Corrosion mechanisms of AZ31 magnesium alloy: Importance of starting pH and its evolution

Abstract

This article aims at improving the understanding of the corrosion mechanism of AZ31 magnesium alloys during a long period of immersion in an aqueous electrolyte. In particular, the influence of the starting pH of the electrolyte on the oxidation of AZ31 alloy and its evolution due to corrosion phenomena were investigated. Several electrolytes with different pH values containing or not chlorides were used. The electrochemical properties of the metal substrate in these electrolytes were studied as a function of immersion time by electrochemical impedance spectroscopy. The kinetics of the reactions were determined by potentiodynamic polarization as well as dihydrogen evolution measurements by eudiometry. All these tests were carried out while the sample surface and electrolyte volume remained constant. The corrosion products generated on the alloy surface were analyzed by X‐ray photoelectrons spectrometry. This study has shown that the evolution of the corrosion rate and the corrosion products depends greatly on the initial pH and the nature of the used electrolyte. Alkaline electrolyte leads to a passive protective layer that can be locally destroyed by chloride ions. For a neutral chloride electrolyte, the strong increase of the pH due to the magnesium corrosion is not able to form a protective layer by precipitation of corrosion products. The kinetic and the corrosion extent are affected by the pH changes.

Tribo- and Tribocorrosion Properties of Magnesium AZ31 Alloy

“…The samples have been chemically pickled in nitric acid and rinsed in deionized water and air before testing. [7] The substrates have been characterized in terms of roughness, structure, and microstructure. Their roughness has been determined by a Zeiss 119 Surfcom 1400D-3DF apparatus (Oberkochen, Germany).…”

“…The primary issues with these metals are their extremely poor corrosion and wear resistances. The literature attributes the behaviour to the strong chemical activity of magnesium as well as to the unstable, imperfect natural oxide layer that coats its surface [6,7]. Understanding not just the wear and corrosion properties but also the tribocorrosion properties of magnesium is essential given the potential of the magnesium alloys.…”

Evolution of Microstructure, Mechanical Properties, and Corrosion Resistance of Mg–2.2Gd–2.2Zn–0.2Ca (wt%) Alloy by Extrusion at Various Temperatures

“…No inductive loop was observed in the present study. This is basically because the EIS tests were conducted at a lower limit frequency of 0.1 Hz, and below this limit, some non-stationarities are likely generated due to the fast dissolution of magnesium, resulting in an appearance of a pseudo-inductive loop [60,61]. Although there have been disagreements among the researchers about the interpretation of time constants in EIS spectra of magnesium alloys, recent studies by Wang et al [62,63] have shed light on the origin of the capacitive loops in Nyquist plots, where the high-frequency capacitive loop results from the barrier properties of the surface film (oxide/hydroxide layer) and the middle-frequency capacitive loop is due to the charge transfer process and the respective double-layer structure at the electrode/electrolyte interface.…”