Isothermal Corrosion Research Articles

CMAS Corrosion Resistance of Plasma-Sprayed YSZ and Yb2O3-Y2O3-Co-Stabilized ZrO2 Coatings under 39–40 KW Spraying Power

This study uses atmospheric plasma spraying (APS) technology to prepare thermal barrier coatings (TBCs) with yttrium-stabilized zirconia (YSZ) and Yb2O3-Y2O3-co-stabilized ZrO2 (YbYSZ) materials at different spraying powers. It analyzes the differences and changes in the microstructure, thermodynamic properties, and mechanical properties of the TBCs. The CaO-MgO-Al2O3-SiO2 (CMAS) resistance of coatings was tested using thermal cycling-CMAS experiments and isothermal corrosion experiments. Compared to YSZ coatings, YbYSZ coatings have lower thermal conductivity, a higher hardness and elastic modulus, a longer lifetime under thermal cycling-CMAS conditions, and lower penetration and degradation depths. Under thermal cycling-CMAS coupling conditions, the optimal power range for the longest thermal cycling lifetime for both coatings is 39–40 kW. Overall, compared to the YSZ material, the YbYSZ material exhibits superior properties.

Investigating the CMAS corrosion resistance and thermal cycling failure behavior of porous thermal barrier coatings with different morphologies prepared by plasma spraying

This study investigates the influence of two-phase powder feeding spacing on the thermal cycling and CMAS corrosion resistance of the thermal barrier coatings (TBCs). Porous thermal barrier coatings with varying microstructures were prepared using the embedded micro agglomerated particle (EMAP) coating preparation method, which is based on conventional yttrium stabilized zirconia (YSZ) coatings. The method involved changing the powder feeding spacing. TBCs' failure mode and microstructure are investigated under complex service conditions. The corrosion resistance, thermal cycling life, hardness, and elastic modulus of TBCs are investigated by thermal cycling-CMAS experiments and isothermal corrosion tests. The EMAP coating has a good service life in the thermal cycling-CMAS tests, with a lower hardness, lower elasticit modulus and larger surface roughness than conventional coatings. This research aims to provide a strategy to optimize structure for TBCs with excellent comprehensive performance.

High-temperature corrosion behavior of C276 alloy, 1.4529 steel and laser-cladding 1.4529 coating under the synergistic action of deposited chloride salt and HCl-containing atmosphere

The corrosion behavior and mechanism of C276 alloy, 1.4529 steel (9 S) and its laser-cladding coating (9 C) in a simulated waste incineration environment were studied by isothermal corrosion tests. The results show 9 C and 9 S have comparable corrosion resistance to C276 at 550 °C, and they are well protected by the mixed product layer rich in Fe and Cr. A loose and porous product layer caused the corrosion resistance of C276 to be lower than expected. At 700 °C, all three materials underwent severe high-temperature electrochemical corrosion and activated oxidation, forming a Ni-rich micro-nano porous surface and multilayer corrosion products.

Phase stability, thermophysical properties, thermal shock behavior and CMAS resistance of Sc2O3-CeO2 co-stabilized ZrO2 TBCs

In this study, 16 mol% CeO2–2 mol% Sc2O3 co-stabilized ZrO2 (4S16CZ) coating was prepared by atmospheric plasma spraying (APS) technology. Parameters related to the application of 4S16CZ as a thermal barrier coatings (TBCs) were studied, including high-temperature stability, thermal insulation performance, coefficient of thermal expansion (CTE), thermal shock behavior and CaO-MgO-Al2O3-SiO2 (CMAS) resistance. After heat treatment at 1400 °C for 100 h, the 4S16CZ coating maintained a single non-transformable tetragonal phase (t'), exhibiting excellent high-temperature stability. Additionally, at the test temperatures (25 °C–1000 °C), the thermal conductivity of the 4S16CZ coating ranged from 0.79 W/(m·K) to 1.02 W/(m·K), lower than that of the ZrO2 coating stabilized by 7.6 wt% Y2O3 (YSZ, 0.88–1.13 W/(m·K)), The CTE of the 4S16CZ coating was from 8.3 × 10−6 K−1 to 11.2 × 10−6 K−1, which was comparable to the YSZ. Moreover, the thermal shock behaviors of 4S16CZ TBCs and 4S16CZ/YSZ TBCs were evaluated at 1050 °C. The thermal durability of the 4S16CZ/YSZ TBCs was higher than that of 4S16CZ TBCs due to the buffering effect of the YSZ layer. The main reasons for the failure of both TBCs were the oxidation of the bond coat, thermal mismatch stress, low fracture toughness and sintering effects. In CMAS isothermal corrosion tests, CMAS exhibited low wettability on the 4S16CZ coating. Compared with the YSZ coating, the 4S16CZ coating showed better resistance to CMAS attacks.

