• Corrosion Science and Technology
• /
• v.15 no.3
• /
• pp.120-124
• /
• 2016

The effect of alloying element Ca (0, 1, and 2 wt%) on corrosion resistance and bioactivity of the as-received and anodized surface of rolled plate AM60 alloys was investigated. A plasma electrolytic oxidation (PEO) was carried out to form anodic oxide film in $0.5mol\;dm^{-3}\;Na_3PO_4$ solution. The corrosion behavior was studied by polarization measurements while the in vitro bioactivity was tested by soaking the specimens in Simulated Body Fluid (1.5xSBF). Optical micrograph and elemental analysis of the substrate surfaces indicated that the number of intermetallic particles increased with Ca content in the alloys owing to the formation of a new phase $Al_2Ca$. The corrosion resistance of AM60 specimens improved only slightly by alloying with 2 wt% Ca which was attributed to the reticular distribution of $Al_2Ca$ phase existed in the alloy that might became barrier for corrosion propagation across grain boundaries. Corrosion resistance of the three alloys was significantly improved by coating the substrates with anodic oxide film formed by PEO. The film mainly composed of magnesium phosphate with thickness in the range $30-40{\mu}m$. The heat resistant phase of $Al_2Ca$ was believed to retard the plasma discharge during anodization and, hence, decreased the film thickness of Ca-containing alloys. The highest apatite forming ability in 1.5xSBF was observed for AM60-1Ca specimens (both substrate and anodized) that exhibited more degradation than the other two alloys as indicated by surface observation. The increase of surface roughness and the degree of supersaturation of 1.5xSBF due to dissolution of Mg ions from the substrate surface or the release of film compounds from the anodized surface are important factors to enhance deposition of Ca-P compound on the specimen surfaces.

• Proceedings of the Korean Institute of Surface Engineering Conference
• /
• 2017.05a
• /
• pp.96.2-96.2
• /
• 2017

This research was conducted to investigate the electrochemical properties of the thin air-formed oxide film-covered AZ31 Mg alloy. In this work, native air-formed oxide films on AZ31 Mg alloy samples were prepared by knife-abrading method and the changes in the electrochemical properties of the air-formed oxide film were investigated in seven different electrolytes containing the following anions $Cl^-$, $F^-$, $SO{_4}^{2-}$, $NO_3{^-}$, $CH_3COO^-$, $CO{_3}^{2-}$ and $PO{_4}^{3-}$. It was observed from open circuit potential (OCP) transients that the potential initially decreased before gradually increasing again in the solutions containing only $CO{_3}^{2-}$ or $PO{_4}^{3-}$ ions, indicating the dissolution or transformation of the native air-formed oxide film into new more protective surface films. The Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) showed that there was growth of new surface films with immersion time on the air-formed oxide film-covered specimens in all the electrolytes; the least resistive surface films were formed in fluoride and sulphate baths whereas the most protective film was formed in phosphate bath. The potentiodynamic polarization curves illustrated that passive behaviour of AZ31 Mg alloy under anodic polarization appears only in $CO{_3}^{2-}$ or $PO{_4}^{3-}$ ions containing solutions and at more than $-0.4V_{Ag/AgCl}$ in $F^-$ ion containing solution.

• Korean Journal of Metals and Materials
• /
• v.47 no.10
• /
• pp.644-651
• /
• 2009

The passivation (or deactivation) of a metal surface during oxide film formation has been quantitatively explored for a ferritic stainless steel by using dynamic electrochemical impedance spectroscopy (DEIS). For this purpose, the electrochemical impedance spectra were carefully examined as a function of applied potential in the active nose region of the potentiodynamic polarization curve, to separate the charge transfer resistance and oxide film resistance. From the discrepancy in the potential dependence between the experimental charge transfer resistance and the semi-empirically expected one, the degree of passivation could be quantitatively estimated. The sensitivity of passivation of the steel surface to anodic potential, which might be the measure of the quality of the oxide film formed under unit driving force or over-potential, decreased by 31% when 3.5 wt% NaCl was added to a 5 wt% $H_2SO_4$ solution.

