances of the two coating systems are compared regarding the VHTR application.     

Corrosion of high temprature alloys in the primary circuit helium of high temperature gas cooled reactors. – Part I: Theoretical background

The interaction of Ni and Fe-Ni base alloys with the reactive impurities H₂O, CO, H₂ and CH₄ in simulated cooling gas of the primary circuit of the High Temperature Gas Cooled Reactor (HTGR) causes corrosion effects that can significantly influence the mechanical properties. Apart from the formation of surface scales (oxides, carbides or mixed oxides/carbides), structural changes of the alloys are observed; depending on gas composition, gas supply rate and test temperature, carburization or decarburization can occur. In this report it is shown that an interpretation of the basic corrosion effects is possible on the basis of a modified stability diagram for chromium provided that - the kinetics of elementary gas metal reactions are incorporated in the expressions for carbon activity and oxygen partial pressure of the atmosphere and - the gradients of the potentials across the surface scales are taken into account. The interpretation allows the derivation of the corrosion behaviour of NiCr-base alloys in different HTGR helium compositions and enables the limits for the formation of protective chromia surface scales to be given. The influence of alloying elements other than chromium can be explained qualitatively. The results can be transferred to other reactive gas mixtures, which are characterized by an oxygen partial pressure near to the dissociation pressure of the scale forming oxides.     

Experimental analysis and computer‐based simulation of nitridation of Ni‐base alloys—the effect of a pre‐oxidation treatment

Nickel‐base superalloys are commonly used for high‐temperature applications in power‐generation industry, e.g., gas‐turbine blades or heat exchangers. They are designed to resist high creep loading and severe corrosion attack during operation. Nitridation is one of these corrosion processes, in particular when the alloys need to be exposed to a N₂ atmosphere. Based on past assumptions, a dense oxide layer should be an efficient barrier against N₂ ingress. But is this really the case? This work is focused on the nitridation behavior of commercial Ni‐base alloys and the influence of a pre‐oxidation treatment. To model the growth of the internal‐nitridation zone, the diffusion processes were solved using the numerical implicit finite‐difference method in combination with the subroutine ChemApp for thermodynamic calculations.     

Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

It is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr₂O₃ with some FeCr₂O₄ at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.     

The Gaseous Carburisation of Austenitic Steels

The paper is concerned with the corrosion behaviour of materials in high temperature gaseous carburisation environments and initially reviews the current ‘state-of-the-art’ in this field. Results of investigations carried out on three 25 Cr-20 Ni steels exposed to carburising H₂-CH₄ mixtures with a controlled carbon activity of 0.8 at 825°C and 1000°C are then presented. Kinetic data for exposure periods of up to 1100 hours are given and it is shown that gross carburisation of these alloys has occurred after only short exposures at 1000°C. The importance of alloy composition and surface condition, particularly during exposure at 825°C, is demonstrated. The results of surface and cross-sectional examinations using a range of optical, electron and X-ray techniques are reported. The use of a nuclear microprobe technique has enabled accurate carbon concentration profiles to be established and diffusion coefficients for the three alloys have been determined. In addition, the use of a magnetic technique for the non-destructive monitoring of the extent of sub-surface carburisation has been investigated.     

High temperature Corrosion behaviour of iron aluminides and iron-aluminium-chromium alloys

The high temperature corrosion of different iron aluminides and iron-aluminium-chromium alloys containing between 6 and 17 wt% aluminium, 2 and 10 wt% chromium and additions of mischmetal has been investigated in air as well as in carburising, chlorinating and sulphidising environments. It was found that all alloys showed excellent corrosion resistance to both oxidation in air and carburisation in CH₄/H₂ up to at least 1100°C and to sulphidation in SO₂/air up to at least 850°C. In these environments the corrosion behaviour is not influenced by the concentrations of aluminium and chromium. In oxygen deficient H₂S-atmospheres, however, the corrosion behaviour depends sensitively on the aluminium and chromium concentration. At least 12 wt% aluminium in chromium-free alloys or 10 wt% aluminium in alloys containing 10 wt% chromium are required to provide sulphidation resistance at 550°C. The chlorination resistance of iron-aluminium-chromium alloys is low due to their formation of volatile aluminium chlorides.     

