High Temperature Corrosion Behavior Research Articles

An Investigation on the Chemical Variation in TLP Joint Line and Its Effect on High-temperature Corrosion Behavior of the Bonded IN738-Lc

Turbine blade face with crack generation during high-temperature application. Due to its high cost of manufacturing, repair of the damaged section is usually more desirable than its replacement. transient liquid phase (TLP) bonding is a common joining procedure used for maintenance of a gas turbine damaged vein and blade. This paper describes how the TLP joint area resist hot corrosion at high working temperatures. Microstructure and metal loss were studied from scanning electron microscope (SEM) images, and corrosion products were identified by XRD and EDS analyses. SEM observations revealed that the TLP area is composed of three distinguished regions: a thermally solidified zone with about 15 µm thickness, the isothermally solidified zone with about 32 µm thickness, and diffusion affected zone with about 54 µm thickness. Exposing test samples to Na2SO4/NaCl (3:1 wt%) salt mixture at 750 °C, creates a 40 µm of porous oxide scales containing the outer layer of chromium oxide and the inner layer of chromium–nickel oxides. Moreover, 160 µm of internal attacks were observed and were identified as sulfide attack. The corrosion attacks in the diffusion affected area were observed in the form of micro-cracks and internal sulfide island. The morphological together with cross-sectional studies on corroded samples showed that the thickness of corrosion product in TLP joint was more than that of area far from TLP centerline. Surfaces on the side of TLP line acted as the short-circuit path to deliver chromium and nickel to the corrosion exposure. Moreover, cracks which were formed during hot corrosion in TLP area, acted as short-circuit inward diffusion of sulfur, resulting in internal sulfidation. It is also suggested that the formation of boride and silicide intermetallics in the TLP area also weakens the corrosion resistance.

High-Temperature Corrosion Behavior of Al-Coated Ni-Base Alloys in Lithium Molten Salt for Electroreduction

The electrolytic reduction of a spent oxide fuel involves the liberation of oxygen in a molten salt LiCl–Li2O electrolyte, which creates a corrosive environment for typical structural materials. In this study, the corrosion behaviors of Al–Y-coated specimens in a Li molten salt kept under an oxidizing atmosphere at 650 °C for 72 and 168 h were investigated. The weight loss fraction of the coated specimen to bare specimen was approximately 60% for 3% Li2O and 54% for 8% Li2O at 72 h, and approximately 38% for 3% Li2O and 30% for 8% Li2O at 168 h. Corrosion was induced in the LiCl–Li2O molten salt by the basic oxide ion O2− via the basic flux mechanism, and the corrosion product was found to be dependent on the activity of the O2− ion. The increase in weight loss may have been caused by the increase in the O2− concentration due to the increase in the Li2O concentration rather than being because of the increased reaction time. The Al–Y coating was found to be beneficial for hot corrosion resistance, which can be useful for handling high-temperature lithium molten salt under an oxidizing atmosphere.

High-Temperature Corrosion Behaviors of Structural Materials for Lead-Alloy-Cooled Fast Reactor Application

The corrosion of nuclear-grade steels in lead–bismuth eutectic (LBE) complicates the realization of high coolant temperatures. Corrosion tests of T91, HT9, and SS316L were performed in static cells at 600 °C for 2000 h at an oxygen level of 10−6 wt.%. The obtained corrosion surfaces of post-processed samples were characterized by several microscopy methods. Up to 1000 h, all the alloys exhibited an evolution of duplex oxide layers, which were spalled until 2000 h due to their increased thickness and decreased integrity. Following the spallation, a thin internal Cr-rich oxide layer was formed above the Cr-depleted zone for T91 and HT9. SS316L was penetrated by LBE down to 300 μm in severe cases. A comparison on the corrosion depths of the materials with regard to the parabolic oxidation law with abundant literature data suggests that it may lose its validity once the duplex layer is destroyed as it allows LBE to penetrate the metal substrate.