In Situ High-Temperature TEM Observation of Inconel Corrosion by Molten Chloride Salts with N2, O2, or H2O

In situ transmission electron microscopy (TEM) diffraction and imaging techniques are used to monitor and quantify corrosion of Inconel-625 by pure molten chloride salts (MgCl2 − NaCl − KCl) at 500 °C–800 °C in 1.0 atm inert N2 or pure O2, or by salts which are controllably hydrated in a high vacuum chamber. The isothermal corrosion rate R in inert N2 increases from 203 ± 30 μm year−1 at 700 °C to 463 ± 30 μm year−1 at 800 °C. An oxygen ambient causes a six-fold increase to R = 1261 ± 170 μm year−1 at 700 °C. Salt hydration dramatically accelerates corrosion to R> 3 × 105 μm year−1 at 700 °C while it leads to a more moderate R = 95 ± 20 and 486 ± 30 μm year−1 at 500 °C and 600 °C, respectively. These isothermal corrosion rates indicate that the molten chloride corrosion is significantly accelerated by salt hydration at temperatures above 600 °C, where corrosion is aggravated by increased generation and solubility of corrosive HCl gases. Hence, to reduce rate of corrosion it is important to both avoid incorporation of H2O into the system at each stage and ensure proper flushing of the system before increasing the temperature beyond 600 °C. Compositional analysis of the corroded cells indicate that corrosion in O2 ambient is dominated by oxidation of metals by O2 gas dissolved in the chloride melt, but corrosion in H2O ambients is caused by chlorination of metals by dissolved HCl gas and MgOH+ ions. So, to reduce rate of corrosion, steps should be taken to tailor chloride melt compositions that has low solubility for HCl and O2. All of our corroded samples exhibit passive-protective oxide layers of Cr, Mg, and Ni. In addition, distinct volatile compounds of Ni, Mo and Cr involving NiCl2, (Na,K)2MoO4 and CrO2(OH)2 are detected in N2, H2O, and O2 ambients, respectively. We believe that corrosion acceleration can be minimized by minimizing formation of volatile by-products or promoting reactions that could convert these volatile compounds to solid phases, as these volatile compounds led to destruction of protective oxide layers.

Activity gradient driven mass transport in molten fluoride salt medium

The molten salt-cooled reactor concept has garnered significant interest and one of the current challenges limiting the deployment of these reactor concepts is the complex corrosion phenomenon observed in molten salt environments. One of these phenomena is activity gradient mass transport, which has been shown to affect dissimilar materials submerged in the same salt medium even when best efforts have been made to electrically isolate dissimilar materials from one another. This mechanism while shown experimentally, has not been predictively studied through a modeling approach. In this study, activity gradients in several 316L-X materials systems have been modeled and the mass transport predicted by the model has been confirmed through static isothermal corrosion testing in a molten fluoride salt medium.

Phase stability, thermal shock behavior and CMAS corrosion resistance of Yb2O3-Y2O3 co-stabilized zirconia thermal barrier coatings prepared by atmospheric plasma spraying

Rare-earth element doping has been extensively employed as one of the effective methods to enhance the durability of thermal barrier coatings (TBCs). In present study, ytterbia and yttria co-stabilized zirconia (YbYSZ) coatings and corresponding double-ceramic-layer (DCL) coatings comprised of a dense layer were produced by atmospheric plasma spraying. And the service capability including phase stability, thermal shock behavior and CMAS resistance were systemically evaluated. After heat treatment at 1500 °C for various durations, less monoclinic phase can be found in YbYSZ specimen, indicating excellent phase stability at high temperature. In the thermal shock test, the conventional YbYSZ coatings show a longest thermal shock lifetime, which increases by ~10% as compared with that of conventional yttria-stabilized ZrO 2 (YSZ) coatings. CMAS corrosion resistance of DCL coatings was studied using an isothermal corrosion test at 1300 °C. The results show that YbYSZ systems with a dense layer performed a high resistibility to CMAS corrosion, and less damage can be observed in coatings as well as smaller infiltration depth within the same time. • A double-ceramic-layer (DCL) coating comprised of dense layer was prepared. • The YbYSZ material exhibits higher resistance to high temperature. • The YbYSZ coatings have a longest thermal shock lifetime comparatively. • The DCL-YbYSZ coatings show enhanced CMAS resistance due to high chemical inertness.

