High-Temperature Exposure Studies of HVOF-Sprayed Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-25(NiCr)/(WC-Co) Coating

In this research, development of Cr₃C₂-25(NiCr) + 25%(WC-Co) composite coating was done and investigated. Cr₃C₂-25(NiCr) + 25%(WC-Co) composite powder [designated as HP2 powder] was prepared by mechanical mixing of [75Cr₃C₂-25(NiCr)] and [88WC-12Co] powders in the ratio of 75:25 by weight. The blended powders were used as feedstock to deposit composite coating on ASTM SA213-T22 substrate using High Velocity Oxy-Fuel (HVOF) spray process. High-temperature oxidation/corrosion behavior of the bare and coated boiler steels was investigated at 700 °C for 50 cycles in air, as well as, in Na₂SO₄-82%Fe₂(SO₄)₃ molten salt environment in the laboratory. Erosion-corrosion behavior was investigated in the actual boiler environment at 700 ± 10 °C under cyclic conditions for 1500 h. The weight-change technique was used to establish the kinetics of oxidation/corrosion/erosion-corrosion. X-ray diffraction, field emission-scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS), and EDS elemental mapping techniques were used to analyze the exposed samples. The uncoated boiler steel suffered from a catastrophic degradation in the form of intense spalling of the scale in all the environments. The oxidation/corrosion/erosion-corrosion resistance of the HVOF-sprayed HP2 coating was found to be better in comparison with standalone Cr₃C₂-25(NiCr) coating. A simultaneous formation of protective phases might have contributed the best properties to the coating.     

Superheater fireside corrosion mechanisms in MSWI plants: Lab‐scale study and on‐site results

Combustion of municipal waste generates highly corrosive gases (HCl, SO₂, NaCl, KCl, and heavy metals chlorides) and ashes containing alkaline chlorides and sulfates. Currently, corrosion phenomena are particularly observed on superheater's tubes. Corrosion rates depend mainly on installation design, operating conditions i.e., gas and steam temperature and velocity of the flue gas containing ashes. This paper presents the results obtained using an innovative laboratory‐scale corrosion unit, which simulates MSWI boilers conditions characterized by a temperature gradient at the metal tube in the presence of corrosive gases and ashes. The presented corrosion tests were realized on carbon steel at fixed metal temperature (400 °C). The influence of the flue gas temperature, synthetic ashes composition, and flue gas flow pattern were investigated. After corrosion test, cross sections of tube samples were characterized to evaluate thickness loss and estimate corrosion rate while the elements present in corrosion layers were analyzed. Corrosion tests were carried out twice in order to validate the accuracy and reproducibility of results. First results highlight the key role of molten phase related to the ash composition and flue gas temperature as well as the deposit morphology, related to the flue gas flow pattern, on the mechanisms and corrosion rates.     

Accelerated corrosion of five commercial steels under a ZnCl2-KCl deposit in a reducing environment typical of waste gasification at 673-773 K

The corrosion of five Fe-Cr commercial steels containing 0-18 wt.% Cr at 673-773 K has been studied in a reducing H₂-HCl-CO₂ atmosphere under a ZnCl₂-KCl deposit typical of waste gasification environments. In comparison with the behavior of the same steels in a similar gas mixture without salt deposit, all steels suffered from accelerated corrosion induced by the salt and formed porous scales with poor adherence to the underlying steels. Some Cl was detected close to the steels/scale interface, indicating that Cl-containing species were able to go through the scale down to the metal matrix. Even though the corrosion rates generally decreased with increasing Cr content, the high-Cr stainless steel SS304 was still unable to provide a good corrosion resistance against the ZnCl₂-KCl deposit. The reaction mechanisms are discussed on the basis of thermodynamic considerations and of the “active oxidation” model.     

Corrosion characteristics of boronized AISI 8620 steel in oil field water containing H<Subscript>2</Subscript>S

The corrosion behaviours of non-boronized and boronized AISI 8620 steels in both oil field water and H₂S-saturated oil field water have been investigated by means of immersion test, electrochemical method, X-ray diffraction and scanning electron microscopy. The experimental results show that boronized steel has better corrosion resistance to as-received oil field water and H₂S-saturated oil field water than the non-boronized steel. Both non-boronized and boronized steels have reacted with H₂S and form corrosion film of FeS, which could not retard the corrosion process due to pores and cracks in the FeS film, and large scale of pitting corrosion is found on non-boronized AISI 8620 steel surface. The immersion corrosion of non-boronized AISI8620 in both corrosion solutions can be divided into two stages: the rapid corrosion stage with high slope and the gradual corrosion stage with low slope, corresponding to uniform corrosion and corrosive product scaling off the surface, respectively. The better corrosion resistance shown by boronized AISI 8620 steel is ascribed to lower corrosion current as compared with the non-boronized AISI8620 steel.     

