Hydrogen production and utilisation involve H2O(g) at high temperatures. However, the effect of hydrogen on chromia-forming heat resistant alloys in H2O is still unclear. In this work, Fe-20Cr and Fe-20Cr-(1Si, 2Mn or 2Al) (wt%) were exposed in Ar-xH2-10H2O gases (x = 2, 10 and 20 vol%) at 650 degrees C for up to 310 h. The binary alloy experienced breakaway oxidation in all gases. Increasing hydrogen content reduced corrosion rates and altered oxide morphologies. Adding Mn, Al or Si to the alloy suppressed oxidation to some extent. The effects of both hydrogen and alloying on the diffusion processes supporting oxidation are discussed.

Most high-temperature alloys are protected by chromia scales formed on their surfaces during service. Water vapour negatively affects the performance of these scales, an effect this work seeks to overcome. Binary Fe-Cr alloys containing 10 or 20 wt.

Model alloy, Fe-20Cr (wt%), was exposed in Ar-(5, 20)H2O, Ar-(5, 20)H2O-5H2 and Ar-20O2 (vol%) at 750 and 850 degrees C. The alloy formed Cr2O3 scales plus Fe-rich oxide nodules in H2O-containing gases, but a Cr2O3 scale in Ar-20O2 at 750 degrees C. The alloy formed an outer (Cr,Fe)-rich oxide layer and an inner, porous Cr2O3 layer in Ar5H2O-5H2, whereas a dense Cr2O3 scale grew in Ar-5H2O at 850 degrees C. Application of Wagner's theory successfully differentiated effects of oxide grain sizes and oxygen partial pressures of gases on scaling rates. Chemically inert SiO2 markers showed that Cr2O3 scales grew by outward metal diffusion.

Alloys of Ni–25Cr–(2Mn–1Si) under mixed deposits of ash + (0, 10, 50 and 90) wt

Model alloy, Fe-20Cr (wt%), was oxidized in two gas mixtures Ar-5H2O-(5H2) (vol%) at 850 °C. The alloy formed Cr2O3 scales in both gases. The Cr2O3 scale developed faster in Ar-5H2O-5H2 and contained fine pores, whilst that grown in Ar-5H2O was dense. Experiments with inert SiO2 marker revealed that the Cr2O3 scale growth in Ar-5H2O-(5H2) was controlled mainly by outward Cr diffusion. When adjusted for grain boundary diffusion effects, Wagner’s theory was successful in describing the hydrogen effect, provided that the Cr2O3 scales are n-type.

Alloys of Ni-25Cr-(2Mn-1Si) under deposits of sulphate or ash+ sulphate were exposed to a wet CO2 gas with or without 0.5 vol.% SO2 at 650 °C for 300h. Without SO2, multi-layered reaction products formed. Under sulphate, they were external NiO, inner NiCr spinel+ Cr2O3 band, internal oxidation zone and internal CrxS precipitates. Under ash+ sulphate, they were external NiO+ Ni above the internal oxidation zone. Introducing SO2 in the gas promoted alloy passivation, forming thin chromia scales under both deposits. Alloying with Mn and Si produced only minor improvements. These effects are examined in light of gas-deposit-alloy interactions.

Model alloy, Fe-20Cr (wt.%), was oxidized in two gas mixtures Ar-5H2O-(5H2) (vol.%) at 850 oC. The alloy formed Cr2O3 scales in both gases. The Cr2O3 scale developed faster in Ar-5H2O-5H2 and contained fine pores, whilst that grown in Ar-5H2O was dense. Experiments with inert SiO2 marker revealed that the Cr2O3 scale growth in Ar-5H2O-(5H2) was controlled mainly by outward Cr diffusion. When adjusted for grain boundary diffusion effects, Wagner’s theory was successful in describing the hydrogen effect, provided that the Cr2O3 scales are n-type.