CO2 High Temperature Corrosion and Its Prevention of Chromia Forming Fe-base Alloys

High temperature corrosion of chromia forming Fe-base alloys by CO_2 produces not only oxidation but also carburisation. The corrosion kinetics in CO_2-rich gas is found to be increased compared with that in air or oxygen. As a result, higher alloy chromium levels are required to achieve protective chromia formation in CO_2. Corrosion reaction mechanisms in CO_2 are examined and the internal carburisation of alloys in low carbon activity CO_2 gases are analysed based on the variation of p_(O_2) at the interface of oxide and metal. Carbon penetration through chromia oxide scale has been revealed by atom probe tomography. The strategies to resist CO_2 corrosion are reviewed by alloying of Si and/or Mn, forming additional diffusion barrier layers, and by adding sulphur to modify oxide grain boundaries to reduce carbon diffusion along the grain boundaries.     

Corrosion of a Vented Stainless-Steel Tube Exposed to CO2 Containing 1 Co in a Nuclear Reactor

Abstract

A thin-walled 18Cr/10Ni/Ti stabilised austenitic steel tube was exposed to CO₂ containing 1 vol.- % CO and 0·1 vol. % CH₄ and 80–90 ppm H₂O at 20 atm (285 psig) and 600 610° for 1000 hours in an Advanced Gas Cooled Reactor. The external surface of the tube over which the gas flowed remained in good condition, but in the bore, where the gas was effectively static, grossoxidation and carburisation occurred. Microscopical examination and thermodynamic considerations lead to the conclusion that static gas conditions favoured the attack observed in the bore but poor surface finish may have initiated it.     

Factors affecting the high-temperature carburisation behaviour of chromium-nickel alloys in gaseous environments

The paper is concerned with the corrosion behaviour of a range of commercial and simpler “model” high chromium-nickel alloys exposed to gaseous carburising environments of low oxygen activity, at temperatures within the range 825°C to 1000°C. H₂-CH₄ mixtures have been used and carbon activities of 0.3 and 0.8 studied by varying the proportion of CH₄ in the mixture. The surface condition of the specimens and its effect upon carburisation behaviour has been investigated as a variable by the use of standardised surface preparation techniques. The kinetics of the corrosion process have been studied by the use of conventional gravimetric techniques and cross-sectional microstructural examinations have aided in establishing the factors of primary importance in the degradation of materials by carburisation. The contributions made by the temperature and carbon activity of the gaseous environment have been highlighted and the important material variables such as metallurgical form, composition and surface condition have been investigated in order to evaluate their significance.     

Carbon Permeability of Nickel and Ni–Cu Alloys

A Ni–20Cr alloy and variants containing 5, 10 and 20Cu (all in wt.%) were carburised in H₂-5% CH₄ at 1,000 °C. All alloys formed internal carburisation zones containing Cr₃C₂ and Cr₇C₃. The Ni–20Cr alloy also developed a surface deposit of graphite, but the copper-bearing alloys did not. Measured parabolic rate constants for intragranular carburisation were used to calculate carbon permeabilities from Wagner’s diffusion analysis. The value obtained for Ni–20Cr was in good agreement with the product of independently measured carbon solubility and diffusion coefficient values for nickel. Permeabilities found for copper-bearing alloys were similar, showing that the presence of copper had little effect on carbon diffusion in nickel. This finding is used in analysing the mechanism by which nickel undergoes metal dusting.     

High temperature corrosion of cast heat resisting steels in CO + CO2 gas mixtures

Two commercial variants of the cast heat resistant grade HP40Nb (Fe-25Cr-35Ni, Nb modified) were exposed to CO/CO₂ gases at 982 and 1080 °C in order to simulate exposure to the carbon and oxygen potentials realised in steam reformers under normal and overheated conditions. Both alloys developed external chromium-rich oxide scales, intradendritic silica precipitates and interdendritic oxide protrusions where primary, interdendritic carbides were oxidised in situ. Surprisingly, the lower silicon content alloy developed a more continuous internal silica layer, thereby slowing external scaling. Intradendritic oxidation was fast in both alloys, and is attributed to interfacial oxygen diffusion. Both alloys underwent rapid internal carburisation, indicating that their oxide scales failed to prevent carbon access to the underlying alloys under these reaction conditions.     

