Guang-Ling Song

Corrosion Mechanisms of Magnesium Alloys

The high strength to weight ratio of magnesium alloys makes them extremely attractive for applications in transport or aerospace technology. However, their corrosion behavior is a major issue and one reason why they are still not as popular as aluminum alloys. This papers reviews the corrosion mechanisms of magnesium and provides the basis for the design of new alloys with improved corrosion properties.

Influence of microstructure on the corrosion of diecast AZ91D

The corrosion of die cast AZ91D was studied and related to its microstructure. For comparison and to more fully understand the behaviour of die cast AZ91D, corrosion studies and microstructural examinations were also carried out using slowly solidified high purity AZ91, Mg-2%Al, Mg-9%Al, low purity magnesium and high purity magnesium. Corrosion was studied in 1N NaCl at pH 11 by (1) observing the corrosion morphology, (2) measuring electrochemical polarisation curves and (3) simultaneously measuring both the hydrogen evolution rate and the magnesium dissolution rate. The skin of die cast AZ91D showed better corrosion resistance than the interior. This is attributed to a combination of(1) a higher volume fraction of the beta phase, (2) a more continuous beta phase distribution around finer alpha grains, and (3) lower porosity in the skin layer than in the interior of the die casting. This study showed that the casting method can influence the corrosion performance by its influence on the alloy microstructure

Corrosion behaviour of AZ21, AZ501 and AZ91 in sodium chloride

The corrosion behaviour of AZ21, AZ501 and AZ91 was studied in 1 N NaCl at pH 11 by measuring electrochemical polarization curves, electrochemical AC impedance spectroscopy (EIS) and simultaneously measuring the hydrogen evolution rate and the: magnesium dissolution rate. The corrosion rates increased in the following order: AZ501 < AZ21 < AZ91. The: corrosion behaviour was related to alloy microstructure as revealed by optical and electron microscopy. The beta phase was very stable in the test solution and was an effective cathode. The beta phase served two roles, as a barrier and as a galvanic cathode. If the beta phase is present in the alpha matrix as intergranular precipitates with a small volume fraction, then the beta phase mainly serves as a galvanic cathode, and accelerates the corrosion of the alpha matrix. If the beta Fraction is high, then the beta phase may mainly act as an anodic barrier to inhibit the overall corrosion of the alloy. The composition and compositional distribution in the alpha phase is also crucial to the overall corrosion performance of dual phase alloys. Increasing the aluminum concentration in the alpha phase increases the anodic dissolution rate and also increases the cathodic hydrogen evolution rate. Increasing the zinc concentration in the alpha phase may have the opposite effect

THE ANODIC DISSOLUTION OF MAGNESIUM IN CHLORIDE AND SULPHATE SOLUTIONS

The electrochemical behaviour of magnesium was studied in representative chloride and sulphate solutions including NaCl, Na2SO4, NaOH and their mixed solutions, HCl, and H2SO4: (1) by measuring electrochemical polarisation curves, (2) by using electrochemical impedance spectroscopy (EIS), and (3) by simultaneous measurement of hydrogen gas evolution and measurement of magnesium dissolution rates using inductively coupled plasma atomic emission spectrophotometry (ICPEAS). These experiments showed that a partially protective surface film played an important role in the dissolution of magnesium in chloride and sulphate solutions. Furthermore, the experimental data were consistent with the involvement of the intermediate species Mg+ in magnesium dissolution at film imperfections or on a film-free surface. At such sites, magnesium first oxidised electrochemically to the intermediate species Mg+, and then the intermediate species chemically reacted with water to produce hydrogen and Mg2+. The presence of Cl- ions increased the film free area, and accelerated the electrochemical reaction rate from magnesium metal to Mg+

The electrochemical corrosion of pure magnesium in 1 N NaCl

An electrochemical investigation was carried out to study the corrosion of pure magnesium in 1 N NaCl at different pH values involving electrochemical polarisation, scanning tunnel microscopy (STM), measurement of hydrogen gas evolution and measurement of the elements dissolved from the magnesium specimen which were determined by inductively coupled plasma atomic emission spectrophotometry (ICPAES). A partially protective surface film was a principal factor controlling corrosion. Film coverage decreased with increasing applied electrode potential. Application of a suitable external cathodic current density was shown to inhibit magnesium dissolution whilst at the same time the hydrogen evolution rate was relatively small. This showed that cathodic protection could be used to significantly reduce magnesium corrosion. A new definition is proposed for the negative difference effect (NDE)

Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc

An investigation was carried out into the galvanic corrosion of magnesium alloy AZ91D in contact with zinc, aluminium alloy A380 and 4150 steel. Specially designed test panels were used to measure galvanic currents under salt spray conditions. It was found that the distributions of the galvanic current densities on AZ91D and on the cathodes were different. An insulating spacer between the AZ91D anode and the cathodes could not eliminate galvanic corrosion. Steel was the worst cathode and aluminium the least aggressive to AZ91D. Corrosion products from the anode and cathodes appeared to be able to affect the galvanic corrosion process through an alkalisation, passivation, poisoning effect or shortcut effect

