Abstract
Polished samples of austenitic stainless steel AISI 321 were chemically oxidized in 0.1â0.6 M aqueous solutions of chromic and sulphuric acids at 90 °C for an oxidation time of 25 min, as well as for oxidation times ranging from 10 to 32 min in 0.6 M solution. Chemical oxidation of the stainless steel produced interference films ranging in colour from light-blue to deep-blue, purple-red and peacock-green. Measurements of solar absorptance ( α s ) and total hemispherical thermal emittance ( Ï” h ) showed that all the oxidized surfaces were spectrally selective. The most favourable spectrally selective surface was deep-blue in colour and it had values of α s = 0.85 ± 0.01 and Ï” h = 0.18 ± 0.01. This surface was prepared after an oxidation time of 12 min at a concentration of 0.6 M of chromic and sulphuric acids. For the unoxidized polished stainless steel, the α s and Ï” h were 0.38 ± 0.01 and 0.15 ± 0.01 respectively. No significant changes in the optical properties of the films were observed after 6 months exposure to highly humid tropical weather conditions.
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