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ptical interferometry techniques were used for the first time to measure the volume
resistivity/conductivity of carbon steel samples in seawater with different concentrations of a corrosion
inhibitor. In this investigation, the real-time holographic interferometric was carried out to measure the
thickness of anodic dissolved layer or the total thickness , Utotal , of formed oxide layer of carbon steel
samples during the alternating current (AC) impedance of the samples in blank seawater and in 5-20 ppm
TROS C-70 inhibited seawater, respectively. In addition, a mathematical model was derived in order to
correlate between the AC impedance (resistance) and to the surface (orthogonal) displacement of the
surface of the samples in solutions. In other words, a proportionality constant ( resistivity (ρ) or
conductivity(σ)=1/[ resistivity (ρ)]) between the determined AC impedance (by EIS technique) and the
orthogonal displacement (by the optical interferometry techniques) was obtained. The value of the
resistivity of the carbon steel sample in the blank seawater was found similar to the value of the resistivity
of the carbon steel sample air, around 1x10-5 Ohms-cm. On the contrary, the measured values of the
resistivity of the carbon steel samples were 1.85x107 Ohms-cm , 3.35.x107 Ohms-cm, and 1.7x107 Ohmscm
in 5ppm,10ppm, and 20ppm TROS C-70 inhibited seawater solutions, respectively. Furthermore, the
determined value range of the ρ of the formed oxide layers, 1.7x107 Ohms-cm to 3.35.x107 Ohms-cm, is
found in a reasonable agreement with the one found in literature for the Fe Oxide-hydroxides, i.e.,
Goethite(α-FeOOH) and for the Lepidocrocite (γ-FeOOH), 1x109 Ohms-cm. The ρ value of the Fe
Oxide-hydroxides, 1x109 Ohms-cm, was found slightly higher than the ρ value range of the formed
oxide layer of the present study. This because the former value was determined by a DC method rather
than by an electromagnetic method, i.e., holographic interferometry, with applications of EIS, i.e., AC
method. As a result, erroneous measurements were recorded due to the introduction of heat to Fe oxidehydroxides.
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