Microstructure and corrosion behaviour of Al2O3–13 wt-% TiO2 laser alloyed magnesium alloys

Nanostructured Al₂O₃–13?wt-% TiO₂ was prepared on AZ91D magnesium alloy surface by laser surface alloying to improve its corrosion resistance. The microstructure of the laser surface alloyed specimens before and after corrosion tests was characterised by scanning electron microscope and optical microscope (OM). The phase and element composition were investigated by X-ray diffractometer and energy-dispersive spectrometry. An electrochemical workstation was used to evaluate the corrosion behaviour of the specimens. Results showed that the laser surface alloyed layer was primarily composed of Mg and Mg₁₇Al₁₂. Al₂O₃ and TiO₂ existed in the form of agglomerated particles. The corrosion resistance was improved after laser surface alloying.     

Improving the properties of cold-rolled Al–6%Ni sheets by alloying and heat treatment

The mechanical properties of a cold-rolled binary eutectic Al–Ni alloy can be considerably improved by additional alloying with Zr and heat treatment of a billet. Two-stage annealing of billets results in the favourable morphology changes in the eutectics structure and in the substantial hardening with participation of Al₃Zr dispersoids. The cold-rolled material starts to soften only at temperatures as high as 350 °C. The main reason of softening at higher temperatures is the beginning of recrystallization with the formation of subgrains and grains of submicron size.     

Electrodeposition of multi-layer Pd–Ni coatings on 316L stainless steel and their corrosion resistance in hot sulfuric acid solution

Pd–Ni coating shows good corrosion resistance in strong corrosion environments. However, in complex aggressive environments, the performance of the coatings is limited and further improvement is necessary. The effects of the applied plating current density on the composition, structure and properties of Pd–Ni coatings were studied. By changing the current density in the same bath, multi-layer Pd–Ni coatings were prepared on 316L stainless steel. Scanning electronic microscopy, weight loss tests, adhesion strength, porosity and electrochemical methods were used to study the corrosion resistance of the films prepared by different coating methods. Compared with the single layer Pd–Ni coating, the multi-layer coatings showed higher microhardness, lower internal stress, lower porosity and higher adhesive strength. The multi-layer Pd–Ni coating showed obviously better corrosion resistance in hot sulfuric acid solution containing Cl^?.     

The improvement of high-temperature oxidation of Ti–50Al by anodic coating in the phosphoric acid

The high temperature cyclic oxidation resistance of Ti–50Al can be improved by anodic coating in phosphoric acid aqueous solution (4 wt% H₃PO₄) at 18°C. Sparking occurs sporadically on the surface as the voltage is over 300 V and the instantaneous current density after 45 min of anodization increases with increasing voltage. The anodic films are amorphous and contain substantial amount of phosphorus. Cyclic oxidation test indicates that the anodization can remarkably reduce the oxidation in air at 800°C and the improvement increases with increasing anodizing voltage up to 400 V, at which the parabolic oxidation rate constant can be reduced to about 1/600 of that for as-homogenized Ti–50Al. Raman spectra show that the anodic film can slow down the formation of rutile and α-Al₂O₃ during oxidation. The doping effect of phosphorus ions in titanium oxide accounts for the improvement of high temperature oxidation of Ti–50Al.     

Microstructure and corrosion behaviour of ferritic steel–Zr-based metal waste form alloys in simulated ground water

ABSTRACT

The microstructure and corrosion behaviour of the metal waste form (MWF) alloys based on ferritic steel with Zr content in the range of 3–15?wt-% were investigated. The MWF alloys are composed of α-Fe and Fe-Zr phases and with the increase of Zr content, α-Fe phase gradually decreases and the relative content of Fe–Zr intermetallic phase also increases. TEM and XRD confirmed the presence of Fe₂Zr and Fe₂₃Zr₆ intermetallics. Potentiodynamic polarisation curves showed The MWF alloys exhibited passivation behaviour in the simulated Kalpakkam (KGW) and Rajasthan ground water (RGW) media. Electrochemical impedance spectra revealed improved passive film stability in RGW than in KGW which is related to the formation of more stable adherent insoluble passive film in RGW. Higher Zr containing MWF alloy exhibited higher corrosion resistance than lower Zr containing MWF alloys. The relative content of Fe–Zr intermetallics is attributed for the corrosion resistance of The MWF alloys.     

