Microstructure and corrosion characteristics of laser-alloyed magnesium alloy AZ91D with Al–Si powder

Blown-powder laser surface alloying was performed on the magnesium alloy AZ91D with Al–Si alloy powder to improve corrosion resistance. Characterization by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD) analysis revealed that intermetallic compounds (IMCs) of Mg₂Si, Al₁₂Mg₁₇ and Al₃Mg₂ were formed in the matrix of α-Mg and Al solid solutions in Al–Si alloyed layers. The anodic polarization test in 3.5% NaCl aqueous solution showed that preferential corrosion occurred in the α-Mg matrix of the AZ91D base metal. The Al–Si alloyed layers exhibited a lower corrosion rate and a higher polarization resistance than AZ91D. The compactly dispersed dendritic Mg₂Si phase, and the dendritic and angular phases of Al₁₂Mg₁₇ and Al₃Mg₂ in the alloyed microstructure were observed to be corrosion-resistant, constituting a barrier that retards corrosion. Corrosion initiated at the interface between IMCs and the solid solution matrix, and at substructures of the matrix, subsequently pervaded into the surrounding microstructure.     

Corrosion properties and corrosion evolution of as-cast AZ91 alloy with rare earth yttrium

The corrosion resistance property and the corrosion evolution of as-cast AZ91 alloy with rare earth Y addition are investigated by using immersion tests, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). The results show that the proper amount of Y in the alloys can improve the corrosion resistance of AZ91 alloys effectively. With the increment of Y, the corrosion rate of the modified AZ91 alloys by Y addition was markedly less than that of AZ91 alloy. The corrosion rate of AZ91 alloy with 0.3 wt.% Y was the slightest, but further addition of Y content over 0.3 wt.% make the corrosion heavier. The XPS analysis suggests that the compound film of AZ91 alloy with 0.3 wt.% Y is mainly composed of Mg(OH)₂ and MgCO₃ without any Al(OH)₃ and Al₂O₃, in addition, Y₂O₃ phase is found in the compound film of AZ91 alloy with 0.3 wt.% Y, which benefits to stabilize the surface film.     

In-situ hydrothermal crystallization Mg(OH)2 films on magnesium alloy AZ91 and their corrosion resistance properties

Mg(OH)₂ films have been fabricated on magnesium alloy AZ91 substrates by an in-situ hydrothermal method. AZ91 alloy substrates act as both the source of Mg²⁺ ion and the support for the Mg(OH)₂ film in synthetic process. The effect of pH value and hydrothermal treatment time on the morphologies and corrosion resisting properties of Mg(OH)₂ film is studied. The obtained Mg(OH)₂ films are uniform and compact. The adhesion between the films and the substrate is strong due to the in-situ growth process, which enhances their potential for practical applications. Potentiodynamic polarization measurements showed that the Mg(OH)₂ films obtained at pH 10, 3 h exhibits the highest increase in corrosion potential at −0.7097 V and lowest i_corr, which suggests that it is the best effective film in improving the corrosion resistance of AZ91in all obtained films.     

Corrosion resistance of AZ91D magnesium alloy with electroless plating pretreatment and Ni–TiO2 composite coating

In this paper, a protective multilayer coating, with electroless Ni coating as bottom layer and electrodeposited Ni–TiO₂ composite coating as top layer, was successfully prepared on AZ91D magnesium alloy by a combination of electroless and electrodeposition techniques. Scanning electron microscopy and X-ray diffraction were employed to investigate the surface, cross-section morphologies and phase structure of coatings, respectively. The electrochemical corrosion behaviors of coatings in 3.5 wt.% NaCl solutions were evaluated by electrochemical impedance spectroscopy, open circuit potential and potentiodynamic polarization techniques. The results showed that the corrosion process of Ni–TiO₂ composite coating was mainly composed of three stages in the long-term immersion test in the aggressive media, and could afford better corrosion and mechanical protection for the AZ91D magnesium alloy compared with single electroless Ni coating. The micro-hardness of the Ni–TiO₂ composite coating improved more than 5 times than that of the AZ91D magnesium alloy.     