• Proceedings of the Korean Institute of Surface Engineering Conference
• /
• 2017.05a
• /
• pp.150.2-150.2
• /
• 2017

This research was conducted to investigate in detail the effect of $PO_4{^{3-}}$, $CO_3{^{2-}}$ and $F^-$ anions on the electrochemical properties of the thin air-formed oxide film-covered AZ31 Mg alloy. In this work, native air-formed oxide films on AZ31 Mg alloy samples were prepared by knife-abrading method and the changes in the electrochemical properties of the air-formed oxide film were investigated in electrolytes containing 0.01 M, 0.05 M and 0.1 M of $PO_4{^{3-}}$, $CO_3{^{2-}}$ and $F^-$ anions. It was observed that the trend of open circuit potential (OCP) transients changed only in the solution containing $PO_4{^{3-}}$ ions. The Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) showed that the resistance of the new surface films formed in fluoride ion containing bath increased with the increase in concentration of fluoride ions but the resistance of surface films formed in carbonate ion containing bath decreased with the increase in concentration of carbonate ions. The potentiodynamic polarization curves illustrated that under anodic polarization, there was growth of porous passive layer only in fluoride ion containing solution while the surface layer formed in phosphate and carbonate ion containing solutions lost its passivity at high anodic potential of $2.5V_{Ag/AgCl}$.

• Proceedings of the KWS Conference
• /
• 2002.10a
• /
• pp.65-69
• /
• 2002

An Al film deposited on the Kovar alloy substrate was anodically-bonded to the borosilicate glass, and the bond interfaces was closely investigated by transmission electron microscopy. Al oxide was found to form a layer ~l0 nm thick at the bond interface, and fibrous structure of the same oxide was found to grow epitaxially in the glass from the oxide layer. The fibrous structure grew with the bonding time. The mechanism of the formation of this fibrous structure is proposed on the basis of the migration of Al ions under the electric field. Penetration of Al into glass beyond the interfacial Al oxide was not detected. The comparison of the amount of excess oxygen ions generated in the alkali depletion layer with that incorporated in the Al oxide suggests that the growth of the alkali-ion depletion layer is controlled by the consumption of excess oxygen to form the interfacial Al oxide.

• Corrosion Science and Technology
• /
• v.6 no.6
• /
• pp.316-321
• /
• 2007

The effect of annealing on the pitting corrosion resistance of anodized Al-Mg alloy (AA5052) processed by equal-channel angular pressing (ECAP) was investigated by electrochemical techniques in a solution containing 0.2 mol/L of $AlCl_3$ and also by surface analysis. The Al-Mg alloy was annealed at a fixed temperature between 473 and 573 K for 120 min in air after ECAP. Anodizing was conducted for 40 min at $100-400A/m^2$ at 293 K in a solution containing 1.53 mol/L of $H_2SO_4$ and 0.0185 mol/L of $Al_2(SO_4)_3$. The internal stress generated in anodic oxide films during anodization was measured with a strain gauge to clarify the effect of ECAP on the pitting corrosion resistance of anodized Al-Mg alloy. The time required to initiate the pitting corrosion of anodized Al-Mg alloy was shorter in samples subjected to ECAP, indicating that ECAP decreased the pitting corrosion resistance. However, the pitting corrosion resistance was greatly improved by annealing after ECAP. The time required to initiate pitting corrosion increased with increasing annealing temperature. The strain gauge attached to Al-Mg alloy revealed that the internal stress present in the anodic oxide films was compressive stress, and that the stress was larger with ECAP than without. The compressive internal stress gradually decreased with increasing annealing temperature. Scanning electron microscopy showed that cracks occurred in the anodic oxide film on Al-Mg alloy during initial corrosion and that the cracks were larger with ECAP than without. The ECAP process of severe plastic deformation produces large internal stresses in the Al-Mg alloy; the stresses remain in the anodic oxide films, increasingthe likelihood of cracks. It is assumed that the pitting corrosion is promoted by these cracks as a result of the higher internal stress resulting from ECAP. The improvement in the pitting corrosion resistance of anodized AlMg alloy as a result of annealing appears to be attributable to a decrease in the internal stresses in anodic oxide films

