Al-rich Layer Research Articles

The Transition from Transient Oxide to Protective Al2O3 Scale on Fe–Cr–Al Alloys During Heating to 1000 °C

The transition behavior of an Al-rich amorphous oxide layer to an external Al2O3 layer on Fe–(4, 24)Cr–(6, 10)Al (at.%) alloys was investigated during heating to 1000 °C at a heating rate of 50 °C/min, by means of in situ high-temperature X-ray diffraction measurement and TEM observation. In the alloy containing 6Al, internal amorphous Al2O3 was initially developed below the Al-rich amorphous surface layer. The amorphous internal precipitates transformed to be crystalline and grew laterally with time. The internal precipitates subsequently connected with each other to form a continuous α-Al2O3 scale. In the case of 10Al alloy, an Al-rich amorphous layer transitioned to a crystalline α-Al2O3 layer from the interface between transient/amorphous layers during heating. The Al2O3 scale developed on high Al alloys contained Fe and Cr with relatively higher contents, but that formed on low Al alloy contained low Fe and Cr. The effect of Cr on promoting an external Al2O3 scale formation was found to be weaker for alloys with higher Al content compared to the alloys with lower Al content, if Al2O3 scale was directly transitioned from the amorphous layer.

Improvement of the electrochemical performance of an NCA positive-electrode material of lithium ion battery by forming an Al-rich surface layer

A positive-electrode material of LiNi0.80Co0.15Al0.05O2 (NCA) having an Al-rich surface layer was synthesized, and its characteristics and electrochemical performance were investigated and compared with those of pristine NCA prepared by a conventional solid-state reaction. The Al-rich surface layer with its compositional gradient was formed by a wet-coating method using a fluidized bed coating technique and a subsequent heat treatment: fluidized NCA powder and a mist of 2-propanol solution containing an aluminium oxide sol were mixed in a chamber, followed by heat treatment at 600 °C in an oxygen atmosphere. For the electrochemical measurement, the positive electrode fabricated using the positive-electrode material was assembled into a 2032-type coin cell with lithium metal as an anode and LiClO4 in ethylene carbonate-diethyl carbonate as an electrolyte. The measurement revealed that the Al-rich surface layer effectively contributed to the improvement of charge-discharge cycle life performance; NCA with the Al-rich surface layer retained 96% of the initial capacity after 100 cycles, compared to the 90% retained by pristine NCA. For a better understanding of the improvement, the structure of the positive-electrode materials after 100 cycles was examined by scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM). High-angle annular dark-field signal images obtained from STEM showed no clear difference between these materials in terms of the growth of inactive rock-salt-like structure layer near the surface of the primary particles. On the other hand, low-magnification SEM observation revealed that the pristine NCA had widened microcracks among the primary particles in the secondary particles, while the NCA with the Al-rich surface layer did not. Therefore, the Al-rich surface layer improved the fracture strength of the secondary particles during the charge-discharge cycling, thus preventing the formation of isolated primary particles, which do not contribute to the charge-discharge behaviour.

Aluminide formation on Alloy 800 by plasma spraying and heat treatment

ABSTRACTSpecimens of Fe–Ni–Cr based Alloy 800 were subjected to atmospheric plasma spraying using aluminum powder. Quantitative microscopic analysis of as-sprayed specimen revealed formation of aluminum-rich oxygen-containing layer (∼100 µm) on the uppermost surface, while adjoining layer (∼25 µm) comprising oxygen-rich Fe–Ni–Cr–Al-containing phase. Microscopy with quantitative analysis of as-sprayed + heat-treated (at 1273 K) substrate revealed formation of FeAl type layer (∼80 µm) on the uppermost surface with an Al-rich intermediate layer (∼25 µm) adjacent to substrate. Knoop hardness number was obtained as 764 (±20), 477 (±10), and 210 (±10) for FeAl layer, Al-rich intermediate layer, and substrate, respectively. During scratch tests at 2, 4, and 6 N loads, friction coefficient for the aluminides was recorded in the range of ∼0.05–0.15, while that was ∼0.18 for the substrate. Scratched surfaces at 2 and 4 N load tests revealed neither cracking nor peeling off at aluminides or interfaces indicating good adherence, whereas cracking occurred at aluminides during 6 N load test. Penetration depths for the aluminides were recorded as lower than that of the substrate during the scratch tests. X-ray photoelectron spectroscopy on thermally oxidized FeAl layer revealed formation of Al2O3 type oxide on the surface.

