Carbon steel specimens exposed to ordinary and sheltered environments at Choshi, Chiba, and Miyakojima, Okinawa, for 1.5 years were examined using a hyperspectral camera. SAM analysis was used to visualize the spatial distributions of surface corrosion products. Reflectance spectra were acquired over a wavelength range of 400–1100 nm, and each corrosion product was identified by evaluating spectral similarity at 500 and 760 nm using reference spectra of four known corrosion products. In Fig. 1, α-FeOOH, β-FeOOH, γ-FeOOH, and Fe₃O₄ are represented by blue, yellow, red, and gray, respectively. At both sites, γ-FeOOH was predominantly detected under ordinary exposure, whereas Fe₃O₄ was predominantly detected under sheltered exposure, revealing differences in surface rust associated with the exposure environment. The detection of Fe₃O₄ suggests an association with prolonged wet conditions. The surface fraction of each corrosion product was calculated from the number of pixels assigned to its corresponding color, enabling evaluation of the distributions and fractions of corrosion products that are difficult to distinguish visually.

The surface fraction of each corrosion product was then compared with the increase in corrosion loss over the subsequent year (Figure 2). For example, the increase was calculated as the difference in corrosion loss between specimens exposed for 0.5 and 1.5 years. A one-year comparison interval was used to reduce seasonal effects. While no clear correlations were observed for γ-FeOOH or Fe₃O₄, the surface fraction of α-FeOOH showed a positive correlation with the increase in corrosion loss, whereas that of β-FeOOH showed a negative correlation. These results suggest that the surface fractions of α-FeOOH and β-FeOOH may serve as indicators of subsequent corrosion progression. Further work is needed to improve the identification accuracy of mixed rust and validate the quantitative assessment of surface fractions. Validation across a wider range of environments is also necessary to clarify the relationship between corrosion products and subsequent corrosion progression.