Fig. 1. THz reflected signal path and received results. (a) THz signal path of monolayer; (b) reflected THz signal of monolayer (epoxy resin+steel plate sample as an example); (c) THz signal path of double layers; (d) reflected THz signal of double layers (epoxy resin+corroded steel plate sample as an example)

Fig. 2. Detection of corroded steel plates. (a) Reflection THz-TDS; (b) vernier caliper

Fig. 3. Samples for transmission THz-TDS. (a) Corrosion products; (b) epoxy resin; (c) rubber; (d) paste

Fig. 4. Time-domain waveforms signals

Fig. 5. Refractive index of samples. (a) Refractive index; (b) relationship of refractive index and delay time difference

Fig. 6. Absorption coefficient of the samples. (a) Corrosion products; (b) epoxy resin; (c) rubber; (d) paste

Fig. 7. Detection of corrosion thickness. (a) Reflected THz signals; (b) comparison of detection results by THz-TDS and vernier caliper

Fig. 8. Reflected THz signals of different coating materials. (a) Un-corroded steel plates; (b) steel plates corroded for 28 d