Oxidation of an austenitic stainless steel with or without alloyed aluminum in O2 + 10% H2O environment at 800 °C

Oxidation behavior in O2+10% H2O at 800°C of an austenitic stainless steel with or without aluminum is investigated. The Al-free steel enters into breakaway oxidation stage after only 15h exposure, while the one with 2.7wt% aluminum shows high oxidation resistance. This minor amount of aluminum is lower than the critical content to form an intact alumina scale, but it promotes the formation of a thin Al-rich inner layer which is favor in prohibiting holes formation due to volatilization of CrO2(OH)2 and Kirkendall effect. A compact scale develops and protects the Al-containing steel very well.

The Effect of Cr on the Lifetime of Al-Rich Amorphous Oxide Layer Formed on Fe–Cr–Al Alloys at 650 °C

The oxidation behavior of Fe–6Al with different Cr contents (0–24 at.%) at 650 °C in air was investigated so as to clarify the role of Cr in the oxidation resistance of the Al-rich amorphous oxide layer. The time to breakdown of the Al-rich amorphous layer was found to increase with increasing alloy Cr content. This corresponds to the time to increase the rate of oxidation by formation of Fe3O4. Such a beneficial effect of Cr to maintain the protective Al-rich amorphous layer for a longer oxidation time is attributed to the enhancement of outward diffusion flux of Al by positive “cross-term effect” of Cr in the Al- and Cr-depleted zone.

Effect of sodium chloride on the electrochemical corrosion of Inconel 625 at high temperature and pressure

The effect of sodium chloride (NaCl) on the electrochemical corrosion of the Inconel 625 at 300 ± 2 °C and 16 ± 1 MPa was investigated by polarization curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). The experimental results showed that NaCl stimulated the Inconel 625 electrochemical corrosion and the oxide film on the surface of corroded Inconel 625 became less stable with increasing NaCl concentration. The oxide film was duplex with an inner Mg, Al-rich layer of MgO+Al2O3, with probably some Cr2O3, and an outer Ni-rich layer, principally Ni(OH)2/NiO. The inner layer was more protective than the outer layer due to the precipitation of magnesium and aluminum oxide in the presence of Cl−. In order to explain the corrosion behavior of the Inconel 625, an oxide film growth mechanism was discussed in this research.

High-Temperature Oxidation Behavior of Refractory High-Entropy Alloys: Effect of Alloy Composition

The high-temperature oxidation behavior of a new family of refractory high-entropy alloys (HEAs) with compositions of W–Mo–Cr–Ti–Al, Nb–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al was studied at 1000 and 1100 °C. Based on these equimolar starting compositions, the main incentive of this study was to select the most promising alloy system whose properties may then be successively improved. Despite the high amount of refractory elements, Ta–Mo–Cr–Ti–Al showed good oxidation resistance at 1000 and 1100 °C. Moderate values of mass gain and complex oxidation kinetics were observed for the W- and Nb-containing HEAs. These alloys formed inhomogeneous oxide scales possessing regions with thick and porous layers as well as areas revealing quite thin oxide scales due to the formation of discontinuous Cr- and Al-rich scales. The most promising behavior was shown by the alloy Ta–Mo–Cr–Ti–Al which followed the parabolic rate law for oxide growth due to the formation of a thin and compact Al-rich layer.

