Simulation of galvanic corrosion of magnesium coupled to a steel fastener in NaCl solution

The galvanic corrosion of magnesium alloy AZ91D coupled to a steel fastener was studied using a boundary element method (BEM) model and experimental measurements. The BEM model used the measured polarization curves as boundary conditions. The experimental program involved measuring the total corrosion rate as a function of distance from the interface of the magnesium in the form of a sheet containing a mild steel circular insert (5 to 30 mm in diameter). The measured total corrosion rate was interpreted as due to galvanic corrosion plus self corrosion. For a typical case, the self corrosion was estimated typically to be similar to 230 mm/y for an area surrounding the interface and to a distance of about I cm from the interface. Scanning Kelvin Probe Force Microscopy (SKPFM) revealed microgalvanic cells with potential differences of approximately 100 mV across the AZ91D surface. These microgalvanic cells may influence the relative contributions of galvanic and self corrosion to the total corrosion of AZ91D.

Boundary element method predictions of the influence of the electrolyte on the galvanic corrosion of AZ91D coupled to steel

This research investigated the galvanic corrosion of the magnesium alloy AZ91D coupled to steel. The galvanic current distribution was measured in 5% NaCl solution, corrosive water and an auto coolant. The experimental measurements were compared with predictions from a Boundary Element Method (BEM) model. The boundary condition, required as an input into the BEM model, needs to be a polarization curve that accurately reflects the corrosion process. Provided that the polarization curve does reflect steady state, the BEM model is expected to be able to reflect steady state galvanic corrosion.

Corrosion of magnesium alloys in commercial engine coolants

A number of magnesium alloys show promise as engine block materials. However, a critical issue for the automotive industry is corrosion of the engine block by the coolant and this could limit the use of magnesium engine blocks. This work assesses the corrosion performance of conventional magnesium alloy AZ91D and a recently developed engine block magnesium alloy AM-SC1 in several commercial coolants. Immersion testing, hydrogen evolution measurement, galvanic current monitoring and the standard ASTM D1384 test were employed to reveal the corrosion performance of the magnesium alloys subjected to the coolants. The results show that the tested commercial coolants are corrosive to the magnesium alloys in terms of general and galvanic corrosion. The two magnesium alloys exhibited slightly different corrosion resistance to the coolants with AZ91D being more corrosion resistant than AM-SC1. The corrosivity varied from coolant to coolant. Generally speaking. an oraganic-acid based long life coolant was less corrosive to the magnesium alloys than a traditional coolant. Among the studied commercial coolants. Toyota long, life coolant appeared to be the most promising one. In addition. it was found that potassium fluoride effectively inhibited corrosion of the magnesium alloys in the studied commercial coolants. Both general and galvanic corrosion rates were significantly decreased by addition of KF, and there were no evident side effects on the other engine block materials, such as copper, solder. brass. steel and aluminium alloys, in terms of their corrosion performance. The ASTM D 1384 test further confirmed these results and suggested that Toyota long life coolant with 1%wt KF addition is a promising coolant for magnesium engine blocks.

The corrosion performance of anodised magnesium alloys

The corrosion performance of anodised magnesium and its alloys, such as commercial purity magnesium (CP-Mg) and high-purity magnesium (HP-Mg) ingots, magnesium alloy ingots of MEZ, ZE41, AM60 and AZ91D and diecast AM60 (AM60-DC) and AZ91D (AZ91D-DC) plates, was evaluated by salt spray and salt immersion testing. The corrosion resistance was in the sequential order: AZ91D approximate to AM60 approximate to MEZ >= AZ91D-DC >= AM60-DC > HP-Mg > ZE41 > CP-Mg. It was concluded the corrosion resistance of an anodised magnesium alloy was determined by the corrosion performance of the substrate alloy due to the porous coating formed on the substrate alloy acting as a simple corrosion barrier. (c) 2006 Elsevier Ltd. All rights reserved.

In vivo degradation of magnesium plate/screw osteosynthesis implant systems: Soft and hard tissue response in a calvarial model in miniature pigs.

Biodegradable magnesium plate/screw osteosynthesis systems were implanted on the frontal bone of adult miniature pigs. The chosen implant geometries were based on existing titanium systems used for the treatment of facial fractures. The aim of this study was to evaluate the in vivo degradation and tissue response of the magnesium alloy WE43 with and without a plasma electrolytic surface coating. Of 14 animals, 6 received magnesium implants with surface modification (coated), 6 without surface modification (uncoated), and 2 titanium implants. Radiological examination of the skull was performed at 1, 4, and 8 weeks post-implantation. After euthanasia at 12 and 24 weeks, X-ray, computed tomography, and microfocus computed tomography analyses and histological and histomorphological examinations of the bone/implant blocks were performed. The results showed a good tolerance of the plate/screw system without wound healing disturbance. In the radiological examination, gas pocket formation was found mainly around the uncoated plates 4 weeks after surgery. The micro-CT and histological analyses showed significantly lower corrosion rates and increased bone density and bone implant contact area around the coated screws compared to the uncoated screws at both endpoints. This study shows promising results for the further development of coated magnesium implants for the osteosynthesis of the facial skeleton.