Novel multilayer Mg–Al intermetallic coating for corrosion protection of magnesium alloy by molten salts treatment

A novel multilayer Mg–Al intermetallic coating on the magnesium alloy was obtained by AlCl₃–NaCl molten salt bath treatment. The molten salt was treated at 400 °C, which is lower than the treatment temperature of solid diffusion Al powder. The thick Al₁₂Mg₁₇, Al_0.58Mg_0.42 and Al₃Mg₂ multilayer Mg–Al intermetallic coating forms on the magnesium alloy. The corrosion resistance of AZ91D alloy with and without coating by multilayer of Mg-Al intermetallic compound was evaluated by electrochemical impedance spectroscopy measurements in 3.5% (mass fraction) NaCl solution. The polarization resistance value of the multilayer coating on the magnesium alloy by molten salt bath treatment is greater than that of the uncoated one, which is attributed to the homogenously distributed intermetallic phases.     

Study of the pitting corrosion of superduplex stainless steel and X-65 carbon steel during erosion–corrosion by cyclic polarisation technique

Erosion-enhanced corrosion behaviour of X-65 carbon steel and UNS S32750 superduplex stainless steel was investigated by electrochemical cyclic polarisation. The tests were performed using a jet slurry device coupled with a potentio-galvanostat at various jet velocities of 4, 6.5 and 9?m?s^?1 and impingement angles of 30 and 90? in a 3.5?wt-% NaCl water containing 6?wt-% silica sand particles. The results showed that increasing the jet velocity and impingement angle increased the corrosion rate of both alloys. Negative hysteresis and greater E_rp than OCP were observed for superduplex stainless steel in all erosion–corrosion conditions that indicated the pitting resistance of the alloy. However, the low resistance of carbon steel against pitting during erosion–corrosion was demonstrated by positive hysteresis in the cyclic polarisation curves as well as SEM images of the eroded surfaces.     

Thermomechanical treatment and corrosion resistance correlation in the AA2198 Al–Cu–Li alloy

ABSTRACT

The influence of T3, T8 and T851 thermomechanical treatments on the microstructure and corrosion resistance of the AA2198 was investigated. Differential scanning calorimetry and scanning electron microscopy were used for microstructural characterisation, whereas electrochemical methods were employed to analyse the corrosion behaviour of the alloy. The morphology and composition of constituent particles were similar for the T3 and T8 thermomechanical treatments but varied in the T851. There was an inverse relation between T1 phase density and corrosion resistance. The T3 treatment with the highest corrosion resistance was the one with the lowest density of T1 phase. The mechanisms of corrosion varied with the thermomechanical treatments.     

The effect of heat treatment on properties of Ni–P–SiO<Subscript>2</Subscript> nano-composite coating

In this study, the surface morphology of Ni–P–SiO₂ composite coating was investigated by field emission scanning electron microscopy (FESEM). The amount of SiO₂ in the coating was examined by energy dispersive analysis of X-ray (EDX) and the Corrosion behavior of coating was evaluated by electrochemical impedance spectroscopy (EIS) and polarization techniques, showing the corrosion resistance of Ni–P–SiO₂ diminished after heat treatment. The results showed that in the coating with 12.5 g/L SiO₂, the coating hardness enhanced from 453VH to 980 VH before and after heat treatment. Furthermore, the wear behavior of the coating was analyzed before and after heat treatment.     

Enhanced high-temperature corrosion resistance of (Al2O3–Y2O3)/Pt micro-laminated coatings on 316L stainless steel alloy

A 7-layer (Al₂O₃–Y₂O₃)/Pt micro-laminated coating was successfully prepared on 316L stainless steel alloy by magnetron sputtering. High-temperature cyclic oxidation and hot corrosion tests were adopted to investigate the high-temperature corrosion resistance of the coating. It is revealed that the (Al₂O₃–Y₂O₃)/Pt micro-laminated coating which effectively suppressed the inward diffusion of oxygen and corrosive fused salt to an extremely low level can significantly improve the high-temperature corrosion resistance of alloy substrate. The great mechanical properties of such coating were attributed to the brittle/ductile laminated composite structure by means of multilayer toughening and release mechanisms.     

