Effect of gas composition on coking and metal dusting of 2.25Cr–1Mo steel compared with iron

The reaction of 2.25Cr–1Mo at 650 °C with CO–H₂–H₂O gas was investigated. Gas compositions were varied with respect to p_CO and nominal carbon activity which calculated assuming equilibrium of the synthesis gas reaction. After an induction period, the steel grew a cementite scale, covered by a graphite deposit containing cementite particles. Carbon uptake kinetics were roughly linear. The rates did not correlate with the carbon activity, but indicated the existence of three parallel reaction paths. Individual rate constants were greater than those for pure iron, an effect attributed to faster cementite scale disintegration caused by the presence of chromium-rich oxide particles.     

Initial corrosion protection of Zn–Mn alloys electrodeposited from alkaline solution

The effects of a deposition current density (c.d.) on the corrosion behaviour of Zn–Mn alloy coatings, deposited from alkaline pyrophosphate solution, were investigated by atomic absorption spectrophotometry (AAS), X-ray diffraction (XRD), atomic force microscopy (AFM), optical microscopy, electrochemical impedance spectroscopy (EIS) and measurement of corrosion potential (E_corr). XRD analysis disclosed that zinc hydroxide chloride was the main corrosion product on Zn–Mn coatings immersed in 0.5 mol dm⁻³ NaCl solution. EIS investigations revealed that less porous protective layer was produced on the alloy coating deposited at c.d. of 30 mA cm⁻² as compared to that deposited at 80 mA cm⁻².     

Hydrogen gas production during corrosion of copper by water

This paper considers the corrosion of copper in water by: (1) short term, open system weight measurements and (2) long term, closed system immersion in distilled water (13,800 h) without O₂ at 21–55 °C. In the latter experiments, the hydrogen gas pressure is measured above the immersed copper and approaches ∼10⁻³ bar at equilibrium. This pressure is mostly due to copper corrosion and greatly exceeds that in ambient air. Accordingly, this measured hydrogen pressure from copper corrosion increases with temperature and has the same dependency as the concentration of OH⁻ in the ion product [OH⁻] [H⁺].     

Internal oxidation and metal dusting of Fe-Si alloys

Iron and Fe-Si alloys (1, 2, 3 and 5 wt.% Si) were reacted at 680 °C with a gas mixture of 68% CO, 26% H₂ and 6% H₂O (a_C = 2.9, p_O2 = 2 × 10⁻²³ atm). Alloy reaction products consisted of internally precipitated SiO₂, an external scale of Fe₃C + SiO₂ and surface coke deposits containing cementite and silica particles. Coking and metal dusting rates both increased with alloy silicon level. This is proposed to be related to an increase in graphite nucleation sites at Fe₃C/SiO₂ phase boundaries, and the volume expansion accompanying graphite precipitation.     

Accelerated Corrosion of Fe–xCr–10Al Alloys Containing 0–20 at.% Cr Induced by Sulfur and Chlorine in a Reducing Atmosphere at 600 °C

The corrosion behavior of five Fe–xCr–Al alloys with a constant Al content of 10 at.% and Cr contents ranging from 0 at.% to 20 at.% was examined at 600 °C in a H₂–HCl–H₂S–CO₂ gas mixture providing 3.7 × 10⁻²² atm O₂, 2.4 × 10⁻¹⁴ atm Cl₂ and 3.9 × 10⁻⁹ atm S₂. All the alloys formed duplex scales containing an outermost layer of iron oxide plus an inner layer composed of mixtures of the oxides of all the alloy components. Besides, a region of internal attack of Al or Al + Cr, whose depth decreased with increasing Cr content, formed in all the alloys. The simultaneous presence of chlorine and sulfur in the gas mixture significantly accelerated the corrosion of all the alloys with respect to their oxidation in a simpler H₂–CO₂ mixture providing the same oxygen pressure, by forming thick and cracked scales. The effect was particularly large for the high-Cr alloys due to their inability to form external protective alumina scales in the present gas mixture.     

Direct electrosynthesis of polyaniline–montmorrilonite nanocomposite coatings on aluminum alloy 3004 and their corrosion protection performance

Homogeneous and adherent polyaniline–montmorrilonite (MMT) nanocomposite coatings were electrosynthesized on aluminum (Al) alloy 3004 (AA 3004) by using the galvanostatic polarization method. The synthesized coatings were characterized by UV–Vis absorption spectrometry, Fourier transform infrared spectroscopy, X-ray diffraction patterns and scanning electron microscopy. The corrosion protection effect of the coatings was demonstrated by performing a series of electrochemical experiments of potentiodynamic and impedance measurements on Al in 3.5 wt% aqueous NaCl electrolytes. The corrosion current (i_corr) values decreased from 6.55 μA cm⁻² for uncoated Al to 0.102 μA cm⁻² for nanocomposite-coated Al under optimal conditions.     