Effect of neodymium on microstructure and corrosion resistance of AZ91 magnesium alloy

The corrosion behavior of a new Mg–9Al–1Zn (AZ91) magnesium alloy containing neodymium (Nd) is investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), immersion tests and electrochemical experiments. The results indicate that Nd decreases the size and volume fraction of the β (Mg₁₇Al₁₂) phase and forms Al₂Nd in the alloy. In addition, during corrosion, Nd is incorporated into corrosion film in the form of Nd₂O₃. AZ91 alloy containing 1.0 wt.% Nd possesses an outstanding passivation property and excellent corrosion resistance. The corrosion resistance enhancement is attributed to the reduction in the size and volume fraction of the β phase and the incorporation of Nd₂O₃ in the corrosion film.     

Preparation of spherical zirconium salt particles by homogeneous precipitation

Spherical basic zirconium sulphate particles were prepared by homogeneous precipitation in mixed solutions of zirconium sulphate and urea. Values of [SO ₄ ^2– ]/[Zr⁴⁺] and [urea]/[Zr⁴⁺] in starting mixed solutions and cooling rate may affect the formation of spherical particles. Complexes such as [Zr(OH)_n]^4–n could prevent the formation and thus lead to gel precipitation. In addition, spherical particles could only be obtained in the presence of SO ₄ ^2– ; for NO^3– and Cl^–, only gel precipitation occurred.     

Microstructure and corrosion characteristics of laser-alloyed magnesium alloy AZ91D with Al–Si powder

Abstract

Blown-powder laser surface alloying was performed on the magnesium alloy AZ91D with Al–Si alloy powder to improve corrosion resistance. Characterization by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD) analysis revealed that intermetallic compounds (IMCs) of Mg₂Si, Al₁₂Mg₁₇ and Al₃Mg₂ were formed in the matrix of α-Mg and Al solid solutions in Al–Si alloyed layers. The anodic polarization test in 3.5% NaCl aqueous solution showed that preferential corrosion occurred in the α-Mg matrix of the AZ91D base metal. The Al–Si alloyed layers exhibited a lower corrosion rate and a higher polarization resistance than AZ91D. The compactly dispersed dendritic Mg₂Si phase, and the dendritic and angular phases of Al₁₂Mg₁₇ and Al₃Mg₂ in the alloyed microstructure were observed to be corrosion-resistant, constituting a barrier that retards corrosion. Corrosion initiated at the interface between IMCs and the solid solution matrix, and at substructures of the matrix, subsequently pervaded into the surrounding microstructure.     

Corrosion and impedance studies on magnesium alloy in oxalate solution

Corrosion behavior of AZ91E alloy was investigated in oxalate solution using potentiodynamic polarization and electrochemical impedance measurements (EIS). The effect of oxalate concentration was studied, where the corrosion rate increases with increasing oxalate concentration. The effect of added ions (Br⁻, Cl⁻ or SiO₃²⁻) on the electrochemical behavior of magnesium alloy in 0.1 M Na₂C₂O₄ solution at 298 K, was investigated. It was found that the corrosion rate of 0.1 M oxalate solution containing silicate ion is lower than the blank (0.1 M Na₂C₂O₄). This was confirmed by scanning electron microscope (SEM) observations. However, for the other added ions Br⁻ or Cl⁻, the corrosion rate is higher than the blank.     

Influence of phytic acid concentration on performance of phytic acid conversion coatings on the AZ91D magnesium alloy

In this study, the phytic acid conversion coating, a new environmentally friendly chemical protective coating for magnesium alloys, was prepared. The influences of phytic acid concentration on the formation process, microstructure, chemical state and corrosion resistance of the conversion coatings on AZ91D magnesium alloy were investigated by means of weight gain measurement, field-emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, potentiodynamic polarization method and electrochemical impedance spectroscopy (EIS), respectively. And the depth profile of all elements in the optimal conversion coatings was analyzed by auger electron spectroscopy (AES).The results show that the growth, microstructure, chemical state and corrosion resistance of the conversion coatings are all obviously affected by the phytic acid concentration. The concentration of 5 g l⁻¹ corresponds to the maximum weight gain. The main elements of the coating are Mg, Al, O, P, and C, which are distributed gradually in depth. The functional groups of conversion coatings formed in higher concentration phytic acid solution are closer to the constituent of phytic acid than those formed in lower concentration phytic acid solution. The coatings formed in 1–5 g l⁻¹ are integrated and uniform. However, those formed in 20–50 g l⁻¹ have some micro-cracks on the α phase. The coating formed in 5 g l⁻¹ has the best corrosion resistance, whose open circuit current density decreases about six orders than that of the untreated sample, although the coatings deposited in 1–20 g l⁻¹ can all improve the corrosion resistance of AZ91D.     