In the oil and gas industry, produced water with a high dissolved salt content is a common byproduct of hydrocarbon extraction from conventional and unconventional wells. Other than salts, corrosive gases such as CO2 are abundant in the production stream, which dissolve and acidify the solution, posing a risk of internal pipeline corrosion. To mitigate this issue, injection of corrosion inhibitors has emerged as a cost-effective approach. In various aggressive conditions, heterocyclic molecules that contain nitrogen atoms have proven to be highly effective corrosion inhibitors for many alloys. In this study, tetrahydropyrimidinium (THP-C14) inhibition efficiencies were investigated at temperatures of 25°C, 55°C, and 80°C using electrochemical methods, including linear polarization resistance and potentiodynamic sweeps. Corrosion inhibition data were then correlated with THP-C14 concentration, using the five adsorption isotherms: Langmuir, Temkin, Frumkin, Flory-Huggins, and Dhar-Flory-Huggins models. These isotherms utilize different assumptions to establish the correlation between coverage and inhibitor concentration. The suitability of these five isotherm models for describing the corrosion inhibition behavior of THP-C14 was examined. In addition, the thermodynamic parameters (Kad, ΔadGo) of adsorption for THP-C14 at 25°C, 55°C, and 80°C were calculated and compared using the aforementioned adsorption isotherm models. Finally, a mechanism was proposed for the adsorption behavior of the THP-C14 corrosion inhibitor model compound. Chloride ions were important for inhibitor adsorption.

Produced water can cause internal corrosion in oil pipelines. The flow of hydrocarbons and water can result in intermittent surface wetting, impacting corrosion inhibition. However, this is still poorly understood. This research uses electrochemical techniques to study the impact of intermittent wetting and the wettability of the steel surface on corrosion inhibition. An inhibitor model compound and model oil were used at 25°C and 55°C in CO2-saturated solutions. Corrosion inhibition varied with temperature and steel surface hydrophobicity, altering inhibition significantly after intermittent contact with hydrocarbon at 55°C. Electrochemical impedance spectroscopy and wettability results revealed distinct protective mechanisms when oil was present.

Two ferritic alloys, Fe-25Cr and Fe-25Cr-2Mn-1Si, and two austenitic alloys, Fe-25Cr-20Ni and 310SS, coated with a thin film of Na2SO4-K2SO4 mixture were exposed to a flowing wet CO2 gas at 650 and 750 °C for up to 300 h. In addition to oxide scaling, a new manifestation of Fe oxide fluxing into the salt layer was observed. The effects of alloy composition and reaction temperature on this phenomenon were determined. The process is attributed to formation of a low melting Na2O-Na2SO4 solution in the early stages of reaction, followed by basic fluxing of FeO and/or Fe3O4.

Produced water can cause internal corrosion in oil pipelines. The flow of hydrocarbons and water can result in intermittent surface wetting, impacting corrosion inhibition. However, this is still poorly understood. This research uses electrochemical techniques to study the impact of intermittent wetting and the wettability of the steel surface on corrosion inhibition. An inhibitor model compound and model oil were used at 25 degrees C and 55 degrees C in CO2-saturated 2-saturated solutions. Corrosion inhibition varied with temperature and steel surface hydrophobicity, altering inhibition significantly after intermittent contact with hydrocarbon at 55 degrees C. Electrochemical impedance spectroscopy and wettability results revealed distinct protective mechanisms when oil was present.

The occurrence of localized corrosion in carbon steel pipelines, even when the uniform corrosion rate remains low, is a major concern in the hydrocarbon production and transmission industry. The propagation of these pits, caused by the galvanic coupling between the inhibited surface and the active pit, can lead to serious consequences such as financial loss, environmental damage, production interruption, and even loss of life. To better understand this phenomenon, this work focuses on using the potentiostatic technique to evaluate the tendency of localized corrosion propagation. The experiments were conducted using a primarily imidazolinium-based corrosion inhibitor in produced water conditions (5 wt% NaCl, pH 4.5, CO2-saturated) at 55°C and 80°C. The baseline results were obtained through linear polarization resistance and potentiodynamic polarization tests. The potentiostatic experiments were then conducted to artificially simulate different levels of galvanic coupling that could exist in case of active localized corrosion. The results showed that, at certain anodic potentials, increased inhibitor dosage was necessary to significantly decrease the current. However, at high current levels, further injections were insufficient, indicating that substrate dissolution may affect the adsorption of the inhibitor. This work provides insights into the role of inhibitors and important factors in stopping the propagation of localized corrosion of carbon steel. Further research, such as designing a proper zero-resistance ammeter setup, will be necessary to fully understand this complex phenomenon. The results show that the potentiostatic methodology can be a rapid and easy alternative to obtain electrochemical information and improve understanding of localized corrosion propagation.