Formation of Al2O3/Fe-Al layers on SS 316 surface by pack aluminizing and heat treatment

Formation of different iron aluminide (Fe-Al) layers during pack aluminizing of stainless steel 316, and subsequent heat treatments has been studied in detail. The single Fe2Al5 layer formed during pack aluminization carried out in the temperature regime 450–650 °C is transformed into FeAl/Fe(Al) layers in the course of post aluminizing heat treatments conducted at higher temperatures. The iron aluminide layers were characterized using SEM, BSE, EDS and EBSD techniques. The precipitates of Cr5Al8 are formed mainly in the Al-rich Fe2Al5 layer, and they get completely dissolved in FeAl layer developed after heat treatment at 1050 °C. The concentration of porosities in the FeAl or Fe(Al) is reduced when the heat treatment temperature is lowered from 1050 to 900 °C. Controlled oxidation approach has been adopted to study the formation and phase transition (γ → θ → α) behavior of alumina (Al2O3) scale in the temperature regime of 700–1050 °C.

Microstructure evolution of a Zn-Al coating co-deposited on low-carbon steel by pack cementation

In this study, Zn–Al coatings were co-deposited on AISI 1020 steel by pack cementation. The as-prepared coatings were characterised by scanning electron microscopy, energy–dispersive X–ray spectroscopy, and X–ray diffraction. The evolution mechanism of the coating was modelled and analysed in detail. Furthermore, the corrosion resistance of the as-received coatings was investigated by a potentiodynamic polarisation test in a 3.5% NaCl solution. We observed that the relatively loose and porous structure of those coatings was a function of the Al content in the packed powders. The regions with a natural porous structure evidently increase with an increase in Al concentration. The Zn-rich layer was formed because of the dominant inward diffusion of Zn with a small diffusion of Fe in the substrate. Thus, the Zn-rich layer/substrate interface moved into the ferrous substrate. As the reaction continued, the deposition of Al dominated. The interdiffusion of Al and Zn in the Zn-rich layer and the diffusion of Fe into the Zn-rich layer towards the Al-rich layer led to the nucleation and growth of the Fe–Al phases and the consumption of the Zn-rich layer. As a consequence, the Al-rich layer/Zn-rich layer interface moved inside the Zn-rich layer. At higher deposition temperatures, when the heat was released because of the Fe–Al phase formation, the evaporation of Zn resulted in void formation in the Al-rich layer. The potentiodynamic polarisation test results show that both Zn and Zn–Al coatings significantly enhance the corrosion resistance of a ferrous substrate, but Zn–Al coatings seem to be more efficient.

High-rate and long-life performance of a truncated spinel cathode material with off-stoichiometric composition at elevated temperature

A truncated octahedral structure spinel cathode material with a nominal composition of Li1.08Al0.08Mn1.85Co0.05O3.9F0.1 (LAMCOF) and an Al-rich surface layer is successfully prepared by a sol-gel route combined with a temperature-controlled heat-treatment process. Concentration gradient of Mn4+ ions is found to decrease through the surface to the interior of the as-prepared sample. As the consequence, the LAMCOF sample delivers high rate capability and excellent cycling stability at elevated temperature, showing an initial discharge capacity of 111.1mAhg−1 and capacity retention of 70.5% over 850 cycles at 1C rate under 55°C. Even at 5C rate under 55°C, it also displays a high capacity of 102.5mAhg−1 with capacity retention of 80.0% over 850 cycles. These results reveal the importance of an off-stoichiometric strategy and a temperature-controlled heat-treatment process for the preparation of LiMn2O4-based spinel cathode material with unique crystal orientation and Mn4+-rich surface layer, which both decrease the Mn dissolution and provide interfacial stability while preserving fast Li+ diffusion, resulting in improvement of cycle performance and rate capability of the as-prepared material. This proposed synthesis method does not need a supplementary coating process and is likely suitable to prepare other high performance cathode materials.