Electrochemical characterization of the corrosion of a Mg-Li Alloy

The corrosion characteristic of a novel Mg-Li alloy with RE in alkaline NaCl solution was investigated by electrochemical means, such as open circuit potential vs time curves, potentiodynamic polarization curves and electrochemical impedance spectroscopy(EIS). The result showed that Cl⁻ concentration and pH value affected the corrosion of Mg-Li alloy, and in high Cl⁻ concentration solution Cl⁻ concentration was the major factor. Corrosion of the alloy was slighter in the stronger alkaline solution, because corrosion current(I_corr) reduced, corrosion potential (E_corr) turned to positive direction and the capacitive loops enlarged. When Cl⁻ increased, I_corr increased and capacitive loops shrinked, this means that corrosion of the alloy was more serious with the increase in Cl⁻ concentration.     

Crystallization of salts M3[NpO4(OH)2]·nH2O (M = Na, K, Rb, Cs) from concentrated alkali solutions at low temperatures. Synthesis and structure of a new Np(VII) compound, Na3[NpO4(OH)2]·6H2O

Precipitation of salts M₃[NpO₄(OH)₂]·nH₂O (M = Na, K, Rb, Cs) from concentrated alkali solutions at low temperatures (about ?10°C) was studied. From solutions with [OH^?] > 9.5 M, these compounds are isolated as coarse black crystals in high yield without impurity of other phases. The K, Rb, and Cs salts crystallize in the form of the previously studied compounds K₃[NpO₄(OH)₂]·2H₂O and M₃[NpO₄(OH)₂]·3H₂O (M = Rb, Cs). In the case of Na, a new hydrate Na₃[NpO₄(OH)₂]·6H₂O was obtained, and its crystal structure was determined. Crystals of the hexahydrate consist of centrosymmetrical tetragonal-bipyramidal anions [NpO₄(OH)₂]^3?, crystallographically independent Na(1) and Na(2) cations, and water molecules. The coordination surrounding of the Np atom is characterized by noticeable difference (Δ = 0.0203 Å) in the Np-O bond lengths in the equatorial plane of the bipyramid. The [NpO₄(OH)₂]^3? anions are combined with the Na(2) cations to form infinite chains [Na(2)NpO₄(OH)₂(H₂O)₂]^2? in such a manner that the lateral edges of the anion are simultaneously the lateral edges of the Na(2) coordination polyhedron. Incorporation of one of the two crystallographically independent O atoms of the NpO₄ group into the Na(2) coordination surrounding is responsible for a noticeable difference in the Np-O bond lengths in the equator of the [NpO₄(OH)₂]^3? anion. The types of hydrogen bonding in the structures of Na3[NpO₄(OH)₂]·nH₂O (n = 0, 2, 4, 6) are compared.     

The surface chemistry of 3-mercaptopropyltrimethoxysilane films deposited on magnesium alloy AZ91

Magnesium and its alloys have desirable physical and mechanical properties for a number of applications. Unfortunately, these materials are highly susceptible to corrosion, particularly in the presence of aqueous solutions. The purpose of this study is to develop a uniform, non-toxic surface treatment to enhance the corrosion resistance of magnesium alloys. This paper reports the influence of the coating bath parameters and alloy microstructure on the deposition of 3-mercaptopropyltrimethoxysilane (MPTS) coatings on magnesium alloy AZ91. The surface chemistry at the magnesium/MPTS interface has also been explored. The results indicate that the deposition of MPTS onto AZ91 was influenced by both the pH and MPTS concentration in the coating bath. Furthermore, scanning electron microscopy results showed that the MPTS film deposited uniformly on all phases of the magnesium alloy surface. X-ray photoelectron spectroscopy studies revealed that at the magnesium/MPTS interface, the molecules bond to the surface through the thiol group in an acid-base interaction with the Mg(OH)₂ layer, whereas in the bulk of the film, the molecules are randomly oriented.     

