Quantitative Analysis of Iron Rust Using Terahertz Time-Domain Spectroscopy

The insight into the composition of iron corrosion products is critical for corrosion studies. A novel and non-destructive method is proposed in this paper to quantitatively analyse the binary iron oxides mixtures based on the terahertz time-domain spectroscopy combined with the effective medium theory. The utility of this method is exemplified by analysing the THz refractive index for a set of samples, including hematite, magnetite, and their mixtures at several volume ratios. The effective medium theory is applied to the raw data of single-component samples to remove the dielectric influence of dilution polymer matrix and trapped air voids. The refractive index of hematite and magnetite is determined to be 4.01 ± 0.04 (1.00%) and 5.6 ± 0.2 (3.6%). The measured refractive index of two iron oxides is further adopted to construct an effective medium model for the binary mixture. The analytical model can predict the volume ratios of two iron oxides within the mixture. Comparison between the theoretical predictions and experimental measurements reveals a relative difference of less than 10.0%.