• Corrosion Science and Technology
• /
• v.6 no.6
• /
• pp.275-281
• /
• 2007

The effect of annealing on the pitting corrosion resistance of anodized Al-Mg alloy (AA5052) processed by equal-channel angular pressing (ECAP) was investigated by electrochemical techniques in a solution containing 0.2 mol/L of $AlCl_3$ and also by surface analysis. The Al-Mg alloy was annealed at a fixed temperature between 473 and 573 K for 120 min in air after ECAP. Anodizing was conducted for 40 min at $100-400A/m^2$ at 293 K in a solution containing 1.53 mol/L of $H_2SO_4$ and 0.0185 mol/L of $Al_2(SO_4)_3$. The internal stress generated in anodic oxide films during anodization was measured with a strain gauge to clarify the effect of ECAP on the pitting corrosion resistance of anodized Al-Mg alloy. The time required to initiate the pitting corrosion of anodized Al-Mg alloy was shorter in samples subjected to ECAP, indicating that ECAP decreased the pitting corrosion resistance. However, the pitting corrosion resistance was greatly improved by annealing after ECAP. The time required to initiate pitting corrosion increased with increasing annealing temperature. The strain gauge attached to Al-Mg alloy revealed that the internal stress present in the anodic oxide films was compressive stress, and that the stress was larger with ECAP than without. The compressive internal stress gradually decreased with increasing annealing temperature. Scanning electron microscopy showed that cracks occurred in the anodic oxide film on Al-Mg alloy during initial corrosion and that the cracks were larger with ECAP than without. The ECAP process of severe plastic deformation produces large internal stresses in the Al-Mg alloy; the stresses remain in the anodic oxide films, increasingthe likelihood of cracks. It is assumed that the pitting corrosion is promoted by these cracks as a result of the higher internal stress resulting from ECAP. The improvement in the pitting corrosion resistance of anodized AlMg alloy as a result of annealing appears to be attributable to a decrease in the internal stresses in anodic oxide films

• Journal of the Korean institute of surface engineering
• /
• v.35 no.6
• /
• pp.383-390
• /
• 2002

To investigate behaviors of Ti and O elements and microstructures of anodic titanium oxide films, the films were prepared by anodizing pure titanium in $H_2$S $O_4$, $H_3$P $O_4$, and $H_2O$$_2$ mixed solution at 180V. The microstructures and chemical states of the elements were analyzed using SEM, X-ray mapping, AFM, XRD, XPS (depth profile). The films formed on a titanium substrate showed porous layers which were composed of pore and wall, And with increasing anodizing time a hexagonal shape of cell structures were dominant and solace roughness increased. From the XRD result the structure of the Ti $O_2$ layer was anatase type of crystal on the whole. In the XPS spectra it was found that Ti and O were chemically binded in forms of Ti $O_2$, TiOH, $Ti_2$ $O_3$ at Ti 2p, and Ti $O_2$, $Ti_2$ $O_3$, $P_2$ $O_{5}$, S $O_4^{2-}$ at O ls respectively. Concentration of Ti $O_2$ decreased as the depth increased from the surface of the oxide film towards the substrate, but to the contrary concentrations of TiOH and $Ti_2$ $O_3$ increased.d.

• Journal of the Korean institute of surface engineering
• /
• v.55 no.5
• /
• pp.292-298
• /
• 2022

In this study, PEO (plasma electrolytic oxidation) film formation behavior of AZ31 Mg alloy under application of 300 Hz pulse current was studied by the analyses of V-t curve, arc generation behavior, PEO film thickness and morphology of PEO films with treatment time in 0.05 M NaOH + 0.05 M Na₂SiO₃ + 0.1 M NaF solution. PEO films was observed to grow after 10 s of application of pulse current together with generation of micro-arcs. PEO film grew linearly with treatment time at a growth rate of about 5.58 ㎛/min at 200 mA/cm² of pulse current but increasing rate of film formation voltage became lowered largely with increasing treatment time after passing about 250 V, suggesting that resistivity of PEO films during micro-arc generation decreases with increasing film formation voltage at more than 250 V.

• Journal of the Korean Magnetics Society
• /
• v.14 no.3
• /
• pp.105-108
• /
• 2004

Array of magnetic Ni nanostructures has been fabricated on Si substrate by using nanoporous alumina film as a mask during deposition. The nanostructures are truncated cone-shape and the lateral sizes are comparable to height. While the continuous film shows well-defined in-plane magnetization, the nanostructure shows perpendicular component of magnetization at remanence. The hysterectic behavior of nanostructures is dominated by the demagnetizing field instead of interaction among them.