Corrosion behaviour of AZ80 Mg/beryllium–bronze–alloy galvanic couples in typical marine environment

A high-strength AZ80 Mg alloy was prepared via multi-direction forge, thermal extrusion and peak-aged heat treatment. The corrosion behaviour of AZ80 Mg/beryllium–bronze–alloy galvanic couples in a typical marine environment was investigated. Beryllium–bronze alloy (QBe1.7 Cu) acted as cathode, accelerating the corrosion of AZ80 Mg alloy. With the increase in contact areas, the corrosion rates and galvanic effect also increased, and the galvanic potentials moved positively. However, with a prolonged exposure time, the galvanic potentials and current densities decreased. The corrosion products mainly consisted of MgCl₂, Mg₅(CO₃)₄(OH)₂·5H₂O and MgSO₄·7H₂O. According to the standard of galvanic corrosion sensitivity from the Air Force Materials Laboratory (AFML), AZ80 Mg alloy was not allowed to make contact with QBe1.7 Cu alloy without effective protection.     

Influence de l'oxygene et des ions chlorure sur le comportement electrochimique d'un alliage Fe---Ni (36 % Ni) en milieu acide et neutre

An investigation of the behaviour of a Fe---Ni alloy (64-36) was undertaken in sulfate solutions of pH from 1.7 to 9, and the effects of Cl⁻ ions and dissolved oxygen were examined. In the absence of these two species, the alloy is in the active state in the whole pH range and only becomes passive in neutral solutions at high concentrations of oxygen. Pitting was found to occur only in acidic solutions in the presence of Cl⁻ ions.     

Effect of bismuth and silver on the corrosion behavior of Sn-9Zn alloy in NaCl 3 wt.% solution

The effect of silver (Ag) and bismuth (Bi) on the corrosion resistance of Sn-9Zn alloy in NaCl 3 wt.% solution was investigated using electrochemical techniques. The results showed that the addition of Bi and Ag lead to the increase of corrosion rate and the corrosion potential E_corr is shifted towards less noble values. After immersion, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive of spectroscopy (EDS) analysis of the corroded alloy surface revealed the nature of corrosion products. EDS and XRD analyses confirmed the oxide of zinc (ZnO and Zn₅(OH)₈Cl₂H₂O) as the major corrosion product formed on the outer surface of in the tested three solder alloys.     

Electrochemical behaviour of Cu24Zn5Al alloy in alkaline medium in the presence of chloride ions and benzotriazole

This paper deals with electrochemical behaviour of Cu24Zn5Al alloy in a sodium tetraborate solution (borax), in the presence of chloride ions and benzotriazole. It was found that during anodic polarization of the investigated alloy, in a sodium tetraborate solution, at lower potentials, copper (I)-oxide formed on the alloy surface. The voltammograms show peak potential shifts corresponding to the formation of Cu₂O towards more positive values with longer immersion time. It was found that chloride ions had an activating effect in a sodium tetraborate solution containing various concentrations of chloride ions (0.001, 0.005, 0.010, 0.050 and 0.100 mol dm⁻³ Cl⁻). It was observed that Cu24Zn5Al alloy corroded more intensely in more concentrated solutions and with longer exposure to Cl⁻. Investigations of the effect of inhibitor concentrations (8.4 × 10⁻⁶, 8.4 × 10⁻⁵, 8.4 × 10⁻⁴ and 8.4 × 10⁻³ mol dm⁻³ BTA in 0.1 mol dm⁻³ borax solution) showed that BTA had a good protective effect. The inhibiting effect of BTA was also confirmed with various times of immersion of this alloy in a 1.7 × 10⁻² mol dm⁻³ solution of this inhibitor.     

Corrosion and wear resistance of AZ91D magnesium alloy with and without microarc oxidation coating in Hank’s solution

Immersion test, electrochemistry test and block-on-cylinder type wear test have been applied to study the corrosion and wear resistance of AZ91D Mg alloy with and without microarc oxidation (MAO) treatment in Hank’s solution. Through MAO, a ceramic coating is directly formed on the surface of AZ91D Mg alloy, by which its corrosion and wear resistances are greatly improved. The immersion test results show the mass loss of untreated AZ91D Mg is 15 times of that of MAO ones after 21 days immersion test. The electrochemical corrosion experiments show that the corrosion potential of Mg alloy is improved from −1.5786 V to −0.43019 V through MAO surface treatment, the corrosion current is reduced from 0.028703 A/cm² to 2.0456 × 10⁻⁷ A/cm², and the polarization resistance is improved from 1.2753 × 10⁻⁵ Ω/cm² to 0.90886 Ω/cm². The lubricate sliding wear test results show the mass loss of untreated AZ91D Mg is 1.5 times of that of MAO ones.     

