Fireside Corrosion Research Articles

Creep stress induced pit-to-crack transition of Super304H in fireside corrosion

Parallel experiments on creep-rupture and corrosion of Super304H were conducted at 650 ℃ in a simulated ultra-supercritical fireside environment to study the impact of creep stress on fireside corrosion. The corrosion products, corrosion pits, grain structure and phase composition, and surface cracks of creep-corrosion and corrosion samples were compared. Results show that stresses not only affect the formation and growth of corrosion products, but also widen the recrystallization zone in corrosion affected area. A model of surface cracking from corrosion pits under the synergy of fireside corrosion and creep stress was proposed.

High Temperature Air and Steam Oxidation and Fireside Corrosion Behavior of 304HCu Stainless Steel: Dichotomous Role of Grain Boundary Engineering

High Temperature Air and Steam Oxidation and Fireside Corrosion Behavior of 304HCu Stainless Steel: Dichotomous Role of Grain Boundary Engineering

Studying the formation of nodules in a Ni-Fe-Cr based superalloy used in fireside corrosion under the impact of creep stress

Creep rupture tests on a Ni-Fe-Cr based superalloy were conducted in a custom-built rig at 750 °C under simulated fireside corrosion environment. The impact of creep stress on corrosion was investigated by controlling load level (without loading, loads of 120–195 MPa). The phase composition, microstructure, and morphology of surface corrosion products and corrosion-affected area were studied. The mechanism responsible for the production of surface nodules and the observed uniform oxide scale during fireside corrosion are discussed. Results show that the presence of creep stress promotes the development of surface nodules and increases the corrosion-affected area.

Corrosion and isothermal interdiffusion properties of Ni–Al coatings under simulated biomass combustion conditions

The main issue with using biomass fuels in power plants is fireside corrosion. Applying corrosion-resistant coatings is one potential choice. The current study examines how a Ni–Al diffusion coating on austenitic stainless steel reacts to corrosion and interdiffusion (TP347H). Nickel was electrolytically deposited to create Ni–Al coatings, which were then pack aluminized at 650 °C. With an internal layer of Ni and a Ni2Al3 outer layer, a homogenous and dense Ni–Al coating was created. Samples were heated to 560 °C for 168 h in an environment designed to simulate the combustion of biomass. A localized corrosion attack was the result. By using isothermal heat treatment in static air at 650 °C or 700 °C for up to 3000 h, interdiffusion was examined. In the course of the interdiffusion process, the Ni2Al3 eventually changed into NiAl and Ni3Al. At both temperatures, porosity formed at the intersection of the Ni–Al coating and the Ni layer and grew with time.

Creep rupture of a Ni-Fe based model alloy under simulated fireside corrosion in coal-fired boilers

The influence of fireside corrosion on creep rupture of a Ni-Fe based model alloy was investigated by comparing the rupture life, fractography and microstructure of specimens in fireside corrosion and air. Results showed that the rupture life at 140 MPa in fireside corrosion is only 40% of that in air. The internal sulfidation along grain boundaries promoted the initiation of intergranular cracks, and was responsible for the premature rupture. The surface collapse in fireside corrosion was attributed to the outward propagation of internal cracks, while surface cracks in air originated from the formation of recrystallized grains in the surface layer.

Impact of Fireside Corrosion on the Creep Rupture Life and Oxide Scale Structure of a Super304H Boiler Tube

The creep rupture life of commercial Super304H in fireside corrosion (coal ash and flue gas) and static air environment at 650°C was investigated. Results showed that the creep rupture life under fireside corrosion conditions decreases by 26.5% to 83.3% at different stress levels, compared with that in air. The corrosion products, including their composition, morphology, and distribution, and the microstructure of substrate were characterized to research the impact of the fireside corrosion on the creep rupture life. The exfoliation of oxide scales and serious surface cracking resulted from fireside corrosion were detrimental for the creep properties of the alloy. The formation of internal sulfides promoted the initiation and propagation of intergranular cracks in the substrate during creep tests. These degradations of Super304H in fireside corrosion contributed to premature creep rupture.

