Copper tube is generally used in water supplying systems, however the copper tube rarely suffers from a type I pitting-corrosion. To begin with, the effect of components dissolved in tap water on copper-corrosion rate was investigated in this paper. Copper specimens were immersed in 20 samples of tap water. After 48 hours immersion tests, electrochemical impedance spectroscopy (EIS) measurements were carried out. The copper-corrosion rate was estimated by the result of the EIS measurement. A relationship between the copper-corrosion rate and components dissolved in the tap water was calculated from a multiple regression analysis. This analysis shows that the copper corrosion-rate increases with concentration of a free carbon dioxide. Polarization curves of the copper specimens were also measured in the tap water samples. Cathodic polarization curves in 20 waters have various profiles in the range of 0 to -0.4V vs. SSE. The slope (ΔV/mV decade-1) of the cathodic polarization curve at -0.25V vs. SSE decreased as the copper corrosion-rate decreased. The profile of the cathodic polarization curve might depend on a state of the oxide film on the surface of copper. It is considered that protective oxide film prevents reduction of dissolved oxygen and that copper corrosion-rate decreases with inhibition of the reduction reaction.
Using a Kelvin probe, the corrosion potential distribution on the surfaces of two types of steel, ordinary carbon steel and type 430 stainless steel, under thin solution layer has been investigated. On the surface of carbon steel covered with corrosion products, which sucked in water, noble and less noble potentials were separately observed in the potential distribution because of a large IR drop in the absorbed water phase. The former potential area worked as a cathodic site and the latter potential area, as an anodic site. On the other hand, the cathodic areas around pits of stainless steel showed less noble potential compared with other areas. This indicated that the farther from the pits the location became, the more the cathodic current density decreased because of a large IR drop.
In order to study whether acoustic emission (AE) can be monitored from atmospheric rust of steel structure, AE monitoring during wet-dry rusting test by five kinds of electrolytes was attempted. It revealed a linear increase of AE counts with rust thickness. AE amplitudes from thicker rusts than 100μm were large enough to be monitored as the in-water longitudinal wave by resonant type AE sensors (450kHz center frequency) mounted on the cylindrical cell wall on the steel plate. The frequencies of these Lamb wave AEs through the steel plate are in the range from 0.3 to 0.5MHz. Mechanical fracture of rust film by four point bending produced AEs with the frequency range 0.6 to 1MHz. AEs were produced due to elastic strains of substrate steel for thicker rust.
The practical and accurate method for corrosion life prediction and anti-corrosion design of automobiles have been newly developed. A number of case studies on corrosion occurrence from automobiles aged from 4 to 10years in a real environment were collected. The neural-network-based nonlinear multivariate analysis method was applied to derive implicit relations of corrosion in automobiles from the collected case studies. Firstly, the method was used for the corrosion occurrence prediction about the SILL OUTER parts. Success rate of the prediction was over 90%. Secondly, the method was used to predict the E-coat thickness on the inner side of the SILL OUTER parts required for corrosion protection. Actual and estimated E-coat thickness values agreed reasonably well with each other, i.e. with a high linear correlation factor of 0.9.
Recently, the risks such as AC corrosion, overprotection and corrosion inside casing pipes are increasing on cathodically protected pipelines due to the changes in burial situations, application of new construction techniques and so on. Authors have proposed new cathodic protection (CP) criteria based on DC and AC current densities measured using probes that provide an appropriate assessment of those risks together with effective measures to eliminate those risks. The design of CP systems for newly constructed pipelines, on which those risks were likely to occur, was then successfully performed based on the new CP criteria. In this paper, actual procedures to design CP systems with respect to the new CP criteria are demonstrated. In addition, the methodology to eliminate the risk of AC corrosion is presented.