Corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy in NaCl solution

The corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy was investigated in air and 3.5 wt%NaCl solution. Epoxy coating as a new method was used to improve the corrosion fatigue property of the material.Results show that the fatigue limit(FL) of the coated specimens is higher than that of the uncoated specimens in3.5 wt% NaCl solution because of the strengthening and blocking functions of the epoxy coating. The FL of the coated specimens in 3.5 wt% NaCl solution is as high as that in air. It implies that the coated specimens are not as sensitive to the environment as the magnesium alloy. The low tensile strength and the short elongation of the pure epoxy coating lead to that the fatigue crack of the coated specimen is always initiated from the epoxy-coating film Pores and pinholes accelerate the fatigue crack initiation process. Pinholes are caused by the corrosion reactions between the epoxy coating and the NaCl solution.     

Enhanced corrosion resistance of Zn–Cu–Ti alloy with addition of Cu–Ti amorphous ribbons in 3.5% NaCl solution

In the present study, Zn–0.3Cu–0.3Ti alloy (sample I) was fabricated by a simple low-temperature melting method using Cu–50Ti amorphous alloy ribbons for corrosion in 3.5% NaCl solution. As a comparison, crystalline Cu–50Ti master alloy was used to prepare Zn–0.3Cu–0.3Ti alloy (sample II). Sample I comprising Zn, TiZn₃, and TiZn₁₅ phases exhibits an equiaxed microstructure with subgrain structure. Large TiZn₃ particles show cluster feature, whereas intermittent small TiZn₁₅ particles exist at grain boundaries and subgrain boundaries. In sample II, the Zn matrix with typical dendritic microstructure is observed and no large particles are found. Compared with sample II, sample I shows lower weight gain and corrosion current density and a higher slope of cathode polarization curve. The weight gain for sample I is only 0.59 mg·cm⁻², but for sample II, this value reaches 0.70 mg·cm⁻². After 8 days of corrosion, corrosion products are mainly Zn₅(OH)₈Cl·H₂O and ZnO, showing loose particle shape. As corrosion time increases from 2 days to 8 days, corrosion layer thickness increases from about 15 to 24 μm for sample 1.     

Influence of various aging treatments on microstructure,strength and corrosion behaviour of high Zn content Al–Zn–Mg–Cu alloy

In this paper, the influence of T6, T74 and RRA aging treatments on microstructure, strength and corrosion behaviour of high Zn content Al–Zn–Mg–Cu alloy was investigated by tensile properties tests, inter-granular corrosion (IGC) tests, exfoliation corrosion (EXCO) tests, polarisation tests, metallographic microscope and transmission electron microscopy (TEM) analysis. The results show that the T74 and RRA temper can increase the size and the distribution discontinuity of the grain boundaries precipitates (GBPs), thus leading to improvement of the corrosion resistance. However, with the coarser matrix precipitates (MPs) relative to T6 treatment, RRA and T74 temper both have a decrease in strength. Besides, all the performances (including mechanical properties and corrosion properties) of the RRA treatment show an intermediate level relative to T6 and T74. Therefore, we can select the appropriate heat treatment process according to the different performance requirements in the industrial production.     

Enhancing the intergranular corrosion resistance of high Mg-alloyed Al–Mg alloy by Y addition

Microalloying is thought to improve the performance of Al–Mg alloys commonly used in transport applications. The effect of Y addition (0–0.4%) on the microstructure, mechanical properties, and corrosion resistance of Al–9.2Mg–0.7Mn alloy is investigated for potential use in engineering applications. The generation of the β-Al₃Mg₂ phase along the grain boundaries is suppressed in the as-cast alloy due to the formation of the AlMgY ternary phase. The average intergranular corrosion mass loss of the alloy with 0.1% Y addition decreases about 53.1% almost at no expense of mechanical performance in the as-rolled alloy after annealing. Moreover, the alloy with 0.1% Y addition shows the corrosion mass loss about 30.2% lower than the Y-free alloy in the sensitized state. The enhanced corrosion resistance of the alloy can be ascribed to the reduced β-Al₃Mg₂ precipitation along the grain boundaries associated with Y addition.     

Low temperature superplasticity in a friction-stir-processed ultrafine grained Al–Zn–Mg–Sc alloy

Friction stir processing (FSP) was used to create a microstructure with ultrafine grains (0.68 μm grain size) in an as-cast Al–8.9Zn–2.6Mg–0.09Sc (wt.%) alloy. The ultrafine grained alloy exhibited superplasticity at relatively low temperatures and higher strain rates. Optimum ductility of 1165% at a strain rate of 3 × 10⁻² s⁻¹ and 310 °C was obtained. Enhanced superplasticity was also achieved at a temperature as low as 220 °C. Experimentally observed parametric dependencies and microstructural examinations indicated that the operating deformation mechanism might be the Rachinger grain boundary sliding accommodated by intragranular slip. The FSP microstructure became highly unstable at 390 °C onwards, thus, affecting ductility adversely. In situ transmission electron microscopy heating was used to understand the instability phenomenon, which has been attributed to the drop in particle pinning forces due to the dissolution of metastable precipitates and microstructural heterogeneity.     

