Corrosion of Binary Fe–Si Alloys in Reducing Oxidizing and Sulfidizing–Oxidizing Atmospheres at 700 °C

The corrosion behavior of three Fe–Si alloys containing approximately 5, 9 and 13 at.% Si was studied at 700 °C in an H₂–CO₂ gas mixture providing 10^?20 atm O₂ as well as in an H₂–H₂S–CO₂ gas mixture providing the same oxygen pressure coupled to an S₂ pressure of 10^?8 atm. All the alloys followed complex kinetics which were mostly linear for Fe–5Si, but showed one or two parabolic stages for the other two alloys. Simple oxidation produced essentially two-layered scales in which Si was confined to the alloy consumption zone in the form of silicon oxide and iron-silicon double oxide. Corrosion in the oxidizing–sulfidizing gas mixture produced scales composed of a thick external zone of pure FeS followed by an internal region containing complex mixtures of FeS with Si and Fe oxides. Internal oxidation of silicon was only observed for the oxidation of Fe–5Si in both environments. The extent of corrosion decreased in both gas mixtures with an increase in the Si content of the alloys. Finally, the addition of sulfur produced a significant increase of the overall mass gains for each alloy.     

Sulfidation behaviour of nickel aluminides

The sulfidation behaviour of four nickel aluminium alloys containing 25 to 45 at.% Al was studied over the temperature range of 750 to 950°C in a gas mixture of H₂-H₂S (0.1 to 10 vol.%). The sulfidation kinetics were determined using a continous weight gain system. The corrosion products were examined by SEM, EDX and XRD. Sulfidation in H₂-H₂S gas mixtures formed bilayered scales consisting of an outer layer of Ni₃S₂ and an inner layer of NiAl_3,5S_5,5 on all alloys regardless of the different aluminium contents. In H₂-H₂S gas mixtures the sulfidation kinetic generally followed the parabolic rate law for all alloys. The influence of aluminium content on corrosion rate was relatively low. The influence of low oxygen partial pressure on sulfidation was investigated in H₂-H₂S-H₂O mixtures. In these atmospheres the corrosion mechanism is completely different. Severe attack by rapid internal oxidation destroyed all the alloys except Ni25Al (25 at.%Al). The internal oxidation zone consisted of a mixture of γ-Ni₃Al and Al₂O₃. On the alloys containing 36 and 45 at.% Al local attack occurred, fast growing pocks were observed after an incubation period. Nickel aluminides show this corrosion phenomena only in H₂-H₂S-H₂O mixtures. An interruption of the H₂S gas flow stops the running internal oxidation. In flowing H₂-H₂O atmospheres no internal oxidation was observed. These facts prove that H₂S is necessary for starting and maintaining the internal oxidation of the nickel aluminides.     

Electric Current Effects on the Corrosion Behaviour of High Chromium Ferritic Steels

The high-temperature corrosion behaviour of the Cr containing ferritic alloys Crofer 22 APU and Crofer 22 H were investigated for their potential application as interconnects in planar-type solid oxide electrolysis cells (SOECs) operating at 800 °C for syngas production in steam/CO₂ co-electrolysis mode. To simulate the operating conditions for this application, oxidation tests in relevant atmospheres with and without electric current were conducted. The corrosion behaviour was influenced by the electric current resulting in accelerated oxidation on the negative side and suppressed oxidation on the positive side. The scale structure was influenced by a combination of atmosphere and electric current effects. The modified oxidation of the interconnect steels due to the electric current effect could have detrimental impact for the O₂ side and beneficial effect for the CO₂/H₂O side in an SOEC stack operating in co-electrolysis mode.     

High temperature Corrosion behaviour of iron aluminides and iron-aluminium-chromium alloys

The high temperature corrosion of different iron aluminides and iron-aluminium-chromium alloys containing between 6 and 17 wt% aluminium, 2 and 10 wt% chromium and additions of mischmetal has been investigated in air as well as in carburising, chlorinating and sulphidising environments. It was found that all alloys showed excellent corrosion resistance to both oxidation in air and carburisation in CH₄/H₂ up to at least 1100°C and to sulphidation in SO₂/air up to at least 850°C. In these environments the corrosion behaviour is not influenced by the concentrations of aluminium and chromium. In oxygen deficient H₂S-atmospheres, however, the corrosion behaviour depends sensitively on the aluminium and chromium concentration. At least 12 wt% aluminium in chromium-free alloys or 10 wt% aluminium in alloys containing 10 wt% chromium are required to provide sulphidation resistance at 550°C. The chlorination resistance of iron-aluminium-chromium alloys is low due to their formation of volatile aluminium chlorides.     