Mixed oxidant corrosion in nonequilibrium gasifier environments

The major use of high-temperature steel alloys in gasifiers is in heat exchangers for cooling hot syngas, consisting mainly of CO and H₂ with lesser amounts of H₂O and CO₂ and minor quantities of H₂S and HCl. Metal temperatures range from 250 to 600°C, gas temperatures from 250 to 1200°C. Because of rapid cooling the composition of the gas does not change with temperature. Therefore the gas is not in equilibrium at the metal surface. Calculations show that such gases have lower oxygen and sulfur pressures than equilibrated gases at the same temperature. This makes the results of previous laboratory studies less appropriate for predicting mixed oxidant corrosion in gasifiers. For this reason the present study was carried out using nonequilibrium gas mixtures, similar to gases, produced in entrained-slagging gasifiers. Most corrosion experiments were carried out at 540°C, as this is a common temperature for superheaters and hot-gas cleanup systems. Iron-base model alloys containing 35% Ni, 20% Cr, and various minor alloying additions were studied. Three corrosion regimes were identified over the range of conditions studied, depending on the sulfur-to-oxygen pressure ratio of the gas and the alloy composition. At high P_{S 2}/P_{O 2} ratios a somewhat protective FeCr₂S₄ scale formed on all alloys. Below this layer internal oxidation and sulfidation occurred at a slow rate. At lower P_{S 2}/P_{O 2} ratios nonprotective Fe(Ni, Cr)S external scales formed. These allow rapid internal oxidation of the chromium in the alloy, resulting in high corrosion rates. Under the same conditions very low corrosion rates are obtained when silicon is added to the alloy, because the presence of SiO₂ precipitates makes the internal-oxidation layer protective. Thus, the same corrosion model is operative in all three corrosion regimes: external sulfidation of iron and nickel, together with internal oxidation of chromium and other strong-oxide formers.     

High temperature alloy chloridation at 850°C. Part I: Comparison of Ni‐based and Fe‐based alloy behaviour

Eight alloys were tested under Ar/Cl₂ atmosphere at 850°C for 15 min and 1 h. Their gross and net weights were evaluated together with the base metal consumption. Macroscopic and microscopic micrographs, associated with chemical analyses and X‐ray diffraction gave the composition and microstructure of the corrosion products. Huge differences were observed if one compared the nickel based alloy behaviour to that of the iron based alloy. Molybdenum and tungsten could also play a role, but it was not clearly defined until now. A tentative evaluation of the best candidates will be given, according the experimental conditions of this work and the chosen criteria. A corrosion index was established after each observation and helped to classify the different material behaviour under chloridation. The Ni‐based alloys possessed the best chloridation behaviour in the present conditions, followed by the Ni‐based alloys containing more Cr and finally by the Fe‐based materials. The chloridation behaviours were discussed according to the volatility of the chlorides species formed on the different alloys.     

Corrosion behavior of NiCr alloys in HCl-containing oxidation atmosphere at 700-800 ℃

Corrosion behaviors of pure Ni and three NiCr alloys were investigated in an HCl-containing oxidizing atmosphere at 700 ℃ and 800 ℃. All materials suffer from accelerated corrosion at both temperatures. NiCr alloys show an initial mass loss due to the formation of volatile CrCl3 and CrO2Cl2. Some chlorides are detected at the scale/substrate interface and many voids are also found there. NiCr alloys with higher chromium content have better corrosion resistance. However, Ni50Cr is inferior to Ni25Cr due to its two-phase structure, which makes it easy for chlorine to diffuse along grain boundary and to occur inner oxidation. The relevant corrosion mechanism was also discussed.     