High Temperature Corrosion of HVOF Coatings in Laboratory-Simulated Biomass Combustion Superheater Environments

This study examines the fireside corrosion of FeCrAl, NiCr, NiCrAlY and A625 coatings applied by ‘high velocity oxy fuel’(HVOF) and exposed to simulated biomass firing conditions (gas composition CO2, N2, SO2 and HCl). The coatings and a typical base steel alloy (T92) were exposed to simulated conditions at 600 °C for 1000 h in a laboratory scale furnace. Samples were coated with a potassium chloride deposit. Samples were then cold mounted in a low-shrinkage epoxy resin and then cross-sectioned. Corrosion was assessed by dimensional metrology comparing the coating thickness change of the samples. The cross-sections of the ‘worst’ and ‘best’ coatings were examined. Results show that all but one coating (HVOF NiCr) outperformed the T92 alloy. No coating composition or method was conclusively better. Evidence of Cr depletion as well as the formation of a sulphidation layer have been found in the exposed samples with coatings. The formation of a K2SO4 layer has also been observed on all coated specimens.

Fireside corrosion and deposition on heat exchangers in biomass combustion systems

ABSTRACT To address climate change, power plants need to switch to greener fuels. One possible fuel is biomass; a carbon neutral/low carbon fuel. However biomasses’ chemistries are both different from coal’s and vary depending on their sources, containing unique levels of the trace elements (e.g., Cl and S) capable of altering the degradation of heat-exchangers. As such, an understanding of the effects of these variations on fireside corrosion is needed. Laboratory testing exposed alloys T91 and TP347HFG in a simulated agricultural product combustion environment at 600°C (up to 1000h; 100h cycles). Three different deposits mixtures were investigated (comprised of KCl, K2SO4, Na2SO4, CaSO4 indifferent percentages) mimicking accelerated corrosion from different biomasses. Corrosion behaviour was found to be dependant on both alloy and deposit chemistries, with the two materials showing different responses. The deposit with lowest KCl showed lowest corrosion damage, while the highest KCl deposit showed more aggressive behaviour.

A comparative evaluation of waste wood and herbaceous biomass fireside corrosion behaviours

Biomass/biomass waste (BBW) is a renewable energy source fired in combustion power plants. However, challenges associated include accelerated ‘fireside’ corrosion of heat exchangers. Fireside corrosion is fuel dependent, e.g. virgin wood fuels have lower fireside corrosion risks; as such their demand and cost is high. Waste wood fuels (WWF) and herbaceous grass biomass (HGB), generate different combustion environments, influencing heat exchanger fireside corrosion damage.This paper compares the fireside corrosion of WWF and HGB fuels to improve understanding of their attack mechanisms on T91 and 374HFG. The species generated in combustion have been investigated by thermodynamic modelling. Corrosion damage has been evaluated by high temperature corrosion furnace tests at 600 °C, employing the well-established deposit recoat method based on these evolved species. Vapour condensation fluxes have been compared for both fuel categories; corrosion rates increase with this condensation flux. Deposition fluxes from HGB fuels are higher than for WWF. For example, the average KCl deposition fluxes calculated are 1566 µg/cm2h and 295 µg/cm2h per kg fuel feed respectively, explaining the comparatively greater fireside corrosion rates experienced in power plants combusting HGB fuels. The deposit chemistries generated are also different: K-containing compounds only (for HGB fuels) versus K-, Na- and Ca-containing compounds (for WWF).Evaluation of corroded samples exposed to the different deposit chemistries show increased corrosion attack to both steels under simulated WWF firing conditions than for the HGB at matched deposit flux (100 µg/cm2h).

Application of Computational Fluid Dynamics and Process Modeling to Investigate Low-Load Operation of a Subcritical Utility-Scale Boiler

Abstract To enable the inclusion of intermittent renewable energy sources on existing power grids requires base-load coal-fired power plants to operate flexibly, including fast load changes and continuous low-load operation. Low-load operation of a 620 MWe subcritical boiler is analyzed with the aid of a cosimulation methodology that incorporates a detailed 3D computational fluid dynamics (CFD) model together with a one-dimensional process model. A discretized one-dimensional model of the water/steam circuit was developed for the furnace, the radiant superheaters, and the convective/back pass heat exchangers. This is coupled with a detailed CFD model incorporating the furnace and radiant superheaters using an Eulerian–Eulerian reference frame. The coupled model is used to investigate the combustion stability, water/steam side effects, and the radiant heat-exchanger operational limits for six different burner firing arrangements at a low boiler load of 32% of maximum continuous rating. The results show that certain firing arrangements can lead to a high risk of fire-side corrosion and overheating of heat-exchanger components. Based on the analyses of combustion stability, boiler efficiency, and the safe operation of heat-exchanger components, a mixed firing arrangement with a higher secondary air mass flowrate for nonfiring burners was selected as the best operational strategy at this low load for the boiler under investigation.