Bio-corrosion characterization of Mg–Zn–X (X = Ca, Mn, Si) alloys for biomedical applications

The successful applications of magnesium-based alloys as biodegradable orthopedic implants are mainly inhibited due to their high degradation rates in physiological environment. This study examines the bio-corrosion behaviour of Mg–2Zn–0.2X (X = Ca, Mn, Si) alloys in Ringer’s physiological solution that simulates bodily fluids, and compares it with that of AZ91 magnesium alloy. Potentiodynamic polarization and electrochemical impedance spectroscopy results showed a better corrosion behaviour of AZ91 alloy with respect to Mg–2Zn–0.2Ca and Mg–2Zn–0.2Si alloys. On the contrary, enhanced corrosion resistance was observed for Mg–2Zn–0.2Mn alloy compared to the AZ91 one: Mg–2Zn–0.2Mn alloy exhibited a four-fold increase in the polarization resistance than AZ91 alloy after 168 h exposure to the Ringer’s physiological solution. The improved corrosion behaviour of the Mg–2Zn–0.2Mn alloy with respect to the AZ91 one can be ascribed to enhanced protective properties of the Mg(OH)₂ surface layer. The present study suggests the Mg–2Zn–0.2Mn alloy as a promising candidate for its applications in degradable orthopedic implants, and is worthwhile to further investigate the in vivo corrosion behaviour as well as assessed the mechanical properties of this alloy.     

A Spectrophotometric Study of the Interaction of Np(VI) with Orthosilicic Acid and Polymeric Silicic Acids in Aqueous Solutions

The interaction of NpO ₂ ²⁺ ions with orthosilicic acid Si(OH)₄ and polymeric silicic acids (PSAs) in aqueous solutions was studied spectrophotometrically. The interaction at pH ≤ 4.5 is described by the equation NpO ₂ ²⁺ + Si(OH)₄ = NpO₂OSi(OH) ₃ ⁺ + H⁺ with the equilibrium constant log K = − 2.88±0.12 at the ionic strength I = 0.1–0.2 (log K ⁰ = −2.61±0.12 recalculated to I = 0); the stability constant of the complex NpO₂OSi(OH) ₃ ⁺ (I = 0) is log β⁰ = 7.20± 0.12. At pH > 5, a second complex of NpO ₂ ²⁺ with PSAs of the presumed composition NpO₂(≡ SiO)₂(≡SiOH) _{m − 2}, where (≡SiOH)_m denotes a PSA molecule with surface Si-OH groups, is formed. The absorption spectra of the complexes NpO₂OSi(OH) ₃ ⁺ and NpO₂(≡ SiO)₂(≡SiOH) _{m − 2} were obtained. In contrast to the hydroxo complexes, they have pronounced maxima at 560 – 600 nm with the molar extinction coefficients of about 25–30 l mol⁻¹ cm⁻¹, which is several times higher compared to the Np(VI) aqua ion.__________Translated from Radiokhimiya, Vol. 47, No. 4, 2005, pp. 322–327.Original Russian Text Copyright © 2005 by Yusov, Shilov, Fedoseev, Astafurova, Delegard.     

Superplastic behavior and microstructural stability of friction stir processed AZ91C alloy

Superplastic behavior of a solution treated and friction stir processed (FSP) AZ91C alloy is studied. These studies are conducted in the temperature range of 300–375 °C and strain rates (SRs) in the range of 1 × 10^?4–3 × 10^?3 s^?1. Microstructural stability of the FSP alloy is also studied in comparison to the AZ31, AZ61, and AZ91 alloys processed by various routes. High SR sensitivity in the range of 0.33–0.39 and grain size stability till 350 °C is observed for the FSP alloy. The FSP AZ91C alloy showed better thermal stability in comparison to AZ31 and AZ61 alloys. Kinetics of superplastic deformation of the FSP alloy is found to be slower as compared to AZ31 and AZ61 alloys processed by various routes, which is due to the presence of significant amount of second phase precipitates, such as, β-Mg₁₇(Al,Zn)₁₂, Mg₂Si, and Al₈Mn₅ in the FSP alloy. However, these precipitates contributed for better thermal stability of the microstructure of FSP AZ91C alloy.