The high temperature wetting and corrosion mechanism analysis of Nb by TiAl alloy melt

High temperature corrosion behavior of Nb containing a TiAl melt is studied connecting with grain boundary wetting (GBW). Three intermetallic compounds (IMC) layers are formed and corroded step by step via the TiAl melt. The interfacial morphology and wetting angle evolution are investigated theoretically and experimentally. The morphology of the grooves controlled by interfacial tension, while nonzero wetting angles θd controlled by surface diffusion are established at the roots. By quantitative assessment, when θd is more than 140°, the grooves terminating on a thin sheet, which is responsible for the effective inhibition of further wetting and corrosion.

High Temperature Corrosion Behavior of Eutectic Structured NiAl-34Cr and NiAl-28Cr-6Mo Alloys Produced by Electric Current Activated Sintering

In this study, NiAl-34Cr and NiAl-28Cr-6Mo eutectic alloys were produced by electric current assisted sintering (ECAS) method in a 3500-4200 A current range with a waiting time of 47 minutes. Phase examinations of the obtained samples were carried out with the help of X-ray diffraction analysis (XRD). While determining NiAl and Cr phases in NiAl-34Cr alloy from XRD patterns; It was determined that NiAl-28Cr-6Mo alloy consists of two phases, together with Mo residues, NiAl and CrMo. According to the Archimed principle, the relative density of NiAl-34Cr and NiAl-28Cr-6Mo alloys was determined as 96.2%, 97.9% respectively. The hardness values of NiAl-34Cr and NiAl-28Cr-6Mo samples were approximately 275 ± 13 HB and 255 ± 20 HB detected. In addition, the corrosion properties of the samples were examined by hot corrosion tests at 800, 900 and 1000°C for 165 hours (15 cycles) in 25% wt. K2SO4 + 75% wt. Na2SO4 salt medium. Weight changes, microstructure (SEM-EDS) and phase analysis of the samples after corrosion were carried out, and the corrosion properties of NiAl-34Cr alloy were found to be better compared to the Mo-added alloy.

Dynamic simulation on high-temperature corrosion behaviour of tube surface with fouling in utility boiler fired by high-chlorine coal

A dynamic model on high-temperature corrosion with fouling-type ash deposition was established to predict the thickness distribution of corrosion layer and the outer/inner layer of ash deposit. It provides an observation on variation of heat transfer characteristics of the alloy test probe over time. This model involves the combustion characteristics of high-chlorine coal, the process of formation, migration, and deposition of fly ash particles, and the corrosion of alloy test probe. In this study, the dynamic model has been integrated into the numerical simulation of high-chlorine coal combustion in a 50 kW long-period corrosion test furnace. The prediction results show that the amount of fly ash deposition on the windward surface is the most while the least amount appears on the side surface. There is a great correlation between the corrosion layer and the inner layer of ash deposit. Because the inner layer of ash deposit and the alloy substrate are spatially adjacent, the high temperature corrosion process is greatly affected by the growth and property of fly ash deposit layer. The simulation results show that the heat flux of probe surface beneath ash deposit is 2.5 kW/m2 which is 50% of the clean surface (5 kW/m2). The changing pattern of thermal resistance versus time at different positions of the alloy surface is consistent with the variation of thickness of the corrosion layer and ash deposit layer. The windward face has the highest thermal resistance and the least heat flux due to the highest thickness of the corrosion layer and ash deposit layer. With the increase of probe wall temperature, the ratio of thermal resistance of corrosion layer to total heat transfer resistance increases, and the ratio at the side face is larger than that at other locations. And the proportion of the corrosion layer in thermal resistance for alloy TP347H reaches 8% at the probe wall temperature of 973 K.

High temperature corrosion behaviour of stainless steels and Inconel 625 in hydroxide salt

Biomass gasification had proven to be an alternative source of energy to coal gasification. However, it requires high temperatures of about 1000°C for biomass drying and reduction. On the other hand, to reduce the oxidation and corrosion of gasifier structural materials it is important to keep the gasifier working temperature as low as possible. One effective way of keeping the reduction temperature low is to use molten salts as catalyst during biomass gasification. However, by virtue, molten salts cause several corrosion issues in ferrous alloys. In this context, the present study investigates the effect of hydroxide molten salt on the corrosion behaviour of stainless steels such as 316 and 310 and Inconel 625. The samples exposed to the salt at 700°C for about 48 h was analysed for corrosion using weight loss method. A scanning electron microscopy analysis of the exposed samples revealed the depth of corrosion and change in microstructure due to molten salt attack. Although all the selected materials suffered severe corrosion, among all, Inconel 625 show higher corrosion resistance.