Wear and corrosion studies of Fe–B–Cr alloy coating on en 24 steel by HVOF thermal spray method

This work investigate the wear behavior of Fe–B–Cr coatings on medium carbon steel (EN24) substrate is used for several automotive parts. The high velocity oxy-fuel (HVOF) method was used to create the new crystalline coating of Fe–B–Cr (composition of 59%Fe–26%B–15%Cr in wt %) on a medium carbon steel substrate (AISI 4340). The characteristics of powder and coating are investigated using scanning electron microscopy (SEM) merged with energy dispersive spectroscopy (EDS), optical microscopy (OM) and thermogravmentric analysis (TGA) which were undertaken in the partial characterization of the coating. The phase contents of both powder and coatings were studied by X-ray diffraction (XRD). The coatings consist of melted and un-melted particles identified in the coatings. Moreover, oxides and micro-cracks were observed at the surface. The mechanical property of the coatings was characterized using a microhardness test. The hardness value increased three times more than the substrate. The coated surface showed lower levels of porosity. Moreover, the electrochemical investigation found Fe–B–Cr coating on medium carbon steel. The corrosion test was carried out in an environment with 0.5 M of NaCl, which showed that the corrosion resistance improved by coating.     

Oxidation of Al2O3-dispersion chromizing coating by pack-cementation at 800℃

Preparation and oxidation of an Al2O3-dispersed chromizing coating were investigated by chromizing an aselectrodeposited Ni-Al2O3 nanocomposite film using a conventional pack-cementation method at a greatly decreased temperature (800℃).For comparison,chromizing was also performed with the same condition on an as-deposited Ni film without Al2O3 nanoparticles.Oxidation at 900℃ indicates that,compared with the Al2O3-free chromizing coating,the Al2O3-dispersed chromizing coating exhibits a increased oxidation resistance,due to the formation of purer and denser chromia scale.The effect of Al2O3 on the coating formation and the coating oxidation behavior was discussed in details.     

Non-equilibrium phase synthesis in Al2O3–Y2O3 by spray pyrolysis of nitrate precursors

The phase evolution in the Al₂O₃–Y₂O₃ system has been studied for 4, 10, 15 and 37.5% Y₂O₃ amorphous powders prepared by spray pyrolysis of nitrate precursor solutions. Two distinct metastable transformation sequences were identified for the amorphous powders upon heat treatment. Crystallisation accompanied by partitioning leads to a mixture of hexagonal YAlO₃ and γ-Al₂O₃ (spinel structure). Partitionless crystallisation, on the other hand, leads directly to a γ-Al₂O₃ solid solution in dilute alloys, and garnet at temperatures as low as 800°C in the stoichiometric composition provided segregation is avoided during decomposition. Further heat treatment of a yttria-supersaturated γ-Al₂O₃ leads to the precipitation of the orthorhombic YAlO₃ that is stable up to temperatures as high as 1600°C, while hexagonal YAlO₃ converts to garnet, Y₃Al₅O₁₂. A rationalisation of the phase evolution sequence has been attempted on the basis of kinetic considerations, cation coordination and semi-quantitative free energy–composition curves for the various competing phases.     

Effects of MgO/Al2O3 ratio on viscous behaviors and structures of MgO–Al2O3–TiO2–CaO–SiO2 slag systems with high TiO2 content and low CaO/SiO2 ratio

The effects of MgO/Al₂O₃ ratio on the viscous behaviors of MgO–Al₂O₃–TiO₂–CaO–SiO₂ systems were investigated by the rotating cylinder method. Raman spectroscopy was used to analyze the structural characteristics of slag and Factsage 7.0 was adopted to demonstrate the liquidus temperature of slag. The results show that the viscosity and activation energy for viscous flow decrease when the MgO/Al₂O₃ ratio increases from 0.82 to 1.36. The break point temperature and liquidus temperature of slag initially decrease and subsequently increase. The complex viscous structures are gradually depolymerized to simple structural units. In conclusion, with the increase of MgO/Al₂O₃ ratio, the degree of polymerization of slag decreases, which improves the fluidity of slag. The variations of liquidus temperature of slag lead to the same changes of break point temperature.     

Improvement in the Oxidation Resistance of an Al-Deposited Fe–Cr–Al Foil by Preoxidation

The oxidation kinetics of conventional Fe–20Cr–5Al (in mass %) foil, Al-deposited foil and Al-deposited and preoxidized foil was studied at 1373 K in air. All the foils were 50-m thick and contained minor additions of rare-earth elements. The oxide scales were observed with SEM and TEM combined with EDS and were characterized with X-ray diffractometry and electron diffraction. The deposition of Al onto the foil from the vapor phase improves oxidation resistance. The details regarding this matter were reported elsewhere. The combination of the Al deposition and the subsequent preoxidation at 1173 K for 90 ks in air further increases the oxidation resistance, i.e., the smallest parabolic rate constant among the three kinds of foils, and excellent scale adherence. Preoxidation enhances the growth of -Al₂O₃, which transforms to -Al₂O₃ during subsequent oxidation. However, such -Al₂O₃ grains are much larger than those formed on the conventional foil of similar chemical composition. Small closed voids and small spinel-type, oxide particles appear in -Al₂O₃ grains with the progress of oxidation. The former is explained in terms of the volume decrease accompanying the phase transformation and the latter by the low solubility of Fe in -Al₂O₃.     

