Synergistic effect of polyaspartic acid and iodide ion on corrosion inhibition of mild steel in H2SO4

The inhibition effect of polyaspartic acid (PASP) and its synergistic effect with KI on mild steel corrosion in 0.5 M H₂SO₄ solution are studied by weight loss and electrochemical methods. The inhibition efficiency increases with the concentration of PASP and increases further with the presence of 1 mM KI. Result of the zero charge potential measurement shows that iodide ion promotes the film formation of PASP greatly. The mild steel surfaces after immersion test were analyzed using scanning electron microscopy and X-ray photoelectron spectroscopy. An adsorption model is proposed to elucidate the synergistic mechanism of synergistic effect.     

Effects of ZrB2 and SiC dual addition on the oxidation resistance of graphite at 1600–2000 °C

The effects of ZrB₂ and ZrB₂ + SiC additions on the oxidation kinetics of graphite at 1600–2000 °C in air were investigated. The ZrB₂ + SiC dual addition improves the oxidation resistance of graphite more effectively than the ZrB₂ single addition, because the oxide scale formed on C–ZrB₂–SiC is denser and thinner due to the existence of glassy SiO₂. As the oxidation temperature increases, the oxidation rate of C–ZrB₂–SiC gradually increases and oxide scales with layered microstructures form on its surface due to the greatly enhanced active oxidation of SiC at higher temperatures.     

Initial oxide-film growth on Mg-based MgAl alloys at room temperature

The initial, thermal oxidation of bare, Mg-based MgAl alloys (containing up to 7.31 at.% Al) at 304 K in the partial oxygen pressure range of 10⁻⁶ ⩽ pO₂ ⩽ 10⁻⁴ Pa was investigated by angle-resolved X-ray photoelectron spectroscopy and real-time in situ spectroscopic ellipsometry. The chemical constitution of the initially grown oxide film resembles that of a MgO-type of oxide with, adjacent to the alloy/oxide interface, Al enrichments in both the oxide film and the subsurface region of the alloy substrate. The Al-to-Mg content of the oxide films is governed by the alloy composition in the subsurface region, which deviates from the bulk alloy composition due to sputter cleaning prior to oxidation. Continued oxide-film growth proceeds by the transformation of a defective surface-oxide structure into “bulk” oxide upon reacting with outwardly diffusing Mg cations under the influence of a surface-charge field. The effective rate of depletion of Mg from the alloy subsurface region is governed by the competing processes of preferential oxidation of Mg and interfacial segregation of Mg.     

Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-silane coating

Anodic coatings on magnesium ZE41 alloy were formed by DC plasma electrolytic oxidation (PEO) in spark regime in solution composed of NaOH, Na₂SiO₃ and KF. The positive effect of poly(ethylene oxide) addition into the anodizing electrolyte on PEO process, anodic film porosity and its protective performance was described. Anodic films were sealed with hybrid epoxy-silane formulation. The corrosion behavior of the coated ZE41 was studied through electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. Resulting duplex PEO/epoxy-silane coating provides good protective performance without significant signs of corrosion during 1 month of immersion test.     

Analysis of the acid,base and air oxidized carbon microspheres synthesized in a single step from waste engine oil

Carbon microsphere (C-μS) ball bearings are prone to corrode after long exposure to oxidizing environments. ∼4.2 μm diameter C-μSs synthesized from waste engine oil (at 600 and 900 °C) by dry autoclaving, were exposed to nitric acid (30 and 60 vol.%), piranha (H₂SO₄:H₂O₂ 60:40 and 90:10), base (NH₄OH:H₂O₂ 15:30 and 30:15) and air oxidation at 300 and 450 °C for 12–48 h. The oxidation processes were monitored by weight loss analyses, scanning electron microscopy, X-ray diffraction, Raman spectroscopy, chemical functionalization and electrical conductivity measurements. Crystallinity and higher graphitic content in the C-μSs results in better resistance to oxidation.     

Effects of pH value on characteristics of oxide films on 316L stainless steel in Zn-injected borated and lithiated high temperature water

The effects of pH values from 6.9 to 7.4 on oxide films for 316L stainless steel in borated and lithiated high temperature water at 573.15 K without and with Zn injection were examined by in situ potentiodynamic polarization curves, electrochemical impedance spectra and ex situ X-ray photoelectron spectroscopy (XPS) analysis. The composition of oxide films appears slightly pH dependent: rich in chromites and ferrites at pH = 6.9 and pH = 7.4, respectively. The corrosion rate decreases significantly in the high pH value solution with Zn injection due to the formation of compact oxide films. The solubilities and structural model of oxides are proposed and discussed.     

