Variations of the photocacoustic signal are observed to the surface oxidized stainless steels of three different oxidizing temperatures $500^\circ{C}$, $750^\circ{C}$, and $1000^\circ{C}$. The amplitudes and the phase delays of the signals of these samples are compared to those of the unoxidized stainless steel at various chopping frequencies.
Photocacoustic signal is affected by thermal and optical properties of a sample. These thermal and optical properties changs due to the degree fo surface oxidation. The amplitude of the signal of the $1000^\circ{C}$ sample is much larger than that of the unoxidized sample. The $500^\circ{C}$C and the $750^\circ{C}$ samples are very simlar to the unoxidized sample in the variation of their amplitudes.
In the behavior of the phase delays, the $1000^\circ{C}$ sample is different from the other samples. These experimental results show that the thermal and optical properties of the $1000^\circ{C}$ sample is different from the other three samples. Thus it is conclude that the $1000^\circ{C}$ sample is highly oxidized and has thick oxidized surface layer but the degrees of oxidation of the $500^\circ{C}$ and the $700^\circ{C}$ samples are very low and these samples have very thin oxidized surface layers.
