N. Birbilis

Electrochemical Characteristics of Intermetallic Phases in Aluminum Alloys

This paper presents a survey of corrosion potentials, pitting potentials, and electrochemical characteristics for intermetallic particles commonly present in high-strength aluminum-based alloys. Results from relevant pure metals and solid solutions are also presented. It is seen that corrosion potentials and pitting potentials vary over a wide range for various intermetallics. Elaboration of the results reveals that the electrochemical behavior of intermetallics is more detailed than the simple noble or active classification based upon corrosion potential or estimated from the intermetallic composition. Intermetallics capable of sustaining the largest cathodic current densities are not necessarily those with the most noble Ecorr, similarly those with the least noble Ecorr will not necessarily sustain the largest anodic currents. The data herein was collected via the use of a microcapillary electrochemical cell facilitating electrode investigations upon intermetallic particles in the micrometer-squared range. This survey may be used as a tool for clarification of localized corrosion phenomena in Al alloys.

Assessing the corrosion of biodegradable magnesium implants: a critical review of current methodologies and their limitations.

Magnesium (Mg) and its alloys have been intensively studied as biodegradable implant materials, where their mechanical properties make them attractive candidates for orthopaedic applications. There are several commonly used in vitro tests, from simple mass loss experiments to more complex electrochemical methods, which provide information on the biocorrosion rates and mechanisms. The various methods each have their own unique benefits and limitations. Inappropriate test setup or interpretation of in vitro results creates the potential for flawed justification of subsequent in vivo experiments. It is therefore crucial to fully understand the correct usages of each experiment and the factors that need to be considered before drawing conclusions. This paper aims to elucidate the main benefits and limitations for each of the major in vitro methodologies that are used in examining the biodegradation behaviour of Mg and its alloys.

Effect of Grain Size on Corrosion: A Review

Abstract Grain refinement is known to lead to improvements in strength and wear resistance. Inherent processing involved in grain refinement alter both the bulk and the surface of a material, leading to changes in grain boundary density, orientation, and residual stress. Ultimately, these surface changes can have an impact on electrochemical behavior and, consequently, corrosion susceptibility as evidenced by the large number of studies on the effect of grain size on corrosion, which span a range of materials and test environments. However, there has been limited work on developing a fundamental understanding of how grain refinement and more generally how grain size affects the corrosion resistance of an alloy. Existing literature is often contradictory, even within the same alloy class, and a coherent understanding of how grain size influences corrosion response is largely lacking. A survey of previous work related to the relationship between grain size and corrosion resistance for a number of light meta...

Review of Corrosion-Resistant Conversion Coatings for Magnesium and Its Alloys

Abstract The corrosion susceptibility of Mg and its alloys in humid and aqueous environments limits their widespread application. The performance of several conversion coating technologies is reviewed herein. In addition to the assessment of performance based on the literature to date, thermodynamic analysis has been used to compare coating processes. Ongoing research reveals that the search for appropriate conversion coatings to replace toxic Cr-based coatings continues. It is seen that the variability between coatings of the same technology was greater than the averages between coating technologies. Additionally, coating pretreatment also appears to be more significant than the choice of coating technology itself.

Investigation and Discussion of Characteristics for Intermetallic Phases Common to Aluminum Alloys as a Function of Solution pH

This paper presents results for corrosion potentials, pitting potentials, and electrochemical characteristics for intermetallic particles commonly present in high strength aluminum-based alloys, for tests conducted in a 0.1 M NaCl solution of varying pH via the use of a microcapillary electrochemical cell. The intermetallics investigated were Mg2Si, MgZn2 ,A l7Cu2Fe, Al2Cu, Al2CuMg, and Al3Fe. Elaboration of the results reveals that the electrochemical behavior of such compounds varies markedly with pH, with attendant ramifications for localized corrosion and protection in Al alloys. Examples of this are shown for AA7075-T651, where it is shown that the localized corrosion morphology is drastically different upon the bulk alloy depending on the pH of the test environment. A stochastic pitting is observed at an acid pH, near-neutral conditions result in a deterministic-type pitting, and a general corrosion is observed at an alkaline pH.