The effect of grain refinement on the adhesion of an alumina scale on an aluminide coating

To understand the effect of grain refinement on the thermally grown alumina scale adhesion to the metal substrate, two δ-Ni₂Al₃ coatings, one coarse-grained (∼70 μm) and the other ultrafine-grained (generally below ∼500 nm), were prepared. The cyclic oxidation in air at 1100 °C shows that the ultrafine-grained (UFG) coating is better oxidation resistant than the coarse-grained (CG) coating due to the formation of a more adherent alumina scale. The latter is intrinsically correlated with the fact that the aluminide grain refinement helps to increase the oxide/metal strength through a route to prevent the formation of large-sized voids at the interface.     

Oxidation behavior and phase transition of ZrB2–SiCw–ZrO2f ceramic

The oxidation behavior and phase transition of ZrB₂–SiC_w–ZrO_2f ceramic had been investigated by in situ high-temperature XRD, XPS, SEM, EDS and TEM measurements. The initial oxidation temperature of most ZrB₂ was 1000 °C and no significant oxidation of SiC was found up to 1200 °C. The oxidation products formed at lower temperatures would penetrate into the pores and flaws on the surface, which was beneficial to crack healing. In order to improve the oxidation resistance of this system, it should be focused on decreasing the oxygen diffusivity and the volume expansion caused by phase transition.     

Oxidation properties of self-propagating high temperature synthesized niobium disilicide

NbSi₂ monoliths were prepared by self-propagating high temperature synthesis (SHS) and hot pressing (HP) and their oxidation behavior was investigated at various temperatures (823–1123 K) in air. The combustion mode of SHS reaction was steady state combustion, and the combustion product was single-phase NbSi₂. Oxidation studies show that the highest mass gain was 0.95675 kg m⁻² at 1023 K. In cyclic oxidation, the oxidation rate was reduced and the mass gain was only 0.15507 kg m⁻². A dense protective amorphous SiO₂ scale formed at 823 K and 923 K whereas a porous multilayer SiO₂ and α/β-Nb₂O₅ oxide scales formed at and above 1023 K and spalled off. Pest oxidation of NbSi₂ monoliths was not observed in hot pressed NbSi₂ monoliths.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al₆₅Cr₂₇Fe₈ and Al₈₀Cr₁₅Fe₅ was studied in the 600–1080 °C range. Formation of transient alumina layers is obtained up to 900 °C. On Al₆₅Cr₂₇Fe₈ transient to α-phase transformations occur when performing oxidation at 1000 °C, together with the possible appearance of (Al_0.9Cr_0.1)₂O₃. At 1080 °C, direct formation of α-alumina is obtained. On Al₈₀Cr₁₅Fe₅, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 °C, revealing thermal etching of the underneath alloy surface. At 1050 °C the α-Al₂O₃ scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.     

High-temperature oxidation behavior of minor Hf doped NiAl alloy in dry and humid atmospheres

Oxidation behavior of NiAl and 0.05 at.% Hf doped NiAl alloys were investigated at 1100 °C in dry and humid atmospheres. Hf doping significantly improved the cyclic oxidation resistance. Water vapour promoted the formation of voids at the scale/alloy interface and accelerated scale spallation. Also, water vapour had effect on the phase transformation from θ- to α-Al₂O₃ at the initial oxidation stage. In humid atmosphere, more Hf segregated at the scale/alloy interface to form oxide pegs. Pre-oxidation process in O₂ + Ar could compromise the effect of humid atmosphere on the oxidation kinetics of the NiAl alloys.     

Oxidation of Fe–10Cr in O2 and in O2 + H2O environment at 600 °C: A microstructural investigation

Oxidation of Fe–10Cr in dry and wet O₂ was studied at 600 °C for up to 168 h. Oxide microstructure was investigated by STEM/EDX, FIB/SEM and TEM. Oxidation in dry O₂ gives a Cr-rich protective (Fe_1−xCr_x)₂O₃ scale. The same protective oxide initially forms in O₂ + H₂O environment, but after an incubation period scale breakdown is triggered by CrO₂(OH)₂ evaporation that depletes the substrate in Cr and converts (Fe_1−xCr_x)₂O₃ to FeCr spinel oxide. Internal oxidation occurs after breakaway. Alternating external and internal oxidation result in the inward-growing scale showing a characteristic banded morphology.     

Effects of oxygen partial pressure and atomic oxygen on the microstructure of oxide scale of ZrB2–SiC composites at 1500 °C

The oxidation behaviour of ZrB₂-20 vol.% SiC composites was investigated based on the microstructural evolution of oxide scale under different oxygen partial pressures at 1500 °C, and the similar experiment was performed in atomic oxygen for comparison. The thickness of the oxide scale increases first and then gradually decreases as the pressure decreases, which is strongly dependent on both total pressure and oxygen partial pressure. The atomic oxygen significantly enhances the oxidation of ZrB₂–SiC composites, but has little effect on the microstructure of oxide scale. The oxidation mechanism of ZrB₂–SiC composites is also discussed in detail.     

