When the exhaust gas of marine engine is passing the supercharger, it needs to cool the water way of the supercharger by sea water or fresh water for preventing over heat. A lot of corrosion product was formed in its water way. Sometimes, it blocked the stream of cooling water and made corrosion hole. Present paper is the study of comparison the corrosion products and oxidation processes of cast iron between flowing sea and fresh water. The test pieces were made of the same cast iron as the supercharger, and the corrosion tests were performed as a function of the surface temperatures of the test pieces and the rate of cooling water in the both test equipments of sea and fresh water. The crystalline iron oxides in both corrosion products were analyzed by the powder method of X-ray diffractometer. The intensity of diffracted X-ray of a certain iron oxide was expressed by the percentage intensity in the total counts of iron oxides measured. A temperature of the cooling water 60°C and the surface temperature of test pieces 80°C, hematite, Fe(OH)₂ and goethite presented less than 20%, respectively, and the intensities of them decreased with increasing in cooling water rate in sea water. These iron oxides were a few in fresh water. The quantities of γ-Fe₂O₃, Fe(OH)₃ and magnetite were a little larger in fresh water than those in sea water. The corrosion rate in sea water was about 1.7 times larger than that in fresh water.
The oxidation processes of iron were proposed as follows; Fe³⁺→Fe(OH)₃→goethite→hematite and Fe²⁺→Fe(OH)₂→magnetite→lepidocrocite→γ-Fe₂O₃→hematite. The relationships of diffracted X-ray intensities between the neighboring iron oxides along with the oxidation process were examined. At 45°C of unheated test piece surface in sea water, the processes of magnetite→lepidocrocite and lepidocrocite→γ-Fe₂O₃ were estimated to be 1% level of significance, and were considered as predominant processes. At the temperature range of 45°-55°C heated, the process of magnetite→lepidocrocite→γ-Fe₂O₃ were estimated to be 1% level of significance. At the temperature range of 60°-80°C heated, lepidocrocite was not detected in sea water and the process of Fe(OH)₃→goethite→hematite were estimated to be 1% level of significance. In the fresh water at the temperature range of 80°-100°C, the oxidation process of Fe(OH)₃→goethite←hematite were estimated to be 1% level of significance. From these results, it was found that the corrosion products and the oxidation processes of iron were different depending on sea and fresh waters and the temperatures of test pieces.

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The degree of sensitization of austenitic stainless steels (Type 304, 316, 321, 347) heat-treated at different conditions were evaluated by EPR test, ASTM A262-A, A262-B, A262-C and A262-E tests to study the correlation between the reactivation ratio obtained by EPR test and the intergranular corrosion (IGC) susceptibility obtained by the corrosion tests (A262-A-E tests). To clarify the influence of precipitaties on the results of EPR test and A262-A-E tests, the morphology of the precipitates were observed by Transmission Electron Microscope and Energy Dispersive X-ray Spectrometer, and the dissolution mode was observed by optical microscope. The critical reactivation at which the specimens start to show the IGC susceptibility in the A262E test increased with the following order; Type 316<Type 304<Type 321<Type 347. The causes of this dependency of the critical reactivation ratio on the kinds of steels was discussed in terms of the soluble carbon content and Cr profile near grainboundaries. In the EPR test, the dissolution initiates at the Cr depleted zone, but not at the precipitates.

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Corrosion protection activities of thin films formed on iron by electrochemical polymerization of organic compounds were studied by electrochemical measurements, spectroscopy, and chemical analysis. Pyrrole, aniline, phenol and their derivatives were polymerized by electrooxidation on an iron disk electrode in nonaqueous solvents, and vinyl compounds and organic halides by electroreduction. Corrosion protection effects of the films thus formed were examined by the polarization measurements for the electrodes with rotating at 500rpm in an aerated aqueous 0.5M Na₂SO₄ solution (pH7). Among all of the examined compounds, α, α′-dibromo-p-xylene afforded the most protective film when it was cathodically electropolymerized in the presence of oxygen. The role of oxygen for the corrosion protection was investigated by the comparison of the films prepared from α, α′-dibromo-p-xylene between in air and under nitrogen, by chemical analysis, polarized reflection IR spectroscopy, and scanning electron microscopy. It was found that oxygen was reduced to form O₂^-· and this radical was involved in the polymerization reaction to result in the construction of packed polymer structure containing C=O and C-O moieties besides the main chain of (-CH₂-C₆H₄-CH₂-)_n.

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In order to study the effect of oxidation conditions on the metal release of Type 304SS in high temperature pure water, the specimens oxidized in various environments such as air, steam or water were immersed in deionized and deaerated water of 180°C, and metal release level was measured by weight loss. The behavior of metal release was also discussed in relation to the surface film which was formed by oxidation treatment. As the results, the oxidation treatments in air at 425°C and in steam at 350°C were found to be specially effective to decrease the metal dissolution compared with those of mechanical polishing and the oxidation in water. The results corresponded well with the structure of surface films.

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The effect of ash compositions on the coal ash corrosion of SUS 347 H and 17-14 CuMo steel which were the candidate materials of superheater tubes was investigated. The sodiumpottasium molar ratio (Na₂NO₄/K₂SO₄) in coal ash deposit influenced the corrosion rate. When the content of Na₂SO₄ in alkali constituents exceed 90% the corrosion rates were remarkably decreased. Al₂O₃ and SiO₂ which were the main constituents of coal ashes did not influence the corrosion rates. However CaO and MgO remarkably suppressed the corrosion rates, because these oxides inhibited the formation of alkali iron trisulfate, and decreased the acidity of the molten salt. The alkali content in coal ash deposits significantly influenced the corrosivity of ash. Consequently, the corrosion rates increased with the increase of alkali content in the ash deposit.

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Theories of passive film destruction and pitting corrosion are reviewed with a prospect for the future development of kinetic theories concerning pitting phenomena. The structure and chemical composition, the electrical and mechanical properties of passive films are reviewed briefly, to provide the knowledge of structural change as a precursor of pitting. Also the specific nature of pitting corrosion known experimentally, such as pitting potential, critical concentration of aggressive ions, the induction time, pit localization and a stochastic aspect of pit initiation, is noted. The existing models which have been proposed for the action of aggressive anions in the course of pit initiation are compared and some of promising models are considered in connection with the relevant kinetic theories of pitting corrosion. Future development of atomistic models are recommended since pitting as a kinetic process needs clarification from microscopic point of view.

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