High-Temperature Corrosion Behaviors of Ni-Based Alloys in Carbon Dioxide

High-Temperature Corrosion Behaviors of Ni-Based Alloys in Carbon Dioxide

Synthesis and high temperature corrosion behaviour of nearly monolithic Ti3AlC2 MAX phase in molten chloride salt

Nearly monolithic MAX phase containing 95 wt.% Ti3AlC2 and 5 wt.% TiCx was synthesized by spark plasma sintering under vacuum sintering conditions. Corrosion behaviour of Ti3AlC2 was investigated in molten LiCl–KCl salt at 600 °C under a dry Ar atmosphere. Evolution of microstructure and surface chemistry of the exposed sample was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and glancing angle X-ray diffraction (GAXRD). Results showed that Al dissolution led to delamination of the layered structure which favoured the ingress of chlorine and its subsequent intercalation into the Al–site plane to form a Ti3C2Cl2 exfoliation layer. De-twinning of the Ti3C2 layers possibly due to Cl− anions substitution by O results in non-stoichiometric TiC0.67 formation.

High-temperature corrosion behavior of hot-pressed Al-based Gd2O3-W shielding materials used in spent fuel storage

The high-temperature corrosion behavior of new types of Al-based Gd2O3-W composites prepared by hot-pressing and used in spent fuel storage were investigated. The samples were irradiated by Fe+ to simulate neutron and γ-ray irradiations and then corroded in a high-temperature boron-lithium-water environment. An outer flocculent AlO(OH) layer with a limited protective effect and an inner amorphous Al2O3 layer were formed on the surface. Pitting initiated from the Al-only portions and was inhibited by Gd-containing particles. The corrosion could bypass dispersed particles and led to a general corrosion. Irradiation resulted in more active corrosion sites and caused severe corrosion.

The influence of Si on the primary protection of lean FeCrAl model alloys in O2 and O2+H2O at 600 °C—A microstructural investigation

The present study investigates the influence of Si on the high-temperature corrosion behavior of lean FeCrAl model alloys in O2 and O2+H2O at 600 °C. The addition of Si prevented breakaway oxidation in O2+H2O which may be explained by the increased Al-content and reduced Cr-content in the oxide (O2) as well as the Si-enrichment in the outer part of the scale (O2+H2O). A proposed explanation for the impact of Si-addition on the Cr- and Al-content was given by thermodynamic calculations which showed that the presence of Si increases the activity of Al in the alloy while reducing the activity of Cr.

Formation of Sulfide Deposits and High-Temperature Corrosion Behavior at Fireside in a Coal-Fired Boiler

Because of adopting a high air-staging combustion ratio for Chinese coal-fired boilers, the water walls currently suffer from severe sulfidic corrosion and slagging. This work aims to understand th...

High-Temperature Corrosion Behaviour of CNT-reinforced Zirconium Yttrium Coatings on Boiler Tube Steel in Coal-Fired Boiler of Thermal Power Plant

Corrosion behaviour of carbon nanotubes (CNT)-reinforced zirconium yttrium coatings at high temperature under cyclic thermal loading has been investigated in actual coal-fired boiler environment of thermal power plant. The amount of CNT was varied from 0.5 to 6 wt.%, and the coating was developed by the plasma spray technique. The comparative effects of variation in CNT content on hot corrosion behaviour were studied by weight change measurements. The corroded products were then analysed with XRD, FE-SEM with EDAX and X-sectional analysis techniques. The results revealed that the variation in CNT percentile enhances the hot corrosion resistance of boiler steel in actual boiler environment at elevated temperature. The reinforced coatings showed the lower weight gain measurements with the formation of protective scale of oxides during the experiment. Corrosion rate was observed to be reduced with the increased content of CNT in the coating composite.