A Comparative Analysis of Ternary Element Addition on Corrosion Behavior of Aluminide Coatings in Harsh Environmental Conditions

Increasing hot corrosion durability of aluminide coatings is important to extend the lifetime of turbine blades. The addition of hafnium, yttrium, zirconium, chromium, platinum, and cobalt improves the performance of aluminide coating by increasing oxide adherence and selective oxide formation rate. In this research, the effect of adding ternary elements (Y, Cr, Y/Cr, Zr, and Hf) on type-1 hot corrosion behavior of aluminide coatings was investigated by an accelerated isothermal corrosion test at 900°C for up to 400 h. To simulate harsh environmental conditions, Na2SO4- and V2O5-containing solutions were applied to the substrate surface. Subsequently, for 1 h, 50 h, 100 h, 200 h, and 400 h exposure times, the oxide layer thicknesses, spallation time, coating layer depletion time, and elemental analysis of each set were analyzed and their performances compared.

Performance Comparison Between Isothermal Hot Corrosion And In Situ Cyclic Hot Corrosion of Nickel-Based Superalloys

Although a lot of work has been done to understand both major mechanisms of hot corrosion, namely type I (high-temperature hot corrosion) and type II (low temperature hot corrosion), there is very little information available on more representative cyclic performance in these regimes. This work addresses this by assessing the performance of isothermal (type I and type II) hot corrosion tests against combined (short and long) cyclic corrosion tests. Single-crystal alloy PWA 1484 and directionally solidified alloy MAR-M247 were assessed in all test regimes. Pre- and post-exposure dimensional metrology was used to quantify the corrosion damage and characterised using SEM/EDX. This paper highlights that the results of short cycle test conditions are more damaging compared to long cycle and standard isothermal type I and II test conditions. The cast nickel-based alloy MAR-M247 was found to be a better performer compared to PWA 1484 single-crystal alloy.

In Situ Transmission Electron Microscopy of High-Temperature Inconel-625 Corrosion by Molten Chloride Salts

This paper describes an approach to monitor high temperature molten chloride (MgCl2-NaCl-KCl) salt corrosion of Inconel-625 alloy in real time at high spatial resolution. The approach is based on a micro-environmental-cell assembly integrated into a transmission-electron-microscope goniometer to examine in situ the salt-alloy interface during corrosion, employing real time electron diffraction and imaging. It establishes procedures to minimize incorporation of H2O or O2 from atmosphere in the chloride salts during sample fabrication and corrosion, which is critical to understanding the fundamental corrosion mechanisms. A clustering algorithm and a 2D Gaussian fit function are used to determine diffraction spot intensities in in situ diffraction patterns, to quantify alloy corrosion. This facilitates quantitative observation of the evolution of individual grains, in contrast to conventional macroscopic corrosion rate quantification. The isothermal corrosion rate of Inconel-625 in an anhydrous, unoxidized salt-stack is 220 ± 30 μm year-1 at 700 °C and 350 ± 20 μm year-1 at 800 °C. However, the corrosion rate at 700 °C increases five-fold to 1000 ± 170 μm year-1 when the salt stack is air-exposed, indicating the dominant effects of hydrated or oxidized impurities on corrosion acceleration. Real time imaging of the microstructure evolution suggests that corrosion is initiated at grain boundaries.

Isothermal and cyclic corrosion behaviour of Ni-based alloys in an electrochemical reduction process

ABSTRACT In this study, the isothermal and cyclic corrosion behaviours of Haynes 282 and Haynes 214 in a lithium molten salt were investigated at 650°C for 168 h and 7 thermal cycles, respectively; this was achieved by measuring the mass changes, surface and cross-sectional morphologies with their elemental distributions, and compositional changes in the subscale, substrate and spalled oxide scale. The weight loss of Haynes 282 comprising an external corrosion layer was approximately four times lower than that of Haynes 214 with a spalled external corrosion layer, depending on the effects of their alloying elements and the preservation, spallation and dissolution of the corrosion layer. After 168 h and 7 thermal cycles, corrosion products of Haynes 282 were Cr2O3 and CoCr2O4 and those of Haynes 214 were Cr2O3, LiFeO2 and FeCr2O4. In addition, the internal corrosion layers of Haynes 282 and Haynes 214 exhibited localised and uniform corrosion behaviours, respectively.