Effect of Pre-oxidation on Carburisation and Metal Dusting of Nano-crystalline Ni–Cr Alloys

Nanocrystalline (NC) Ni–Cr coatings, containing 5, 10 and 20 wt% Cr were prepared using magnetron sputtering deposition, on the substrates of the same composition materials. These alloys were tested in 47 %CO–47 %H₂–6 %H₂O for 50 h at 650 °C. Weight gain kinetics showed that increasing Cr content decreased the carburisation kinetics. After reaction, the NC coatings containing high Cr (10 and 20 wt%) remained, with the formation of surface and inner Cr₂O₃ and internal precipitates of fine carbon deposits for Ni–10Cr and Cr₇C₃ for Ni–20Cr. In contrast, the Ni–5Cr coated sample suffered a severe metal dusting with whole coating scale was destroyed completely. Preoxidation of these alloys and their miro-grained counterparts was conducted before metal dusting. It was found that preoxidation significantly reduced weight gain kinetics. This reduction effect is more significant for NC Ni–Cr alloys than the micro-grained alloys. The critical chromium concentration for protective chromia scale formation and for internal chromium carbide formation were discussed using Wagner’s analysis and product solubility calculation, respectively. The effects of preoxidation and grain size on oxide formation and carburisation/metal dusting were also discussed.     

Corrosion behavior of nickel alloys in wet hydrofluoric acid

Wet hydrofluoric acid at concentrations below approximately 60% is highly corrosive to glass, reactive metals, carbon steel and stainless steels. Nickel alloys offer moderate corrosion resistance over a wide range of acid concentration and temperature. The corrosion behavior of eleven commercial alloys was quantified through laboratory testing. Variables that were studied included testing time, acid concentration, temperature, vapor and liquid phases and the presence of residual stresses. Results show that the corrosion rate of a Ni‐Cu and a Ni‐Cr‐Mo‐Cu alloy increased with the acid concentration and the temperature. However, both for increasing acid concentration and temperature, the corrosion rate of the Ni‐Cu alloy increased faster than the corrosion rate of the Ni‐Cr‐Mo‐Cu alloy, especially in the vapor phase. Even in unstressed coupons, nickel alloys showed internal penetration in presence of wet HF; the mode of this internal penetration varied from alloy to alloy. Considering all the studied variables that influence corrosion, the highest ranked material for wet HF service was a Ni‐Cr‐Mo‐Cu alloy.     

The resistance of 20Cr/25Ni steels to carbon deposition. III. Cold work and selective pre-oxidation

Specimens of annealed 20 Cr/25Ni/Nb, selectively pre-oxidised 20Cr/25Ni/Nb, cold worked 20Cr/25Ni/Nb and TiN-strengthened 20Cr-25Ni steels have been exposed to a 75% CO/CO₂ gas atmosphere for periods up to 2000 hrs at 923 K (650°C). Whereas the cold-worked and TiN-strengthened steels show high resistance to this environment, the annealed Nb-stabilised alloy exhibited large amounts of carbon deposition. Associated with these were extensive regions of oxide pitting and gross carburisation of the substrate. By contrast, the selectively preoxidised specimens suffered much less deposition and no evidence for carburisation was found. It was also shown that small quantities of defects in the original preoxidised layer increased the propensity for deposition but did not result in local carburisation.     

Corrosion phenomena of alloys and electrode materials in Molten Carbonate Fuel Cells

