The initiation process of stress-corrosion crack is composed of the initiation process of non-propagating micro cracks as the initial stage, followed by a second stage consisting of the coalescence of adjacent micro cracks formed in the first stage. Then, the self propagating third stage of cracks that can grow steadily is established after cracks larger than a certain critical size are formed as a result of the crack coalescence process. This paper discusses the distribution of the initiation time of the steady-state crack propagation expressed by an exponential distribution model, while the two random processes of the first two stages are assumed to be Poisson stochastic processes. A random-process model for the initiation of stress-corrosion cracks, and the initiation of crack propagation, based on two Poisson stochastic processes connected sequentially had been discussed, and the statistical analysis results corresponded quite well to Monte Carlo simulation results. Results of statistical analyses of crack initiation rates vs. applied stress, and of lower-bound lives vs. applied stress were shown indicating that the statistical models fit the experimental data.
To clarify mechanism of corrosion resistance in outdoor exposure tests for 55% Al-Zn-plated steel sheets, behavior in edge creep development was observed in various cyclic corrosion tests. One of cyclic corrosion tests with a relatively longer highly humid period showed similar corrosion behavior to that of an outdoor exposure test conducted in heavily corrosive area with salt damage. Then, electrochemical impedance of the test panels was measured after the cyclic corrosion tests to control the edge creep development. The edge creep development was strongly depended on the following two factors. One is reaction between an anticorrosive pigment and the substrate where the plating has already dissolved away and the other is electrochemical barrier effect given by tightly adhered coating materials. A schematic diagram indicating the edge creep development was proposed and capability of the corrosion control by the used anticorrosive pigments was discussed with the diagram.
In Japan, the Sn-Zn coating steel sheet (ECOKOTE-T) has been widely used for fuel tanks. To extend the useful life of the fuel tanks as critical safety parts, we have modified the structure of Sn-Zn coating by controlling the solidification process. The newly developed Sn-Zn coating steel sheet (ECOKOTE-S) has very finely dispersed zinc, which contributes to a far superior corrosion resistance. No composition change has been made in this structural modification, attributed to a pressing, welding and painting ability equivalent in comparison to ECOKOTE-T. In addition, ECOKOTE-S is sufficiently durable for new alternative fuel, such as an ethanol/gasoline mixture and FAME (Fatty Acid Methyl Ester)