Hot Corrosion Performance of Plasma-Sprayed Multiwalled Carbon Nanotube–Al2O3 Composite Coatings in a Coal-Fired Boiler at 900 °C

The present study investigates the high-temperature corrosion behavior of plasma-coated boiler steel for 1000 h in a 110-MW water tube boiler, which had encountered severe corrosion and the tube failure. The coating was carried out by alumina and MWCNTs (multiwalled carbon nanotubes)-reinforced alumina composite powders. The uncoated and coated steel substrates were hung at a platen superheater at 900 °C. After the exposure, thermogravimetric analysis was carried out to evaluate the kinetics of corrosion. XRD, SEM, TEM and EDS analysis was carried out to evaluate the corrosion products at the end of the cyclic high-temperature corrosion. The present research work concludes that CNT-reinforced alumina coatings (4.0 wt.% CNT + Al2O3 coating) have decreased the corrosion rate to 30.72 mpy, which was 113.53 mpy for conventional alumina coatings and 376.96 mpy for bare T11 boiler steel in the boiler environment. This manuscript submits the failure of tube at the platen superheater, exposure conditions and the analysis of the coated and uncoated samples of the steel tubes.

High-temperature corrosion mechanism of a promising scandium tantalate ceramic for next generation thermal barrier coating under molten calcium–magnesium-aluminosilicate (CMAS)

Hot corrosion caused by molten calcium–magnesium-aluminosilicate (CMAS) is one of the significant failure factors of thermal barrier coating (TBC). Scandium tantalite (ScTaO4) is expected to be a next generation of TBC material because of its great thermophysical characteristics. This study focused on high-temperature corrosion behaviors of ScTaO4 ceramic exposure to molten CMAS at 1300 °C. During corrosion experiments, a clear but discontinuous reaction layer containing CaTa2O6 and Sc2SiO5 was observed, which was formed by dissolution reprecipitation. In addition, ScTaO4 showed good phase stability and maintained monoclinic wolframite structure with P2/c (13) space group in the corrosion tests. Moreover, Ca, Mg and Al were all kept away from the ScTaO4 ceramic due to low solubility of Sc3+ in the CMAS. Therefore, ScTaO4 ceramic owns great ability to resist the corrosion of CMAS. Good phase stability enables ScTaO4 to stably exist at high temperature, reducing volume change, thus extending the service life of the coating. Meanwhile, great corrosion resistance of CMAS reduces the consumption of ScTaO4 under adverse environmental conditions, further extending the service life of the coating. In summary, ScTaO4 is a new kind of potential thermal barrier material to replace YSZ ceramic.

High-temperature corrosion of an Fe–Ni-based alloy HR6W under various conditions at 750 °C and 810 °C: Effect of the temperature, water vapor, simulated ash and SO2

The influences of temperature, SO2, water vapor and simulated ash on the high-temperature corrosion of an Fe–Ni-based alloy HR6W at 750 °C and 810 °C for 1000 h was systematically investigated. The weight gains and characterization of the corrosion products formed on the HR6W alloy in various environments were used to obtain the high-temperature corrosion behaviors of the alloy. The results show that weight gains of alloy HR6W under various gas conditions at 750 °C and 810 °C was much lower than that of the samples coated with salt, indicating that alloy HR6W exhibited excellent corrosion resistance due to a Cr2O3-rich scale. The SO2 promoted crackings of the Cr-rich oxide scales at 750 °C and 810 °C. After exposure to air containing water vapor and SO2, the corrosion products were mainly composed of an outer layer of Fe2O3, a small amount of NiO and an inner layer of Cr2O3. Sulfidation of alloy HR6W underneath the Cr-rich oxide scale was clearly observed on the samples exposed to SO2 and on all samples coated with salt. Additional NiO was found on the samples coated with salt exposed to air containing SO2. Alloy HR6W coated with salt exposed to air containing SO2 at 750 °C and 810 °C experienced the most severe corrosion among all samples herein due to the high causticity of the alkali-iron-tri-sulfates. The key elements affecting the high-temperature corrosion of alloy HR6W in the various gas environments were temperature > SO2>water vapor, in the order. Under the gas-molten salt conditions, the SO2 played the most important role in determining the corrosion rate. The key elements were SO2> temperature > water vapor, in that order.