Model Ni-25Cr and Ni-25Cr-2Mn-1Si (wt%) alloys coated with an industrial fly ash were exposed to a wet CO2 gas at 750 °C. It was found that the outer thermally-grown NiO scale encapsulated ash particles at an accelerated rate. TEM combined with TKD analysis revealed that nano structured NiO grains on the ash surface contributed to the observed NiO encapsulating effect.

A Ni-25Cr (wt.%) alloy coated with the salts and mixtures of these salts with an industrial coal ash was exposed to Ar-60CO(2)-20 H2O at 650 degrees C for 300 h. The alloy without any deposit developed a uniform internal oxidation zone (IOZ) and an external metallic nickel layer surmounted by a thin NiO scale. Salt deposits changed the scale constituents and morphology. Predominantly, a duplex scale of NiO over a Cr2O3 layer grew with an underlying IOZ containing large Cr-rich oxide precipitates. Under a deposit of ash plus salts, the basic reaction morphology was the same as in the gas-only case, but the oxide layer was non-uniform. Local voids were formed within the alloy beneath chloride plus ash deposits. Under ash with sulphates, the alloy formed both partially protected areas and non-protective multi-layered scales. Spherical ash particles were enveloped by NiO in both salt-ash deposits.

Silicon-bearing vapour at low pressure was used to deposit SiO 2 during exposure of pure chromium, Fe-30Cr (wt.%) and Kanthal APM alloy to an Ar-10H 2 O-0.1HCl mixture (vol.%) at 650 °C. Pure Cr formed porous chromia blades on top of a polycrystalline Cr 2 O 3 scale containing SiO 2 . A model Fe-30Cr alloy formed a scale consisting of a thick, amorphous (Cr,Si)O x outer layer and a thin, polycrystalline Cr 2 O 3 inner layer. Kanthal alloy formed a thin scale containing a Si-rich oxide outer layer and an inner layer of Fe, Cr, and Al oxides. Amorphous (Cr,Si)O x nodules formed locally on top of (Cr,Fe) 23 C 6 precipitates at the Kanthal surface.

The corrosion behaviour of Fe-25Cr-20Ni, 310SS, Ni-25Cr, and Ni-25Cr-2Mn-1Si (all in wt. %) with and without NaCl-KCl deposits in Ar-60%CO2-20%H2O gas at 650oC was investigated. Without salt deposits, both Fe-25Cr-20Ni and Ni-25Cr formed thick internal oxidation zones (IOZs) and external oxide scales. The addition of Si+Mn significantly improved the corrosion resistance in this case. The presence of NaCl-KCl deposits accelerated the corrosion, forming porous external oxide scales and IOZs, with no protective effect provided by Si+Mn alloying. The effect of chlorides is discussed in terms of oxide volatilization and the accelerated reaction of chlorine with oxides in the IOZs.

Corrosion behaviours of Fe–25Cr, Fe–25Cr–2Mn–1Si, Fe–25Cr–20Ni, 310SS, Ni–25Cr, and Ni–25Cr–2Mn–1Si (all in wt%) with and without NaCl–KCl deposits in Ar–60%CO 2 –20%H 2 O gas at 750 °C were studied. Without salt deposits, Fe–25Cr performed protectively, while Fe–25Cr–20Ni and Ni–25Cr underwent breakaway oxidation with multilayered scales formed. Adding alloy elements Si + Mn increased the corrosion resistance of all alloys by forming additional Mn-rich oxides and silica. Surface deposits of NaCl–KCl accelerated corrosion, forming porous Fe-rich oxide nodules for Fe–25Cr and thick, porous scales and internal oxidation zones for all other alloys. The protective effect of Si + Mn alloying disappeared in the presence of chlorides. Limited intergranular carbides were observed for all alloys in the gas-only condition. The extent of carburisation increased with the presence of chloride deposits for all Fe-based alloys, but remained unchanged for Ni-based alloys. Corrosion of these alloys at 750 °C is compared with that at 650 °C. The effect of chlorides in volatilising metals at 750 °C is discussed.

