Basic Study on Underground Corrosion Properties of Steel
Kenichiro IMAFUKU, Kazumi MATSUOKA and Nobuaki OTSUKI
Abstract:Researches into underground corrosion of steel structures are very few because of its mildness. And corrosion rates in underground corrosion are given only qualitatively or as a certain value based on statistical analysis of mass exposure test result without evaluation of soil condition. So we started to try to evaluate quantitatively the effect of environmental factors on underground corrosion. In this research, the basic underground corrosion properties of steel could be clarified. High temperature acceleration tests were carried out with the parameters of soil grain size, pH of solution, chloride ion concentration and aeration condition. The conclusions are as follows;(1) Under aerated condition : 1) Corrosion rate of steel in clay, containing no chloride ion, is much smaller than that in aerated water, and can be regarded almost constant between pH 2 to pH10. 2) Corrosion rate of steel in clay and in fine sand, containing chloride ion, is almost constant regardless of the chloride concentration. And uniform corrosion is observed in fine sand while ununiform corrosion is observed in clay, and the corrosion rate is much higher in clay. With electrochemical hydrogen permeation method, pH on the surface of steel goes down from almost 8 to almost 4 in Sumi clay. Furthermore, at the result of polarization resistance measurement, Rp in clay and that in fine sand are not so much difference. 3) In clay, the aeration has little effect on the corrosion rate because of the low oxygen diffusion.(2)Under deaerated condition : 1) Effect of pH without chloride ion : i) In spite of the soil type the effect of pH to corrosion rate is small from pH 2 to pH 12 under soil environment, ii) Under non-soil environment the effect of pH to corrosion rate is small in pH 6 and over, while corrosion rate become very large under pH 2. iii) Corrosion rate is almost the same regardless of soil existence in 6 and over. 2) Larger the grain size, the corrosion rate increases in pH 1, while corrosion rate is almost the same from pH 2 to pH 12 regardless of the grain size with the exception of clay. Key Words:Underground corrosion, Corrosion rate, Steel, Acceleration test, Hydrogen ion permeation, Polarization esistance