Influence of the applied stress rate on the stress corrosion cracking of 4340 and 3.5NiCrMoV steels under conditions of cathodic hydrogen charging

Stress corrosion cracking (SCC) of as-quenched 4340 and 3.5NiCrMoV steels was studied under hydrogen charging conditions, with a cathodic current applied to the gauge length of specimens subjected to Linearly Increasing Stress Test (LIST) in 0.5 M H₂SO₄ solution containing 2 g/l arsenic trioxide (As₂O₃) at 30 °C. Applied stress rates were varied from 20.8 to 6 × 10⁻⁴ MPa s⁻¹. Both the fracture and threshold stress decreased with decreasing applied stress rate and were substantially lower than corresponding values measured in distilled water at 30 °C at the open circuit potential. The threshold stress values correspond to 0.03–0.08 σ_y for 4340 and 0.03–0.2 σ_y for the 3.5NiCrMoV steel. SCC velocities, at the same applied stress rate, were an order of magnitude greater than those in distilled water. However, the plots of the crack velocity versus applied stress rate had similar slopes, suggesting the same rate-limiting step. The fracture surface morphology was mostly intergranular, with quasi-cleavage features.     

Metal release from stainless steel, Co–Cr–Mo–Ni–Fe and Ni–Ti alloys in vascular implants

A seven-day immersion test using several solutions was conducted on stainless steel, Co-based alloy, and Ni–Ti alloy, which are used for stents and stent grafts. The quantitative data on the release of each metal ion and the correlation between metal ion release rate and pH were obtained. The quantities of Fe and Ni released from stainless steel gradually decreased with increasing solution pH (pH 2–7.5). For Co–Cr–Mo–Ni–Fe alloy, the quantity of Cr released steadily increased as pH decreased (pH ≤ 6) and reached nearly zero at pHs higher than 6 (pH 6–7.5). Co release was slightly affected by a variation in pH. The quantities of Ni and Ti released from Ni–Ti alloy markedly increased with decreasing pH (pH ≤ 4) and they leveled off from pH 4 (pH 4–7.5). Although the rapid increases were observed at approximately pH 2, the quantities were even higher than that of Co released from the Co–Cr–Mo and Co–Cr–Mo–Ni–Fe alloys. For further investigation of the rapid increase in the quantities of metals released at pH 2, an anodic polarization test was employed to study the passive and transpassive behaviours of Ni–Ti alloy. The critical current density for the passivation of Ni–Ti alloy markedly increased as pH decreased (pH ≤ 4) and was low (1.4 μA/cm²) at pH higher than 4 (pH 4–7.5). The potential at a current density of 10 μA/cm², by contrast, markedly rose with decreasing pH (pH ≤ 2), and was 1.2 V from pH 2 (pH 2–7.5).     

Effect of copper on metal dusting of austenitic stainless steels

Three steels, 304SS, 310SS and 800H, were alloyed with 5%, 10%, and 20% (by weight) copper, and then exposed to 68%CO-31%H₂-1%H₂O gas at 680 °C (a_C = 19 and p_O2=5.4×10^-25 atm) under thermal cycling conditions. Kinetic measurements showed that copper-free alloys all dusted, with 304SS experiencing the greatest metal wastage. Copper additions did not have any effect on metal wastage of 304SS, but reduced the attack on 310SS and 800H markedly at levels of 5% and 10%. However, increasing the copper content to 20% produced large copper-rich precipitates which accelerated dusting by promoting internal graphitisation.Dusting was associated with surface coking. When pitting occurred, on copper-free alloys and on copper containing 304SS, large coke structures grew above the pits. Internal grain boundary carburisation always took place, and intragranular carbides also precipitated when dusting occurred. A lamellar surface layer of internally precipitated spinel and austenite also developed in association with dusting. The copper effect is discussed in terms of its alloy solubility and its known beneficial effect in Ni-Cu binaries.     

Dissolution mechanism of 316L in lead–bismuth eutectic at 500 °C

In the frame of the Accelerator Driven System (ADS) cooled by liquid lead–bismuth eutectic (LBE), the austenitic stainless steel 316L is considered as a possible structural material for the reactor. However, the corrosion of 316L in this liquid alloy environment can be substantial, especially when a dissolution process occurs. In order to understand the dissolution process and to obtain a modelling of the 316L corrosion rate by LBE, an experimental dissolution kinetics of 316L is carried out in stagnant LBE at 500 °C up to 3000 h. A Ni preferential dissolution of the 316L is observed, leading to the formation of a ferritic layer at the 316L surface. A discussion on the various steps occurring in dissolution process leads to the conclusion that only the Ni dissolution reaction rate can control the 316L dissolution kinetics. The dissolution reaction rate constant, k_d, calculated from this study experimental points is equal to 4.2 × 10⁻¹¹ mol cm⁻² s⁻¹.     