Comparison of High-Temperature Corrosion Behavior of a FeCrAl Anode Current Collector in Liquid Ag and O2 for the Solid Oxide Membrane Electrolysis Process

The high-temperature corrosion behavior of anode current collectors in liquid Ag under an oxidizing atmosphere was investigated. The weight gain in oxygen was higher than that of liquid Ag because of the effective barrier formed by Ag-rich layer coated on the corrosion layer. The thickness of the corrosion layer under oxygen was approximately twice that of the layer developed under liquid Ag because of the effective barrier originating from the Ag-rich layer. The corrosion products on the surface of the specimens in both oxygen and liquid Ag were Al2O3 and FeAl2O4 from 24 to 168 h. The surface morphology that was developed on the scale of the corroded layer represented a buckle structure in liquid Ag and a convoluted structure in oxygen, which resulted from the lateral growth of the oxide. This study proved the improved stability and cost-effectiveness of using FeCrAl alloy as a current collector in a solid oxide membrane system.

Elevated temperature molten salt corrosion study of SS304L austenitic boiler steel

The aim of this study is to investigate the high-temperature corrosion behavior of 304L stainless steel in the environment of various molten salt mixtures in the temperature range of 550–850°C. Weight change measurement has been done and characterization of corroded specimen was carried out using XRD, SEM/EDS techniques. Findings indicate that high chloride concentration salts affect the corrosion resistance adversely. The protective behavior of Cr2O3 layer deteriorated and its passivation is hindered at higher temperature owing to the chlorination-oxidation process. Fe2O3 and (Fe, Cr)xOy were found to be the major phases in the corrosion product. The salts were found to significantly affect the corrosion of 304L stainless steel in the following order: K2SO4 + 60% NaCl > Na2SO4 + 50% NaCl > 40% K2SO4 + 40% Na2SO4 + 10% KCl + 10% NaCl > Na2SO4 + 60% V2O5 > Air.

High-temperature corrosion behavior of high-temperature and high-pressure cavitation processed Cr–Mo steel surface

In this paper, long-term high-temperature corrosion at 500 °C and high-temperature corrosion at the melting temperature of a corrosive ash mixture were examined because the use of high-temperature equipment such as boilers and gas turbines increases year over year. To investigate the optimum cavitation processing conditions for the specimens used in high-temperature corrosion tests, the surface properties of each processed specimen were examined. In specimens processed using multifunction cavitation (MFC), the compressive residual stress was high when the processing time was 10 min and the Cr content on the surface was greater than on the surface of an unprocessed specimen. On the other hands, in specimens subjected to water-jet peening (WJP), the compressive residual stress was high when the processing time was 10 min. In the present study, the processing time was selected to be 10 min and all high-temperature corrosion tests were conducted by the coating method. In the case of long-term high-temperature corrosion at 500 °C, the corrosion loss of the MFC-processed and WJP-processed specimens was small, whereas the corrosion loss of the unprocessed specimen was large.

High Temperature Corrosion Behaviour of Aluminide-Coated Cast Iron for an Exhaust Manifold Application

To reduce the pollution emission from vehicles, an improvement on the combustion process is expected, leading to increased exhaust gas temperature. As a result, the development of new materials for an exhaust manifold used at higher temperatures is required. A cost-effective cast iron exhaust manifold treated by aluminising pack cementation was developed in the present work to combat the high temperature corrosion. Its kinetics under cyclic oxidation in N2–12%O2–10%H2O at 850 °C was parabolic with the rate constant (kp) of 5.66 × 10−12 g2 cm–4 s–1, about two orders of magnitude lower than that of the bare cast iron, which indicated the protectiveness of the applied coating. These results relate to the protective alumina formation for the aluminised cast iron and the formation of the less protective iron oxides for the bare cast iron after oxidation, as evidenced by the XRD and Raman spectroscopy results. The addition of 10% water vapour to N2–12%O2 thickened the aluminide layer from 344 μm for the sample oxidised in dry atmosphere to 409 μm for the sample oxidised humidified one. It accelerated the oxidation rate of the aluminised cast iron as the kp value increased by 8.5 times, and also increased the hardness of the aluminised surface, as it was 364 HV for the sample exposed to dry atmosphere and 420 HV for the sample exposed to humidified one. The latter result implied the possibility of the hydrogen dissolution into the metal surface. The roles of hydroxyl ion and dissolved hydrogen on the oxidation and evolution of the aluminide layer after exposure to water vapour were proposed.