Role of Adsorption Phenomena in Cubic Tricalcium Aluminate Dissolution.

The workability of fresh Portland cement (PC) concrete critically depends on the reaction of the cubic tricalcium aluminate (C3A) phase in Ca- and S-rich pH >12 aqueous solution, yet its rate-controlling mechanism is poorly understood. In this article, the role of adsorption phenomena in C3A dissolution in aqueous Ca-, S-, and polynaphthalene sulfonate (PNS)-containing solutions is analyzed. The zeta potential and pH results are consistent with the isoelectric point of C3A occurring at pH ∼12 and do not show an inversion of its electric double layer potential as a function of S or Ca concentration, and PNS adsorbs onto C3A, reducing its zeta potential to negative values at pH >12. The S and Ca K-edge X-ray absorption spectroscopy (XAS) data obtained do not indicate the structural incorporation or specific adsorption of SO42- on the partially dissolved C3A solids analyzed. Together with supporting X-ray ptychography and scanning electron microscopy results, a model for C3A dissolution inhibition in hydrated PC systems is proposed whereby the formation of an Al-rich leached layer and the complexation of Ca-S ion pairs onto this leached layer provide the key inhibiting effect(s). This model reconciles the results obtained here with the existing literature, including the inhibiting action of macromolecules such as PNS and polyphosphonic acids upon C3A dissolution. Therefore, this article advances the understanding of the rate-controlling mechanism in hydrated C3A and thus PC systems, which is important to better controlling the workability of fresh PC concrete.

Dense and high-stability Ti2AlN MAX phase coatings prepared by the combined cathodic arc/sputter technique

Ti2AlN belongs to a family of ternary nano-laminate alloys known as the MAX phases, which exhibit a unique combination of metallic and ceramic properties. In the present work, the dense and high-stability Ti2AlN coating has been successfully prepared through the combined cathodic arc/sputter deposition, followed by heat post-treatment. It was found that the as-deposited Ti-Al-N coating behaved a multilayer structure, where (Ti, N)-rich layer and Al-rich layer grew alternately, with a mixed phase constitution of TiN and TiAlx. After annealing at 800°C under vacuum condition for 1.5h, although the multilayer structure still was found, part of multilayer interfaces became indistinct and disappeared. In particular, the thickness of the Al-rich layer decreased in contrast to that of as-deposited coating due to the inner diffusion of the Al element. Moreover, the Ti2AlN MAX phase emerged as the major phase in the annealed coatings and its formation mechanism was also discussed in this study. The vacuum thermal analysis indicated that the formed Ti2AlN MAX phase exhibited a high-stability, which was mainly benefited from the large thickness and the dense structure. This advanced technique based on the combined cathodic arc/sputter method could be extended to deposit other MAX phase coatings with tailored high performance like good thermal stability, high corrosion and oxidation resistance etc. for the next protective coating materials.

Oxidation protection of several intermetallic TixAly alloys by fluorine

The oxidation resistance of γ-TiAl alloys can be dramatically improved by the fluorine effect. The Al content is a critical factor. It has to be above at least 40 at.% to get the fluorine effect to operate. Alloys with an Al content near this value or below have to be enriched with Al in advance. For this purpose, aluminisation is one possibility. The Al content within the diffusion layers can be determined by variation of the pack parameters, i.e. Al activity, time and temperature. These Al-rich diffusion layers form a protective alumina layer during high-temperature exposure in oxidising environments, but they have to be treated subsequently with fluorine to stabilise their protective effect. Results of high-temperature exposure tests of several untreated and treated alloys will be presented in this paper. Post experimental metallographic investigations reveal the formed oxide scales so that the effects can be rated. Finally, the results will be discussed in view of the known fluorine effect for technical TiAl alloys.