Method for determining reaction rate of mild steel containers during melting of magnesium-aluminium alloys and effect of aluminium content on directionally solidified microstructures

A magnesium alloy of eutectic composition (33 wt-'%Al) was directionally solidified in mild steel tubes at two growth rates, 32 and 580 mum s(-1,) in a temperature gradient between 10 and 20 K mm(-1). After directional solidification, the composition of each specimen varied dramatically, from 32'%Al in the region that had remained solid to 18%Al (32 mum s(-1) specimen) and 13%Al (580 mum s(-1) specimen) at the plane that had been quenched from the eutectic temperature. As the aluminium content decreased, the microstructure contained an increasing volume fraction of primary magnesium dendrites and the eutectic morphology gradually changed from lamellar to partially divorced. The reduction in aluminium content was caused by the growth of an Al-Fe phase ahead of the Mg-Al growth front. Most of the growth of the Al-Fe phase occurred during the remelting period before directional solidification. The thickness of the Al-Fe phase increased with increased temperature and time of contact with the molten Mg-Al alloy. (C) 2003 Maney Publishing.

Morphology and crystallography of Mg24Y5 precipitate in Mg-Y alloy

Orientation relationships between Mg24Y5 precipitates and matrix in a Mg-Y alloy were accurately determined using Kikuchi line diffraction. The Burgers relationship with habit planes of {10 (1) over bar0}(H) and {31 (4) over bar0}(H) were observed for all precipitates. Compared with the Mg17Al12 precipitate in AZ91, the precipitation hardening effect in this alloy was significantly increased. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Corrosion behaviour of magnesium in ethylene glycol

Corrosion of magnesium engine components by coolant is an important issue in the automotive industry where magnesium alloys may be used. It is of significance to understand the corrosion behaviour of pure magnesium in ethylene glycol solutions, as this can provide a basis for developing new coolants for magnesium alloy engine blocks. In this paper, through corrosion and electrochemical tests, it was found that the corrosion rate of magnesium decreased with increasing concentration of ethylene glycol. Individual contaminants, such as NaCl, NaHCO3, Na2SO4 and NaCl can make aqueous ethylene glycol solution more corrosive to magnesium. However, in NaCl contaminated ethylene glycol, NaHCO3 and Na2SO4 showed some inhibition effect. The solution resistivity played an important role in the corrosion of magnesium in ethylene glycol solutions, and the competitive adsorption of ethylene glycol and the contaminants on the magnesium surface was also responsible for the observed corrosion behaviours. The corrosion of magnesium in ethylene glycol can be effectively inhibited by addition of fluorides that react with magnesium and form a protective film on the surface. (C) 2003 Elsevier Ltd. All rights reserved.

Anodizing treatments for magnesium alloys and their effecton corrosion resistance in various environments

This paper reviews various aspects of anodizing of magnesium alloys, such as the basics, processes, properties and applications. It systematically summarises the existing fundamental studies and technical developments of anodizing of magnesium alloys, and concludes that new anodizing processes based on electrolytic plasma anodizing that convert the surface of a magnesium alloy into a hard ceramic coating in an electrolytic bath using high energy electric discharges can offer improved wear and corrosion resistance. These new anodized coatings are often claimed to perform better than the traditional ones obtained through older anodizing processes, such as DOW17 or HAE. The new anodizing techniques are chromate free and hence environment friendly. It is expected that more cost-effective, environment-friendly and non-toxic anodizing techniques will be developed and applied to magnesium alloy components in the future.

Fatigue crack propagation in a sintered 2xxx series aluminium alloy

Many potential applications for sintered aluminium are limited by the poor fatigue properties of the material. In order to increase understanding of the fatigue mechanisms in sintered aluminium, fatigue tests were carried out on a sintered 2xxx series aluminium alloy, AMB-2712. The alloy has a fatigue endurance strength of approximately 145 MPa (R = 0.1). Three regions were identified on the fatigue fracture surfaces. Region I contains the initiation site and transgranular crack propagation. When the size of the cyclic plastic zone ahead of the crack becomes comparable to the grain size, microstructural damage at the crack tip results in a transition to intergranular propagation. Region 2 mainly contains intergranularly fractured material, whilst the final fracture area makes up Region 3, in the form of dimple coalescence and intergranular failure. Transgranular fractographic features observed on fatigued specimens include fissure-type striations, cross-hatched grains, furrowed grains and grains containing step-like features. (c) 2006 Elsevier B.V. All rights reserved.

