Electrochemical Estimation of the Corrosion Rate of Magnesium/Aluminium Alloys

The corrosion rate of AZ31, AZ80, and AZ91D magnesium/aluminium alloys immersed in 3.5 wt.% NaCl was determined comparing gravimetric and electrochemical measurements. The findings revealed that, for all investigated materials, a fraction of the metallic surface exposed to the corrosive medium did not reveal a normal electrochemical response to the applied signal. This may be associated with phenomena such as partial disintegration of specimens into fine metallic particles, electrochemical formation of ions, and/or anomalous chemical attack occurring simultaneously with the normal electrochemical corrosion attack. The abnormal electrochemical behaviour was more evident for lower amounts of aluminium in the bulk composition of the investigated materials. Thus, the electrochemical estimates of pure Mg and the AZ31 alloy were not reliable and tended to underestimate corrosion losses.Peer Reviewe

Combined effect of composition and surface condition on corrosion behaviour of magnesium alloys AZ31 and AZ61

The work is an attempt to learn more about the role of several experimental variables in the corrosion of magnesium alloys in immersion tests carried out in 0.6 M NaCl. The effect of as-received and polished surface conditions, geometrical characteristics of the exposed area and different aluminium contents in the magnesium based alloys is considered. Results indicated that polished surfaces and AZ61 surfaces tend to develop corrosion slower than the respective as-received and AZ31 surfaces; these tendencies can change by prolonging exposure time. Filiform and localised corrosion are influenced by the presence of cut surfaces and small exposed areas, respectivelyfinancial support for this work from the Spanish Ministry of Science and Innovation (MAT 2009-13530)Peer reviewe

Correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys

X-ray photoelectron spectroscopy (XPS) was used in order to investigate the correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys exposed to 98% relative humidity at 50 °C. Commercially pure magnesium, used as the reference material, revealed MgO, Mg(OH)2 and tracers of magnesium carbonate in the air-formed film. For the AZ80 and AZ91D alloys, the amount of magnesium carbonate formed on the surface reached similar values to those of MgO and Mg(OH)2. A linear relation between the amount of magnesium carbonate formed on the surface and the subsequent corrosion behaviour in the humid environment was found. The AZ80 alloy revealed the highest amount of magnesium carbonate in the air-formed film and the highest atmospheric corrosion resistance, even higher than the AZ91D alloy, indicating that aluminium distribution in the alloy microstructure influenced the amount of magnesium carbonate formedThanks to the MCYT for the financial support given to this work (Project MAT2006-13179-C02-01-02)Peer reviewe

Electrochemical Impedance Spectroscopy for the Measurement of the Corrosion Rate of Magnesium Alloys: Brief Review and Challenges

From a technological point of view, measurement of the corrosion rate of magnesium (Mg) and its alloys is critical for lifetime predictions of Mg-based structures and for comparative assessments of their corrosion protection ability. Whilst weight loss, hydrogen evolution, and polarization curves methods are frequently used for measuring the corrosion rate, the determination of values by electrochemical impedance spectroscopy (EIS) is relatively scarce and has only been realized recently. This technique seems to be the most suitable for monitoring corrosion rate values due to its “non-destructive” character, its reproducibility, and its reliable determination of small corrosion rates, much lower than those measured by other techniques. This review aims to picture the state-of-the-art technique of using EIS for measuring the corrosion rate of Mg. This paper starts by introducing some fundamental aspects of the most widely used methods for monitoring the corrosion rate of Mg/Mg alloy and continues by briefly explaining some of the fundamental concepts surrounding EIS, which are essential for the user to be able to understand how to interpret the EIS spectra. Lastly, these concepts are applied, and different approaches that have been proposed to obtain quantitative values of corrosion rate since the 1990s are discussed.This research was funded by the Spanish Ministry of Economy and Competitiveness (project MAT2015-65445-C2-1-R).Peer reviewe

Sodium enrichment of an Al–Mg alloy surface after alkaline etching

This paper reports a case of sodium enrichment of the surface of an Al–Mg alloy that contains sodium impurities in its bulk and has been subjected to etching treatment in an alkaline solution. This phenomenon has only occurred in the case of the aforementioned Al–Mg alloy and has not been observed in other tested Al–Cu, Al–Mg–Si or pure Al type alloys, despite the fact that these also contain sodium as an impurity. The origin of the sodium ions incorporated in the aluminium oxide film that covers the surface of the Al–Mg alloy is discussed, and an explanation is suggested for this difference in behaviour compared with the other alloysfinancial support from CICYT (Spain) within the framework of Project MAT2003-02217Peer reviewe