Incorporation of transition metal ions and oxygen generation during anodizing of aluminium alloys

Enrichment of nickel at the alloy/film interface and incorporation of nickel species into the anodic film have been examined for a sputtering-deposited Al-1.2at.%Ni alloy in order to assist understanding of oxygen generation in barrier anodic alumina films. Anodizing of the alloy proceeds in two stages similarly to other dilute aluminium alloys, for example Al-Cr and Al-Cu alloys, where the Gibbs free energies per equivalent for formation of alloying element oxide exceeds the value for alumina. In the first stage, a nickel-free alumina film is formed, with nickel enriching in an alloy layer, 2 nm thick, immediately beneath the anodic oxide film. In the second stage, nickel atoms are oxidized together with aluminium, with oxygen generation forming gas bubbles within the anodic oxide film. This stage commences after accumulation of about 5.4 × 10¹⁵ nickel atoms cm⁻² in the enriched alloy layer. Oxygen generation also occurs when a thin layer of the alloy, containing about 2.0 × 10¹⁹ nickel atoms m⁻², on electropolished aluminium, is completely anodized, contrasting with thin Al-Cr and Al-Cu alloy layers on electropolished aluminium, for which oxygen generation is essentially absent. A mechanism of oxygen generation, based on electron impurity levels of amorphous alumina and local oxide compositions, is discussed in order to explain the observations.     

Accelerated corrosion of MoO3-deposited copper

The accelerated oxidation kinetics of MoO₃-deposited copper were studied in the temperature range of 480–700 °C in air. The accelerated oxidation followed the parabolic-rate law, indicating that the process was diffusion-controlled. Oxygen diffusion along liquid channels in the oxide scale was inferred to be the rate-limiting step in the overall mechanism. The rate constant increased from 9.2 × 10⁻⁶ to 3.8 × 10⁻⁵ kg² m⁻⁴ s⁻¹ with increasing the deposit mass from 0.3 to 0.9 kg m⁻² at 700 °C.     

Effect of tin on high-temperature sulphidation of Fe-Cr alloys in H2/H2S atmospheres

The high temperature sulphidation behaviour of Fe-46Cr-xSn (x = 0; 0.2; 0.5; 1; 2) alloys has been studied at temperatures of 1073, 1173 and 1273 K in H₂/H₂S mixtures with different sulphur vapour partial pressures of 10⁻¹, 10⁻³ and 10⁻⁵ Pa. Thermogravimetric studies in combination with scanning electron microscope (SEM), with energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques, have displayed a significant influence of the sulphur partial pressure on the composition and growth rate of the sulphide scale. The results have shown that addition of tin increases the sulphidation rate of Fe-46Cr alloys but not considerably (except at temperatures of 1073 and 1173 K combined with sulphur partial pressure of 10⁻⁵ Pa). The metallic core of the studied samples was enriched in tin and iron, moreover tin was found in the internal layer close to the metallic core as metallic Fe_xSn_y inclusions with tin concentrations of up to 12 at.%.     

High temperature corrosion of cast heat resisting steels in CO + CO2 gas mixtures

Two commercial variants of the cast heat resistant grade HP40Nb (Fe-25Cr-35Ni, Nb modified) were exposed to CO/CO₂ gases at 982 and 1080 °C in order to simulate exposure to the carbon and oxygen potentials realised in steam reformers under normal and overheated conditions. Both alloys developed external chromium-rich oxide scales, intradendritic silica precipitates and interdendritic oxide protrusions where primary, interdendritic carbides were oxidised in situ. Surprisingly, the lower silicon content alloy developed a more continuous internal silica layer, thereby slowing external scaling. Intradendritic oxidation was fast in both alloys, and is attributed to interfacial oxygen diffusion. Both alloys underwent rapid internal carburisation, indicating that their oxide scales failed to prevent carbon access to the underlying alloys under these reaction conditions.     

