CIE L*a*b* Colorimetry as a Non-Destructive Indicator of Rust-Layer Stage and Corrosion Rate in Carbon Steel Under a Modified Cyclic Corrosion Test

The rust layer on carbon steel controls how the metal beneath it corrodes, but its state is usually assessed by destructive analysis. This work followed uncoated cold-rolled carbon steel through 30 cycles (240 h) of a modified ISO 14993 cyclic corrosion test in which ultrapure water replaced the NaCl spray. Two specimens were retrieved at each of nine intervals. Surface color and rust coverage were measured from photographs and compared with SEM/EDS, XRD, linear polarization resistance and confocal topography. Three stages were resolved. Rust coverage stayed below 6% up to cycle 5. Between cycles 10 and 15, coverage rose to about 50%, γ-FeOOH appeared and polarization resistance fell from 441.1 to 301.3 Ω∙cm2. Coverage then leveled off near 63% while the instantaneous corrosion rate kept rising. Among the color coordinates, b* correlated most strongly with corrosion rate (Spearman ρ = 0.867, exact p = 0.005, 95% CI 0.46–0.97). These correlations rest on nine intervals under one condition, and they partly reflect exposure time. No predictive model was built or validated. Within these limits, surface color serves as a non-destructive indicator of rust-layer stage.