Subsurface microstructural degeneration of Super304H boiler tube under the synergy of fireside corrosion and creep stress

The deterioration mechanism of creep rupture life of Super304H in a lab simulated fireside corrosion environment at 650 °C is investigated. This includes in-depth study of microstructural evolution in grain structure, elemental distribution and precipitates. Results reveal a two-thirds reduction in creep life by corrosive media. The major reason for this degradation under the synergy of high-temperature corrosion and creep stress is considered to be more serious dynamic recrystallization and martensitic transformation occurring in the surface layer. The fractography of the rupture specimens indicates creep voids coarsening is responsible for fracture in both conditions.

Investigation of the fireside corrosion of a new wrought Ni-Fe-Cr-based alloy in advanced ultra-supercritical steam boilers

The gas-phase and coal ash-related fireside corrosion behavior of a new type of Ni-Fe-Cr based superalloy GH984G and two other contrast alloys were investigated for its application under advanced ultra-supercritical (A-USC) conditions. Results showed that the oxidation activation energy of the three alloys at 600–780 °C in the flue gas environment can be ranked from large to small: C-HRA-1>GH984G>Alloy 625. In the coal ash environment, GH984G and C-HRA-1 exhibited satisfactory corrosion resistance, while Alloy 625 had weaker corrosion resistance due to the presence of molybdenum. The corrosion mechanism of GH984G and the influence of constituent elements were elucidated.

High Temperature Corrosion Stability of Ceramic Materials for Magnetohydrodynamic Generators

Abstract Corrosion by alkali metals and their compounds poses a significant challenge to the long-term material stability and service life for both metal alloys and ceramics at high operating temperatures. Chemical reactivity between alkali metals and materials underlie numerous industry challenges ranging from fireside corrosion in biomass-fired boilers to reaction with silica-based refractory ceramics in glass furnaces. The problem is particularly significant in magnetohydrodynamic (MHD) generators, where potassium or cesium compounds are introduced to improve the electrical conductivity of the working fluid. Thus, resistance to attack by alkali metal vapor is an important consideration in the selection and fabrication of ceramic electrodes and insulators for MHD generators. We evaluated several refractory ceramics to assess phase stability and known reactions with potassium and its compounds at high temperatures (T > 1,200 °C). Refractory ceramics were tested for potassium vapor corrosion using a modified ASTM standard test method with in situ monitoring of gas composition. Unlike other materials, the magnesia (MgO) and ceria (CeO2) samples did not exhibit corrosion, and no phase or mass changes were observed. This indicates that CeO2 and MgO could exhibit long lifetimes as plasma facing components in MHD generators. Ultimately, this development provides valuable data in evaluating critical materials performance issues that can attest to the viability of high temperature direct fired MHD generator applications.

A Review on the Material Development and Corresponding Properties for Power Plant Applications

Abstract Over the past decade, increased demand for energy generation at affordable rates and in a clean state has been the driving force for material development for energy generation. In developing countries, coal-fired thermal power plants are the major source of electricity generation. However, coal-fired plants induce a higher carbon footprint and contribute to greenhouse gas emissions. Hence, several strategies have been formulated to mitigate environmental pollution while maintaining high energy output. This review comprehensively presents the historical material development process that led to an increase in the efficiency of a coal-fired power plant. Candidate material identification and application, essential alloying elements, and their impact on service life are comprehensively reviewed. Adapting the ultra-supercritical (USC) and advanced ultra-supercritical (A-USC) power generation technology requires the development of new alloys that can withstand harsh operating conditions (700–725°C and 25–35 MPa), which demands the replacement of conventional materials (ferritic/austenitic steel) with better ones. The candidate alloys should be evaluated across several key issues such as high-temperature material response to loading, creep strength, high-temperature oxidation, and component fabricability. The evaluation of creep strength data suggests that nickel-based alloys like Inconel alloy 282 are preferred because of the high stability of the γ′ phase. Moreover, at higher temperatures, nickel-based alloys are preferred over ferritic/austenitic steel alloys because of their superior corrosion resistance. The fireside corrosion and high-temperature oxidation resistance are governed by the formation of different oxide layers depending on the temperature, pressure, and alloy content. Furthermore, the weldability of different candidate alloys is discussed in detail, along with the microstructural evolution in the heat-affected zone. This review also focuses on the cost of constructing a greenfield USC/A-USC power plant. Lastly, a perspective on new alloy development for A-USC/USC application is presented, and the microstructure and property relationships of these alloys are discussed.