Imitating seasonal temperature fluctuations for the H2S corrosion of 304L and 316L austenitic stainless steels

Temperature fluctuations are inevitable in sour oil and gas production. In this study, the H₂S corrosion of 304L and 316L alloys was investigated at pH 3 and temperatures of 20–60 °C using DC and AC electrochemical techniques. Two-fold increases in the corrosion rates of both alloys were reported with increases in temperature to 60 °C. In the 304L alloy, the surface layer was observed to be 3% rougher and 34% thicker than that of the 316L alloy. The two alloys exhibited different corrosion behaviors in the temperature ranges of 20–40 °C and 40–60 °C. Although the 316L alloy revealed a greater corrosion resistance at the free potential condition, the passivation on the 304L alloy was significantly greater than that of the 316L alloy at 40 °C and 15 ppm H₂S. The FeS₂ and combined FeS₂-MoS₂ compounds contributed to the surface layer constituents in the 304L and 316L alloys, respectively. The increase in temperature kinetically provided more favorable conditions for FeS₂ than MoS₂ formation, i.e. it had a relatively constructive effect on the 304L alloy passivation.     

Oxidation-Sulphidation of iron aluminides at high temperature

An investigation has been carried out into the oxidation-sulphidation resistance of a series of novel iron aluminides, containing 5 wt% Cr, 0.2 wt% Zr, from 0 to 0.3 wt% Y and from 8 to 16 wt% Al, at 500°C to 700°C. The test environments were H₂/H₂O/H₂S gas mixtures, giving oxygen and sulphur partial pressures in the ranges of 10⁻²⁴ to 10⁻³¹ atm and 10⁻⁹ to 10⁻¹¹ atm respectively. The effects of preoxidation on the degradation resistance have also been studied. For comparison, several standard Cr₂O₃-forming and Al₂O₃-forming alloys were included in the programme. In general, the Al₂O₃-forming intermetallics and alloys were much more resistant to degradation than the Cr₂O₃-forming alloys while, in all cases, preoxidation increased the time to the onset of breakaway-type corrosion resulting from the initiation and growth of sulphides. In environments in which the Cr₂O₃-forming alloys developed thick sulphide scales, it was found that the iron aluminides developed and retained protective Al₂O₃-rich scales more effectively than the conventional Al₂O₃-forming alloys. At the relatively low temperatures of these tests, it was more difficult to establish the protective scale than at high temperatures, particularly on the alloys of relatively low aluminium concentration. Thus, an iron aluminide containing 16 wt% Al showed good degradation resistance under all conditions while an aluminide containing 8 wt% Al developed more extensive sulphides at 500°C that at 700°C. Overall, the resistance of the intermetallics to degradation increased with increasing aluminium concentration, although none of them showed excessive sulphidation in the 50 h test programmes.     

Influence of the Oxygen Partial Pressure on the High Temperature Corrosion of 38Ni–34Fe–25Cr Alloy in Presence of NaCl Deposit

In biomass gasification processes, some molten salts formed during the process can promite high temperature corrosion. In this study the chromia-forming austenitic alloy Haynes^® HR-120 was oxidized with a deposit of sodium chloride for 96 h at 825 and 900 °C. Two different atmospheres were selected; one with a high oxygen partial pressure (Ar/O₂ 90/10 %vol.) and one, named syngas, with a low oxygen partial pressure (CO/H₂/CO₂ 45/45/10 %vol.). While at 900 °C the behaviour of the alloy in presence of sodium chloride was catastrophic in high oxidizing conditions, the impact of sodium chloride was insignificant in the syngas atmosphere. When exposed to the Ar/O₂ mixture, the catastrophic oxidation was attributed to the setting up of an active oxidation. At 900 °C under the syngas atmosphere, the protective behaviour of the alloy seems linked to the association of a faster evaporation of the salt and a very low oxygen partial pressure. At 825 °C a catastrophic behaviour is observed under the syngas atmosphere as the NaCl evaporation rate is much slower.     

Microstructure and Properties of Aluminum-Containing Refractory High-Entropy Alloys

A new metallurgical strategy, high-entropy alloying (HEA), was used to explore new composition and phase spaces in the development of new refractory alloys with reduced densities and improved properties. Combining Mo, Ta, and Hf with “low-density” refractory elements (Nb, V, and Zr) and with Ti and Al produced six new refractory HEAs with densities ranging from 6.9 g/cm³ to 9.1 g/cm³. Three alloys have single-phase disordered body-centered cubic (bcc) crystal structures and three other alloys contain two bcc nanophases with very close lattice parameters. The alloys have high hardness, in the range from H _v = 4.0 GPa to 5.8 GPa, and compression yield strength, σ _0.2 = 1280 MPa to 2035 MPa, depending on the composition. Some of these refractory HEAs show considerably improved high temperature strengths relative to advanced Ni-based superalloys. Compressive ductility of all the alloys is limited at room temperature, but it improves significantly at 800°C and 1000°C.     

