Hot Corrosion Rate Research Articles

A simplistic model to study the influence of film cooling on low temperature hot corrosion rate in coal gas/syngas fired gas turbines

The present paper is an attempt to establish an analytical study of low temperature hot corrosion (LTHC) in the context of high temperature turbines using coal gas or syngas with trace amount of sulfur in the fuel. LTHC in the presence of film cooling is explored using a simple analytical approach and heat and mass transfer analogy with film cooling air temperatures from 450 °C to 550 °C, hot gas stream temperature of 1425 °C and 0.5% of sulfur concentration in the fuel. For all the cooling air temperatures studied here, film cooling augments corrosion when the mainstream velocity is high. However, reduction in the mainstream velocity results in the suppression of corrosion after employing film cooling. A sharp peak in corrosion rate close to the cooling hole (<10 s) is also seen. As the base super alloy is vulnerable to hot corrosion in this region, designers should consider the high corrosion rate seriously. The present model provides a simple baseline prediction methodology and sets direction for optimization needs.

Influence of pre-oxidation on the hot corrosion of Ti 3SiC 2 in the mixture of Na 2SO 4–NaCl melts

Polycrystalline Ti 3SiC 2 suffered from serious hot corrosion attack in the mixture of 75wt.%Na 2SO 4 + 25wt.%NaCl melts at 850 °C. In order to improve the hot corrosion resistance of this material, pre-oxidation treatment was conducted at 1200 °C in air for 2 h. A duplex oxide scale with an outer layer of TiO 2 and an inner layer of a mixture of TiO 2 and SiO 2 was formed during the pre-oxidation. Because the outer oxide layer of the pre-oxidation treated specimens could inhibit hot corrosion process, they exhibited good hot corrosion resistance in the mixture of 75wt.%Na 2SO 4 + 25wt.%NaCl melts at 850 °C for 50 h. However, during the hot corrosion the outer layer of TiO 2 would degrade gradually. Once the outer layer damaged, the hot corrosion rate increased sharply, the corrosion behavior was similar to Ti 3SiC 2 corroded under the same conditions. The microstructure and phase compositions of the hot corrosion samples were investigated by SEM/EDS and XRD.

Hot corrosion of carbon-alloyed Fe 3Al-based iron aluminides

The oxidation and hot corrosion behavior of two Fe 3Al-based iron aluminides, Fe–25Al and Fe–27.5Al–3.7C (at.%) have been studied at 1100, 1225 and 1330 K. Hot corrosion studies were conducted by coating the specimen surfaces with 2.5±0.2 mg cm −2 of Na 2SO 4 prior to exposure in pure oxygen. The oxidation kinetics of the carbon-alloyed iron aluminide were generally slower than that of the binary alloy. Alumina was identified in the scale after oxidation of both the alloys. The rates of hot corrosion were generally higher than the rates of oxidation for both the alloys. The presence of α-Fe 2O 3 in addition to alumina was indicated by X-ray diffraction analysis of the scales present on the surface of the samples after hot corrosion. Fourier transform infrared spectra from the spalled scales in hot corrosion divulged the presence of α-Al 2O 3, α-Fe 2O 3 and sulfate. Cross-sectional microscopy revealed that the scale–metal interfaces were pitted under hot corrosion conditions and the pits contained aluminum sulfide. Aluminum sulfide was also identified along the grain boundaries in the binary aluminide matrix below the scale–metal interface. The hot corrosion process has been explained based on sulfide formation and its subsequent oxidation. The lower rate of hot corrosion in the carbon-alloyed iron aluminide has been related to the blocking effect of carbides, present along the grain boundaries, for the penetrating sulfur.

Hot corrosion of Fe3Al

The oxidation and hot corrosion behavior of the binary iron aluminide, Fe-25Al (at.%), has been studied at 1100 K, 1225 K and 1330 K. Hot corrosion studies were conducted by coating the specimen surfaces with 2.5 ± 0.2 mg/cm2 of Na2SO4 prior to exposure in pure oxygen. Parabolic rate constants were obtained from weight gain data. The faster kinetics in the initial stages of oxidation have been related to the formation of θ-Al2O3 and the slower kinetics in the later stages of oxidation, to the formation of α-Al2O3. The overall rate of hot corrosion was higher than that of oxidation at all the temperatures. The presence of α-Fe2O3 in addition to alumina was indicated by XRD analysis of the scales present on the surface of the samples after hot corrosion. FTIR spectra from the spalled scales in hot corrosion divulged the presence of α-Al2O3, α-Fe2O3 and sulfate. Cross-sectional microscopy revealed that the metal-scale interfaces were pitted in hot corrosion conditions and the pits contained aluminum sulfide. Sulfides were also identified along the grain boundaries in the intermetallic near the scale-metal interface. The hot corrosion process has been explained based on sulfide formation and its subsequent oxidation.

