Understanding magnesium corrosion - A framework for improved alloy performance

The purpose of this paper is to provide a succinct but nevertheless complete mechanistic overview of the various types of magnesium corrosion. The understanding of the corrosion processes of magnesium alloys builds upon our understanding of the corrosion of pure magnesium. This provides an understanding of the types of corrosion exhibited by,magnesium alloys, and also of the environmental factors Of most importance. This deep understanding is required as a foundation if we are to produce magnesium alloys much more resistant to corrosion than the present alloys. Much has already been achieved, but there is vast scope for improvement. This present analysis can provide a foundation and a theoretical framework for further, much needed research. There is still vast scope both for better fundamental understanding of corrosion processes, engineering usage of magnesium, and also on the corrosion protection of magnesium alloys in service.

Electrochemical study of UNS S32550 super duplex stainless steel corrosion in turbulent seawater using the rotating cylinder electrode

The cathodic and anodic characteristics of freshly polished and pre-reduced UNS S32550 (ASTM A479) super duplex stainless steel in a filtered and conductivity-adjusted seawater have been investigated under controlled flow conditions. A rotating cylinder electrode was used together with both steady and non-steady-state voltammetry and a potential step current transient technique to investigate the electrode reactions in the fully characterized electrolyte. Both oxygen reduction and hydrogen evolution were highly irreversible and the material exhibited excellent passivation and repassivation kinetics. Relative corrosion rates were derived and the corrosion mechanism of the alloy was found to be completely independent of the mass-transfer effects, which can contribute to flow-induced corrosion.

Electrochemical corrosion of unalloyed copper in chloride media–a critical review

The literature dealing with the electrochemical corrosion characteristics of unalloyed copper in aqueous chloride media is examined. The enormous quantity of polarisation and mixed/corrosion potential data that has been made available in the literature over the last 50 years has been compiled and discussed in a comprehensive review. For a wide range of electrode geometries, the importance of the chloride ion and the mass transport of anodic corrosion products on the corrosion behaviour of copper are made clear for both freshly polished and 'filmed' surfaces. (C) 2003 Elsevier Ltd. All rights reserved.

Stress corrosion cracking and hydrogen embrittlement of an Al-Zn-Mg-Cu alloy

The age hardening, stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of an Al-Zn-Mg-Cu 7175 alloy were investigated experimentally. There were two peak-aged states during ageing. For ageing at 413 K, the strength of the second peak-aged state was slightly higher than that of the first one, whereas the SCC susceptibility was lower, indicating that it is possible to heat treat 7175 to high strength and simultaneously to have high SCC resistance. The SCC susceptibility increased with increasing Mg segregation at the grain boundaries. Hydrogen embrittlement (HE) increased with increased hydrogen charging and decreased with increasing ageing time for the same hydrogen charging conditions. Computer simulations were carried out of (a) the Mg grain boundary segregation using the embedded atom method and (b) the effect of Mg and H segregation on the grain boundary strength using a quasi-chemical approach. The simulations showed that (a) Mg grain boundary segregation in Al-Zn-Mg-Cu alloys is spontaneous, (b) Mg segregation decreases the grain boundary strength, and (c) H embrittles the grain boundary more seriously than does Mg. Therefore, the SCC mechanism of Al-Zn-Mg Cu alloys is attributed to the combination of HE and Mg segregation induced grain boundary embrittlement. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Corrosion behaviour of magnesium in ethylene glycol

Corrosion of magnesium engine components by coolant is an important issue in the automotive industry where magnesium alloys may be used. It is of significance to understand the corrosion behaviour of pure magnesium in ethylene glycol solutions, as this can provide a basis for developing new coolants for magnesium alloy engine blocks. In this paper, through corrosion and electrochemical tests, it was found that the corrosion rate of magnesium decreased with increasing concentration of ethylene glycol. Individual contaminants, such as NaCl, NaHCO3, Na2SO4 and NaCl can make aqueous ethylene glycol solution more corrosive to magnesium. However, in NaCl contaminated ethylene glycol, NaHCO3 and Na2SO4 showed some inhibition effect. The solution resistivity played an important role in the corrosion of magnesium in ethylene glycol solutions, and the competitive adsorption of ethylene glycol and the contaminants on the magnesium surface was also responsible for the observed corrosion behaviours. The corrosion of magnesium in ethylene glycol can be effectively inhibited by addition of fluorides that react with magnesium and form a protective film on the surface. (C) 2003 Elsevier Ltd. All rights reserved.

Corrosion resistance of aged die cast magnesium alloy AZ91D

The corrosion behaviour of die cast magnesium alloy AZ91D aged at 160degreesC was investigated. The corrosion rate of the alloy decreases with ageing time in the initial stages and then increases again at ageing times greater than 45 h. The dependence of the corrosion rate on ageing time can be related to the changes in microstructure and local composition during ageing. Precipitation of the beta phase (Mg17Al12) occurs exclusively along the grain boundaries during ageing. The beta phase acts as a barrier, resulting in a decreasing corrosion rate in the initial stages of ageing. In the later stages, the decreasing aluminium content of alpha grains makes the alpha matrix more active, causing an increase in the corrosion rate. Electrochemical testing results also confirm the combined effects of the changes in alpha and beta phases on the corrosion resistance of the aged die cast AZ91D alloy. (C) 2003 Elsevier B.V. All rights reserved.