Microstructure and properties of Al0.3CrFe1.5MnNi0.5Ti x and Al0.3CrFe1.5MnNi0.5Si x high-entropy alloys

In this article, the microstructure, hardness, and corrosion resistance of the Al0.3CrFe1.5MnNi0.5Tixand Al0.3CrFe1.5MnNi0.5Six(x = 0, 0.2, 0.5, 1.0) high-entropy alloys were investigated via X-ray diffraction(XRD)scanning electron microscopy(SEM), digital display Vickers hardness tester, and electrochemical technique These alloys are mainly composed of BCC solid-solution structure. When adding high content of Ti or Si elemen(x C 0.5), some intermetallic compounds are found in the microstructure, which makes the alloys have a high hardness, high brittleness, and easy cracking. While the alloys with low content of Ti or Si(x = 0.2) have a hardness of HV 420–HV 430, and its hardness increases about 14 %compared with that of Al0.3CrFe1.5MnNi0.5. Electrochemical results in 3.5 % NaCl solution show that the alloying elements Ti and Si have a negative influence on the corrosion resistance of the Al0.3CrFe1.5MnNi0.5alloys.     

Effect of Pressure on Metal Dusting Initiation on Alloy 800H and Alloy 600 in CO-rich Syngas

The effect of pressure on metal dusting initiation was studied by exposing conventional alloys 600 and 800H in CO-rich syngas atmosphere (H₂, CO, CO₂, CH₄, H₂O) at ambient and 18 bar total system pressure and 620 °C for 250 h. It was verified that, at constant temperature, increasing the total system pressure increases both oxygen partial pressure (pO₂) and carbon activity (a _C), simultaneously. Both samples exposed at ambient pressure showed very thin oxide scale formation and no sign of metal dusting. By contrast, samples exposed in the high-pressure experiment showed severe mass loss by metal dusting attack. Iron- and chromium-rich oxides and carbides were found as corrosion products. The distinct pressure-dependent behavior was discussed by considering both thermodynamic and kinetic aspects with respect to the protective oxide formation and pit initiation.     

Effect of Different Degrees of Sensitization on the EIS Response of 316L and 316 SS in Transpassive Region

Different heat treatments were conducted on 316L and 316 stainless steels, and the sensitized specimens were characterized using anodic polarization and EIS tests in 0.5 M H₂SO₄ containing 0.01 molar KSCN. The potential ranges related to the transpassive region related to each specimen were determined. The EIS experiments were conducted at different potentials in that region, and the results showed the presence of three different regions, namely the anodic dissolution of the passive layer, dissolution of the grain boundaries, and the occurrence of pitting corrosion owing to the variations in the anodic potential. The higher the applied sensitization temperature, the lower the obtained charge-transfer resistance (R _ct) values, but healing effect was observed at the temperatures above 600 °C for these alloys.     

Evaluation of High Temperature Corrosion in Simulated Waste Incinerator Environments

In this study, high temperature reactions of Fe–Cr alloys at 500 and 600 °C were investigated using an atmosphere of N₂–O₂ 8 vol% with 220 vppm HCl, 360 vppm H₂O and 200 vppm SO₂; moreover the following aggressive salts were placed in the inlet: KCl and ZnCl₂. The salts were placed in the inlet to promote corrosion and increase the chemical reaction. These salts were applied to the alloys via discontinuous exposures. The corrosion products were characterized using thermo-gravimetric analysis, scanning electron microscopy and X-ray diffraction.The species identified in the corrosion products were: Cr₂O₃, Cr₂O (Fe_0.6Cr_0.4)₂O₃, K₂CrO₄, (Cr, Fe)₂O₃, Fe–Cr, KCl, ZnCl₂, FeOOH, σ-FeCrMo and Fe₂O₃. The presence of Mo, Al and Si was not significant and there was no evidence of chemical reaction of these elements. The most active elements were the Fe and Cr in the metal base. The Cr presence was beneficial against corrosion; this element decelerated the corrosion process due to the formation of protective oxide scales over the surfaces exposed at 500 °C and even more notable at 600 °C; as it was observed in the thermo-gravimetric analysis increasing mass loss. The steel with the best performance was alloy Fe9Cr3AlSi3Mo, due to the effect of the protective oxides inclusive in presence of the aggressive salts.     