Effect of chromium on the corrosion behavior of low alloy steel in sulfuric acid

The effects of a chromium (Cr) addition on the corrosion resistance of low alloy steel used in flue gas desulfurization systems were examined by electrochemical (potentiodynamic polarization tests, linear polarization measurements and electrochemical impedance spectroscopy) and weight loss measurements in a 10 wt% H₂SO₄ solution at room temperature. All measurements revealed a decrease in corrosion rate with increasing Cr content. SEM, EPMA and XPS examinations of the corroded surfaces after the immersion test indicated that 0.6% Cr addition decreased corrosion damage to the steels because protective Cr oxides formed in all the rust layers and Fe oxides dominated over Fe sulphate compounds in the inner rust layers.     

High-Temperature Corrosion of A210-C Carbon Steel in Simulated Coal-Combustion Atmospheres

Corrosion of A210 C carbon steel was investigated under three different conditions: (1) using synthetic gas mixtures with varying amounts of O₂, SO₂, and HCl; (2) in the flue gas introduced from a coal-fired fluidized-bed combustor (FBC), with and without a deposit cover; and (3) within the freeboard of the FBC firing two different coals. Generally, the oxide scale formed in the temperature range of 370–560°C was mainly Fe₂O₃. The oxidation rate was significantly increased with an increase in temperature. In the synthetic gas mixtures SO₂ and HCl caused scale damage by weakening of the scale-metal interface. The combination of the gases can greatly accelerate the metal corrosion. In the FBC flue gas (condition 2), deposit additions exerted a significant effect on accelerating the metal corrosion. In the FBC freeboard tests (condition 3), the atmospheres containing a higher content of SO₂ and HCl increased the metal corrosion in comparison to corrosion in low-SO₂ and HCl-containing atmospheres. In the former case, an S-enriched phase or pits formed in the residual wastage at or near the metal. This may be the cause of wastage spallation. The HCl effect is discussed but is not conclusive. Moreover, in the FBC system, erosion and deposition appeared to play important roles in exaggerating metal recession.     

Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

Oxidation Behavior of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Spinel-Coated Interconnects in Wet Air

Corrosion of boilers and heat exchangers is accelerated in the presence of vanadium, sodium, and sulfur from low-grade fuels. Several iron- and nickel-based alloys were immersed in 60 mol% V₂O₅–40Na₂SO₄ salt for 1000 h in order to investigate their degradation behavior at 600 °C in air. Materials performance was analyzed by means of substrate recession rate and metallographic characterization. Their corrosion mechanism is characterized by the formation of a sulfide/oxide layer adjacent to the metal, the dissolution of scale oxides in the molten deposit, and their precipitation near the outer surface of the deposit. High Ni- and Cr-containing alloys show the lowest metal loss rates. Al addition was detrimental due to low-melting eutectic AlVO₄–V₂O₅ formation. Fe–Cr-based alloys showed the highest metal loss rates. In such alloys, high Cr additions (above 20%) did not improve the performance due to the negative synergetic effect by simultaneous dissolution of Fe₂O₃ and Cr₂O₃. The predominant salt composition at the corrosion front varied from vanadate rich to sulfate rich during the exposure. This change in the attacking salt makes it difficult to find a protective material for mixed sulfate–vanadate-induced corrosion.     

Condensed phase corrosion of P91 and DSS 2205 steels at advanced oxygen-fired pressurized fluidized bed combustion plants

To achieve the targets of high energy efficiency and reduced CO₂ emission, advanced oxygen-fired pressurized fluidized bed combustion technology is being developed. The generated flue gas condensates are very corrosive, but very limited information is available to select appropriate alloys for the cost-effective construction and long-term safe operation of flue gas components. Thus, this study investigated the corrosion performance of P91 and DSS 2205 steels in the simulated condensates at 60°C–150°C. The dominant reactions on the two steels were considerable oxide formation and high chemical dissolution of the formed oxides instead of localized pitting. The increase in temperature leads to an exponential increase in the long-term corrosion rates of the steels. Benefited from its high Cr and Mo contents, DSS 2205 steel exhibited much better corrosion resistance, and the formed surface scales consisted of inner Fe-enriched and outer Cr-enriched oxides in which Cr₂O₃ was transformed into Cr(OH)₃ with the increase in temperature. The corrosion products on P91 steel consisted of inner Cr–Fr–Mo oxides and outer Fe-enriched oxides, which were porous and unable to protect the steel.     

