Oxidation of phosphated iron powders

The present work deals with the influence of phosphating treatment on the o xidation resistance of iron powders. Iron powders with particles diameter s ize of approximately 100 mum, 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 mum is formed at the metal surface. XRD spectrum is characte ristic of an amorphous or nanocrystalline coating. The oxidation is perform ed by in situ thermogravimetric experiments in artificial air (20% O-2-80% N-2). For oxidation times of 24 or 48 h, between 350 and 700 degreesC the k inetics are recorded for both untreated and phosphated powders. At all temp eratures, the phosphate acts as a protective barrier layer. Between 350 and 450 degreesC the coated powders present two parabolic stages in their oxid ation kinetics and progressively with increasing temperature, the first sta ge disappears. On the other hand, uncoated iron powders show only one parab olic 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 va lue of approximately 1.8 eV is the same that for the uncoated iron. That se cond parabolic stage for the coated powders can be compared with the oxidat ion of cast iron while in the first stage another mechanism may be involved . XRD studies of the oxidised powders at 350 degreesC for 48 h show that Fe 3O4 and Fe2O3 are formed both on uncoated iron and on the coated powder. Fo r phosphated powders, the amount of oxide is less important. As a result th e phosphate layer acts as a diffusion barrier that slows down the oxidation of iron. (C) 2000 Elsevier Science B.V. All rights reserved.