Long-term Hot Corrosion Behavior of Boiler Tube Alloys in Waste-to-Energy Plants

Accelerated corrosion of candidate alloys was induced by metal chlorides/sulfates at 500 °C. Results suggest that the corrosivity of the studied metal chlorides increases in the order CaCl₂ < NaCl < KCl < ZnCl₂ < PbCl₂ < FeCl₂. Mechanisms to explain the different impacts of chlorides were proposed. It is believed that materials exposed to chloride salts corrode through vicious cycles, in which a shorter path of the cycle leads to a higher corrosion rate. Experimental results confirmed that FeCl₂ with the shortest path of the corresponding vicious cycle has the highest corrosion rate. It is also confirmed that the sulfates of Zn and Pb are less corrosive than their chlorides for the alloys tested. A kinetic study on the hot corrosion of T22, Esshete 1250 and Sanicro 28 was carried out under simulated waste-to-energy (WTE) ashes at 500 °C for 1000 h. Results from the kinetic study show that T22, Esshete 1250, and Sanicro 28 exhibited comparable performance for short-term exposure; however, the degradation thickness presented a clear trend after the 1000-h exposures in terms of decreasing resistance to corrosion: T22 > Esshete 1250 > Sanicro 28. EDX maps confirmed the role of Ni/Cr for slowing the corrosion kinetics of these three alloys.     

The corrosion behavior of several heat resistant materials in air + 2% Cl2 at 300 to 800 °C. Part 2 – Nickel base alloys

The corrosion behavior of the alloys 59, C‐2000 and HR‐160 was investigated in dry air and in air with 2% Cl₂ at temperatures of 300 to 800 °C. Up to 500 °C Alloy 59 and Alloy C‐2000 do not exhibit any significant attack. At 650 °C in particular HR‐160 is subjected to a marked increase of the corrosion rates. In the latter case the higher amount of C compared to the other two alloys seems to decrease corrosion resistance. At 800 °C the resistance of C‐2000 is inferior to that of Alloy 59 which is attributed to differences in the microstructure consisting of Cr/Mo precipitates in the Ni‐base matrix which are primarily attacked.     

The Effect of Temperature and Acid Gas Loading on Corrosion Behavior of API 5L X52 Carbon Steel in Amine Unit

The effect of temperature and H₂S concentration on amine corrosion of API 5L X52 carbon steel in a CO₂-saturated 25 wt.% diethanolamine solution was investigated via electrochemical techniques. It was found that increase in temperature from 25 to 80 °C resulted in severe increase in corrosion rate from 0.88 to 16.24 mpy due to increase in degradation rate of amine. Also, it was concluded that increase in H₂S concentration led to increase in corrosion rate because of formation of more heat stable amine salts. The effect of temperature on corrosion rate was more significant than acid gas loading.     

Corrosion behavior of three iron-based model alloys in reducing atmospheres containing HCl and H2S at 600 °C

The corrosion of three Fe-xCr alloys (x = 8, 12, 18 wt.%) was examined at 600 °C in reducing atmospheres containing HCl and H₂S with two different H₂S contents and compared with the behavior of the same materials in H₂S-free H₂-CO₂ and H₂-HCl-CO₂ mixtures producing similar oxygen and chlorine pressures. Exposure to the low-H₂S gas mixture had only a reduced effect on the behavior of Fe-8Cr and Fe-18Cr, but increased significantly the corrosion rate of Fe-12Cr. Increasing the H₂S level accelerated the corrosion of all the alloys, but particularly that of Fe-18Cr. In both cases only minor amounts of sulfur and chlorine were present close to the alloy/scale interface. An increase of the Cr content reduced the corrosion rate in both H₂S gas mixtures, especially in the range 8-12 wt.% Cr, due to a larger volume fraction of Cr₂O₃ in the scale. The results have been discussed on the basis of the thermodynamic stability diagrams of the Fe-O-Cl-S and Cr-O-Cl-S systems.     