Fireside Corrosion of Heat Exchanger Materials for Advanced Solid Fuel Fired Power Plants

To address the challenge of climate change, future energy systems need to have reduced greenhouse gas emissions and increased efficiencies. For solid fuel fired combustion plants, one route towards achieving this is to increase the system’s steam temperatures and pressures. Another route is to co-fire renewable fuels (such as biomass) with coals. Fireside corrosion performance of two candidate superheater/reheater alloys has been characterised at higher heat exchanger surface temperature. Samples of the alloys (a stainless steel, Sanicro 25 and a nickel-based alloy, IN740) were exposed in fireside corrosion tests at 650 °C, 700 °C and 750 °C, in controlled atmosphere furnaces using the ‘deposit recoat’ test method to simulate superheater/reheater exposure for 1000 h. After exposure, the samples were analysed using dimensional metrology to determine the extent and distributions of corrosion damage in terms of surface recession and internal damage. At 650 °C, the stainless steel and nickel-based alloy performed similarly, while at 700 °C and above, the median damage to the steel was at least 3 times greater than for the nickel-based alloy. Optical and electronic microscopy studies were used to study samples’ damage morphologies after exposure. Intergranular damage and pits were found in sample cross sections, while chromium depletion was found in areas with internal damage. For high-temperature applications, the higher cost of the nickel-based alloy could be offset by the longer life they would allow in plant with higher operating temperatures.

Non-Destructive Diagnostic Methods for Fire-Side Corrosion Risk Assessment of Industrial Scale Boilers, Burning Low Quality Solid Biofuels—A Mini Review

The use of low-emission combustion technologies in power boilers has contributed to a significant increase in the rate of high-temperature corrosion in boilers and increased risk of failure. The use of low quality biomass and waste, caused by the current policies pressing on the decarbonization of the energy generation sector, might exacerbate this problem. Additionally, all of the effects of the valorization techniques on the inorganic fraction of the solid fuel have become an additional uncertainty. As a result, fast and reliable corrosion diagnostic techniques are slowly becoming a necessity to maintain the security of the energy supply for the power grid. Non-destructive testing methods (NDT) are helpful in detecting these threats. The most important NDT methods, which can be used to assess the degree of corrosion of boiler tubes, detection of the tubes’ surface roughness and the internal structural defects, have been presented in the paper. The idea of the use of optical techniques in the initial diagnosis of boiler evaporators’ surface conditions has also been presented.

Comparing fuel additives for fireside corrosion inhibition in pulverised fuel boilers using thermodynamic modelling

This study presents a method for comparing corrosion inhibiting fuel additives using the thermodynamic modelling software FactSageTM. Two biomass ashes were investigated while using a range of loadings of three additives: Fe-based additive and two coal ashes, one with an 80% Si and Al contents and one with less than 50% of Si and Al contents but having a significant Ca content. The metrics used for analysis were the formation of various corrosive compounds and by-products. The Fe-based additive could inhibit the formation of corrosive species but not as well as either of the coal ashes, as it was key to increase the Si and Al content of the deposits. The coal ash with the greatest Si and Al content proved most capable of inhibiting fireside corrosion, while the contaminants present in the other coal ash proved detrimental to reducing the production of certain corrosive species. This method can be used as a qualitative predictive tool by research and development teams to quickly and economically understand the consequences of utilising fuel additives on ash chemistry or of changes in proportion/sources of fuel in a power plant.