Hochkorrosionsbeständige Sonderstähle für Rauchgas-Entschwefelungsanlagen

Highly corrosion resistant special steels for flue gas desulfurisation plants Highly corrosion resistant stainless steel grades have been proved under the severe corrosion conditions existing in flue gas desulfurisation scrubbers (FGD). Besides general corrosion pitting, crevice corrosion and eventually stress corrosion cracking can occur. Thus highly alloyed special steels must be used. Steel grades with a minimum content of 2.75% Mo are essential. At higher chloride levels and decreasing pH-values higher alloyed stainless steels containing up to 6% Mo are necessary. Some of these special steels are described in view to their composition and mechanical properties; their corrosion behavior has been tested under laboratory and field conditions. The use of nitrogen alloyed grades has been shown of remarkable advantage. Nitrogen additions enhance the mechanical properties and structure stability. Furthermore the precipitation of deleterious intermetallic compounds during heat treatment will be delayed by nitrogen additions, thus e.g. multi layer weldings can be carried out with higher security in view to corrosion resistance and mechanical properties. Materials selection for the different scrubber systems will be illustrated by examples. Up to now experiences about stainless steel components in FGD plants are taken into consideration. Welding with distinctly higher alloyed filler metal at the medium-touched side has been well proved in view to adequate corrosion properties.     

High temperature corrosion of hot-dip aluminized steel in Ar/1%SO<Subscript>2</Subscript> gas

Carbon steels were hot-dip aluminized in Al or Al-1at%Si baths, and corroded in Ar/1%SO₂ gas at 700-800 °C for up to 50 h. The aluminized layers consisted of not only an outer Al(Fe) topcoat that had interdispersed needle-like Al₃Fe particles but also an inner Al-Fe alloy layer that consisted of an outer Al₃Fe layer and an inner Al₅Fe₂ layer. The Si addition in the bath made the Al(Fe) topcoat thin and nonuniform, smoothened the tongue-like interface between the Al-Fe alloy layer and the substrate, and increased the microhardness of the aluminized layer. The aluminized steels exhibited good corrosion resistance by forming thin α-Al₂O₃ scales, along with a minor amount of iron oxides on the surface. The interdiffusion that occurred during heating made the aluminized layer thick and diffuse, resulting in the formation of Al₅Fe₂, AlFe and AlFe₃ layers. It also smoothened the tongue-like interface, and decreased the microhardness of the aluminized layer. The non-aluminized steel formed thick, nonadherent, nonprotective (Fe₃O₄, FeS)-mixed scales.     

腐蚀产物MgO对Fe基合金在固态和熔融NaCl-MgCl2 中高温耐蚀性能的影响

熔融氯化盐是下一代聚光式太阳能热发电站（第3代CSP）候选传热和储热介质,含MgCl₂的熔融氯化盐对金属传热管道和储热容器腐蚀后在其表面形成MgO,MgO对管道耐腐蚀性能影响尚不清楚。通过对比碳钢和3种Fe-Cr-Ni合金在固态（345 ℃）和熔融NaCl-MgCl₂（445和545 ℃）中的腐蚀行为,分析了MgO对4种试样在不同温度下的腐蚀行为机理。结果表明,在固态NaCl-MgCl₂中,碳钢表面MgO壳致密且连续,可以保护试样免受腐蚀。在熔融NaCl-MgCl₂中,4种试样表面也生成了致密的MgO壳,但它因热应力作用而开裂和剥落,熔融盐沿着氧化膜裂纹渗入MgO/基体界面,发生化学-电化学联合腐蚀反应,不能保护试样免受该熔盐腐蚀。     

Corrosion of HR3C heat exchanger alloy in a biomass fired PF utility boiler

Detailed microscopic examinations have been conducted on two, temperature‐regulated probes (commercial HR3C heat exchanger alloy) after being exposed to biomass flue gas inside a PF boiler for 3770 h at 600°C and 650°C respectively. Corrosion of the tube proceeds via scale formation and internal element depletion. Three characteristic types of internal corrosion have been identified depending on their position relative to the flue gas passage and deposit/flue gas chemistry. Severe, mainly internal corrosion occurs at down‐stream locations where higher potassium chloride content exists within the deposit. Corrosion mechanisms corresponding to each type of internal corrosion have been proposed based on further laboratory tests and thermodynamic analysis. The increased temperature (650°C) causes slightly higher material wastage for the alloy.     