Control of soundness and mechanical properties in aluminium-silicon-magnesium alloys

Previously, specifications for mechanical properties of casting alloys were based on separately cast test bars. This practice provided consistently reproducible results; thus, any change in conditions was reflected in changes in the mechanical properties of the test coupons. These test specimens, however, did not necessarily reflect the actual mechanical properties of the castings they were supposed to represent'. Factors such as section thickness and casting configuration affect the solidification rate and soundness of the casting thereby raising or lowering its mechanical properties in comparison with separately cast test specimens. In the work now reported, casting shapes were developed to investigate the variations of section thickness, chemical analysis and heat treatment on the mechanical properties of a high strength Aluminium alloy under varying chilling conditions. In addition, an insight was sought into the behaviour of chills under more practical conditions. Finally, it was demonstrated that additional information could be derived from the radiographs which form an essential part of the quality control of premium quality castings. As a result of the work, it is now possible to select analysis and chilling conditions to optimize the as cast and the heat treated mechanical properties of Aluminum 7% Silicon 0.3% Magnesium alloy.

An Investigation on Biocompatibility of Bio-Absorbable Polymer Coated Magnesium Alloys

Advances in biomaterials have enabled medical practitioners to replace diseased body parts or to assist in the healing process. In situations where a permanent biomaterial implant is used for a temporary application, additional surgeries are required to remove these implants once the healing process is complete, which increases medical costs and patient morbidity. Bio-absorbable materials dissolve and are metabolized by the body after the healing process is complete thereby negating additional surgeries for removal of implants. Magnesium alloys as novel bio-absorbable biomaterials, have attracted great attention recently because of their good mechanical properties, biocompatibility and corrosion rate in physiological environments. However, usage of Mg as biodegradable implant has been limited by its poor corrosion resistance in the physiological solutions. An optimal biodegradable implant must initially have slow degradation to ensure total mechanical integrity then degrade over time as the tissue heals. The current research focuses on surface modification of Mg alloy (MZC) by surface treatment and polymer coating in an effort to enhance the corrosion rate and biocompatibility. It is envisaged that the results obtained from this investigation would provide the academic community with insights for the utilization of bio-absorbable implants particularly for patients suffering from atherosclerosis. The alloying elements used in this study are zinc and calcium both of which are essential minerals in the human metabolic and healing processes. A hydrophobic biodegradable co-polymer, polyglycolic-co-caprolactone (PGCL), was used to coat the surface treated MZC to retard the initial degradation rate. Two surface treatments were selected: (a) acid etching and (b) anodization to produce different surface morphologies, roughness, surface energy, chemistry and hydrophobicity that are pivotal for PGCL adhesion onto the MZC. Additionally, analyses of biodegradation, biocompatibility, and mechanical integrity were performed in order to investigate the optimum surface modification process, suitable for biomaterial implants. The study concluded that anodization created better adhesion between the MZC and PGCL coating. Furthermore, PGCL coated anodized MZC exhibited lower corrosion rate, good mechanical integrity, and better biocompatibility as compared with acid etched.

The influence of the atmosphere on the sintering of aluminum

Alloys of Al, Al-0.15Mg, and Al-12Sn made using air atomized aluminum powder and pressed to green densities of 75 to 98 pet were sintered under argon or nitrogen. Sintering in argon is only effective at high green densities when magnesium is present. In contrast, highly porous aluminum can be sintered in nitrogen without the need for magnesium. The oxygen concentration in the gas is reduced by the aluminum through a self-gettering process. The outer layers of the porous powder compact serve as a getter for the inner layers such that the oxygen partial pressure is reduced deep within the pore network. Aluminum nitride then forms, either by direct reaction with the metal or by reduction of the oxide layer, and sintering follows.

Liquid phase sintering of aluminium alloys

The principle that alloys are designed to accommodate the manufacture of goods made from them as much as the properties required of them in service has not been widely applied to pressed and sintered P/M aluminium alloys. Most commercial alloys made from mixed elemental blends are identical to standard wrought alloys. Alternatively, alloys can be designed systematically using the phase diagram characteristics of ideal liquid phase sintering systems. This requires consideration of the solubilities of the alloying elements in aluminium, the melting points of the elements, the eutectics they form with aluminium and the nature of the liquid phase. The relative diffusivities are also important. Here we show that Al-Sn, which closely follows these ideal characteristics, has a much stronger sintering response than either Al-Cu or Al-Zn, both of which have at least one non-ideal characteristic. (C) 2001 Elsevier Science B.V. All rights reserved.