The corrosion rates of multiple metals were simultaneously monitored with the multi-channel quartz crystal microbalance sensors, and the measurement-based inverse estimation method for the concentrations of corrosive gases was studied. It was found that the relationship between the corrosion rates and the environmental factors could be formulated by regarding the corrosion rates of Ag, Co, and Cu as the function of SO2, NO2, H2S and relative humidity. The inverse estimation of the concentrations of corrosive gases can be made on the basis of these approximate equations. The dependence of corrosion rate for each metal on the environmental factors is possibly influenced by the kind and the abundance ratio of corrosion products.
Hydraulically expansion-type rock bolts made of steel pipe (hereinafter referred to as rock bolt) have often been used for the construction of tunnels in the natural ground with plentiful spring water or areas where the natural ground is readily displaced by tunnel excavation. Rock bolts hot-dip coated with Zn-6%Al-3%Mg alloy and without coating have been installed in Hida Tunnel on the Tokai Hokuriku Expressway to evaluate the durability under spring water conditions. These rock bolts were collected annually by over-coring method, one, two and three years after the installation and their corrosion behavior was examined. The corrosion rate of rock bolts without coating was found to be nearly linear and therefore the corrosion is thought to be controlled by the diffusion of dissolved oxygen. In the case of rock bolts coated with Zn-6%Al-3%Mg, on the other hand, there exists little corroded area with gray appearance as well as somewhat corroded area with dark gray appearance. However, the corrosion rate of coating in dark gray area decreased with progress of time and became substantially zero after two-year service. The outermost surface of the rock bolt after three-year service has been covered uniformly with Zn-Si-O composite which was estimated to be zinc silicate by analyzing chemical compositions of spring water obtained at the test site. This zinc silicate layer formed on corrosion products may act as a further anti-corrosive barrier and hinder further progress of corrosion.
In order to explain the mechanism that works in microbiologically influenced enhancement of corrosion rates, cathodic currents were measured on type 329J4L stainless steel exposed to seawater, and a biological analysis on biofilms was conducted. The cathodic current densities measured at 0.1 V vs. SHE were ranging from 0.3 μA/cm2 to 3 μA/cm2, after the specimen had been held at 0.1 V vs. SHE in natural seawater. On the other hand, the cathodic current densities at 0.1 V vs. SHE after the exposure to natural seawater under an open circuit condition, were higher than 1 μA/cm2 in the early stage of the measurement. However, they fell to values lower than 0.1 μA/cm2 in 12 h. The current densities measured at 0.2 V vs. SHE were approximately 2 μA/cm2 in the initial stages and the final stages of the measurement, after the beforehand exposure to natural seawater at 0.2 V vs. SHE. The current densities measured at 0.2 V vs. SHE after the beforehand exposure to natural seawater under an open circuit condition, were higher than 1 μA/cm2 in the initial stages of the measurement and lower than 0.1 μA/cm2 in the final stages. In the case of measurements at 0.3 V vs. SHE, the average values of cathodic current densities were approximately 0.2 μA/cm2afer the beforehand exposure to natural seawater at 0.3 V vs. SHE, and lower than 0.01 μA/cm2 after the beforehand exposure to natural seawater in an open circuit state. The cathodic current densities after the exposure to synthetic seawater, were lower than 0.01 μA/cm2 in the cases of measurement at 0.1 V vs. SHE, 0.2 V vs. SHE and 0.3 V vs. SHE. The data on DNA base arrangement detected by the denaturing gradient gel electrophoresis test in a biofilm formed at 0.2 V vs. SHE in natural seawater, was different from the data detected in a biofilm formed under an open circuit condition. It is, therefore, concluded that some specific types of microorganisms selectively attach to steel surfaces under cathodic conditions, and that such microorganisms are the cause of large cathodic currents resulting in high corrosion rates.