PHYSICOCHEMICAL CHARACTERIZATION OF CONVERSION COATINGS DEVELOPED ON REFRACTORY STAINLESS-STEEL AISI 430 AL-TI AFTER THERMAL-OXIDATION BETWEEN 400 AND 900-DEGREES-C

17% ferritic stainless steel containing aluminium (French standard Z8C AT17 - technical composition in wt. % : C 0.008, Al 1.54, Si 0.65, Ti 0.40, Cr 16.57, Mn 0.28, Fe bal) was used for selective surface prepar ation. Conversion coatings were made by chemical treatment. The therma l stability of these ferritic stainless steel conversion coatings for selective solar absorbers were studied in air using X-ray diffraction, FTIR specular reflectance spectrometry at variable incidence, GEMS an d SEM EDS. The coating thickness is around 200 nm and remains constant up to 700 degrees C in spite of the fact that different phases are ob served in samples treated at 600 degrees C and 700 degrees C. Above 70 0 degrees C, the morphology of the coating changes and the oxide layer is characteristic of the alloy oxidation. In the range 400 - 600 degr ees C, the oxidation of the layer as whole brings about the formation of hematite alpha-Fe2O3. Heat treatments at 700 degrees C and beyond 7 00 degrees C induce the oxidation of the metallic substrate and the fo rmation of alumina and mixed iron titanium oxides by reaction between TiO2 and rhombohedral phases. In agreement with the data issued from S IMS depth profiles, the X-ray under low glancing angle and SEM - EDS m easurements at 10 and 15 kV reveal that the scale formed at 900 degree s C consists of an outer oxide layer richer in titanium and inner laye r containing predominantly Al2O3.