Composition, Structure, and Protective Properties of Air-Formed Oxide Films on Magnesium Alloys

Chapter 9In spite of the growing interest in the corrosion mechanisms of magnesium (Mg) and its alloys, key aspects have not been sufficiently clarified to date. Considerable recent research has addressed the relationships among the chemical composition, the presence and quantity of precipitates, inclusions or impurities and the microstructure of magnesium alloys and their corrosion resistance. However, the precise nature and properties of the thin, air-formed oxide film, which is approximately 3 nm thick, that spontaneously grows on the outermost surface in contact with the atmosphere and its effect on the corrosion of commercial magnesium alloys is still subject to debate. Differences in the thickness, uniformity and chemical composition of these films associated with the alloying element content or the surface conditions of the bulk material may provide basic corrosion-related information that is expected to increase the protective properties for Mg alloys. This entry provides an overview of our current understanding of air-formed oxide films on the surface of magnesium alloys and the possible relationship between the surface composition and degradation of the material. The protective capacity of certain of these thin surface films in different corrosive environments is critically discussed

Correlation between the surface chemistry of annealed IF steels and the growth of a galvanneal coating

An attempt is made to establish possible relationships between the quantity of oxides and hydroxides of the alloying elements on the outer interstitial-free steel surface after the annealing process and the characteristics of the galvanneal coating formed. XPS measurements reflect a clear influence of the alloying element contents and the water vapour content or dew point (DP) of the atmosphere on the oxide and hydroxide coverage of the steel surface. In general, much less Fe–Zn intermetallic formation is seen on the Ti steel substrate than on the Ti–Nb steel. The reason for this may be related with the higher oxide/(Fe + Mn) atomic ratio on the external surface of the Ti steel substrate. With the Ti–Nb–P steel, an absence of Fe was observed in the coatings obtained on the surface of this steel annealed in atmospheres with a DP of −45 or −10 °C. A clear direct relationship has been found between the inhibition of growth of Fe–Zn intermetallic compounds and the fraction of the steel substrate surface covered by manganese oxides as a result of the annealing process. For the Ti–Nb–P steel the increase in the DP of the galvannealing atmosphere to 10 °C resulted in the incorporation of a significant Fe content in the coating. XPS analysis suggests a change in the surface film, and the formation of a phosphate enriched layer, instead of manganese oxide. A lower fraction of oxide coverage on the annealed steel substrate resulted in less of an impediment to the diffusion of iron atoms from the steel substrate to the zinc coating in the galvannealing processresearch was supported by ECSC project 7210- PR 119Peer reviewe

EIS study of the corrosion behaviour of zinc-based coatings on steel in quiescent 3% NaCl solution. Part 2: Coatings covered with an inhibitor-containing lacquer

An EIS study has been made of the behaviour of several lacquered and inhibited zinc-based coatings on steel exposed to a 3% NaCl solution in quiescent conditions. Under the hypothesis that the HF-LF arc supplied the Rt value of the corrosion reaction, the possibility of obtaining quantitative information about the effect of the lacquer film and the inhibitor on the progress of attack has been demonstrated. Different values of the constant B in the Stern-Geary equation must be used depending on the specific metallic coating tested. The relatively high interfacial capacitance values of the lacquered coating suggest a highly defective lacquer film which leave a considerable fraction of the surface of the metallic substrate exposed to the corrosive medium. © 2004 Elsevier Ltd. All rights reserved.Peer Reviewe

Passivating oxide film and growing characteristics of anodic coatings on aluminium alloys

The paper studies some aspects of the behaviour of four aluminium alloys under chemical etching by sodium hydroxide solution and during their subsequent anodizing in sulphuric acid solution. A correspondence is seen between etching rate, thickness of the passivating oxide film and porosity of the anodic layer. The possibility of an influence on these properties of precipitates and micro-heterogeneities in the metallic surface is suggestedThis work was supported by the Comisión Interministerial de Ciencia y Tecnología (CICYT) of Spain within the framework of Project MAT2003-02217Peer reviewe

The prediction of atmospheric corrosion from meteorological and pollution parameters-II. Long-term forecasts

Long-term atmospheric corrosion forecasts often rely on the fulfilment of equations of the form C = Atn, where C is the corrosion after t years and A represents corrosion after the first year of exposure. Appropriate values must be assigned to constants A and n. In the first part of the work an analysis was performed on the possibility of expressing A as a function of usually available environmental parameters. In this second part data compiled in a comprehensive literature survey are used to determine whether the exponent n of the above equation can also be expressed as a function of such environmental parameters. © 1993.Peer Reviewe