Corrosion behaviour of Al–29at%Co alloy in aqueous NaCl

Microstructure and corrosion behaviour of a binary Al–29 at%Co alloy have been studied. The alloy was prepared by arc-melting of Al and Co in high purity Ar and rapidly solidified on a water-cooled Cu mould. The alloy chemical composition and microstructure were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Furthermore, the corrosion behaviour was studied by potentiodynamic polarization in aqueous NaCl (0.6 mol dm⁻³) at room temperature. The alloy was found to consist of three phases: hexagonal Al₅Co₂, Z-phase and AlCo (β). The corrosion resistance of different intermetallic phases is characterized. The results are compared to previously published results of Al–TM (TM = transition metal) alloys.     

Effect of surface preparation on the corrosion of austenitic stainless steel 304L in high temperature steam and simulated PWR primary water

The corrosion behavior of 304L grade stainless steel (SS) in high-temperature steam and in a simulated Pressurized Water Reactor (PWR) is studied. The goal was to characterize the nature of the oxide coating generated during 500 h exposure of samples in a 400 °C steam (200 bar) or a 340 °C simulated PWR. Accelerating the effect of the steam environment as well as the influence of surface preparation have been studied. Two initial sample surfaces were used: mechanical polishing and finishing grinding. Oxide coatings were investigated using TEM imaging coupled with EELS spectroscopy and R – SIMS (Secondary Ion Mass Spectroscopy).     

Surface engineering analysis of detonation-gun sprayed Cr3C2–NiCr coating under high-temperature oxidation and oxidation–erosion environments

Detonation-gun (D-gun) spray technology is a novel coating deposition process which is capable of achieving very high gas and particle velocities approaching 4–5 times the speed of sound. This process provides the possibility of producing high hardness coatings with strong adherence. In the present study, this technique has been used to deposit Cr₃C₂–NiCr coating on T22 boiler steel. Investigations on the behaviour of this coating subjected to high-temperature oxidation in air and oxidation–erosion in actual boiler environment at 700 ± 10 °C under cyclic conditions have been carried out. The weight change technique was used to establish the kinetics of oxidation. X-ray diffraction (XRD), field emission-scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS) and EDS elemental mapping techniques were used to analyse the oxidation/oxidation–erosion products. The uncoated boiler steel suffered from a catastrophic degradation in the form of intense spalling of the scale in both the environments. The Cr₃C₂–NiCr coating showed good adherence to the boiler steel during the exposures with no tendency for spallation of its oxide scale.     

First approach for thermodynamic modelling of the high temperature oxidation behaviour of ternary γ′-strengthened Co–Al–W superalloys

In the present work, thermodynamic modelling of the high temperature oxidation behaviour of a γ′-strengthened Co-base superalloy is presented. The ternary Co–9Al–9W alloy (values in at%) was isothermally oxidised for 500 h at 800 and 900 °C in air. Results reveal that the calculated oxide layer sequence (Thermo-Calc, TCNI6) is in good agreement with the formed oxide scales on the alloy surface. Furthermore, prediction of the influence of oxygen partial pressure on Al₂O₃ formation is presented. The modelling results indicate pathways for alloy development or possible pre-oxidation surface treatments for improved oxidation resistance of the material.     

The effects of Cr and Al concentrations on the oxidation behavior of oxide dispersion strengthened ferritic alloys

The oxidation of six oxide dispersion strengthened (ODS) ferritic alloys was investigated at 1050 °C in air up to 200 h. Al plays the dominant role in improving the oxidation resistance of the ODS alloys. Cr and Y are of importance in forming the stable Al₂O₃ scale. To produce the dense alumina layer with enhanced adherence to the metal substrate, the concentrations of Al and Cr should be larger than 2 and 14 wt.%, respectively.     

Erosion–corrosion of HVOF-sprayed Fe-based amorphous metallic coating under impingement by a sand-containing NaCl solution

Erosion–corrosion (E–C) behaviour of a Fe-based amorphous metallic coating (AMC) compared with 304 stainless steel (304 s.s.) was studied under a slurry impingement condition in a sand-containing NaCl solution. AMC exhibited much higher resistance to E–C than 304 s.s. The weight loss reached the maximum at an impact angle of 90° and the critical flow velocity was about 15 m/s for AMC, while 45° and 9 m/s for 304 s.s. The increment in passive current density under impingement for AMC was about 10 times lower than 304 s.s. Passive films formed at different impact velocities were of different thicknesses.