Long-term atmospheric corrosion behaviour of aluminium alloys 2024 and 7075 in urban, coastal and industrial environments

Atmospheric corrosion of alclad and extruded 2024 and 7075 were investigated by weight loss, loss in mechanical properties and depth of pitting over 20 years. The results demonstrated the inner cladding layer on alclad ones had higher corrosion resistance. After 20 years exposure, the cladding had not been penetrated by pitting and those alclads retained their mechanical properties well. Exfoliation occurred on extruded ones in coastal and industrial atmospheres. Especially in coastal atmosphere extruded 2024 suffered severe exfoliation and experienced rapid deterioration of mechanical properties. Furthermore, morphology and chemical compositions of corrosion products were analysed by SEM, XRD and EDS.     

Effect of cooling rate on oxidation resistance and powder ignition temperature of AM50 alloy with addition of yttrium

This paper focused on the effect of cooling rate on oxidation resistance and ignition temperature (T_i) of AM50 alloy. Y addition of 0.0 wt%, 0.15 wt%, 0.28 wt%, 0.45 wt% and 1.00 wt%, respectively was added to the AM50 alloy. The result showed that the oxidation resistance was directly affected by the microstructure. Rapid solidification (RS) had a positive effect on improving the oxidation resistance. It is noticeable that no Al₂Y intermetallic compound was found in the microstructure after RS. Elemental Y dissolved in the solid solution increased with increasing Y addition after RS. It is confirmed that Y addition dissolved in the solid solution and phase distribution were key factors for improving the oxidation resistance.     

Comparison of the dry and wet oxidation at 900 °C of η-Fe2Al5 and δ-Ni2Al3 coatings

η-Fe₂Al₅ and δ-Ni₂Al₃ coatings were correspondingly prepared by aluminizing a carbon steel without and with pre-electrodeposition of a Ni film. Compared to the η-Fe₂Al₅/steel system, the δ-Ni₂Al₃/Ni film/steel system is much better oxidation resistant at 900 °C in the airs without and with 40% water vapor, because of prevention of numerous cavities at the alumina scale/coating interface, and because of mitigation of degradation of the coating due to decreased interdiffusion between the aluminide and the Ni film. Moreover, each aluminide coating has different oxidation kinetics in the dry- and wet air. The reasons for the results are discussed.     

Isothermal oxidation behaviour of Nb-W-Cr Alloys

A study of the effect of Cr content on the microstructure and isothermal oxidation behaviour of four alloys from the Nb-Cr-W system has been performed. Selection of specific alloy compositions has been based on the ternary isothermal sections. Oxidation experiments were conducted in air at 900 and 1300 °C for 24 h under isothermal conditions. Weight gain per unit area as function of the temperature has been used to evaluate the oxidation resistance. The phases present in the alloys and the oxide scales were characterized by XRD, SEM, and EDS. Microstructure consists of Nb solid solution and NbCr₂, Laves phase. The oxidation kinetics follows a parabolic behaviour at 1300 °C; the addition of 30% Cr resulted in the significant reduction of the parabolic oxidation rate. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates in comparison to alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄ and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content and the oxidation temperature. The characterization results delineate the effect of the Cr content on the oxidation mechanisms of these alloys that represent a promising base for high-temperature alloy development.     

Oxidation inhibition of γ-TiAl alloy at 900 °C by inorganic silicate composite coatings

Inorganic silicate composite coatings on γ-TiAl were fabricated by air spraying. The oxidation behaviour of the alloy was investigated at 900 °C. The results indicated that rapid oxidation occurred in the γ-TiAl, and multilayered non-protective TiO₂ and Al₂O₃ scales formed. For coated γ-TiAl alloy, the oxidation was markedly inhibited; a thin Al₂O₃ layer was detected, which improved the oxidation resistance of the alloy. The low oxygen partial pressure at the interface of the coatings and the alloy promotes the preferentially oxidation of Al in the γ-TiAl substrate, and the outward diffusion of Ti to form TiO₂ was retarded.     