Invited Review Paper in Commemoration of Over 50 Years of Oxidation of Metals: Current Aspects of Deposit-Induced Corrosion

Alloys and coatings used in high temperature applications are often subject to surface degradation influenced by the presence of deposits. Typical examples are fireside corrosion in coal-fired boilers and hot corrosion of blades and vanes in the hot sections of gas turbines. Depending on the source, deposit compositions may occur in a wide range from primarily sulfate to primarily oxide and various combinations of the two. There does not seem to be evidence of severe corrosion caused by gaseous Na2SO4. Generally, severe corrosion occurs when the deposits are liquid. However, significant corrosion has been observed in some cases with deposits, which are nominally solid. Corrosion is often caused by liquid deposits, in which negative oxide solubility gradients for alloy components are established across the deposits by rapid interface reactions. Hot corrosion can occur at temperatures near 700 °C by a variety of mechanisms if a phase which allows rapid transport is formed. This includes compounds such as Na2MoO4 (even in atmospheres without SO3), MSO4–Na2SO4 solutions or metastable nanostructured phases. Calling all corrosion in this temperature regime “Type II” can be misleading with regard to mechanism. There are similarities in the underlying mechanisms of some forms of hot corrosion in the 700 °C range and fireside corrosion in that they involve synergistic fluxing. The corrosive species responsible for fireside corrosion of ferrous alloys is a liquid (Na,K)2SO4–Fe2(SO4)3 solution and not alkali iron trisulfate. The propagation mechanism involves a synergistic dissolution process of protective Cr2O3 and Fe2O3. Terms such as “Type I,” “Type II” and “gas-phase-induced acidic fluxing” should be used with care in describing mechanisms of deposit-induced corrosion.

Investigation of the mechanism for fireside corrosion in coal-fired boilers in South Africa

Furnace wall tubes from 600 MW subcritical boilers at three coal-fired power stations were sampled and the fireside deposits examined to determine the mechanism of fireside corrosion. This involved an in-depth investigation into the morphology and composition of the fireside deposits and the conditions of the furnace that enable this type of attack. SEM-EDS analysis revealed high concentrations of oxygen, iron, and sulphur, QEMSCAN and XRD analyses identified the presence of Fe3O4, Fe2O3, FeS, and FeS2. Differential thermal analysis showed thermal activities at temperatures of 500-600°C, 900-1100°C, and 1100-1250°C, which are associated, respectively, with FeS2 oxidation to FeS and Fe2O3, at 475-525°C, formation of aluminosilicates at 925-1100°C, and melting of FeS around 1190°C. The absence of sodium and potassium eliminates the contribution of molten alkali sulphates to the corrosion. The consistent coexistence of iron sulphide and iron oxide is indicative of the substoichiometric conditions in the furnace, while the detection of pyrite suggests that the coal is not completely combusted, which points to a poor combustion process. These observations were affirmed by gas analysis at one of the stations, where very high levels of carbon monoxide were measured at the furnace wall (> 14 000 ppm) and furnace exit (> 3500 ppm). The high CO concentrations are indicative of limited combustion caused by limited O2. These reducing conditions promote the formation of FeS-rich deposit, which is the corrosive species responsible for degradation.

Fireside Corrosion of P92 Steel with Mixed Sulfate Deposit at 650 °C

The fireside corrosion of a martensitic P92 steel with mixed sulfate deposit (Na2SO4/K2SO4 = 8:5, by weight) in the air at 650 °C was studied via X-ray diffraction, scanning electron microscope and electron probe microanalyzer and X-ray photoelectron spectroscopy. The results showed that P92 was highly susceptiblr to fireside corrosion attack. There were three corrosion layers: the Fe2O3 oxides in the outermost layer, the spinel oxides in the middle layer, and the mixed bands with oxide-sulfide between the inner oxide and the substrate. Sulfide particles were also distributed at the interface of martensitic lath in the matrix. There were different corrosion mechanisms in different corrosion layers. On the surface, basic fluxing of the oxide film destroyed the protective performance of the oxide film. At the forefront of the corrosion layer, the existence of sulfide and oxide mixed band and sulfide particles accelerated the mass transfer process and promoted the rapid oxidation of sulfide, making the oxidation rapidly penetrate the matrix. In nodular oxide, the formation of low melting point complex sulfate accelerated the mass transfer and oxidation process.