The effect of zirconium grain refinement on the corrosion behaviour of magnesium-rare earth alloy MEZ

Corrosion performance of sand cast magnesium alloy MEZ was investigated for unrefined (MEZU) and Zr-grain-refined (MEZR) microstructures in 5% NaCl solution using salt spray, immersion, in situ examination of the corrosion morphology, ESEM, electron probe microanalysis, hydrogen evolution and polarisation curves. MEZU demonstrated higher rates of anodic dissolution and cathodic hydrogen evolution than MEZR. The central zirconium-rich areas within the grains of the MEZR microstructure was more corrosion resistant than the outer zirconium-depleted areas of the grains, whereas MEZU showed little difference in corrosion between the centre and the edge of the grains. Based on the analyses of the corrosion process, polarisation behaviour and the microstructure of these two alloys, it is postulated that zirconium acts in a number of ways to improve corrosion resistance. Zirconium stabilises the solid solution and makes it inactive in anodic dissolution, significantly passivates the precipitated particles reducing cathodic hydrogen evolution and increases the barrier effect of the grain boundary phase through a finer grain size and thus a more continuous layer of the grain boundary phase.

Equivalent circuit model for AC electrochemical impedance spectroscopy of concrete

An equivalent circuit model for AC electrochemical impedance spectroscopy (EIS) of concrete has been proposed, which contains parameters R-CCP, the resistance of the continuously connected micro-pores in the concrete; R-CP, the resistance of the discontinuously connected micro-pores, blocked by cement paste layers in the concrete; C-mat, the capacitance across the concrete matrix; and C-DP, the capacitance of the cement paste layers blocking the discontinuously connected micro-pores in the concrete. The proposed model can successfully explain the experimental phenomena observed by other researchers, such as the emergence of the capacitive loops in high-frequency range, the influences of hydration time, silica fume, water/cement ratio on the loops, etc

Corrosion prevention of magnesium alloys

Part 1 Alloying and inhibition: Corrosion behaviour and prevention strategies for magnesium (Mg) alloys Corrosion performance of magnesium (Mg) alloys containing rare-earth elements Corrosion inhibition of magnesium (Mg) alloys. Part 2 Surface treatment and conversion: Surface cleaning and pre-conditioning surface treatments to improve the corrosion resistance of magnesium (Mg) alloys Surface processing and alloying to improve the corrosion resistance of magnesium (Mg) alloys Laser treatment to improve the corrosion resistance of magnesium (Mg) alloys Micro-arc oxidation (MAO) to improve the corrosion resistance of magnesium (Mg) alloys Anodization of magnesium (Mg) alloys to improve corrosion resistance Anodization mechanisms and the corrosion of magnesium (Mg) alloys Corrosion-resistant coatings for magnesium (Mg) alloys. Part 3 Coatings: Corrosion-resistant electrochemical plating of magnesium (Mg) alloys Plating techniques to protect magnesium (Mg) alloys from corrosion Electroless nickel-boron plating to improve the corrosion resistance of magnesium (Mg) alloys Electrodeposition of aluminium on magnesium (Mg) alloys in ionic liquids to improve corrosion resistance Cold spray coatings to improve the corrosion resistance of magnesium (Mg) alloys Electroless electrophoresis coatings to improve the corrosion resistance of magnesium (Mg) alloys Sol-gel coatings to improve the corrosion resistance of magnesium (Mg) alloys Corrosion protection of magnesium (Mg) alloys using conversion and electrophoretic coatings. Part 4 Case studies: Magnesium (Mg) corrosion protection techniques in the automotive industry Control of biodegradation of magnesium (Mg) alloys for medical applications.

Theoretical analysis of the measurement of polarisation resistance in reinforced concrete

Polarisation resistance (Rp) technique based on Stern‐Geary equation is one of the most widely used methods of measuring corrosion rate of reinforcement in the field. With the aid of a ‘‘sensorised guard ring’’, this electrochemical technique is claimed to be able to determine corrosion rate (Icorr) within a given measuring area. However, there are three theoretical problems in the application of this technique: (1) the original Stern‐Geary equation is applicable in a uniform corrosion system at its corrosion potential, whereas the reinforced concrete structure may be subjected to non-uniform corrosion or strong polarisation by macro-cell galvanic eAects or imposed currents; (2) the value of the parameter B in the original Stern‐Geary equation has been estimated to fall within the range 25‐52 mV. This may not be suitable for all the corrosion cases of reinforced concrete structures; (3) The polarised surface area of steel may theoretically not always be fully confined by the sensorised guard ring when the cover concrete is too thick. This paper aims at discussing the theoretical problems. A general relationship between the dissolution rate of steel reinforcement and the measured polarisation resistance is deduced under general conditions. The range of B value is also analysed based on all the possible corrosion situations of reinforced concrete. Furthermore, unsatisfactory confinement by sensorised guard ring on a thick cover concrete is demonstrated. ” 2000 Elsevier Science Ltd. All rights reserved.