Anodic oxidation and dielectric behaviour of aluminium-niobium alloys

The anodizing behaviour of sputtering-deposited Al-Nb alloys, containing 21, 31 and 44 at.% niobium, has been examined in 0.1 M ammonium pentaborate electrolyte with interest in the composition and the dielectric properties of the anodic oxides. RBS and TEM revealed amorphous oxides, containing units of Nb₂O₅ and Al₂O₃ in proportion to the alloy composition. Xenon marker experiments indicated their growth through migration of the Nb⁵⁺, Al³⁺ and O²⁻ species, with cation transport numbers, in the range 0.31-0.35, and formation ratios, in the range 1.35-1.64 nm V⁻¹, intermediate between those of anodic alumina and anodic niobia. Al³⁺ ions migrate slightly faster than Nb⁵⁺ ions, promoting a thin alumina layer at the film surface, although this layer is penetrated by fingers of the underlying niobium-containing oxide of relatively reduced ionic resistivity. The incorporation of units of Nb₂O₅ into anodic alumina increases the dielectric constant from about 9 to the range 11-22 for the investigated alloys.     

High temperature oxidation behaviour of Ni–Fe–Co anodes for aluminium electrolysis

The oxidation behaviour of ternary Ni_xFe_yCo_z alloys (where x/y (wt) = 1 and 1.85; z = 0, 10, 30 and 50 wt.%) was studied in air at 800 °C. Alloys were found to follow complex oxidation kinetics, with the highest oxidation rates observed for alloys having 50 wt.% Co. Significant improvements in oxidation resistance were achieved by addition of 10 and 30 wt.% Co to the Ni–Fe system. The decrease in oxidation rate was associated with suppression of Fe₂O₃ formation in preference for (Co,Ni)_xFe_3−xO₄ growth. The results were discussed in light of the materials requirements for inert anodes for aluminium electrolysis.     

Pourbaix diagrams for titanium in concentrated aqueous lithium bromide solutions at 25 °C

Pourbaix diagrams (electrode potential-pH diagrams) for Ti–Br⁻–H₂O system at 25 °C in the absence and presence of titanium hydrides were developed in 400, 700, 850, and 992 g/L LiBr solutions. The diagrams were compared with the simple Ti–H₂O system at 25 °C. Comparison of the simple Ti–H₂O system with the diagrams of the Ti–Br⁻–H₂O system at 25 °C showed that the titanium solubility range in the acid, neutral, and weak alkaline areas of the diagrams extended slightly to both higher pH values and potentials with increasing bromide ion activity and decreasing water activity.     

Behavior of oxide films on high-temperature alloys in carbonaceous gas atmospheres

Formation and deterioration of oxide films on several contemporary high-temperature alloys was studied in various carbonaceous gas atmospheres. Scanning electron microscopy and metallographic examination were applied to films on Fe-Cr-Ni and Ni-Cr-Al type alloy compositions exposed in CH ₄/H₂ and CO/CO₂ atmospheres at temperatures up to 900° C and pressures up to 900 psi (6.2 × 10⁶ N/m²). The effects of various preoxidation treatments were evaluated. Reduction of certain oxide phases is observed to promote catalytic gas decomposition. Al₂O₃ components in the films are observed to be stable under the reducing conditions experienced. Carbon uptake by various alloys is found to be quite sensitive to surface finish, with an observed increase in penetration with surface roughness.     

Subsurface microstructural changes in a cast heat resisting alloy caused by high temperature corrosion

A cast HP ModNb alloy (Fe-25Cr-35Ni-1Nb, wt.%) was oxidised and carburised in CO-CO₂ corresponding to a_C = 0.1 and p_O2 = 3 × 10⁻¹⁶ atm at 1080 °C. Formation of an external, chromium-rich oxide scale led to depletion of this metal in a deep alloy subsurface zone. Within that zone, secondary chromium-rich carbides dissolved, primary carbides oxidised, solute silicon and aluminium internally oxidised, and extensive porosity developed. Pore volumes correspond to the difference between metal loss by scaling and metal displacement by internal oxidation, assuming the scale-metal interface to be fixed. The pores are concluded to be Kirkendall voids.     

Metal dusting behaviour of Cr-Ni steels and Ni-base alloys in a simulated syngas mixture

Long-term laboratory exposure tests for various Cr and Ni content steels and Ni-base alloys were conducted at 650 °C in a 60vol.%CO-26%H₂-11.5%CO₂-2.5%H₂O gas mixture simulating syngas environments. Upon isothermal heating, alloys with 15% and 20% Cr had many pits on the surface after a brief exposure, while no pit was found on alloys containing of 60% Ni and more than 23% Cr exposed for up to 5000 h. The thermal cycling accelerated pit initiation drastically, resulting that all test specimens except 30%Cr-60%Ni alloy suffered from metal dusting. From a measurement of pit depths, Ni proved to be an effective alloying element to retard the pit growth: growth rate for 75% Ni alloy has achieved double-digit decrease compared to that for 20% Ni. Microscopic observations has revealed that platelet graphite aligned perpendicular at the boundary of gas/metal of pits. The length of the platelet graphite for high Ni alloys was appreciably longer than that for low Ni steels. This can be interpreted from the difference of super saturation of carbon.