Oxidation of phosphated iron powders

Abstract 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 oxidised. 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 oxidised 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.