A novel ultrafine-grained Ni3Al with increased cyclic oxidation resistance

An ultrafine-grained (UFG) Ni₃Al was fabricated by annealing an electrodeposited Ni–Al composite in vacuum at 600 °C for 2 h. The UFG Ni₃Al, compared to a compositional-similar but coarse-grained (CG) alloy prepared by arc-melting, exhibited a greatly increased cyclic oxidation resistance at 900 °C. Microstructural investigation showed that the CG alloy grew a scale with a high susceptibility to buckling and cracking because of the formation of large voids at the scale/metal interface, but that the UFG alloy grew an adherent scale, because its typical structure prevented the formation of the interface void during oxidation.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

Oxidation of Ti2AlC bulk and spray deposited coatings

The oxidation behaviour of Ti₂AlC bulk and high velocity oxy-fuel spray deposited coatings has been investigated for temperatures up to 1200 °C. X-ray diffraction and electron microscopy show that bulk Ti₂AlC forms a continuous layer of α-Al₂O₃ below a layer of TiO₂ at temperatures as low as 700 °C. Oxidation of the Ti₂AlC coatings is more complex, and also involves the phases Ti₃AlC₂, TiC, and Ti_xAl_y, formed during the spraying process. α-Al₂O₃ is observed, however, it is unevenly distributed deep into the material, and does not form a continuous layer essential for good oxidation resistance.     

Effect of yttrium addition on the oxide scale of AM50 magnesium alloy

This paper focused on the effect of yttrium (Y) addition on oxidation resistance of AM50 alloy. The oxide scale on the surface was studied. Elemental Y in solid solution played an important role in improvement of oxidation resistance and formation of oxide scale. Yttrium (0.28 wt%) addition in AM50 alloy could greatly enhance the oxidation resistance. Furthermore, with increasing Y addition, the scale/substrate adherence was increased. However, when Y addition exceeded 0.28 wt%, it was decreased.     

Effect of low copper content and heat treatment on intergranular corrosion of model AlMgSi alloys

Certain 6000-series extrusions may develop susceptibility to intergranular corrosion (IGC) by improper heat treatment, especially if copper is present as an alloying element. Although occurrence of IGC in such cases is documented, the underlying mechanisms are not adequately explained. We present corrosion data for two model alloys, having different Cu content and Mg:Si ratio, showing that the susceptibility to IGC depended primarily on the Cu content and secondly on thermal processing. Low Cu samples (0.0005 wt.% Cu) were essentially resistant to IGC. High Cu samples (0.12 wt.% Cu), which were air cooled after extrusion, exhibited significant IGC. However, IGC susceptibility was reduced significantly as a result of artificial aging to peak strength. Water quenched high Cu samples were essentially resistant to IGC. However, slight IGC susceptibility was introduced after aging. Electron optical characterisation revealed Al₄Mg₈Si₇Cu₂ (Q-phase) grain boundary precipitates on all the variants susceptible to IGC. The susceptibility was attributed to microgalvanic coupling between Q-phase grain boundary precipitates (noble) and the adjacent depleted zone (active).     

Ablation behavior of ZrB2–SiC–ZrO2 ceramic composites by means of the oxyacetylene torch

Ablation behavior of ZrB₂–SiC–ZrO₂ ceramics with two ZrO₂ contents was investigated using oxyacetylene torch. Thermogravimetric analysis demonstrated that ceramic with 10 vol% ZrO₂ showed initial weight change at higher temperature than the one with 20 vol% ZrO₂. After same ablation condition, lighter oxidized microstructure and lower weight loss and line gain were obtained from ceramic with 10 vol% ZrO₂. Ablation mechanism revealed that excessive ZrO₂ would supply much path to the inward transport of oxygen, which led to the dissatisfactory resistance to oxidation and ablation for the ceramic with 20 vol% ZrO₂.     

Effect of high temperature heat treatment on intergranular corrosion of AlMgSi(Cu) model alloy

Copper containing 6000-series aluminium alloys may become susceptible to intergranular corrosion (IGC) as a result of improper thermomechanical processing. Effect of cooling rate after solution heat treatment on the corrosion behaviour of a model AlMgSi(Cu) alloy of nominal composition (wt%) 0.6 Mg, 0.6 Si, 0.2 Fe, 0.2 Mn and 0.1 Cu was investigated. Slow cooling rates were simulated by isothermal treatment for predetermined times in lower temperature baths immediately after solution heat treatment. Treatment for 10-100 s at temperatures below 400 °C introduced susceptibility to IGC. Longer heat treatment at the same temperatures introduced susceptibility to pitting. A corrosion resistant time zone was found between the zones of IGC and pitting at temperatures lower than 350 °C. Quenching in water after solution heat treatment prevented IGC. IGC was related to microgalvanic coupling between the noble Q-phase (Al₄Mg₈Si₇Cu₂) grain boundary precipitates and the adjacent depleted zone. Pitting was attributed to coarse particles in the matrix. Possible mechanisms causing the corrosion resistant intermediate zone are discussed. The results indicate possible methods for obtaining increased corrosion resistance of similar alloys by proper thermal processing.     