Effect of flame rectification on corrosion property of Al–Zn–Mg alloy

The corrosion resistance of Al–Zn–Mg alloy subjected to different times in flame rectification was investigated based on the exfoliation corrosion test. The results indicate that the flame rectification deteriorates the exfoliation corrosion resistance of Al–Zn–Mg alloy. The corrosion resistance of Al–Zn–Mg alloy is ranked in the following order: base metal>two times>three times>one time of flame rectification. The exfoliation corrosion behavior was discussed based on the transformation of precipitates at grain boundaries and matrix. With increasing the number of times in flame rectification, the precipitate-free zones disappeared and the precipitates experienced dissolution and re-precipitation. The sample was seriously corroded after one time of flame rectification, because the precipitates at grain boundaries are more continuous than those in other samples.     

Influence of metallurgical states on the corrosion behaviour of Al–Zn PVD coatings in saline solution

The objective of this study is to define the corrosion behaviour of different Al–Zn coatings, deposited by magnetron sputtering. The coatings exhibiting the best corrosion resistance are then characterised during long immersion tests in neutral 5 wt.% NaCl solution.The results show that the corrosion behaviour is strongly dependent on the zinc content. The evolution of the degradation mechanism is also related to the microstructure of the alloys. These alloys present very interesting properties for steel protection. Nevertheless, the zinc content has to be well defined in order to avoid a high dissolution of the coating.     

Prediction of the morphology of Mg32(Al,Zn)49 precipitates in a Mg–Zn–Al alloy

A geometrical model has been applied to predict the morphology of faceted Mg₃₂(Al, Zn)₄₉ precipitates in a Mg–Zn–Al alloy using the observed orientation relationship (OR) and the lattice parameters of the precipitates and the matrix as inputs. Planes in rational or in irrational orientations with higher densities of good matching sites are more likely to be preferred, which agrees well with experimental observations.     

Mesoscopic structure control of spray formed high strength Al–Zn–Mg–Cu alloys

Several high solute, high strength 7xxx series aluminum alloys with solute contents close to equilibrium solid solubility limits of the Al–Zn–Mg–Cu system have been produced by rapid solidification using spray deposition (the Osprey process). This process yields massive preforms directly from the liquid state by combining atomization and consolidation into one step. Various elements, including chromium, manganese and silver are incorporated to produce a variety of microstructures and mechanical properties. The zinc to magnesium ratio is also varied to see the effect on the strength. Superior strengths in excess of 849 MPa are achieved and are attributed to two major substructures with different scale; nanometer sized η′ metastable precipitates and slightly larger, but finely distributed dispersoids which provide a fiber-like reinforcement. The remarkable strengthening is predominantly attributed to precipitation hardening and a large coherency strain.     

Enhanced corrosion resistance of Ni–W alloy with inclusion of TiN nanoparticles by electrodeposition method

Abstract

Electrodeposition of Ni–W–TiN nanocomposite on mild steel substrate from an ammoniacal citrate bath containing dispersed titanium nitride has been demonstrated. The structure, surface morphology, composition and corrosion resistance properties of the nanocomposite deposits have been characterised by using various techniques. X-ray diffraction analysis (XRD) of the electrodeposited Ni–W–TiN nanocomposite shows that it is fcc crystalline. Scanning electron micrography (SEM) reveals smaller grains and uniform distribution of the titanium nitride in the alloy matrix. The microhardness of the nanocomposite coatings is higher than that of the alloy. The corrosion resistance of the electrodeposited nanocomposite evaluated by electrochemical impedance and Tafel polarization studies showed that the Ni–W–TiN nanocomposite is more corrosion resistant than the Ni–W alloy deposit. The finer grain and uniform distribution of the titanium nitride in alloy matrix favour the enhanced microhardness and corrosion resistance of the nanocomposite.     

Fine-grained Mg–1Mn–0.5Al–0.5Ca–0.5Zn alloy with high strength and good ductility fabricated by conventional extrusion

Mg–1Mn–0.5Al–0.5Ca–0.5Zn (wt.%) alloy was fabricated by conventional extrusion at 673 K with an extrusion ratio of 25:1, followed by aging at 473 K. The microstructure was characterized by scanning electron microscopy, electron back-scattered diffraction, and transmission electron microscopy. The mechanical properties were determined by the tensile test. The peak-aged sample shows fine recrystallized grains with an average grain size of 1.7 μm. Area fraction of Al–Ca particles in the alloy increases significantly after peak aging. Meanwhile, both 〈a〉 and 〈c+a〉 dislocations were observed to remain in the alloy after hot extrusion. Thus, the peak-aged sample exhibits simultaneously high strength and good ductility with the ultimate tensile stress, tensile yield stress, and tension fracture elongation of 320 MPa, 314 MPa, and 19.0%, respectively.     

