Nanomechanical properties of Mg–Al intermetallic compounds produced by packed powder diffusion coating (PPDC) on the surface of AZ91E

A packed powder diffusion coating (PPDC) treatment produced two intermetallic layers on the surface of the commercial magnesium alloy AZ91E. The beta-phase (Mg17Al12) was immediately on top of the AZ91E, on top of which was the tau-phase (Mg-32(Al,Zn)(49)). Nanoindentation showed that the elastic modulus and hardness of each of the intermetallic compounds was significantly greater than that of the AZ91E substrate. Staircase displacement bursts occurred during nanoindentation of the intermetallic compounds, attributed to the combination of incipient plasticity at low loads, and the development of dislocation networks due to dislocation pile ups around the indentation at higher loads. Crystallographic analysis of beta phase orientations using EBSD showed that the nanomechanical properties of the intermetallic compound produced through PPDC treatment were isotropic. (C) 2013 Elsevier B.V. All rights reserved

Calculated phase diagrams, iron tolerance limits, and corrosion of Mg-Al alloys

The factors determining corrosion are reviewed in this paper, with an emphasis on iron tolerance limit and the production of high-purity castings. To understand the iron impurity tolerance limit, magnesium phase diagrams were calculated using the Pandat software package. Calculated phase diagrams can explain the iron tolerance limit and the production of high-purity castings by means of control of melt conditions; this is significant for the production of quality castings from recycled magnesium. Based on the new insight, the influence of the microstructure on corrosion of magnesium alloys is reviewed

Improving the corrosion resistance of MgZn1.2GdxZr0.18 (x =0, 0.8, 1.4, 2.0) alloys via Gd additions

Funding Information: This research was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFB0301101 ), the National Natural Science Foundation of China (Grant No. 51971054 ) and the Fundamental Research Funds for the Central Universities (Grant Nos. N180904006 and N2009006 ). Publisher Copyright: © 2020 Elsevier LtdEffects of Gd addition on microstructure, corrosion behavior and mechanism of cast and extruded MgZn1.2GdxZr0.18 alloys are investigated through microstructure observation, weight loss and electrochemical tests. Increasing Gd from 0 to 2.0 at.%, grains are refined, MgZn2 phase, W-phase and X-phase are formed successively, and basal texture intensity is decreased. The significantly decreased grain size by extrusion and Gd addition induces formation of protective Gd2O3 and MgO layer. The extruded MgZn1.2Gd2.0Zr0.18 alloy shows decreased corrosion rate of 3.72 ± 0.36 mm/year, owing to fine and homogeneous microstructure, dual-role (micro-anode and barrier) of X-phase, compact oxidation layer and basal crystallographic texture.Peer reviewe

CORROSION POTENTIAL OSCILLATIONS

The corrosion potentials of stainless steels in chloride solutions for conditions significantly removed from those leading to pitting and crevice corrosion are constant and to a first approximation largely independent of temperature and chloride ion concentration. With increasing temperature and/or chloride ion concentration or with decreasing corrosion resistance a situation arises where the corrosion potential begins to oscillate, between two values roughly corresponding to the pitting potential and the repassivation potential. These potential oscillations are often associated with pitting and crevice corrosion, although there are clear cases where no corrosion attack was visible even after prolonged exposure. The oscillations are attributed to unstable passivity

Understanding magnesium corrosion, recent progress at UQ

The purpose of this paper is to review the recent advances from our research at UQ. Mg alloys corrosion is often non-linear: the corrosion rate accelerates to a steady state rate after an initial period of low corrosion rate. Plug-in specimens allow simultaneous measurement of the corrosion rate using hydrogen evolution, PH and by Tafel extrapolation of cathodic polarisation curves, P . PH is consistently greater than P ; the data is consistent with the uni-positive Mg ion being a short-lived intermediate. However, Tafel extrapolation needs to be used with caution. Measurements made at short immersion times may not reflect the steady state corrosion rate. Furthermore, the corrosion reaction at the Mg surface may be decoupled from the electrochemical measurement by for example hydrogen gas. Calculated phase diagrams have indicated that the Fe tolerance levels is ∼ 5ppm for heat-treated Mg, which may be of importance for wrought alloys

CORROSION POTENTIAL OSCILLATIONS OF STAINLESS STEELS IN AQUEOUS CHLORIDE SOLUTIONS

The conditions leading to corrosion potential oscillations in aqueous chloride solutions are discussed in terms of chloride ion concentration, oxygen concentration and temperature. The corrosion potentials of stainless steels in chloride solutions for conditions significantly removed from those leading to pitting and crevice corrosion are constant and to a first approximation largely independent of temperature and chloride ion concentration. With increasing temperature and/or chloride ion concentration or with decreasing corrosion resistance there comes a situation when the corrosion potential begins to oscillate, between two end values. These potential oscillations are often associated with pitting and crevice corrosion, although there are clear cases when no corrosion attack is visible even after prolonged exposure. The oscillations are attributed to unstable passivity. A model is developed in terms of the formation and breakup of metastable pit nuclei. (Edited author abstract.

Corrosion resistance of magnesium for implants

Research is outlined that is needed for Mg implant applications. The need for ultra-pure Mg is presented. Our method of producing ultra pure Mg is explained and why ultra pure Mg is likely to produce stainless Mg alloys. A summary is presented on how Nor's solution provides a good model for in vivo corrosion, and how the corrosion in Nor's solution has been shown to be similar to in vivo corrosion. A brief summary is presented on inappropriate methods of measurement of Mg corrosion, and how such measurements have led to the publication of inappropriate results. Recent research at UQ has led to the identification of a new hydride corrosion pathway for Mg as a possible explanation of the values of apparent valence for Mg less than 1.0