The initial stages of oxidation of 9Cr steel in CO 2 , O 2 , CO 2 –O 2 and CO 2 –O 2 –H 2 O is studied by gas phase analysis (GPA) at 550 °C using 13 C 16,16 O 2 , 18,18 O 2 and 2 H 2 16 O isotopic molecules in order to discriminate the reactions of all gas molecules. Protective and non-protective oxide scales are formed on 9Cr steel depending on the exact composition of the gas mixture. In pure CO 2 , 9Cr steel forms a slow growing chromium-rich oxide scale without any carburization. Adding O 2 impurities in CO 2 favors the formation of fast growing iron-rich duplex oxide scale coupled to strong carburization. Adding several % of O 2 in CO 2 favors again the formation of slow growing oxide scale but with different structure and composition than in pure CO 2 . GPA analyses combined with oxide scale analyses demonstrate that the composition and structure of the transient oxide scale formed on 9Cr surface is determined by the rate at which surface adsorbed oxygen atoms are supplied by the gas phase in the first minutes of exposure. The presence of the very oxidizing O 2 molecules in CO 2 increases drastically the surface oxidation rate, favoring formation of a non-protective oxide scale which transmits carbon permitting carburization of the steel. Adding water vapor to a CO 2 gas environment slows carburization. Preferential adsorption of water vapor molecules over CO 2 /CO molecules in the inner oxide scale is proposed to explain this result. A unified mechanism for the formation of the transient oxide scale on 9Cr steel in CO 2 /O 2 /H 2 O gas mixtures is described.

Fe-30Cr (wt%) alloy was exposed to Ar-0.5%SO2 and Ar-10%H2O-0.5%SO2 gases at 650 degrees C, forming Cr2O3 scales. In dry SO2 for 20 h, sulphur enrichment and scattered Cr-sulphides were observed at the scale-alloy interface, together with internal Cr-sulphide precipitates. After 100 h, sulphur disappeared from the interface, while more internal Cr-sulphides were produced. In wet SO2, the sulphur enrichment and Cr-sulphides were maintained in both times, but no internal sulphidation was seen. Atom probe analysis revealed sulphur enrichment on chromia grain boundaries in both gases, to a greater extent in wet SO2. These effects are discussed in terms of chromia grain boundary diffusion.

Abstract The initial stages of oxidation of 9Cr steel in CO2, O2, CO2-O2 and CO2-O2-H2O is studied by Gas Phase Analysis (GPA) at 550°C using 13C16,16O2, 18,18O2 and 2H216O isotopic molecules in order to discriminate the reactions of all gas molecules. Protective and non-protective oxide scales are formed on 9Cr steel depending on the exact composition of the gas mixture. In pure CO2, 9Cr steel forms a slow growing chromium rich oxide scale without any carburization. Adding O2 impurities in CO2 favors the formation of fast growing iron rich duplex oxide scale coupled to strong carburization. Adding several % of O2 in CO2 favors again the formation of slow growing oxide scale but with different structure and composition than in pure CO2. GPA analyses combined with oxide scale analyses demonstrate that the composition and structure of the transient oxide scale formed on 9Cr surface is determined by the rate at which surface adsorbed oxygen atoms are supplied by the gas phase in the first minutes of exposure. The presence of the very oxidizing O2 molecules in CO2 increases drastically the surface oxidation rate and favors the formation of non-protective duplex oxide scale against carburization. Adding water vapor to a CO2 gas environment slows carburization. Preferential adsorption of water vapor molecules over CO2/CO molecules in the inner oxide scale is proposed to explain this result. A unified mechanism for the formation of the transient oxide scale on 9Cr steel in CO2/O2/H2O gas mixtures is described.