The corrosion behavior of different alloys and the electrical conductivity of the growing corrosion scales was investigated under simulated and real molten carbonate fuel cell conditions. The corrosion of the usually used NiO cathode material was also investigated. In several exposure tests in oxidizing atmospheres, the FeCrMnNi steel 1.3965 showed a higher corrosion resistance to the aggressive carbonate media than the FeCrNi alloy 1.4404 (SS316L). This superior corrosion resistance is explained by the formation of a mixed (Fe,Ni,Mn)_xCr_3‐xO₄ spinel layer, which reduces the outward diffusion of iron ions more than the mixed (Fe,Ni)Cr₂O₄ spinel formed on austenitic FeCrNi steels. Oxide debris, which spalls off the current collectors, was investigated by XRD. The corrosion scales spalled off mainly at the curved area of the current collector and not at the cathode/current collector interface. The debris was strongly magnetic and consisted of several, in some cases lithiated iron oxides, whereby α‐Fe₂O₃ (hematite), γ‐Fe₂O₃ (maghemite) and Fe₃O₄ (magnetite) formed most of the debris. The investigations of the electrical conductivity of the corrosion scales have shown that the electrical conductivity is limited by the inner, Cr‐containing oxide of the multi‐layered corrosion scale. Cr‐rich alloys which contain more that 20 wt.% Cr showed extremely high ohmic resistance of the corrosion scale, much higher than that of alloys containing less than 20 wt.% Cr due to the formation of highly conductive mixed spinel layers. Small additions of Al in the alloy increased the ohmic resistance of the corrosion scale by many orders of magnitude. Corrosion tests in the fuel environment showed, that common uncoated stainless steels are not suitable for the use as anodic current collectors. The corrosion resistance in the anodic gas atmosphere is determined by the Cr and the Ni contents of the alloy. Only the model alloy NKK which contains 45 wt.% Ni and 30 wt.% Cr showed an acceptable corrosion resistance. The NiO dissolution and the Ni precipitation was investigated by single cell tests. These tests showed, that the replacement of the metallic Ni by an Al support (which is necessary to avoid cracks inside the ceramic) decreases the amount of metallic Ni in the ceramic matrix significantly. Therefore shorting of a fuel cell having a NiO cathode and a LiAlO₂ matrix with an Al support for the mechanical support is not expected in the target lifetime of 40 000 h. The double layer LiCoO₂‐NiO cathode also showed a significant reduction in Ni precipitation after testing. Due to the improvements and development in materials the MCFC‐lifetime has been trebled in the last few years.     

Effect of Mo on corrosion behavior of Ni20Cr–xMo alloys in air with NaCl–KCl–CaCl2 vapor at 570°C

The effect of Mo on the corrosion behavior of Ni20Cr–xMo alloys in an oxidizing chlorine-containing atmosphere using air mixed with the salt-vapor mixture of NaCl–KCl–CaCl₂ at 570°C was investigated. The results revealed that the corrosion performance of the Ni20Cr alloys in the oxidizing chlorine atmosphere was improved by Mo addition of up to 3 wt%. The Mo-free alloy formed a potassium chromate during corrosion as a result of the reaction between the Cr₂O₃ scale and KCl vapor. The chromate formation increased the chlorine potential at the scale surface and induced the breakdown of the protective Cr₂O₃ scale, resulting in internal chromium chloride precipitates and a Cr-depleted zone. In contrast, the presence of Mo resulted in the formation of a NiO scale, which did not react with the salt vapors and, therefore, prevented the formation of chromates. The beneficial effect of Mo on the high-temperature chlorination of Ni–Cr alloys in salt-vapor-containing atmospheres was ascribed to the suppression of chlorine generation due to NiO scale formation.     

Long‐term exposure of austenitic steels and nickel‐based alloys in lignite‐biomass cofiring

The aim of this study was to assess the long‐term impact that the addition of biomass provokes on superheater materials exposed to fireside corrosion environments. Alloys covering a broad range of commercially available materials were investigated. Their corrosion kinetics under different corrosive deposits and atmospheres was evaluated, and their corrosion products analyzed to deepen understanding of the underlying corrosion mechanisms. Therefore, three nickel‐based alloys and three austenitic steels containing 20–24 wt.% Cr were tested at 650°C for 7,000 hr. The long‐term exposure shows new mechanistic aspects of Type II hot corrosion that were revealed by accelerated material depletion. The formation of Ni–NiS eutectic and the formation of a Cr depleted zone close to the substrate corrosion product interface are indicative of the breakaway occurrence. Differences in the corrosion behavior are related to the balance of Ni, Mo, Co, and Cr and can serve as the material selection argument. The evaluation concluded with the finding that alloys presenting Mo and Ni might be preferentially used in fireside corrosion in the presence of biomass, whereas the use of austenitic steels suffer less corrosion if no biomass is present in the corrosive atmosphere.