Oxidation of phosphated iron powders

The present work deals with the influence of phosphating treatment on the oxidation resistance of iron powders. Iron powders with particles diameter size of approximately 100 μm, have been immersed in phosphoric acid (0.102 mol l-1) in an acetone solution. After half an hour, a phosphate layer of less than 0.1 μm is formed at the metal surface. XRD spectrum is characteristic of an amorphous or nanocrystalline coating. The oxidation is performed by in situ thermogravimetric experiments in artificial air (20% O2-80% N2). For oxidation times of 24 or 48 h, between 350 and 700 °C the kinetics are recorded for both untreated and phosphated powders. At all temperatures, the phosphate acts as a protective barrier layer. Between 350 and 450 °C the coated powders present two parabolic stages in their oxidation kinetics and progressively with increasing temperature, the first stage disappears. On the other hand, uncoated iron powders show only one parabolic stage when oxidized. The oxidation constants kp were plotted for each stage of the curves vs. 1/T. From each Arrhenius plot an activation energy is deduced. For the phosphated powders, an activation energy of less than 0.6 eV is found in the first stage while for the second stage the deduced value of approximately 1.8 eV is the same that for the uncoated iron. That second parabolic stage for the coated powders can be compared with the oxidation of cast iron while in the first stage another mechanism may be involved. XRD studies of the oxidized powders at 350 °C for 48 h show that Fe3O4 and Fe2O3 are formed both on uncoated iron and on the coated powder. For phosphated powders, the amount of oxide is less important. As a result the phosphate layer acts as a diffusion barrier that slows down the oxidation of iron

Structural characterisation of phosphated α-iron oxidised at 400 °C

International audienceα-Iron specimens were treated with phosphoric acid to obtain a conversion surface layer and the oxidation resistance of samples was studied at 400 °C in air. For oxidation time of 48 h, an important decrease in the oxidation kinetics was obtained in comparison with untreated α-Fe. The samples were characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses of the surfaces and these results are related to previous data on the oxidation of α-iron. © 2002 Elsevier Science B.V. All rights reserved

Chronological study of the oxidation of phosphated α-iron

International audienceIn the present work, the morphological evolution of the iron-oxide layers growing at 400 °C on α-Fe and phosphated α-Fe in artificial air at 1 atm is investigated. The oxidation kinetics of α-Fe and phosphated α-Fe are firstly presented for various oxidation times. Then a detailed chronological study is undertaken of the microstructural states in the different stages of the complex oxidation kinetics. Correlations are established between the morphology of the oxides layers and the successive parabolic oxidation stages. It is shown that the phosphate conversion layers seem to modify the growth mechanisms of the oxides. © 2003 Elsevier Science B.V. All rights reserved

Phosphating of bulk a-iron and its oxidation resistance at 400 °C

Fe bulk specimen have been treated with phosphoric acid in order to obtain a conversion surface layer and the oxidation resistance has been studied by means of in situ thermogravimetric experiments in artificial air (20% O2; 80% N2). For oxidation time of 48 h at 400 °C it shows an important decrease of the oxidation kinetic in comparison with α-Fe. Moreover, the kinetic can be described by two successive parabolic stages. This behaviour is respectively related to an X-ray photoelectron spectroscopy (XPS) analysis of the surfaces and to previous results obtained on the oxidation of α-iron