The corrosion of Nb-modified Ti3Al in a combustion gas with and without Na2SO4−NaCl deposits at 600–800°C

The corrosion behavior of a Nb-modified Ti3Al intermetallic compound containing 11 at.% Nb in a simulated combustion gas with and without deposits of a Na2SO4−NaCl mixture was examined at 600–800°C for times up to four days. In the absence of salt deposits the corrosion rates were rather low and increased only slightly with temperature, producing very thin scales of mixed oxides of Ti, Al, and Nb without sulfides. The presence of the salt deposits produced higher weight gains during an initial stage of one to two days at 600 and 700°C, after which the reaction stopped. A more important and longlasting effect was observed instead at 800°C, when the kinetics of hot corrosion became nearly linear. The scales formed by hot corrosion were complex mixtures of various corrosion products at all temperatures and showed a porous outer region containing a mixture of unreacted salts with oxides (mainly TiO2), an intermediate region of a mixture of variable composition of oxides of the three metals, and a TiO2-rich layer beneath it. At 800°C the scales tended to form a thin, discontinuous Al2O3-rich layer in the middle and contained an additional innermost region presenting a large concentration of sulfur, very likely as Nb and Ti sulfides. The high rate of hot corrosion at 800°C is attributed to the appearance of sulfides in the inner region of the scale and to a more efficient scale fluxing.

Hot Corrosion Behaviour of Inconel-600 Alloy in Presence of NaCl and Na,CO, at 850 °C/ Hochtemperaturkorrosion der Legierung Inconel-600 in NaCl und Na,CO, bei 850 °C

The hot corrosion behavior of Inconel-600 industrial alloy has been investigated in presence of NaCl and Na 2 CO 3 at 850 °C in flowing air. The hot corrosion rate and effect of sodium salts deposition were studied. The scale morphologies were determined on the basis of X-ray diffraction and scanning electron microscopy techniques. It has been shown that the specimens corrode at a different rate in presence of NaCl and Na 2 CO 3 at 850 °C

High Temperature Hot Corrosion Control by Fuel Additives (Contaminated Fuels)

Abstract : The potential of fuel additives to minimize corrosion of blade-material has been analyzed by the following series of steps: first, a computer program was employed for an equilibrium-thermodynamic prediction of condensed solution composition; next relevant physico-chemical properties of the molten solution were estimated based on predicted equilibrium composition; then oxide solubility and dissolution rates are calculated for this solution in contact with various solid oxides. Finally, we compare our predicted oxide dissolution-rate results with available experimental hot corrosion rate data, seeking verification of the model for the rate determining process - i.e., oxide dissolution - in hot corrosion. Keywords: Fuel additives, Blade material, Molten solution, Thermodynamic corrosion prediction, Chemical equilibrium.

Transport Considerations in the Hot Corrosion of Ni by Molten Alkali Carbonates

The hot corrosion of Ni induced by molten alkali carbonates has been studied to elucidate the mechanism of attack and in particular to determine whether the Rapp‐Goto theory of fluxing is applicable to this system. Two principal rate‐determining processes were considered in some detail: transport of oxidant from the gas phase through the corrosion products, including the molten salt phase, to the site of active oxidation, and dissolution/reprecipitation of oxide within the corrosion product layer owing to a solubility gradient. To help evaluate these processes, we measured the solubility of in various carbonate salts and we experimentally characterized the scales with respect to average porosity and tortuosity. The kinetics of hot corrosion were measured by TGA under conditions expected to affect oxidant transport and/or fluxing. Experimental rates of oxidation were then compared with calculated maximum rates according to the two rate‐determining processes to evaluate the feasibility of rate control by one or the other process. It was concluded that fluxing was very likely to be rate controlling in the early stages of hot corrosion, but later, when the actual hot corrosion rate had increased considerably, oxidant transport was controlling. The distribution of salt within the pores of the scale changes with time, and this is a key issue in determining the rate of reaction.

A review of recent studies of the role of zinc as an inhibitor of hot corrosion from molten sulphates

Hot corrosion of metals in the temperature range 700–900°C is caused by the presence of sulphate deposits on the surface formed from the reaction between sea salt and sulphur trioxide in marine turbine engines. This paper reviews investigations of the effects of adding zinc in order to inhibit this form of corrosion. The presence of zinc in the salt deposits served to modify the rate of hot corrosion significantly. Its role has been studied with respect to the acid-base fluxing theory and in the presence of excess NaCl. The zinc reduced the ability of the corrosive salt mixtures to dissolve the protective oxide layers on the alloys and also lowered the electrochemical potential of the corrosion reaction itself.

COOPER ALLOY 22W

Abstract Cooper Alloy 22W is a high strength, heat resistant casting alloy with a low creep rate. It is recommended for heat applications where stress and hot gas corrosion rate are very high. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties. It also includes information on corrosion resistance as well as casting, heat treating, machining, joining, and surface treatment. Filing Code: SS-146. Producer or source: Cooper Alloy Corporation.