Non-galvanic mass transport in molten fluoride salt isothermal corrosion cells

Mass transport of corrosion products between Ni-201 test samples and 316 L capsules in a 500-hour FLiNaK corrosion experiment has been investigated. In one experiment, the capsule and samples were electrically connected for galvanic coupling, while in the other they were electrically isolated using boron-nitride. Post-corrosion characterization of Ni-201 from the two experiments did not reveal any remarkable differences. Substantial transport of Fe and Cr from the 316 L to the Ni-201 via the salt was observed, with notable diffusion of these elements in the near-surface regions of the Ni-201 samples, while a Cr-rich film was also observed on their surfaces.

CMAS corrosion of YSZ thermal barrier coatings obtained by different thermal spray processes

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al2O3-SiO2 (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.

Oxidation study of porous metal substrates for metal supported proton ceramic electrolyzer cells

Isothermal corrosion of coated and uncoated porous, ferritic stainless steel (Höganäs), with 20.6 wt% Cr and 30–35 % porosity, has been studied at temperatures from 600 to 800 °C in air and 2.5 %H2O/5 %H2/Ar. Uncoated specimens form protective Cr2O3 scales following parabolic kinetics at 600 and 700 °C in air and at 800 °C in 2.5 %H2O/5 %H2/Ar. Non protective scales were formed in 2.5 %H2O/5 %H2/Ar at 600 and 700 °C and at 800 C in air. The coating overall improves the corrosion behavior. The scaling behavior was explained from Fe-Cr-O2 stability diagrams and ratios between consumption and feed of Cr at the alloy-oxide interface.

High-temperature corrosion characteristics of yttria-stabilized zirconia material in molten salts of LiCl-Li2O and LiCl-Li2O-Li

ABSTRACT The isothermal and cyclic corrosion behavior of yttria (Y2O3)-stabilized zirconia (ZrO2) in a LiCl-Li2O molten salt were investigated at 650 °C in an argon atmosphere. During isothermal and cyclic corrosion tests in the molten salt of LiCl-Li2O for 168 h and 7 thermal cycles, the corrosion rate was very low, whereas under the molten salt of LiCl-Li2O-Li for 168 h, the corrosion rate was almost 10 times higher than that in the molten salt of LiCl-Li2O. No corrosion product was detected until 168 h for the isothermal corrosion test, however, after 7 thermal cycles, a very-low-intensity Li2ZrO3 peak was detected at the beginning stage of the chemical reaction between ZrO2 and Li2O. Additionally, in the molten salt of LiCl-Li2O-Li for 168 h, a large amount of Li2ZrO3 was formed, with evidence of marked cracks, pores, and spallations on the corroded surface. The introduction of Y2O3-stabilized ZrO2 was beneficial in increasing the hot corrosion resistance of the structural materials used to handle molten salts containing Li2O at elevated temperature without forming a lithium at the cathode during the electrolytic reduction process.

Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests

Advanced components in next-generation concentrating solar power (CSP) applications will require advanced heat-transfer fluids and thermal-storage materials that work from about 550°C to at least 720°C, for integration with advanced power-conversion systems. To reach the cost target, less-expensive salts such as molten chlorides have been identified as high-temperature fluid candidates. High-strength alloys need to be identified and their mechanical and chemical degradation must be minimized to be used in CSP applications. Approaches for corrosion mitigation need to be investigated and optimized to drive down corrosion rates to acceptable levels—in the order of tens of micrometers per year—for achieving a long system lifetime of at least 30 years. Surface passivation is a good corrosion mitigation approach because the alloy could then be exposed to both the liquid and the vapor phases of the salt mixture. In this investigation, we pre-oxidized the alumina-forming alloys Inconel 702, Haynes 224, and Kanthal APMT at different temperatures, dwelling times, and atmospheres to produce the passivation by forming protective oxides at the surface. The pretreated alloys were later corroded in molten MgCl2 – 64.41wt% KCl at 700°C in a flowing Ar atmosphere. We performed electrochemical techniques such as open-circuit potential followed by a potentiodynamic polarization sweep and conventional long-term weight-change tests to down-select the best-performing alloy and pre-oxidation conditions. The best corrosion results were obtained for In702 pre-oxidized in zero air at 1050°C for 4h. Metallographic characterization of the pre-oxidized alloys and of the corroded surfaces showed that the formation of dense and uniform alumina scale during the pre-oxidation appears to protect the alloy from attack by molten chloride.

