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In this work, three types of fully dense aluminum-based (Al) coatings (pure Al, AA2219 and AA6061) were deposited on AZ31B magnesium (Mg) alloy by a recently developed in-situ micro-forging assisted cold spray process. The detailed microstructure, coating adhesion and corrosion protection performance were systematically investigated. Results show that the porosities of the pure Al, AA2219 and AA6061 coatings are as low as 0.34 ± 0.11%, 0.23 ± 0.09% and 0.24 ± 0.08%, respectively. Strong adhesion strength (83.6–91.5 MPa) was achieved for the Al-based coatings due to the in-situ micro-forging induced enhanced mechanical-interlocking between Al-based coating and the magnesium alloy substrate. The corrosion current density of the AZ31B Mg alloy decreases to 1.9–5.8 μA cm−2 from 312.8 μA cm−2 after coated with the Al-based coatings. The long-term corrosion tests further indicate that the Al-based coatings are not aqueous solution permeable during the entire 1000 h immersion, thus provided robust long-term corrosion protection of AZ31B Mg alloy substrate. Moreover, the difference in corrosion resistances among the three Al-based coatings was observed and was further explained based on their unique microstructures arising from the coating deposition process. The present in-situ micro-forging assisted cold spray process shows great potential for economical and effective corrosion protection of magnesium alloy and provides more freedom for industries to design and use magnesium alloy parts. Image 1 • Three types of fully dense Al-based coatings were deposited on AZ31B. • Strong adhesion strength (83.6–91.5 MPa) was achieved for the Al-based coatings. • Al-based coatings possess excellent long-term corrosion protection performance. • Key factors of the different corrosion rate among the coatings were clarified. [ABSTRACT FROM AUTHOR]

A 150 μm thick fully dense Ni coating was deposited on AZ31B magnesium alloy by a newly developed in-situ shot-peening-assisted cold spray process. Strong adhesion strength of Ni coating (>65.4 MPa) was achieved. The present Ni coated AZ31B reveals comparable electrochemical behavior with annealed bulk Ni in 3.5 wt.% NaCl aqueous solution. Long-term immersion test (1000 h) and NaCl salt spray test (1000 h) further verified the robust corrosion resistance of the cold sprayed Ni coating. This work provides a novel green approach for corrosion protection of magnesium alloys. [ABSTRACT FROM AUTHOR]

Accelerated environmental (hygrothermal) exposure experiments are performed on organic paints coated on commercial die-cast AZ91D Mg alloys to investigate the effects of contamination on blistering. Specifically, artificial human perspiration spray is used to contaminate the substrate surface. Blistering occurred only for paints that are spread on surfaces with the perspiration present. More blisters gradually form at longer test times, and the volume of blistering increases. Scanning electron microscopy indicates that blistering is initiated by contamination and/or substrate corrosion at the interface of the organic paints and the substrate. Blistering is characterized for two samples exposed to the hygrothermal environments for various times, and is found to be initially empty in the early stages. Hydrophilic chloride contaminants from the perspiration lead to in situ adhesion loss. Simultaneously, the paints volume expands, and the associated compressive stress causes it to bulge. After long-term test exposure, chloride anions corrode the substrate under the films, and MgO, Mg(OH) 2 , and Mg 2 (OH) 3 Cl corrosion products fill the blisters. Finally, a model of blistering evolution is discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Blistering has occurred on the organic coating during their period of store. [•] Source and formation mechanism of blistering were systemically researched. [•] Blistering originated from the interface contaminated. [Copyright &y& Elsevier]

Highlights: [•] Sigma phase was related to the corrosion failure of the SS tube. [•] Sigma phase was formed during the production process of the SS tube due to improper heat treatment. [•] The corrosion of SS tube with sigma phase exhibited an intergranular corrosion. [•] The corrosion of SS tube with sigma phase also exhibited pronounced oxide spallation. [ABSTRACT FROM AUTHOR]

