Effect of surface oxidization on the spectral emissivity of steel 304 at the elevated temperature in air

• Resonances of spectral emissivity are observed.
• Variation of spectral emissivity with heating time is studied.
• Variation of spectral emissivity with temperature is evaluated.
• Polynomial functional form is suitable to fit the measurements.
• Contribution to the spectral emissivity by oxidation is mainly from the first 150 min.

Effect of surface oxidization on the spectral emissivity of steel 304 is studied over the temperature range from 830 to 1100 K at 1.5 μm. The experimental setup is composed of three modules: the optical receiving system, the specimen heating and temperature-controlling system, and the signal-controlling and data-computing system. The temperature of steel 304 specimens is measured by averaging the two R-type platinum–rhodium thermocouples. The radiant energy stemming from the surface specimens is received by an InGaAs photodiode detector. The growth law of oxidization film on the surface of steel 304 specimens is observed carefully. The variation of spectral emissivity with the heating-duration time is studied at a given elevated temperature. The dependence of spectral emissivity on the temperature is evaluated at a definite heating-duration time. The interference enhancement of spectral emissivity appears mainly during the initial heating-duration period, in particular at a relatively low elevated temperature. The interference effect between the radiation stemming from the oxidization film on the specimen surface and the radiation coming from the surface of specimens is discussed, which has been affirmed to be responsible for the interference enhancement of spectral emissivity. The analytic model between the spectral emissivity of steel 304 specimens and the temperature of specimen surface is evaluated at several definite heating-duration times. The conclusion is derived that the polynomial functional form is very suitable for fitting the experimental results obtained at a given heating-duration time at 830–1100 K.