CYCLIC OXIDATION AT 800°C OF AISI 304L STAINLESS STEEL SENSITIZED TO INTERGRANULAR CORROSION AND COATED WITH Ni-P, Ni-P-W AND Ni-P-W-MAGNETITE

Abstract

The process of obtaining self-catalytic coatings of Ni-P, Ni-P-W and Ni-P-W-Magnetite on AISI 304L stainless steel sensitized at a temperature of 700°C to intergranular oxidation were studied, and it was also determined the kinetics of growth of oxide scale when the samples were exposed to cyclic oxidation at a temperature of 800°C, in these kinetic studies the weight changes of the samples were evaluated during 10 cycles of 10 h, allowing the samples to cool slowly at the end of each cycle. By X-ray diffraction studies of the layers formed on the samples, the phases were determined and quantified at the end of the oxidation cycles. The gravimetries showed that the coatings significantly reduced the continuous weight loss observed in the uncoated sensitized steel, because the coatings of Ni-P, Ni-P-W and Ni-P-W-Magnetite favored the growth of layers of iron oxides such as magnetite, hematite and wustite and a series of phases such as FeNi, Ni(Ni0.67Fe0.33)FeP, Cr0.4Ni3.6, CrFe, W, y Ni0.92W0.08 were the ones that acted as an insulating barrier against the oxidizing atmosphere; and that the layer formed on the substrate coated with Ni-P-W-magnetite, composed of magnetite, hematite and nickel tungsten (Ni0.9W0.08) showed the highest protective nature under the conditions of the study, because it caused the least weight loss of the sample.