Highlights: [•] The corrosion process of the failure insert in cold chamber machine was directly presented. [•] The surface morphology and chemical composition were analysed. [•] The mechanism of erosion relies on the depression, plastic deformation and tearing action. [•] The average impact pressure of a liquid drop against a solid plane was calculated. [ABSTRACT FROM AUTHOR]

Abstract: This study investigates the atmospheric corrosion behaviour of ingot and as-extruded AM60 Mg alloys in a polluted city. The corrosion rate of the ingot samples was higher than that of the extruded samples. The size of dust particle conglomerates adsorbed on the sample surfaces was related to the pit sizes. The main corrosion products of both alloys included MgSO4·6H2O and MgSO3·6H2O. The extruded samples had more homogeneous structures and less second-phase contents compared with the ingot samples. As a result, the corrosion films formed on the extruded samples had better protective effects than those formed on the ingot samples. [Copyright &y& Elsevier]

Abstract: At present methods of high energy shot peening (HESP), one of the severe plastic deformation (SPD) has been used to process ultrafine-grained (UFG) (nano- and submicrocrystalline) materials. It has been found that during SPD of AZ91 magnesium alloy the structure of the surface layer develops from a cell structure to a fragmented structure with high angle misorientations and a mean grain size of 40nm. According to X-ray and transmission electron microscopy data, the nanostructure is the single phase which is the supersaturated solid solution by β-phases re-dissolution in Mg matrix. Fast diffusion during deformation is conditioned by the high stresses acting on the grain boundaries and the volumes of the fine grains. During deformation of the nanocrystalline the volume diffusion coefficient is equal to about D v =2.2×10−15 cm2/s. [Copyright &y& Elsevier]

Abstract: The corrosion behaviour of die-cast AZ91D magnesium alloys in sulphate solutions was investigated by SEM, FTIR and polarization measurements. For immersion times less than 48h, no pitting corrosion occurred and only generalized corrosion was apparent. According to the polarization curves, the corrosion rate order of the die-cast AZ91D Mg alloy in three aqueous solutions was: NaCl>MgSO4 >Na2SO4. The main corrosion products were Mg(OH)2 and MgAl2(SO4)4·22H2O in the sulphate solutions and the product film was compact. Precipitation of MgAl2(SO4)4·22H2O required a threshold immersion time. [Copyright &y& Elsevier]

Abstract: In this paper, microstructures and properties of die casting components with various thicknesses made of AZ91D alloy have been investigated by means of scanning electron microscope (SEM), transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM), etc. It is concluded that mechanical properties of the die casting components mainly depend on grain size of α-Mg phase. At the same time, however, the voids formed by entrapping air during die casting process have a remarkable detrimental effect on the mechanical properties. To what extent the effect is caused is determined by quantity, size and shape of the voids, which are related to the thicknesses and position of the component in die. Nanometer particles found on the wall of the voids are magnesium oxide compounds, which are formed when oxygen from the entrapped air reacts with magnesium during die casting process. The compounds have no obvious effect on the mechanical properties even though there is possibly volume change present to form other compounds in the course of the components’ being used. [Copyright &y& Elsevier]

Abstract: In this paper, the microstructures of a thin-wall die-cast commercial magnesium (Mg) alloy, aged at 200°C for various times, were investigated using transmission electron microscopy. A martensite-like phase was observed. The crystal lattice parameters of this phase are a =0.30948nm, c =0.50498nm, and axial ratio c/a =1.6314. Orientation relationship of the martensite-like phase and the parent has been determined as , and or . The driving force of the possible martensitic transformation (MT) might came from the residual microstructure stresses left during die-cast process. [Copyright &y& Elsevier]

Abstract: A component for a mobile phone, made of AZ91D alloy, is manufactured by die cast. After chemical conversion treatment, it was found that there were some bright spots on the component surface. The formation mechanism of the bright spots was studied by means of SEM and metallgraphic microscope. The corrosion resistance of coatings was measured by potentiodynamic polarization test. The reason of bright spots formation is that the coarse primary α is dissolved into a dent during acid activation process because of little netted β and little aluminum. Besides, the coatings on bright spots are thin, the dimensions of primary α grains are large, and the surfaces are smooth relatively, so they are prone to reflecting light and become bright spots. [Copyright &y& Elsevier]

