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Composition, Structure, and Protective Properties of Air-Formed Oxide Films on Magnesium Alloys
Published in Leszek A. Dobrzański, George E. Totten, Menachem Bamberger, Magnesium and Its Alloys, 2020
In an early study in 1923, Pilling and Bedworth29 suggested that the protective nature of the oxide film that naturally forms on a metallic material can be indicated by the Pilling–Bedworth ratio (P-B ratio), which is defined as the ratio of the molecular volume of the oxide to the atomic volume of the corresponding metal from which the oxide is formed. Due to the large difference in the density of the oxide and metal, as indicated by the MgO to Mg volume ratio of 0.81, the oxide scale is subject to increased tension, discontinuous, porous and not compact, and may not protect the substrate from oxidation.16,18–20,23,30 A P-B ratio of 1–2 is typically considered a minimum but is not a sufficient prerequisite for protective film formation.31
Corrosion behaviour and properties of Mg–3.4Y–3.6Sm–2.6Zn–0.8Zr Alloy in 3.5 wt-% NaCl solution
Published in Corrosion Engineering, Science and Technology, 2022
Wenli Wang, Jing Ke, Lintong Guo
In the EIS spectrum of the experimental alloy, the non-inductive loop proves the formation of the film (Figure 9(a)). But the pilling bedworth ratio of the corrosion product film is less than 1, the protection effect is not obvious and it even participates in the corrosion process as a secondary electrode. Under the action of Cl−, the film is easily broken (Ebd in Figure 8). Owing to the high rate of HER, the corrosion penetrates deep into the experimental alloy (Figure 5(d)). Subsequently, the concentration of Cl− in this inward corrosion and pitting corrosion zone aggravated the dissolution of (Mg, Zn)3(Y, Sm). The over-reaction products of Mg2+ and OH− are produced and enter the local area of the solution, reaching a supersaturated state. Therefore, a large amount of corrosion products accumulates at the two-phase interface, and then spread to the entire surface through the process of re-precipitation after dissolution. Local corrosion pits are gradually filled with corrosion products. The increase in the amount of corrosion products also accelerates the corrosion of the remaining (Mg, Zn)3(Y, Sm), which is fully exposed to the former since its cathodic effect.
Mechanistic model for stresses in the oxide layer formed on zirconium alloys
Published in Journal of Thermal Stresses, 2019
Isha Gupta, J. R. Barber, M. D. Thouless, Wei Lu
Zirconium oxide has a higher molar volume than zirconium, so a biaxial compressive stress is developed in the oxide film owing to the constraint exerted by the substrate while the oxide forms. The Pilling–Bedworth ratio, VPB, defined as the ratio of the molar volume of the oxide to the molar volume of the metal, is equal to 1.56 for zirconium oxide [1]. If the volumetric expansion during unrestrained growth is assumed to be isotropic, the resulting compressive strain within the oxide would be given by implying in-plane compressive stresses within the oxide of around 57GPa. It can be seen that substitution of such stress levels into Eq. (8) results in huge plastic strain rates, implying a very rapid relaxation of stresses to more reasonable levels. Previous studies [6, 20] have assumed that the oxide forms at an in-plane compressive stress of about 2GPa, but stresses as high as 4GPa have been measured experimentally [21, 22], suggesting that the value 2GPa is merely an arbitrary temporal point for a rapidly relaxing compressive stress. However, the precise details of how the stresses relax from the theoretical initial value is beyond the scope of this work.