It is important to monitor the corrosion rates of steel in electrolyte environments such as aqueous solution as well as soil in order to evaluate the integrity of the members of facilities. As a nondestructive monitoring method, CIPE (Current Interrupted Polarizing Extrapolation) method was developed. By the CIPE method the corrosion rate is calculated by the Tafel equation obtained from the polarization curve of in-situ measurement. It was found that the CIPE method was useful to monitor corrosion rates of steel in concrete as well as in neutral aqueous solution as presented in the conferences of CORROSION 2018 and 2019. In this paper, results of corrosion rates of carbon steel in sand containing sea water, which were measured by the CIPE method were discussed. As a result, it was confirmed that the CIPE method was also useful to monitor corrosion rates of carbon steel in soil. Furthermore, it was considered that when dissolved oxygen decreased the corrosion rate became negligible and the test piece behaved as a reference electrode without corroding.
It is important to monitor the corrosion rates of steel in electrolyte environments such as aqueous solution as well as soil in order to evaluate the integrity of the members of facilities. As a nondestructive monitoring method, CIPE (Current Interrupted Polarizing Extrapolation) method was developed. 1 This method is called CIPE for the abbreviation of the name in this paper. By CIPE method the corrosion rate is calculated by the Tafel equation obtained from the polarization curves of in-situ measurement. This "Tafel extrapolation method" has been used to determine the corrosion rate of steel in solutions since 1950's.2 However, this method has several problems as follows;
(1) Accurate polarization curve can't be obtained due to IR drops which are generated by the polarizing current and resistance of solution between the tip of the reference electrode and the steel surface.
(2) Polarization curves are distorted due to changes of the steel surface state, which are caused by the accumulation of ions such as Fe2+, H+ and OH− during the polarizing process.
(3) Deciding the Tafel line from the polarization curve is difficult because it is not clear where the Tafel line starts from.