• Three Ni powders were sprayed on AZ31B by cold spray for corrosion protection. • Coating microstructures and corrosion performances were correlated with powder feature. • Powder surface protruding granules will hinder inter-particle intimate contact. • The gas-atomized smooth Ni powder can give > 3000 h corrosion protection for AZ31B. Three nickel (Ni) powders, i.e. gas-atomized spherical Ni (GA-Ni), carbonyl irregular Ni (C-Ni), and electrolytic dendritic porous Ni (E-Ni), were sprayed on magnesium alloys by an in-situ micro-forging assisted cold spray method. Although all deposited coatings present extremely low porosity, the C-Ni and E-Ni coatings reveal poorer inter-particle bonding than GA-Ni coating. Electrochemical tests reveal that the inter-particle gaps will act as fast pathways for the corrosive media to penetrate the coatings, thereby the GA-Ni coating with intimate inter-particle boundaries can isolate the corrosive media for 3000 h and the C-Ni and E-Ni coatings fail after <10 h corrosion. [ABSTRACT FROM AUTHOR]

Surface-porous Mg–Al alloys with different microstructures were fabricated via a new method of electrochemical dealloying in a neutral 0.6M NaCl solution. A bimodal porous structure with 47.57 ± 11.43 μm large pores and 265.60 ± 78.68nm honeycomb-like fine pores was obtained by direct electrochemical dealloying of as-cast Mg 80 Al 20 alloy. Following annealing, the eutectic structure disappeared and a bicontinuous, single-sized, porous structure with 7.10 ± 1.96 μm ligaments was created by an annealing-electrochemical dealloying approach. The porous formation mechanism is governed by selective dissolution of the α-Mg phase, which leaves the Mg 17 Al 12 phase as the porous layer framework. [ABSTRACT FROM AUTHOR]

[Display omitted] • Preparation of new micro arc oxidation-Polydopamine composite coating on magnesium alloy using photocatalytic effect. • Photocatalytic effect of Mg 2 TiO 4 under ultraviolet promotes deposition of dopamine. • The reactive oxygen species of Mg 2 TiO 4 under ultraviolet reduced the destruction of the micro arc oxidation layer by Polydopamine deposition. Micro arc oxidation-Polydopamine (MAO-PDA) composite coating is a functional corrosion resistant coating formed on the surface of magnesium alloy by combining MAO to form a porous ceramic layer and the spontaneous oxidation polymerization characteristics of PDA. The MAO-PDA coating is environmentally friendly and modifiable, providing a basic film layer for achieving self-warning, self-healing, conductivity and other modification functions, which has attracted widespread attention. A new type of MAO-PDA composite coating with good corrosion resistance was deposited on the surface of magnesium alloy by photocatalysis, which expanded the application range of magnesium alloy MAO surface treatment. Surface morphology, coating adhesion and corrosion resistance of the composite coating are characterized. Combined with Density functional theory (DFT) calculation, the possible mechanism is explored. The results indicate that the MAO layer containing Mg 2 TiO 4 exhibits fewer pores and can better deposit PDA under ultraviolet (UV) light conditions, forming a magnesium alloy MAO-PDA composite coating with good corrosion resistance and modification friendliness. Band structure calculations confirm the photocatalytic effect of Mg 2 TiO 4 under UV irradiation. As a result of the photocatalytic effect, reactive oxygen species (ROS) were generated in the pores and on the surface of the MAO sample, thus accelerating the local deposition of PDA. [ABSTRACT FROM AUTHOR]

Abstract: The atmospheric corrosion behaviour of cast and high pressure die-cast AZ91D alloy in a polluted environment were investigated by SEM, XRD and FTIR. The atmospheric corrosion rate of the ingot was higher than that of the die-cast specimen. SO2 gas played an important role in atmospheric corrosion. The effects of dew condensation on the ingot were greater than that on the die-cast specimen. The corrosion was initiated in the less noble α phase of the samples. The β phase, on the other hand, remained and acted as a barrier to corrosion. [Copyright &y& Elsevier]