(Invited) Role of Transient Oxide Scale for Development and Growth of Al2O3

It is well-known that Cr addition to Al2O3 forming alloys and coatings are beneficial to decrease the critical Al content for an external Al2O3 scale formation. This beneficial Cr effect is known as a Third Element Effect, TEE. The TEE explains that initial Cr2O3 scale formation in the transient oxidation period reduces the oxygen potential at the surface of alloy, promoting the exclusive Al2O3 scale formation. However, our previous study of oxidation of Fe-low Cr-Al alloys found that an Al2O3 scale was formed on alloys with lower Al content regardless initial Cr2O3 scale formation, which suggests that decreased oxygen potential due to initial Cr2O3 scale formation is not the essential reason for decreasing the critical Al content for an external Al2O3 scale formation. In this study, we investigated the initial oxidation behavior of Fe-(6~10)Al, Fe-(4~24)Cr, and Fe-(4~24)Cr-(6~10)Al (in at.%) model alloys and tried to understand the role of not only Cr but also Fe and Al on development of the Al2O3 scale. Particular attention was paid for the effect of the initial transient oxide scale on development of an external Al2O3 scale. The formation of initially formed transient oxide(s) and its transition to α-Al2O3 during heating, followed by isothermal oxidation up to 10~60min at 1000°C were characterized by in-situ high-temperature X-ray diffraction by means of Synchrotron radiation. The oxide scale formed at different temperatures during heating was observed by TEM. The distribution of each element in the oxide scale was measured by STEM-EDS. The oxide scale initially observed on all alloys by XRD was α-Fe2O3. The transition from initial transient oxide, Fe2O3, to Cr-rich Cr2O3-Fe2O3 solid-solution during heating on the alloys with higher Cr content was confirmed by an in-situ high-temperature XRD experiments. Beneath this transient oxide scale, Al-rich amorphous oxide scale was found to develop on the alloys with Al addition. No Cr effect was observed in terms of the amorphous layer formation. This Al-rich amorphous layer prevented the growth of outer Fe-rich transient oxide. Therefore, the oxidation mass gain of alloys with Al at relatively lower temperatures, ~700°C was much smaller than that of binary Fe-Cr alloys. Break down of the amorphous layer resulted in rapid oxidation of alloys. The time for break down of the amorphous layer became longer with increasing alloy Cr content, which indicates that Cr helped to maintain the Al-rich amorphous layer for a longer oxidation time at lower temperatures, ~700°C. The amorphous layer crystalized to α-Al2O3 on the alloys with higher Al content, when the sample temperature became about 900 to 1000°C. The crystallization to α-Al2O3 occurred continuously from the interface between an outer transient/amorphous layers to the amorphous/alloy interface. However, on the alloy with lower Al content, amorphous internal Al2O3 precipitates were developed and the amorphous layer disappeared accompanied by formation of internal precipitates. Although the volume fraction of internal precipitates increased with increasing alloy Cr content, higher Cr addition did not prevent internal oxidation of Al. During heating, amorphous internal precipitates transformed to α-Al2O3. After the transformation to α-Al2O3, the internal precipitate grew larger and connected each other, forming an α-Al2O3 scale below the transient oxide scale. No apparent effect of the transient oxide scale on formation of the amorphous internal oxide and its transition to an α-Al2O3 layer was observed on alloys with lower Al content. The Al-rich amorphous layer was found to have important role for preventing Fe-rich oxide scale formation for all alloys with Al. The Fe- or Cr-rich transient oxide, which formed in the initial oxidation stage acted as the template for the transformation from amorphous to α-Al2O3 for the alloys with higher Al content, but it did not have apparent role for formation of the α-Al2O3 scale on the alloys with lower Al content.