A high-specific-strength and corrosion-resistant magnesium alloy

Ultra-lightweight alloys with high strength, ductility and corrosion resistance are desirable for applications in the automotive, aerospace, defence, biomedical, sporting and electronic goods sectors. Ductility and corrosion resistance are generally inversely correlated with strength, making it difficult to optimize all three simultaneously. Here we design an ultralow density (1.4 g cm(-3)) Mg-Li-based alloy that is strong, ductile, and more corrosion resistant than Mg-based alloys reported so far. The alloy is Li-rich and a solute nanostructure within a body-centred cubic matrix is achieved by a series of extrusion, heat-treatment and rolling processes. Corrosion resistance from the environment is believed to occur by a uniform lithium carbonate film in which surface coverage is much greater than in traditional hexagonal close-packed Mg-based alloys, explaining the superior corrosion resistance of the alloy.

Electrochemical Properties of Intermetallic Phases and Common Impurity Elements in Magnesium Alloys

The electrochemical properties of the key intermetallic particles that form in commercial Mg alloys are presented. Results were collected via microcapillary electrochemical testing upon bulk intermetallic analogs in dilute chloride solution. The intermetallics investigated were Mg17Al12 ,M g 2Al3 ,M g 2Ca, Mg12Ce, Mg12La, Mg3Nd, Mg2Si, Mg24Y5, and MgZn2. It was found that the intermetallic phases, with the exception of Mg2Ca, were more noble than Mg, supporting increased levels of cathodic kinetics; however, the variation in electrochemical response between intermetallics was large in terms of corrosion potential, presence of a passive window, and currents sustained over a range of potentials.

Corrosion of magnesium alloys: the role of alloying

Abstract The demand for light-weighting in transport and consumer electronics has seen rapid growth in the commercial usage of magnesium (Mg). The major use of Mg is now in cast Mg products, as opposed to the use of Mg as an alloying element in other alloy systems and there is an emerging market of wrought Mg products and biomedical Mg components – such that the past two decades have seen a significant number of new Mg-alloys reported. None-the-less, the corrosion of Mg alloys continues to be a challenge facing engineers seeking weight reductions by deployment of Mg. Herein, authors review the influence of alloying on the corrosion of Mg-alloys, with particular emphasis on the underlying electrochemical kinetics that dictate the ultimate corrosion rate. Such a review focusing on the chemistry–corrosion link, both in depth and in a holistic approach, is lacking. As such the authors do not describe aspects such as high-temperature oxidation or cracking, but focus on delivering the state-of-the-art with regards to alloying influences on corrosion kinetics. It has been demonstrated that Mg itself will not be thermodynamically passive in environments of pH<11, regardless of the extent and type of alloying and hence corrosion kinetics require unique attention. Authors consolidate the presentation to include essentially all commercially available alloys and in excess of 350 custom alloys with wide variations in composition; in addition to reviewing the range of intermetallic compounds and impurities that form in such alloys systems. An update is also given regarding mechanistic advances and the role of grain size on corrosion of Mg. A wider understanding of the role of chemical effects upon corrosion of Mg is both timely and serves to highlight metallurgical approaches towards kinetically retarding the corrosion problem. The latter is of key relevance to next generation lightweight alloys and rational design of wrought Mg and bio-Mg.

Inhibition of AA2024-T3 on a Phase-by-Phase Basis Using an Environmentally Benign Inhibitor, Cerium Dibutyl Phosphate

the oxygen reduction reaction. 8,9,18 Therefore, in this work, we investigated the effectiveness of Cedbp3 as both an anodic and cathodic inhibitor on intermetallic compounds present as particles in 2024-T3, with the aim of characterizing electrochemically the mechanism of inhibitor action, while adopting a novel approach to achieve this. Ultimately, the inhibitor must reveal mixed properties to approach the inhibition efficiency of chromates. We suggest that Cedbp3 representing an optimized inhibitor of the Ce-based family, based on chemical considerations, will serve as good environmentally benign corrosion inhibitor for AA2024-T3.

Grain character influences on corrosion of ECAPed pure magnesium

Abstract This study seeks to clarify the influence of grain character on the corrosion rate of Mg. There is a special need to understand the largely unknown corrosion–grain size relationship for Mg, which nominally displays poor corrosion resistance, since any efforts to reduce or control the rate of Mg corrosion are of large technological significance. In this work, the microstructure is modified by equal channel angular pressing (ECAP) with up to eight passes, to achieve a range of refined microstructures ranging in grain size from a few hundred micrometres to a few micrometres. Results reveal a significant variation in corrosion rate with the number of ECAP passes, and hence grain size, which is of key significance. The results are discussed in terms of grain size, misorientation, along with resultant surface state produced and electrochemical/polarisation signatures collected.