The Corrosion Performance and Mechanical Properties of Mg-Zn Based Alloys—A Review

Magnesium alloys have shown great potential for applications as both structural and biomedical materials due to their high strength-to-weight ratio and good biodegradability and biocompatibility, respectively. Among them, Mg-Zn based alloys are attracting increasing interest for both applications. As such, this article provides a review of the corrosion performance and mechanical properties of Mg-Zn based alloys, including the influence of environment and processing on both of them. The strategies for tailoring corrosion resistance and/or mechanical properties by microstructure adjustment and surface treatment are discussed

Controllable Degradable Plasma Electrolytic Oxidation Coated Mg Alloy for Biomedical Application

A controllable degradable coating on Mg alloy based on plasma electrolytic oxidation (PEO) process is reported for the first time. The reported results show that introduction of silica nanoparticles into PEO electrolytes leads to their reactive incorporation in coatings and thus influencing the degradation behavior. Dissolution of amorphous phases facilitates chemical reaction with components of simulated body fluid, resulting in self-healing effect via redeposition of insoluble conversion products. The dynamic balance between dissolution of the original coating and reconstruction of corrosion layer is mainly determined by the phase composition of the coating as well as the surrounding corrosive medium

Effect of Heat Treatment on the Corrosion Behavior of Mg-10Gd Alloy in 0.5% NaCl Solution

In 0.5 wt.% NaCl aqueous solution, Mg-10Gd alloy shows promising corrosion resistance. The microstructure of this alloy was modified via heat treatments to understand the effect of accompanying microstructural changes on the corrosion resistance. It was found that corrosion performance depends both on the amount and the distribution of the cathodic intermetallic phases. The T4 heat treatment (24 h at 540 ◦ C) caused the Gd to distribute uniformly in the matrix, which had positive effect on corrosion resistance showing a delay in the time required for the first observation of localized corrosion. The T4 heat treated specimens, specimens aged at 200 ◦ C and 300 ◦ C, showed relatively uniform degradation and thus these heat treatments are not detrimental in terms of corrosion resistance. In contrast, heat treatment at 400 ◦ C seems to increase the formation of small cuboidal particles rich in Gd, most likely to be GdH2 particles, in the matrix, resulting in a detrimental effect on the corrosion behavior

Aluminum anodization in deionized water as electrolyte

Thin oxide films were prepared electrochemically on the aluminum surface using the high-voltage discharge and potentiostatic methods in deionized water as an electrolyte. The growth of continuous films occurred only at potentials lower than the breakdown potential. The films obtained by the discharge method are more uniform and can grow to a higher thickness in comparison to those formed by the potentiostatic mode, as demonstrated by electrochemical impedance spectroscopy (EIS), transmission electron microscopy (TEM), and scanning Kelvin probe force microscopy (SKPFM). The data herein obtained can be used as a reference to understand better the properties of the films produced in conventional electrolytes where apart from water other species are present

The Role of Cu-Based Intermetallic on the Direct Growth of a ZnAl LDH Film on AA2024

The direct ZnAl layered double hydroxide growth on AA2024 is a fast-occurring reaction, yet is characterized by an inhomogeneous film thickness. It has been shown that at the periphery of Cu-rich intermetallic, the flakes tend to be larger and denser. A combination of in situ and ex situ measurements were used to monitor the changes in the layered double hydroxide film grown on the regions of intermetallics. Immediately after immersion, an activation of the intermetallic phases is observed due to the dealloying process with an almost immediate film growth. Dealloying is followed by trenching of the adjacent Al matrix leading to an excessive production of large and dense layered double hydroxide flakes at the periphery of the intermetallic. However, the scanning electron microscopy cross-section images revealed that the trenching process leads to defects in the area surrounding the intermetallic. This could weaken the corrosion resistance performance of the layered double hydroxide conversion coating and lead to adhesion failure of consecutive polymer coatings. Nevertheless, this work highlights a few advantages and drawbacks of the layered double hydroxide conversion coatings and pathways to its potential optimization and improvement

Biodegradation behaviour of Fe-based alloys in Hanks’ Balanced Salt Solutions : part I. Material characterisation and corrosion testing