Corrosion behaviour of Mg–Zn–Y–Mischmetal alloys in phosphate buffer saline solution

The influence of the processing route and chemical composition in the corrosion behaviour of two Mg–Zn–Y–Mischmetal alloys has been evaluated in phosphate buffer saline solution. The corrosion resistance of the alloy processed by conventional techniques was substantially higher than that found for the same alloy processed from atomised powders. Fine homogeneous distribution of the second-phase particles promoted severe attack due to the enhanced number of galvanic microcells. A higher concentration of zinc and a lower content of rare earth additions improved the corrosion resistance of the alloys due to the lower volume fraction of second-phase particles.     

Microstructure,mechanical properties,and corrosion resistance of Mg–9Li–3Al–1.6Y alloy

Mg–9Li–3Al–1.6Y alloys were prepared through mixture method. The microstructure, mechanical properties, and corrosion resistance of the as-cast and asextruded alloys were studied by optical microscopy(OM),scanning electronic microscopy(SEM), X-ray diffraction(XRD), mechanical properties testing, and electrochemical measurement. The as-cast Mg–9Li–3Al–1.6Y alloy with the average grain size of 325 lm is composed of b-Li matrix, block a-Mg, and granule Al_2Y phases. After extrusion, the grain size of the as-cast alloy is obviously refined and reaches to 75 lm; the strength and elongation of the extruded alloy are enhanced by 17.20 % and49.45 %, respectively, owing to their fine microstructure and reduction of casting defects. The as-extruded alloy shows better corrosion resistance compared to the as-cast one, which may be related to the low stored energy and dislocation density in the extruded alloy, also the homogenization treatment before extrusion.     

Influence of microstructural characteristics on corrosion behavior of Mg–5Sn–3In alloy in Hank's solution

The microstructural observation, the mass loss test, potentiodynamic polarization measurements and corrosion morphology examinations were conducted to study the influence of microstructural characteristics on corrosion behavior of Mg–5Sn–3In alloys in Hank's solution after extrusion. The results show that the corrosion rate of the as-cast alloy is similar to that of as-extruded alloy; however, the local corrosion susceptibility is greatly weakened in the as-extruded alloy, especially in the extrusion direction. The relatively uniform corrosion morphology of the as-extruded alloy is attributed to refined Mg₂Sn particles, uniform distribution of Mg₂Sn particles and favorable crystal orientation. Meanwhile, the cytotoxicity tests confirm that the Mg–5Sn–3In alloy exhibits cytotoxicity of Grade 0–1 for NIH3T3 cells, suggesting an acceptable cytotoxicity of this alloy in the vitro assay.     

Comparison of the corrosion resistance of an Al–Cu alloy and an Al–Cu–Li alloy

ABSTRACT

In this study, the corrosion mechanisms of the AA2024-T3 and the AA2098-T351 were investigated and compared using various electrochemical techniques in 0.005?mol?L^?1 NaCl solution. The severe type of corrosion in the AA2098-T351 was intragranular attack (IGA) although trenching and pitting related to the constituent particles were seen. On the other hand, the AA2024-T3 exhibited severe localised corrosion associated with micrometric constituent particles, and its propagation was via grain boundaries leading to intergranular corrosion (IGC). Electrochemical techniques showed that the corrosion reaction in both alloys was controlled by diffusion. The non-uniform current distribution in both alloys showed that EIS was not a proper technique for comparing the corrosion resistance of the alloys. However, local electrochemical techniques were useful for the evaluation of the corrosion resistance of the alloys.     

Enhanced corrosion resistance of micro-arc oxidation coated magnesium alloy by superhydrophobic Mg−Al layered double hydroxide coating

To further enhance the corrosion resistance of the porous micro-arc oxidation (MAO) ceramic layers on AZ31 magnesium alloy, superhydrophobic Mg−Al layered double hydroxide (LDH) coating was fabricated on MAO-coated AZ31 alloy by using in-situ growth method followed by surface modification with stearic acid. The characteristics of different coatings were investigated by XRD, SEM and EDS. The effect of the hydrothermal treatment time on the formation of the LDH coatings was studied. The results demonstrated that the micro-pores and cracks of MAO coating were gradually sealed via in-situ growing LDH with prolonging hydrothermal treating time. Electrochemical measurement displayed that the lowest corrosion current density, the most positive corrosion potential and the highest impedance modulus were observed for superhydrophobic LDH/MAO coating compared with those of MAO coating and LDH/MAO coating. Immersion experiment proved that the superhydrophobic LDH/MAO coating with the active anti-corrosion capability significantly enhanced the long-term corrosion protection for MAO coated alloy.