Degradation behavior of Ni<Subscript>3</Subscript>Al plasma-sprayed boiler tube steels in an energy generation system

Boiler steels, namely, low-C steel, ASTM-SA210-Grade A1 (GrA1), 1Cr-0.5Mo steel, ASTM-SA213-T-11 (T11) and 2.25Cr-1Mo steel, ASTM-SA213-T-22 (T22) were plasma sprayed with Ni₃Al. The alloy powder was prepared by mixing Ni and Al in the stoichiometric ratio of 3 to 1. The Ni-22Cr-10Al-1Y alloy powder was used as a bond coat, with a 150 μm thick layer sprayed onto the surface before applying the 200 μm coating of Ni₃Al. Exposure studies have been performed in the platen superheater zone of a coal-fired boiler at around 755 °C for 10 cycles, each of 100 h duration. The protection to the base steel was minimal for the three steels. Scale spallation and the formation of a porous and nonadherent NiO scale were probably the main reasons for the lack of protection. In the case of T22-coated steel, cracks in the coatings have been observed after the first 100 h exposure cycle.     

Untersuchungen über Korrosionsvorgänge an Stahl nach einmaliger Chlorid-Kontamination

Investigations into the corrosion processes on steel after one-time's chloride contamination Investigation of chloride adsorption and desorption on mild steel St 37 (1.0120) and austenitic stainless steel X 5 CrNi 189 (1.4301) showed chloride adsorption layer, determined radiochemically, depends upon the material, the state of metal surface layer, and its pretreatment. Ground surface of St 37 retains comparatively less chloride ions on washing than an unpretreated specimen. On the other hand the chloride could be removed from the austenitic steel by rinsing twice in unagitated washing medium which lowered the chloride content to below the detection level of 0,02 m?g Cl^?/cm². Corrosion behaviour of specimens of 1.4301 in autoclave showed no difference with regards to corrosion in the test cycles of 300 and 400 h at 150 and 200° C whether or not the specimens were contaminated by chloride. Mild steels whose state of surface is similar to St 37 should be guarded against chloride contamination to prevent delayed damage. In case of steel similar to 1.4301 with regards to their surface characteristics cleaning the surface is relatively easy. Long time tests to assess risks of delayed damage are in progress and will be communicated later.     

Comparison of corrosion scales formed on KO8OSS and N8O steels in CO2/H2S environment

Abstract

Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were applied to analyse the microstructure and composition of the corrosion scale formed on KO80SS and N80 tubes with carbon dioxide (CO₂) and hydrogen sulphide (SO₂). The corrosion scales of both KO80SS and N80 tubes were of the double layer structure, and not only uniform corrosion but also localised corrosion was observed. The crystal of the surface layer is laminar. The main phase in the outer layer is calcium carbonate (CaCO₃), and the inner scale consisted of iron carbonate (FeCO₃) for KO80SS steel and FeS_0·9 with a little amount of FeCO₃ for N80 steel respectively. Additionally, the electrochemical techniques were used to investigate the characteristics of the corrosion scales. The results indicated that the polarisation resistance R_p of KO80SS steel film was nobler than that of N80 steel film. Finally, the corrosion current I_corr of KO80SS steels was lower than that of N80 steels. Corrosion scale of KO80SS tube steels is more protective to the matrix than that of N80 tube steels.     

Simulation of Fe-Cr-X Alloy Exposed to an Oxyfuel Combustion Atmosphere at 600 °C

In coal-fired power plants using oxyfuel combustion process with carbon capture and sequestration, instead of air, a mixture of oxygen and recirculated flue gas is injected in the boiler. A series of steels were exposed to CO₂-SO₂-Ar-H₂O gas mixtures at 600 °C for 1000 h to compare their high temperature corrosion behavior. During the corrosion process, carburization, decarburization and recrystallization were observed underneath the oxide scale depending on the gas mixture and alloy composition. The conditions that lead to carburization are not yet completely understood, but decarburization can be simulated using thermodynamic and kinetic models. In this work, the results of these simulations are compared with measured values for one of the alloys that displayed a decarburized region. Since the mobility of carbon in the scale is not known, two strategies were adopted: simulation of alloy-atmosphere contact; and estimation of the carbon flux to produce the observed decarburization. The second approach might give an insight on how permeable to carbon the scale is.     