Research on Fe-based biodegradable alloys for implant applications has increased considerably over the past decade. However, there is limited information on the influence of testing electrolytes on corrosion product formation and general corrosion progress. In this work, the effect of Hanks’ Balanced Salt Solution (HBSS) with or without Ca2+ on the corrosion of Fe, Fe35Mn and (Fe35Mn)5Ag powder-processed coupons has been studied using potentiodynamic polarisation, Electrochemical Impedance Spectroscopy (EIS), and preliminary localised measurement of pH and dissolved oxygen concentration in close proximity to the metal surface. Both Fe35Mn and (Fe35Mn)5Ag alloys showed accelerated corrosion when compared to pure Fe based on potentiodynamic testing results, with FeMnAg exhibiting the highest corrosion rate in Ca2+-containing HBSS. The results indicate that in Ca2+-containing HBSS, the formation of a partially protective Ca/P layer decelerates the corrosion progress, whereas the Fe- and Mn-phosphates formed in Ca2+-free HBSS do not have the same effect. The Ca/P layer on (Fe35Mn)5Ag experienced a reduction in resistance following several hours of testing, indicating partial loss of its protective effect.peer-reviewe

Biodegradation behaviour of Fe-based alloys in Hanks’ Balanced Salt Solutions : part II. The evolution of local pH and dissolved oxygen concentration at metal interface

Commercially pure Fe, Fe35Mn, and (Fe35Mn)5Ag alloys were prepared by uniaxial pressing of the mixture of individual powders, followed by sintering. The influence of the alloying elements Mn and Ag on the corrosion behaviour of these Fe-based alloys was investigated in Hanks’ Balanced Salt Solution (HBSS). Furthermore, the role of the components in HBSS, particularly Ca2+ ions during alloys degradation was studied. Distribution of local pH and dissolved oxygen concentration was measured 50 μm above the interface of the degrading alloys. The results revealed that 5 wt% Ag addition to Fe35Mn alloy triggered micro-galvanic corrosion, while uniform corrosion dominated in pure Fe and Fe35Mn. Fast precipitation of Ca–P-containing products on the surface of these Fe-based alloys buffered local pH at the metal interface, and blocked oxygen diffusion at the initial stages of immersion. In the (Fe35Mn)5Ag, the detachment or structural changes of Ca–P-containing products gradually diminished their barrier property. These findings provided valuable insights into the degradation mechanism of promising biodegradable Fe-based alloys.peer-reviewe

The Influence of PSA Pre-Anodization of AA2024 on PEO Coating Formation: Composition, Microstructure, Corrosion, and Wear Behaviors

In the frame of the current work, it was shown that plasma electrolytic oxidation (PEO) treatment can be applied on top of phosphoric sulfuric acid (PSA) anodized aluminum alloy AA2024. Being hard and well-adherent to the substrate, PEO layers improve both corrosion and wear resistance of the material. To facilitate PEO formation and achieve a dense layer, the systematic analysis of PEO layer formation on the preliminary PSA anodized layer was performed in this work. The microstructure, morphology, and composition of formed PEO coatings were investigated using scanning electron microscopy (SEM), x-ray diffraction (XRD), and glow-discharge optical emission spectroscopy (GDOES). It was shown that under constant current treatment conditions, the PSA layer survived under the applied voltage of 350 V, whilst 400 V was an intermediate stage; and under 450 V, the PSA layer was fully converted after 5 min of the treatment. The comparison test with PEO formation on the bare material was performed. It was confirmed that during the "sparking" mode (400 V) of PEO formation, the PEO coatings, formed on PSA treated AA2024, were more wear resistant than the same PEO coatings on bare AA2024

How density functional theory surface energies may explain the morphology of particles, nanosheets, and conversion films based on layered double hydroxides

Conversion films based on layered double hydroxides constitute an important and environmentally friendly technology for the corrosion protection of aeronautical structures. Unfortunately, the morphology of layered double hydroxide (LDH) conversion films is still not well understood. In the present work, the structure and driving forces behind the morphology of zinc−aluminum LDH conversion films on aluminum alloy 2024 (AA2024) are explained from the perspective of molecular modeling. Since LDH particles are the core structures of LDH conversion films, the first step in this work was to understand the relation between structure and morphology of the particles themselves and the single-layer nanosheets that constitute them. Results regarding LDH’s crystallites, particles, and conversion films obtained using X-ray diffraction (XRD), dynamic light scattering (DLS), scanning electron microscopy (SEM), and atomic force microscopy (AFM) are interpreted using periodic model density functional theory (DFT) calculations. On the basis of the understanding of the formation of LDH particles and their exfoliation to obtain single-layer nanosheets, for the first time, LDH conversion films have been modeled using periodic model DFT. The results point to a preferential orientation of the cationic layers perpendicular to the surface, thus explaining the film morphology (SEM and AFM) and providing a rational for their crystallization process.publishe

CORDATA: an open data management web application to select corrosion inhibitors

The large amount of corrosion inhibition efficiencies in literature, calls for a more efficient way to organize, access and compare this information. The CORDATA open data management application (https://datacor.shinyapps.io/cordata/) can help select appropriate corrosion inhibitors for application specific challenges.publishe