Effects of alloying elements of nickel-based alloys on the hot-corrosion behavior in an electrolytic reduction process

In this study, the isothermal and cyclic corrosion behaviors of custom-made nickel-based alloys (N105, N107, and N108) with various elemental compositions in a LiCl-Li2O molten salt were investigated at 650 °C in argon atmosphere. The isothermal and cyclic corrosion behaviors were studied by measuring the mass changes, surface morphologies, cross-sectional morphologies with elemental distribution, and the compositional changes of the subscale and substrate as well as the spalled oxide scale. Under the isothermal and cyclic corrosion test, the corrosion rate was in the order of N108 < N105 < N107, and N107 < N105 < N108, respectively, which depended on the morphology and structure of the crystals formed on the surface of the outer scale in contact with the molten salt, and was also related to the region of Al and Cr depletion. After the isothermal corrosion test, the hot corrosion products formed in N105 were Cr2O3, NiCr2O4, and Li2Ni8O10 and those in N107 were Cr2O3 and NiCr2O4, while Cr2O3, NiCr2O4, and LiTiO2 were identified as the hot corrosion products of N108. However, the hot corrosion products of N105, N107, and N108 were Cr2O3, NiCr2O4, and LiFeO2 after the cyclic corrosion test.

Effect of yttria(Y2O3) coating for high temperature oxidation resistance of 9Cr-1Mo steel

This paper investigates the effect of yttria (Y2O3) coating on high temperature oxidation behaviour of low alloy 9 Cr–1Mo steel. The superficial coating is Y2O3 was prepared for experimental investigation. The isothermal corrosion study of uncoated and coated specimens was carried out in air oxidation environment at 973 K for 8 h. The corrosion rate and reaction kinetics were studied and the post corroded scales were characterized in SEM, EDS and XRD. The results clearly indicate that Y2O3 coated specimen improves the high temperature oxidation resistance than uncoated specimens. The improvement of oxidation resistance in presence of Y2O3 coating can be attributed to the changed mechanism of scale growth from outer cation migration to inner anion migration and enhancement of scale adhesion with the substrate. Further, enhancement of scale adhesion with the substrate in case of Y2O3 coating also improves the oxidation resistance. The detail mechanism of the oxidation of Y2O3 coated and uncoated specimen is further discussed in this paper.

Nanostructured CeO2 coating for high temperature oxidation protection

This paper investigates the influence of nanostructured ceria (CeO2) coating on high temperature oxidation behaviour of 2·25 Cr-1 Mo steel. The isothermal corrosion study of the micrometre size and nanostructured CeO2 coated specimens was conducted in air oxidation environment at 973 K for 8 h. The corrosion rate and reaction kinetics were studied, and the post-corroded scales were characterised by field emission scanning electron microscopy and X-ray diffraction. The results clearly indicate that the nanostructured CeO2 coated specimen improves the oxidation resistance in comparison to the micrometre size CeO2 coated specimen. The improvement of oxidation resistance in the case of nanostructured CeO2 coating can be attributed to the formation of semimolten zone, which consists of nanostructured CeO2 particle. On the other hand, in the absence of semimolten nanostructured zone, the micrometre size CeO2 coated specimen shows higher corrosion rate in comparison to the nanostructured CeO2 powder coated specimen.

Ceria (CeO2) based coating for high temperature corrosion protection of 9Cr-1 Mo steel

Ceria (CeO2) based high temperature coating was developed over 9Cr-1 Mo steel substrate. The coating is produced by plasma spray coating method by using robotic arm. Isothermal corrosion study of the CeO2 coated and uncoated specimen was conducted at 973 K for 8 h. The corrosion rate and reaction kinetics were studied and the post corroded scales were characterized in SEM, EDS and XRD. The results clearly indicate that ceria (CeO2) coated specimen improves the high temperature oxidation resistance than uncoated specimens. The kinetic behaviour of both coated and uncoated specimen follows approximate parabolic growth rate which further indicates that the corrosion is diffusion controlled growth. The post corroded scale of the coated specimen shows fine grained oxide scale with no scale cracking and detachment. The improvement of oxidation resistance in presence of CeO2 coating can be attributed to the formation of fine grained oxide on the scale, changed mechanism of scale growth from outer cation migration to inner anion migration and enhancement of scale adhesion with the substrate. The detail mechanism of the oxidation of CeO2 coated specimen and uncoated specimen is further discussed in this paper.