The Techno-Superliner is a new generation, high-speed, hybrid hydrofoil lift/ buoyancy type vessel. The hull structure consists of an upper hull, lower hull, hydrofoils and struts which connect the upper hull to the lower hull and hydrofoils. The group including the author surveyed many different materials, such as aluminum alloys, advanced composite materials, titanium alloys, higher tensile steels, stainless steels and others, including some developing materials. Based on the unique requirements for the structural materials and on the results of extensive investigations, aluminum alloys and higher tensile stainless steels were chosen for the upper hull and the underwater parts, respectively. This paper summarizes the requirements for the structural materials and the basic features of the materials surveyed and outlines the mechanical properties, fabricability, corrosion and erosion resistance of the selected materials.
Small addition of tantalum to Ni-20Cr-20Mo alloy improves the corrosion resistance to sulfuric acid dew point corrosion. The roles of tantalum in enhancing the corrosion resistance in 60 and 80% H2SO4 with active carbon at 120°C was investigated. Dissolution of the alloy containing 2wt% tantalum suppresses in the active region and the alloy is caused to passivate spontaneously. The passive current of the alloy is lower than that of the tantalum free alloy. XPS analysis was carried out to examine the surface films formed on these alloys immersed or polarized in the acids with active carbon. The cationic fraction of tantalum in the surface films on the alloy containing tantalum is so low as it is about 0.02. However, the ratio of O2-/total oxygen in the surface films on the alloy containing tantalum is higher than the ratio on the tantalum free alloy. It is as a result of enhancement of the surface films by the formation of composite chromium-tantalum oxy-hydroxides, which have higher stability.
Stress corrosion cracking (SCC) of austenitic stainless steel type 304 has been investigated as functions of nominal stress, molybdate concentration and test temperature in 0.82 kmol/m3 HCl solution by using a constant load method. It is found that the steady state elongation rate in the SCC-dominated region becomes a useful parameter both for predicting time to failure and for the assessment of SCC susceptibility irrespective of the above factors. On the basis of the criterion for SCC assessment, the critical test temperature and critical molybdate concentration are estimated under a constant nominal stress of 388MPa. No pitting corrosion takes place over the whole molybdate concentrations used, which is different from the case of chromate addition. The difference of the role between chromate and molybdate is discussed in terms of ion selective property, the potency of film formation and so on.
The corrosion rates of rotating aluminum alloy disck were estimated from the measurement of the polarization resistance and electrochemical impedance spectroscopy in 3.5 mass% sodium chloride solution (pH 8.2) at 273K. The corrosion rate of aluminum alloy increased with increase of rotational speed and had a constant value at all immersion times. Corrosion current measured by polarization resistance and electrochemical impedance spectroscopy and corresponded with that from weight loss. It is found that above each technique are a convenient tool for monitaring of the corrosion processes of rotating aluminum alloy.
When stainless steels are immersed in natural sea water, their electrode potential (Esp) ennobles up to about 400mV vs. SCE in summer, while the Esp does not ennoble significantly in winter. In this paper, the relationship between ennoblement of Esp and attached diatoms in biofilm formed on the sample was investigated. Firstly, as Stage I, Type 316 stainless steel was immersed in natural sea water for a few days. The Esp value, Esp, I, at the end of Stage I was controlled to be less than 100mV vs. SCE so that biofilm formed on the sample was mainly consisted of bacteria. As the next stage, Stage II, the steel was transfered to a diatom containing solution in laboratory, which consisted of sterilized sea water, non-oxidizing culture medium and raised diatom suspending in the solution, for 10 days and final Esp value, Esp, II, was determined in relation with amount and kind of attached diatoms on the sample. Without Stage I, Stage II alone could not make significant ennoblement of Esp, II, because diatoms could not attach and develop on the sample. Stage I immersion, even in winter, followed by Stage II immersion in the diatom solution with sufficient density of Chaetoceros gracilis was found to make Esp, II value ennobled as high as in the immersion in natural sea water in summer season. The extent of ennoblement in Esp, II increased with increasing Esp, I value and diatom density in the diatom solution in Stage II. Similar experiments were conducted with other three kinds of diatoms, isolated from natural sea water at Orido Bay, Shizuoka prefecture. Nitzschia sp. A, the most dominant species in summer, showed the highest level of ennoblement in Esp, while Melosira moniliformis, the most dominant species in winter, showed the lowest. This fact could explain the seasonal change of Esp ennoblement in natural sea water from a viewpoint of diatoms.
The effect of strain rate on stress corrosion cracking (SCC) for Type 304L stainless steel was investigated in a 20mass% NaCl aqueous solution containing 10-2kmol m-3 Na2S2O3 under air open condition by using a slow strain rate testing (SSRT) apparatus with a dynamic observation system. This technique can in-situ observe the growth of multiple cracks on a smooth tensile specimen, and can provide the information on the initiation and propagation of an individual crack separately. A decrease in strain rate led to a decrease in the strain where the maximum stress appears in a stress-strain curve, which is one of convenient SCC susceptibility indexes for SSRT. It is clear from the dynamic observation that the cumulative number of pits and cracks at the same strain is similar above the strain rate of 10-6 s-1 and decreases below that rate. In addition, stress intensity factor for SCC (KISCC) was found to show the Weibull probability distribution, and its mode was about 2MN. m-3/2, independent of strain rate. While, crack growth rate showed log-normal probability distribution, and its mode was in a narrow range from 0.8×10-8 to 1.5×10-8m/s, almost independent of strain rate. In comparison with Type 316 L stainless steel, Type 304 L stainless steel had higher SCC susceptibility of the lower KISCC, but had similar susceptibility of almost same crack growth rate. It is concluded that the effect of strain rate on the strain where the maximum stress appears is controlled by crack initiation process, not by crack propagation process.
The effect of metal ion additions on the corrosion behavior of SUS 304 stainless steel in simulated boiling water reactor (BWR) environment was investigated. This exposure test was carried out using a high temperature and high pressure (HT/HP) experimental loop. Each metal ion addition did not affect appreciably the corrosion weight loss (or corrosion rate). The corrosion film in any systems had a double layer structure consisting of a Cr-rich inner layer and an Fe-rich outer layer. The formation mode and thickness of the film were changed by the additions of metal ions.