Relating to the oil spill accident at Japan Sea by Russian tanker Nakhodka in January 1997, steel plate thickness decrease of ship hull due to corrosion was discussed from view point of navigation safety. At first, results of Investigation Committee of Nakhodka accident by Ministry of Transport were shown. Then relation between causes of accidents and decrease in plate thickness was introduced from data of NK (Classification Society of Japan). And examples of plate thickness measurement data in long life ship were shown and the influence of plate thickness decrease to the strength of ship hull was discussed. At last, ships of “flag of convenience” were discussed from view point of international safe navigation of ships.
Corrosion of buried metal structures has mainly been evaluated by the physical and chemical characteristics of a soil. The electrochemical monitoring of the above objects is becoming popular. This paper reviewed the recent progresses and the trend of corrosion monitorings in soils mainly by electrochemical techniques, which included such measurements as corrosion potential, galvanic current and polarization resistance. Fifty nine papers refered in this article were mostly selected from Chemical Abstracts. In the present state, the corrosion potential measurement seemed to deserve the practical field applications. A number of attractive trials were found in the refered papers. This article (Part I) describes the direct monitoring of buried metal structures in the field and the corrosion monitoring with probes. The next article (Part II) describes the correlation of the corrosion rate with a few variables such as corrosion potential, soil resistivity and others. Depending upon the reviewed articles, the authors proposed a systematic way of assessment regarding the underground corrosion of metal structures.
This paper discribes two types of exposure test in Okinawa. One is about 10 years of field observation for SUS 304 and SUS 316 roofs to prove the corrosion perfomance of building materials used stainless steels. The other is exposure test for various kinds of stainless steel specimens attached to model structures, which simulates roof and eaves, to clarify the corrosion characteristics of them in atmosphere. There is no discernible difference in the degree of rusting between SUS 304 and SUS 316. SUS 316 however has a smaller pit depth than SUS 304. The pit depth is not seemed to increase after a few years. The superiority of SUS 316 in the pit depth in comparison with SUS 304 is also observed by the two years exposure test on the simulated roof and eaves model structure. Seawater-resistant stainless steel (20Cr-18Ni-6Mo steel) has superior corrosion resistance on the roof and eaves.
The effect of zinc addition on the corrosion and the cobalt accumulation on type 304 stainless steel exposed to water at 561K has been investigated. The exposure tests were carried out at three different zinc concentrations 0, 0.2 and 0.8μM, and also using three different types of zinc compounds ZnO, ZnSO4, and Zn(NO3)2. The structure and elemental depth distributions of the corrosion surface were analyzed by EPMA, XRD and GDS. The zinc addition suppressed the amount of corrosion and the cobalt accumulation. These effects were related to the zinc concentration, however, no significant change was observed with the zinc constitution. The cobalt accumulation suppression by the zinc addition is due to two separate factors; first, the reduction in amount of corrosion film, and secondly, the suppression of the cobalt concentration increase with time in the corrosion film.
Stainless steels applied to exterior walls and roofs of buildings even in coastal areas. These cases have been increasing lately. In this study, atmospheric corrosion of stainless steels used for eaves of buildings and effects of environmental factors were investigated. Results are as follows. 1) Stainless steels used for the eaves corroded more severely than those for the roof. The average pit depth of specimens on the eaves was deeper and the number of pits was larger than that of specimen on the roof. The steeper the eaves' angle, the more the stainless steel corroded. 2) The corrosion of the stainless steels on the eaves and the roof corresponded to the amount of chloride ion in atmospheric dust on the specimens. Atmospheric dust on the eaves was hardly washed away by the rain. Therefore, the accumulation of chloride ion on the eaves was much more than that on the roof. The humidity around the eaves was more favorable for atmospheric corrosion (RH 33 to 75%). It was though these two environmental factors (the accumulation of chloride ion, the favorable humidity) facilitated the atmospheric corrosion of stainless steels on the eaves.
Stress corrosion cracking (SCC) of austenitic stainless steel type 304 has been investigated as functions of applied stress, chromate concentration and test temperature in 0.82kmol/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. However, pitting corrosion takes place over the whole specimen surface at chromate concentrations of 0.09 and 0.1kmol/m3 and leads to fracture (not SCC). In this case, the steady state elongation rate plays no role in predicting time to failure. The critical chromate concentration and critical test temperature are estimated at a constant applied stress of 388MPa. The results obtained, furthermore, support the SCC mechanism already reported, which has been discussed in terms of corrosion current density at crack tips and the length of crack propagation.
The ceramics, which can withstand corrosion, being heat and abrasion resistant, are drawing the attention of engineers, as they may be successfully applied to the parts of hydraulic machines where cavitation cannot be avoided. However, basic information on the behavior of these materials under cavitation attack is still lacking. In order to select the most suitable testing apparatus for obtaining this information, cavitation erosion tests on ceramics were conducted using three different apparatus, namely a water tunnel, a vibratory unit and a vibratory unit with a stationary specimen. In the water tunnel, the intensity of the cavitation attack was so low and it took such a long time to obtain a test result, that it was judged unsuitable for an accelerated testing. In the vibratory unit, the test results were affected by the vibration amplitude not evenly from a material to another. The vibratory unit with a stationary specimen was selected as the most suitable testing apparatus for ceramics as it is free from these disadvantages. Regarding the preparation of the specimens, it was found that the weaker the material, the thicker specimen should be used. Using the established testing method, cavitation erosion tests were conductedd on six different ceramics and it was revealed that the resistance of ceramics is closely related to their fracture toughness rather than their hardness.
Influence of environmental factors on the outbreak of stains on bright annealed stainless steels was investigated by atmospheric exposure tests for 1-6 months. Atmospheric exposure tests at two coastal-industrial sites along the Japan Sea and the Pacific Ocean showed that the sea-born salt particles from the raging sea presumably caused the stains accompanying pitting corrosions of ferritic stainless steels such as SUS 430. Comparing the corrosion behavior with the weather data, it is assumed that the frequent and much rainfall reduces the stains by washing the salt particles out and the heavy snowfall also reduces the stains by preventing the particles from attaching to the surface of the stainless steel. Very light brown spot stains of diameter about 0.5-1.5mm without pitting sometimes break out on the specimens of SUS304 in the season with little salt particle flying and little rainfall. The acid rain and some corrosive fall dust are suspected to cause the stains.