Possible dissolution pathways participating in the Mg corrosion reaction

The Mg corrosion reaction sequence was considered in terms of possible elementary reaction steps, and their plausibility in relation to their thermodynamic status. The uni-positive Mg+ reaction sequence is thermodynamically favoured, and can occur by a number of elementary steps. There are a number of possible dissolution reactions (which are defined are reactions which contribute no electrons to the corroding Mg electrode). It is impossible at this stage to conclusively determine the reaction mechanism without knowledge of the species present at the magnesium water interface. (C) 2014 Elsevier Ltd. All rights reserved

Stress corrosion cracking of high-strength steels

The mechanisms of stress corrosion cracking (SCC) and hydrogen embrittlement were recently reviewed by Lynch in this journal. The present review, in contrast, focuses on the rate-limiting step of the SCC of low-alloy high-strength steels in water and particularly focuses on the influence of the applied stress rate on the SCC of lowalloy high-strength steels. Linearly increasing stress tests of low-alloy high-strength steels in distilled water indicated that the stress corrosion crack velocity increased with increasing applied stress rate until the maximum crack velocity, corresponding to v in fracture mechanics tests in distilled water. Moreover, the crack velocity was dependent only on the applied stress rate and was not influenced by the steel composition. The rate-limiting step could be the rupture of a surface film, which would control the rate of metal dissolution and/or the production and transport of hydrogen to the crack tip or to the regions ahead of the crack tip

A critical review of the influence of hydrogen on the mechanical properties of medium-strength steels

As medium-strength steels are promising candidates for the hydrogen economy, it is important to understand their interaction with hydrogen. However, there are only a limited number of investigations on the behavior of medium-strength steels in hydrogen. The existing literature indicates that the influences of hydrogen on the tensile properties of medium-strength steels are mainly the following: (i) the steel can be hardened by hydrogen, as demonstrated by an increase in the yield stress or ultimate tensile stress; (ii) some steels can be embrittled by hydrogen, as revealed by lower yield stress or ultimate tensile stress; (iii) in most cases, these steels may experience hydrogen embrittlement (HE), as indicated by a reduction in ductility. The degree of HE mainly depends on the test conditions and the steel. The embrittlement can lead to catastrophic brittle fracture in service. The influence of hydrogen on the fatigue properties of medium-strength steels is dependent on many factors such as the stress ratio, temperature, yield stress of the steel, and test frequency. Generally, the hydrogen influence on fatigue limit is small, whereas hydrogen can accelerate the fatigue crack growth rate, leading to a shorter fatigue life. Inclusions are an important factor influencing the properties of medium-strength steels in the presence of hydrogen. However, it is not possible to predict the influence of hydrogen for any particular steel that has not been experimentally evaluated or to predict service performance. It is not known why similar steels can have different behavior, ranging from good resistance to significant embrittlement. A better understanding of the microstructural characteristics is needed

Influence of Al and Y on the ignition and flammability of Mg alloys

The influence of alloying on the ignition and flammability was studied. One end of a cylindrical specimen was exposed to a free diffusion flame. Ignition required at least partial melting. Burning extinguished once the flame was withdrawn. Specimen tips of pure Mg, AZ61, and AZ91 ignited upon prolonged flame exposure. There was smouldering and delayed ignition for Mg-1Y. There was no ignition for Mg-5Y specimen tips, attributed to a protective surface oxide containing Y. The results indicate that (i) vigorous burning requires a continued supply of Mg vapour, and (ii) a critical alloy concentration is required to change ignition behaviour. (C) 2011 Elsevier Ltd. All rights reserved

Low apparent valence of Mg during corrosion

Our recent data on Mg corrosion has been reanalysed because of the recent criticism that our previous data analysis was inadequate. Re-analysis leads to similar conclusions as previously. The apparent valence of Mg during corrosion was in each case less than 2.0, and in many cases less than 1.0. Moreover, these values were probably over-estimates. The low values were consistent with the evolving hydrogen gas acting as an insulator, so that the corrosion of parts of the specimen could occur isolated from the electrochemical measurement system

The influence of pH on the corrosion rate of high-purity Mg, AZ91 and ZE41 in bicarbonate buffered Hanks' solution

Corrosion of high purity (HP) Mg, AZ91 and ZE41, was studied in CO-bicarbonate buffered Hanks' solution, using mass loss and hydrogen evolution. Corrosion rates were in the following decreasing order: ZE41>HP Mg>AZ91. Corrosion rates of ZE41 were much higher than those of the other Mg alloys. HP Mg and AZ91 had comparable corrosion rates, but HP Mg corroded more quickly. All corrosion rates decreased slightly with increasing pH, attributed to more stable protective films. A slow fluid flow caused a slightly higher corrosion rate and a more-uniform corrosion morphology for all Mg alloys

A model of internal crack extension due to a continuous build-up of hydrogen pressure: Application to a pressure vessel component

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Corrosion and passivation of magnesium alloys

This paper reviews and discusses the possibility of the production of a passive magnesium (Mg) alloy through metallurgical approaches, such as purification, alloying, heat-treatment, mechanical processing and non-equilibrium sputter deposition. High-purity Mg and all existing Mg alloys produced by traditional methods are found to be active in a chloride containing solution. Passivity in a Mg alloy might be produced through a non-equilibrium technique with a sufficiently-high concentration of a strong passivating element supersaturated in the matrix phase. This paper clarifies important concepts regarding the passivity of Mg alloys, and suggests possible approaches to develop a passive, corrosion-resistant Mg alloy

Oxidation of magnesium alloys at elevated temperatures in air: a review

This paper reviews (i) the oxidation of Mg alloys at elevated temperatures in air; (ii) the influence of alloying elements on the oxidation of Mg alloys; and (iii) the progress in the development of oxidation-resistant Mg alloys. Low oxidation rates have been shown by Mg alloys containing the alloying elements, Ca, Be and rare earth elements. However, these alloying elements may also decrease mechanical properties, such as ductility

Cost-Effective Fe-Rich High-Entropy Alloys: A Brief Review

High-entropy alloys (HEAs) have attracted increased attention due to their extraordinary properties. However, the multicomponent characteristic of equiatomic HEAs inevitably leads to high material costs, which thus limits their widespread industrial applications. Although HEAs are claimed to be suitable for applications in extreme environment due to their comprehensive properties, the actual properties of HEAs dramatically vary with compositions and processes. Therefore, the development of cost-effective HEAs with comprehensive properties is indispensable for industrial uses. Till now, although comprehensive review papers on HEAs are available, few works focused on the cost-effectiveness of HEAs, particularly Fe-rich HEAs recently developed. This review thus aims to fill this gap by reviewing the current research progress in Fe-rich HEAs with a focus on the composition design, microstructure, and properties, including mechanical properties, and resistances for oxidation, wear, and corrosion. The challenges for applying cost-effective Fe-rich HEAs into industries are also arising as future research topics