Einfluß von Passivierungs- und Kohlenstoffschichten auf austenitischen CrNiMo-Stählen auf die Beständigkeit gegen Lochfraß und Spannungsrißkorrosion

Effects of passivation and carbon films on austenitic CrNiMo steels on their piting and stress corrosion resistance The influence of passive film and combinations of a passivation and a carbon layer on the resistance to pitting and SCC of austenitic CrNiMo steels has been investigated in physiological sodium chloride solution (Tyrode solution) at pH 6.9 to 7.4 at 37 ± 1°C. The passive film was obtained after electrolytic polishing in H₃PO₄ + H₂SO₄ + C₆H₅NHCOCH₃ + oxalic acid + corrosion inhibitor CS by treatment with 40% nitric acid the carbon film was obtained by CVD. Impurities in the steel (non-metallic inclusions) and the different metallic phases were investigated and the chemical composition of the passive film was determined by quantitative analysis. The resistance to pitting of the steel with and without passive film was determined potentiodynamically in Tyrode's solution at 37 ± 1°C. The resistance to SCC was determined in Tyrode's solution at 37 ± 1°C, in neutral glycerole and in boiling magnesium chloride solution at 154 ± 1°C and evaluated in terms of K_σ and K_τ. The corrosion damage was investigated by optical and scanning electron microscopy. The investigations have revealed that the different surface conditions considerably improve the pitting and SCC resistance of the steels in the media used in this work, so that they make possible the use of these materials as surgical implants.     

Role of chlorides in hot corrosion of a cast Fe–Cr–Ni alloy. Part I: Experimental studies

High alloy stainless steels are often used in corrosive, high temperature applications because they form a protective, adherent Cr₂O₃ scale. When the environment in such applications includes condensed molten salts, especially alkali sulfates and alkali chlorides, these alloys are likely to undergo hot corrosion, even at moderate temperatures compared to their typical maximum application temperature. The chemical (or electrochemical) reactions and transport modes for hot corrosion in a system involving a multi-component alloy and a multi-component salt are complex, but some insight can be gained with the help of a multi-component thermochemical model to identify major reactions. The present work consists of two parts: (a) experimental measurements of hot corrosion rates and characterization of corrosion products on a commercial, cast super-duplex stainless steel (HH), which result from exposure to thin deposits of a mixture of alkali sulfates and alkali chlorides, and (b) an evaluation of possible corrosion reactions by a multi-component thermochemical model. In laboratory hot corrosion tests alloy coupons experienced rapid oxidation rates, penetration of the oxide scale, scale blistering, and scale spallation (on cooling). Compared to simple air oxidation, alkali sulfate deposits increased the corrosion rate by a factor of about 200, but mixtures of alkali sulfate and alkali chloride increased the rate by about 20,000 times. A principal goal of the study was to identify the role of alkali chlorides in accelerating hot corrosion.     

Optimisation of in‐service performance of boiler steels by modelling high‐temperature corrosion

The main objective of the EU OPTICORR project is the optimisation of in‐service performance of boiler steels by modelling high‐temperature corrosion, the development of a life‐cycle approach (LCA) for the materials in energy production, particularly for the steels used in waste incinerators and co‐fired boiler plants. The expected benefits of this approach for safe and cost effective energy production are: ‐ control and optimisation of in‐service performance of boiler materials, ‐ understanding of high‐temperature corrosion and oxidation mechanisms under service conditions, ‐ improvement of reliability to prevent the failure of components and plant accidents and ‐ expanding the limits of boiler plant materials by corrosion simulations for flexible plant operation conditions (steel, fuel, temperature etc.). The technical aim of the EU OPTICORR project is the development of modelling tools for high‐temperature oxidation and corrosion specifically in boiler conditions with HCl‐ and SO₂‐containing combustion gases and Cl‐containing salts. The work necessitates thermodynamic data collection and processing. For development and modelling, knowledge about the corrosion mechanisms and exact data are needed. The kinetics of high‐temperature oxidation and corrosion are determined from laboratory thermo‐gravimetric tests (TG) and multi‐sample exposure tests. The materials studied are typical boiler tubes and fin‐steels: ferritic alloys, the austenitic steel T347 and the Ni‐based alloy Inconel 625. The exposure gases are dry air, air with 15 vol‐% H₂O, and with 2000 ppm HCl and 200 ppm SO₂. The salt deposits used are based on KCl‐ZnCl₂ and Ca, Na, K, Pb, Zn‐sulfates. The test temperatures for exposures with deposits are 320 and 420°C and, for gas exposures, 500 to 600°C. At present the tools being developed are ChemSheet based programmes with a kinetic module and easy‐to‐use interface and a more sophisticated numerical finite‐difference‐based diffusion calculation programme, InCorr, developed for prediction of inward corrosion and internal corrosion. The development of modelling tools for oxidation and high‐temperature corrosion was started with thermodynamic data collection for relevant systems and thermodynamic mappings. Further, there are needs to develop the simulation model and tool for salt‐induced hot corrosion based on the ChemSheet approach. Also, the work on modelling and simulating with the InCorr kinetic modelling tool will be continued to demonstrate the use of the tool for various steels and alloys in defined combustion environments.