Laboratory corrosion tests for simulating fireside wastage of superheater materials in waste incinerators

Laboratory corrosion tests were performed to clarify the effects of relative amounts of fused salts in tube deposits on corrosion rates of superheater materials in WTE plants. All test exposures were at 550 °C and of 100 h duration. The nine synthetic ashes used as corrodents consisted of mixtures of chlorides, sulfates and oxides. The test materials were alloy steel T22, stainless steels TP347H, TP310HCbN, and alloys HR11N and 625. The gas atmosphere consisted of 500 to 3000 ppm HCl‐30 ppm SO₂‐10% O₂‐10% CO₂‐20% H₂O‐bal.N₂. Generally, the relative amount of fused salts in non‐fused ash constituents at 550 °C increased with increasing the chlorine content of the ashes. The corrosion rate of T22 steel did not depend directly on ash chlorine content, but for ashes of 7.7 wt.% Cl, the corrosion rate depended on the calculated amount of fused salt at 500 °C. The corrosion rates of TP347H steel and alloy 625 were maximum for ashes of 6–8 wt.% Cl. For ashes of 7.7 wt.% Cl, the corrosion rates of T22 steel, stainless steels, and alloys increased with ashes having higher amounts of fused salts. Increased HCl content of the gas caused higher corrosion of the stainless steels and high‐nickel alloys, but there was no clear corrosion‐exacerbating effect with T22 steel.     

Corrosion behaviour of X52 pipeline steel in high H2S concentration solutions at temperatures ranging from 25°C to 140°C

Abstract

High temperature and high pressure immersion tests in an autoclave were employed to study the corrosion behaviour of X52 pipeline steel in aqueous solutions containing high concentrations of H₂S. The corrosion products generated were characterised using scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It was seen that at a constant H₂S concentration of 22 g/l, the corrosion rate increased with increasing temperature up to 90°C, thereafter decreased at 120°C and slightly increased again at 140°C while the corrosion rate increased with H₂S concentration at a temperature of 90°C. When the temperature and H₂S concentration increased, the corrosion product converted from iron rich to sulphur rich products in the following sequence: mackinawite→troilite→pyrrhotite, where the microstructure and stability of the corrosion products had an important effect on the corrosion rate. The corrosion film was formed through the combination of the outward diffusion of Fe²⁺ ions and the inward diffusion of H₂S and HS^? species.     

The Corrosion of Cu–Al Binary Alloys in H2/H2S/H2O Atmospheres at 400–900°C

The corrosion behavior of pure Cu and of three Cu–Al alloys containing 1, 5, and 10 wt.% Al was studied at 400–900°C in a H₂/H₂S/H₂O gas mixture. Both Cu–1Al and Cu–5Al alloys had the single-phase structure of α-Cu, while Cu–10Al was the intermetallic compound Cu₃Al. In general, the corrosion behavior of all the alloys followed the parabolic rate law, and the corrosion rate constants generally increased with increasing temperature but decreased with increasing Al content. The scale formed on pure Cu was an exclusive single layer of Cu₂S, while the scales formed on Cu–Al alloys were heterophasic and duplex, consisting of an outer layer of Cu₂S and an inner layer of Cu₂S and CuAlS₂. X-ray diffraction results showed no evidence of oxides and the amount of CuAlS₂ increased with increasing Al content. The formation of Cu₂S and CuAlS₂ on higher Al-content alloys resulted in a subsurface phase transformation from α-Cu (for Cu–5Al) or from Cu₃Al (for Cu–10Al) to Cu₃Al + Cu₉Al₄. The formation of CuAlS₂ in the inner layer of Cu–Al alloys was responsible for the reduction of corrosion rates, as compared to those of pure Cu.     

Die Korrosion von Kupfer-Aluminium-Legierungen in schwefelsaurer Beizlösung

Corrosion of copper-aluminium alloys in sulfuric acid containing pickling solutions Wrought copper aluminium alloys (aluminium contents between 5 and 10 weight-%, additions of Fe, Ni and Mn) have been studied by continous and alternating immersion tests in a solution containing 20% H₂SO₄ and 10% FeSO₄ at 40°C. In the as-extruded state the corrosion of monophasic alpha alloys increases with aluminium content. Larger quantities of ß' martensite exercise a negative effect. Addition of 2 weight-% Ni do not improve the corrosion resistance of the alloys with 5 and 8% Al. Cold reduction of alpha alloys give rise to a pronounced intensification of corrosion. No positive effect can be obtained by a thermal treatment of the alloys CuAl 10 Fe 4 Mn Ni and CuAl 10 Fe 4 Ni 5. The corrosion takes place under the following forms: uniform corrosion (CuAl 5), preferred corrosion of grain boundaries (CuAl*, Cual 9, Mn 2 FeNi and CuAl 10 Fe 4 Ni 5 after thermal treatment) and dealuminisation (CuAl 10 Fe 4 Mn 3 Ni). With a view to corrosion resistance the alloys CuAl 5, CuAl8 CuAl 9 Mn and - probably - CuAl 8 Fe seem to be superior to the others.