Analysis of service stress corrosion cracking in a natural gas transmission pipeline, active or dormant?

Stress corrosion cracks (SCC) had been found in a natural gas transmission pipeline during a dig-up and inspection program. The question was raised as to whether the SCC was active or dormant. This paper describes the resultant investigation to determine if a particular service crack was actively growing. The strategy adopted was to assess the appearance of the fracture surface of the service crack and to compare with expectations from laboratory specimens with active SCC. The conclusions from this study are as follows. To judge whether a crack in the service pipe is active or dormant, it is reasonable to compare the very crack tip of the service crack and a fresh crack in a laboratory sample. If the crack tip of the active laboratory sample is similar to that of the service pipe, it means the crack in the service pipe is likely to be active. From the comparison of the crack tip between the service pipe and the laboratory samples, it appears likely that the cracks in the samples extracted from service were most likely to have been active intergranular stress corrosion cracks. (C) 2003 Elsevier Ltd. All rights reserved.

Corrosion behaviour of a pressure die cast magnesium alloy

High pressure die casting is the most important production method for casting magnesium alloy components, and uniformity of appearance is an important criterion for acceptance of a component by customers. This paper investigates the influence of uniformity in surface appearance of diecast AZ91D plates on their corrosion behaviour. Through immersion, hydrogen collection and weight loss measurements it was found that corrosion is more likely to occur on the areas of the plate that appear to be darker, leading to a non-uniformly corroded surface. Microstructural analysis showed that the non-uniformity in appearance is related to a difference in the morphology and distribution of porosity across the surface of a diecast AZ91D plate. The darker areas of the surface are high in porosity which breaks the continuity of the beta-phase network and provides shortcut paths for corrosion from the surface to the interior of the casting. The brighter shiny areas of the surface are much less porous, with isolated pores being confined by corrosion resistant beta-precipitates thus reducing the corrosion rate.

Influence of the beta phase on the corrosion performance of anodised coatings on magnesium-aluminium alloys

The effect of the beta phase in Mg-Al alloys on the corrosion performance of an anodised coating was studied. It was found that the corrosion resistance of the anodised coating was closely associated with the corrosion performance of the substrate alloy. In particular, Mg alloys with a dual phase microstructure of alpha + beta with intermediate aluminium contents (namely 5%, 10% and 22% Al) after anodisation had the highest corrosion rate and the worst corrosion resistance provide by the anodised coating. The poor performance of an anodised coating was attributed partly to lower corrosion resistance of the substrate alloy and partly to the higher porosity of the anodised coating. (c) 2004 Elsevier Ltd. All rights reserved.

Effect of aging on yield stress and corrosion resistance of die cast magnesium alloy

Corrosion resistance is an important property that could be affected by the ageing process. In order to investigate whether aging affects the corrosion resistance, corrosion rate and yield strength of diecast magnesium alloy AZ91D were measured and analysed after ageing. It was found that the dependence of the corrosion rate on ageing time can be ascribed to the changes in microstructure of the alloy and chemical composition of its matrix. Precipitation of the P phase (Mg17Al12) occurred along the grain boundaries during the initial ageing stages, resulting in a decreasing corrosion rate and an inceasing yield strength. In the later stages, the decreasing aluminium content in the alpha matrix made it more active, causing an increase in the corrosion rate. The decrease in aluminium content in the matrix also leads to a decrease in yield strength.

Atom probe field ion microscope measurements of carbon segregation at an a:a grain boundary and service failures by intergranular stress corrosion cracking

This work reports on a critical measurement to understand the intergranular stress corrosion cracking (IGSCC) of pipeline steels: the atom probe field ion microscope (APFIM) measurement of the carbon concentration at a grain boundary (GB). The APFIM measurement was related to the microstructure and to IGSCC observations. The APFIM indicated that the GB carbon concentration of X70 was similar to 10 at% or less, which correlated with a high resistance to IGSCC for X70. (C) 2005 Elsevier Ltd. All rights reserved.

Improved corrosion resistance of AZ91D magnesium alloy by an aluminium-alloyed coating

An aluminum-alloyed coating was applied onto the surface of magnesium alloy AZ91D. The coating formed in aluminium powder at 420 degrees C is rich in the beta (Mg17Al12) phase. Polarisation curve, AC impedance, salt immersion and salt spray were carried out to investigate the corrosion behaviour and assess the corrosion performance of the coated magnesium alloy. It was found that a coated AZ91D specimen was much more corrosion resistant and harder than an uncoated one. The improved corrosion resistance was mainly ascribed to the high volume fraction of beta phase in the coating. (c) 2004 Elsevier B.V. All rights reserved.

Influence of anodising current on the corrosion resistance of anodised AZ91D magnesium alloy

The thickness, chemical composition and microstructure of anodised coatings formed on magnesium alloy AZ91D at various anodising current densities were measured. It was found that all these parameters could be affected by anodising current density, and hence the coatings formed at different anodising current densities had different corrosion resistances. This suggests that the corrosion performance of an anodised coating could be improved if a properly designed current waveform is used for anodising. In addition, based on the experimental results, some physical, chemical and electrochemical reactions involved in the anodising process were proposed to explain the anodising behaviour in this paper. (c) 2005 Elsevier Ltd. All rights reserved.