Suppression of alloy fluctuations in GaAs-AlGaAs core-shell nanowires

Probing localized alloy fluctuations and controlling them by growth kinetics have been relatively limited so far in nanoscale structures such as semiconductor nanowires (NWs). Here, we demonstrate the tuning of alloy fluctuations in molecular beam epitaxially grown GaAs-AlGaAs core-shell NWs by modifications of shell growth temperature, as investigated by correlated micro-photoluminescence, scanning transmission electron microscopy, and atom probe tomography. By reducing the shell growth temperature from T > 600 °C to below 400 °C, we find a strong reduction in alloy fluctuation mediated sharp-line luminescence, concurrent with a decrease in the non-randomness of the alloy distribution in the AlGaAs shell. This trend is further characterized by a change in the alloy compositional structure from unintentional quasi-superlattices of Ga- and Al-rich AlGaAs layers at high T to a nearly homogeneous random alloy distribution at low T.

Design Considerations for Single-Mode Vertical-Cavity Surface-Emitting Lasers With Impurity-Induced Intermixing

We report on the modeling of 850-nm (Al,Ga)As/GaAs vertical cavity surface-emitting lasers (VCSELs), where the electrical and optical confinement is realized by impurity-induced intermixing. The 3-D simulations of electromagnetic fields are performed by the finite-element method using full vector Maxwell’s equations. An example of a transverse junction VCSEL with Zn-induced intermixing of the areas surrounding the quantum well region is compared with the case of a VCSEL, where the optical modes are confined by selective oxidation of the Al-rich GaAlAs aperture layers. We show that Zn-induced intermixing results in a high efficiency of the leakage of the VCSEL aperture-confined modes into the surrounding intermixed area. Furthermore, there is a strong mode selectivity through the leakage loss between the fundamental and excited transverse modes of the optical cavity. This enables potential realization of a single transverse mode VCSEL at the aperture diameters of 10 $\mu \text{m}$ or above and consequently the realization of high-power single-mode VCSELs. Optimum mode selectivity is realized once the impurity intermixing profile proceeds down to the depth of about 2.6 $\mu \text{m}$ into the bottom distributed Bragg reflector below the quantum well region. Leakage of the optical modes into the intermixed area is accompanied by the evolution of narrow tilted lobes in the far-field emission spectra (at $\sim 60^{\circ }$ tilt angles for the design considered). This effect is similar to the one previously observed in oxide-confined leaky VCSELs and originates from the laterally propagating optical modes outside the non-intermixed aperture region. Where necessary, by applying such phenomenon, a phase-coupled array can be configured. We believe that such single-mode VCSELs may become extremely important in 3-D sensing, printing, and laser illumination. Furthermore, LIDARs with an option of beam steering without moving parts can be realized in 2-D phase-coupled arrays.

Combined Electrochemical and Electron Spectroscopic Investigations of the Surface Oxidation of TiAlN HPPMS Hard Coatings

The surface oxidation of TiAlN High Power Pulsed Magnetron Sputtering (HPPMS) hard coatings was investigated using a combined electrochemical and electron spectroscopic approach. The reversible and irreversible surface redox reactions as well as the resulting surface electronic properties were investigated by means of Cyclic Voltammetry (CV) and Electrochemical Impedance Analysis (EIS). X-ray Photoelectron Spectroscopy (XPS) of the TiAlN near-surface region as oxidised in air after deposition revealed a duplex surface film consisting of oxynitridic TiAl(O,N) covered by a shallow Al rich oxide layer. Complementary EIS revealed metallic conductivity of this TiAl(O,N) phase. A detailed microscopic model of the electrochemical oxidation process highlighting the importance of O interstitials for the oxidation process is presented. The electrochemical oxidation induced by CV resulted into a transition from metallic to n-type semiconducting properties of the surface oxide film. As disclosed by XPS and XPS sputter profiling, a complex multi-layered film was formed comprising segregated a-(TiO2)x(Al(OH)3)y located on top of a more ordered electrochemically inactive oxide film. Further, an intermediary nitrogen doped but segregated a-(TiO2)x(Al(OH)3)y:N phase was also observed. The results are of high importance for understanding the electrochemical oxidation and corrosion resistance of these hard coatings.