Influence of water vapor on the oxidation behavior of aluminized coatings under low oxygen partial pressure

FeCrNi alloy after aluminizing was oxidized at 1000 °C in dry and humid (2.23 vol.% water) H₂. Experimental results showed that H₂ promotes the formation of θ alumina and its transformation to α alumina. The morphology of surface alumina coating does not change significantly, but the oxidation rate of the aluminized layer accelerates by the addition of water vapor. As a result, more cracks are found beneath the alumina layer when water vapor is present. The addition of water vapor seems having a favorable effect on the selective oxidation of Al and concentration of oxygen vacancy in the aluminized alloys.     

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.     

Carburization behaviour of Mn–Cr–O spinel in high temperature hydrocarbon cracking environment

This investigation was initiated to understand chemical stability of MnCr₂O₄ spinel in high temperature carbonaceous reducing environments. MnCr₂O₄ and Cr₂O₃ were comparatively tested at 1050 °C for various time periods up to 100 h. It was found that the resistance to carburization of MnCr₂O₄ is better than Cr₂O₃, and the carbide conversion rate of MnCr₂O₄ was slower than Cr₂O₃. The more porous structure of Cr₂O₃ than MnCr₂O₄ after carburization was observed. MnCr₂O₄ spinel was proved to be more stable than Cr₂O₃ in carburizing environments according to the results of thermodynamic calculation. After further carburization, coke formation was built up in MnCr₂O₄ due to the catalytic effect of Mn₇C₃ converted from MnO.     

Magnetron-sputtered oxidation protection coatings for Mo–Si–B alloys

Oxidation protective Mo–70Al, Mo–37Si–15B and Mo–46Si–24B (at.%) coatings with 5–10 μm thickness were deposited on Mo–9Si–8B alloys by magnetron sputtering; and their oxidation behavior was studied at 800, 1000 and 1300 °C in air. On the Mo–70Al layer a dense aluminum borate scale grew at 800 °C; however, this coating rapidly degraded at 1000 °C linked to substrate oxidation at uncoated areas. The Mo–37Si–15B and Mo–46Si–24B layers provided oxidation protection to the Mo–Si–B alloy at 800 and 1000 °C for up to 100 h due to formation of a borosilicate scale. The latter coating was protective for short times even at 1300 °C.     

Effect of water vapor on the phase transformation of alumina grown on NiAl at 950 °C

This particular study includes the analysis of the effect of H₂O on promoting the phase transformation in thermally grown aluminum oxides formed on NiAl at 950 °C. Oxidation of NiAl is carried out at 950 °C in O₂ and O₂ + 15 vol.% H₂O. It is observed that transient alumina initially formed on NiAl transforms to stable α-alumina in the presence of water vapor which promotes the subject transformation and eventually results in a compact scale, offering superior oxidation resistance. Present study includes the analysis of θ to α-alumina transformation under the effect of temperature and environment.     

Behaviour of the alloy AA2017 in aqueous solutions of NaCl. Part I: Corrosion mechanisms

This paper reports a study carried out on the morphological characteristics the Al-Cu alloy AA2017-T3; its behaviour against corrosion in aqueous solutions of NaCl 0.59 M has been evaluated by immersion during 0-48 h. The techniques employed for this study are SEM and EDS. The results obtained have been verified by electrochemical assays based on monitoring the corrosion potential of the system in OCP, LP and EIS. The behaviour of the intermetallics present in the matrix of the alloy suggests that the design of an effective system of protection should involve the use of cathodic inhibitors.     

Atmospheric corrosion of carbon steel in Colombia

The corrosion behaviour of carbon steel at six test sites in Colombia and its relationship with exposure time and environmental characteristics of each site were investigated. The corrosion products were characterized by XRD, SEM and EDS. It was found that in Barranquilla, the most aggressive site, corrosion depends mainly on chlorides. Furthermore, in the more aggressive environments there was a greater tendency to formation of protective corrosion products. Lepidocrocite and goethite were found as major constituents of rust. A structure not reported in the literature was found, corresponding to strings of several hundred micrometers long and consisting of lepidocrocite plates.