Measurement of surface resistivity/conductivity of carbon steel in 5-20ppm of RA-41 inhibited seawater by optical interferometry techniques

Abstract

Optical interferometry techniques were used for the first time to measure the surface resistivity/conductivity of carbon steel samples in blank seawater and in seawater with different concentrations of a corrosion inhibitor, without any physical contact. The measurement of the surface resistivity/conductivity of carbon steel samples was carried out in blank seawater and in seawater with a concentration range of 5-20ppm of RA-41 corrosion inhibitor, at room temperature. In this investigation, the real-time holographic interferometric was carried out to measure the thickness of anodic dissolved layer or the total thickness, U total , 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 RA-41 inhibited seawater, respectively. In other words, the surface resistivity/conductivity of carbon steel samples was determined simultaneously by holographic interferometry, an electromagnetic method, and by the Electrochemical Impedance (E.I) spectroscopy, an electronic method. 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 (surface resistivity (ρ) or surface conductivity(σ)=1/[surface resistivity (ρ)] between the determined AC impedance (by EIS technique) and the orthogonal displacement (by the optical interferometry techniques) was obtained. Consequently the values ρ and σ of the carbon steel samples in solutions were obtained. Also, the value of ρ from other source were used for comparison sake with the calculated values of this investigation. This study revealed that the thickness of the anodic dissolved layer of the carbon steel sample has been removed from the surface of the sample, in the blank seawater. Therefore, the corresponding value of the resistivity to such layer remained the same as the value of the resistivity of the carbon steel sample in air, around 1x10 -5 Ohms-cm. On the contrary, the measured values of the resistivity of the carbon steel samples were 2.4x10 6 Ohms-cm, 2.2x10 7 Ohms-cm, and 1.1x10 8 Ohms-cm in 5ppm,10ppm, and 20ppm RA-41 inhibited seawater solutions, respectively. Furthermore, the determined value range of the ρ of the formed oxide layers, 2.4x10 6 Ohms-cm to 1.1x10 8 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), 1x10 9 Ohms-cm. The ρ value of the Goethite(α-FeOOH) and of the Lepidocrocite (γ-FeOOH), 1x10 9 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 oxide-hydroxides. This led to higher value of the resistivity of the Goethite(α-FeOOH) and for the Lepidocrocite (γ-FeOOH)), 1x10 9 Ohms-cm, compared to the determined value range of the resistivity of the formed oxide layers, 2.4x10 6 Ohms-cm to 1.1x10 8 Ohms-cm.