Structure and nanomechanical characteristics of Al–Cu–Mg–Si alloy with partly liquated grain boundaries upon heat treatment

The microstructure, phase composition, and mechanical characteristics of the structural constituents of an Al–Cu–Mg–Si alloy in which the liquation of grain boundaries occurred during heat treatment have been studied. Bands of the (Al + Al₁₅(Fe, Mn)₃Si₂) eutectics have been observed at the grain boundaries. An algorithm for calculating the additional pressure, which results from mechanical impact on the metal containing these bands has been described.     

Effect of heat treatment on mechanical and wear properties of Zn–40Al–2Cu–2Si alloy

In order to determine the effect of heat treatment on the mechanical and wear properties of Zn–40Al– 2Cu–2Si alloy, different heat treatments including homogenization followed by air-cooling (H1), homogenization followed by furnace-cooling (H2), stabilization (T5) and quench–aging (T6 and T7) were applied. The effects of these heat treatments on the mechanical and tribological properties of the alloy were studied by metallography and, mechanical and wear tests in comparison with SAE 65 bronze. The wear tests were performed using a block on cylinder type test apparatus. The hardness, tensile strength and compressive strength of the alloy increase by the application of H1 and T6 heat treatments, and all the heat treatments except T6, increase its elongation to fracture. H1, T5 and T6 heat treatments cause a reduction in friction coefficient and wear volume of the alloy. However, this alloy exhibits the lowest friction coefficient and wear volume after T6 heat treatment. Therefore, T6 heat treatment appears to be the best process for the lubricated tribological applications of this alloy at a pressure of 14 MPa. However, Zn–40Al–2Cu–2Si alloy in the as-cast and heat-treated conditions shows lower wear loss or higher wear resistance than the bronze.     

A comparative study of corrosion inhibition of steel and stainless steel in hydrochloric acid by N,N,N′,N′-tetramethyl-p-phenylenediamine

Protection of Metals and Physical Chemistry of Surfaces - The corrosion of mild steel (MS) and AISI type 321 stainless steel (AISI 321) in 1 M HCl solution and the inhibitive mechanism of...     

Development of in situ NiAl–Al2O3 nanocomposite by reactive milling and spark plasma sintering

NiAl–10 vol.% Al₂O₃ in situ nanocomposite has been synthesized by reactive milling and subsequent spark plasma sintering. The synthesized nanocomposites have ～96% of theoretical density after sintering at 1000 °C for 5 min. Microstructural analysis of consolidated samples using TEM has revealed the presence of α-Al₂O₃ particles of 10–12 nm size in NiAl matrix of submicron grain size. Consolidated NiAl–10 vol.% Al₂O₃ nanocomposite has shown very high hardness of 772 HV_0.3 and compressive strength of 2456 MPa with ～14% plastic strain. The high hardness and compressive yield strength are attributed to the presence of nanocrystalline α-Al₂O₃ particles and the appreciable plastic strain is attributed to the submicron grains of NiAl.     

Al2O3–TiO2 composite coatings with enhanced anticorrosion properties for 316L stainless steel

316L stainless steel is used as an important structural material in various industries. However, its service life is limited in the presence of chloride ions due to severe chemical corrosion. Herein, a facile radiofrequency magnetron sputtering process is reported for the synthesis of various Al₂O₃–TiO₂ composite coatings as an anticorrosion layer for 316L stainless steel substrates. The enhanced chemical stability of Al₂O₃–TiO₂ composite coatings was investigated by X-ray photoelectron spectroscopy, electron paramagnetic resonance, and X-ray diffraction measurements. Moreover, the high specific surface area of Al₂O₃–TiO₂ composite coatings displayed better hydrophobic property which can be confirmed by scanning electron microscopy and contact angle measurements. Finally, the direct characterization of anticorrosion properties was carried out using electrochemical tests. All of the above results exhibited the enhanced anticorrosion properties of Al₂O₃ coating after the incorporation of TiO₂. Significantly, the Al₂O₃–TiO₂ composite coatings with 15.56% Ti content provided the best corrosion resistance for 316L stainless steel.