The author's group has studied the corrosion mechanisms of metals by using various electrochemical methods. In this article, some results concerning with the iron dissolution mechanism by a channel flow double electrode (CFDE) and an electrochemical impedance spectroscopy (EIS) are introduced. The contents of the present article are as follows. (1) Transient analysis by CFDE in the time and frequency domains The transient current for active dissolution of iron was separated to the currents for the formations of Fe (II) and the intermediate, calculating the Fe (II) detection current by CFDE in the frequency domain. The fractional coverage of the adsorbed intermediate on the iron surface was estimated from the calculated currents. (2) Application of laser technique to CFDE The adsorbed species on the polarized iron electrode were flushed out by the laser impact. The reformation processes of the adsorbed species were investigated. (3) Determination of dissolved ions by spectrophotometry combined with CFDE The CFDE was combined with the spectrophotometry with an optical fiber system. The Fe (II) and Fe (III) dissolved from the corroding iron can be determined simultaneously. (4) EIS study on the iron dissolution The Fe(II) dissolution efficiency Φdiss was measured with the electrochemical impedance by CFDE. The Φdiss is the frequency dependent parameter regarding with the time constant of the adsorption. The physical meanings of the Φdiss and the Faradaic impedance for iron dissolution were revealed.
Analyses of plastic strain caused by a quasi-static or dynamic intrusion of a hard steel ball were carried out to consider fundamental mechanisms of erosion by solid particle impact. Regular square grids with a pitch of 80μm were printed on the cross-sectional surface of two pieces of the specimens by using a photoengraving method. The specimens were tightly vised with two pairs of bolts and nuts, then indentation tests were conducted at the parting line at angles of 30, 60 and 90 degrees. Principal shearing strain distributions on the corss sectional surface around indentations were obtained for commercially pure aluminium and iron. It was found that both the type of materials and intrusion processes affected the strain distribution behaviour. The elasto-plastic boundary on the cross sectional surfaces and lip height on the surfaces, also studied in this paper, showed a good correlation to the results of principal shearing strain distributions. A finite element method was carried out to simulate plastic strain distributions under the same conditions of the quasi-static indentation test. The experimental and computated results suggested that the strain accumulation beneath the indentation is not an incitement to erosion damage.
The aim of this paper is to examine plastic strain distributions around the indentations and to consider the mechanisms of erosion damage caused by solid particle impact. Measurements of principal shearing strain distributions around indentations were performed on surfaces for aluminium, iron and cast iron at impact angles from 20 to 90 degrees at impact velocities from 50 to 200ms-1. A WC ball or an angular SiC particle of 3mm in diameter was used to obtain the effect of particle shape on plastic strain. It was found that the coincidence of the impact angle dependence was roughly recognized between the maximum principal shearing strain and erosion damage. Measurements of volume ratios of lips to craters proved that material removal did not occur by single particle impact of a WC ball, while occurred at low angle impact of a SiC particle. The surface topography of the impact craters suggested that depth, surface area and volume of indentation are affected by the particle density and the hardness of both a particle and a target material. It is concluded that the origin of erosion is probably attributed by the high plastic strain and by the cutting action caused by the particle impact.
It has well been recognized that buried steel pipelines can be effectively protected against corrosion by the combined use of high resistivity coatings and cathodic protection (CP). Pipe-to-soil potential survey has been conducted periodically to evaluate the CP levels. Recently the risks such as AC corrosion, overprotection and corrosion inside casing pipes are increasing due to changes in burial situations, application of new construction techniques and so on. It is effective to measure DC and AC current densities using probes for the appropriate assessment of those risks. New CP criteria based on DC and AC current densities measured using probes have been developed in order to eliminate the risks such as AC corrosion, overprotection and corrosion inside casings.
The quality of tap water in West Iburi district (the southern part of Hokkaido) where the moundless pitting has frequently occurred has been investigated. It is found that the electric conductivity of tap water in the pitting area is strongly correlated with total hardness, SO42- and residuals. The electric conductivity in the pitting area shows a high value in winter and a low value in summer. We propose that the electric conductivity in winter will be utilized for an estimation of the pitting occurrence rate. A hazard map for the pitting corrosion of copper tube was drawn using electric conductivity measurement.