Annealing effects on the microstructure and properties of an Fe-based Metallic-Intermetallic Laminate (MIL) composite

430SS MIL composites were initially fabricated under semi-solid reaction at 655°C and then annealed at high temperature (1000°C) to obtain the ductile Fe-rich intermetallic phases. The microstructure evolution and micro-hardness distribution across the intermetallic/metal interfaces were investigated, as well as the experimentally and computationally evaluating the growth kinetics of the Fe-rich intermetallic layers. A multilayer structure (namely Al-rich layer, Fe-rich layer and B2 phase layer) with a complex mix of intermetallic phases (FeAl2, Cr5Al8 and FeAl) form in the high temperature annealed 430SS MIL composites. The FeAl phase with a B2 structure formed in the intermetallic layer provides a more gradual transition of mechanical properties across the intermetallic/metal layer. The growth mechanism of the Fe-rich layer is interface controlled, while the growth of the B2 phase layer is diffusion controlled based on both the experimental and numerical analysis. A relationship between the processing-microstructure-properties of the MIL composites has been established and applied to predict and tailor the microhardness distribution across the multi-layered structure of the 430SS MIL composites.

Effect of Mg content on microstructure and corrosion behavior of hot dipped Zn–Al–Mg coatings

In this article, Zn–Al–Mg coatings were prepared by hot dipping method. The surface morphology, cross–section microstructure, microhardness, composition, corrosion behaviour of ZAM coatings were investigated by using X–ray diffraction (XRD), Optical microscope, Environmental scanning electron microscopy equipped with EDS (FESEM–EDS), Microhardness tester and Electrochemical analysis respectively. Corrosion test was also performed in a standard salt fog spray chamber. Microstructure studies indicates that Zn grain size was refined and eutectic areas at Zn grain boundary areas increased with increasing Mg content. ZA5M1.5 and ZA5M2 coatings have two distinct layers. Mg tends to exist in the outer layer while Al is in the inner layer. The inner layer is composed of Al5Fe2Zn0.4 intermetallic, which may to contribute to the microhardness. The outer layer is Zn grains surrounded by Zn–Mg etutectics, which may improve the corrosion resistance. The microhardness is more than 700 HV50g for Al-rich layer and around 151 HV25g for Mg-rich layer. The improved corrosion resistance of Zn–5%Al-1.5%Mg coating comes from the corrosion product of flocculent type simonkolleite, which prolongs the micro-path and impedes the movement of O2 and H2O, ultimately retards the overall corrosion process.

The effect of pre-oxidation treatment on the corrosion behavior of amorphous Al1−xZrx solid-solution alloys

The electrochemical behavior of as-deposited and pre-oxidized amorphous Al1−xZrx solid-solution alloys (x=0.25, 0.32, 0.49, 0.65, 0.74) in 1M HCl was investigated by the micro-electrochemical technique. It was found that, for higher Zr alloying contents, x≥0.49, the as-deposited Al1−xZrx alloys exhibit passive corrosion behavior due to the formation of a protective Zr-rich passive film on the Zr-rich alloy surface. The native oxide layer on the as-deposited Al-rich Al1−xZrx alloys (x<0.49) is much less stable, which rapidly results in a total break-down of the protective character of the native Al-rich oxide layer upon anodization in 1M HCl. The pre-oxidized am-Al1−xZrx alloys all exhibited superior corrosion behavior as compared to their as-deposited pendants, attaining passive current densities as low as 3.5×10−2μA/cm2 at 1000mV. The superior corrosion resistance of the pre-oxidized alloys is attributed to a combination of characteristics of the thermally-grown amorphous (Al0.33Zr0.67)O1.83 overlayer: (i) a relatively high Zr fraction, (ii) a structurally and chemically uniform microstructure, (iii) a stable, minimal residual growth defect structure (in the surface region) established by a sufficiently large time of oxidation.