An algorithm to construct the potential-pH diagram of metal-ion-water systems by using a computer was established and it is applied to the metal-iodine-water system to explain the corrosion behavior of metals in the environment corresponding to the first stage reaction of the Magnesium-Iodine Cycle of thermochemical hydrogen production process. It is concluded that Ta, Zr, and Ti are thermodynamically protected owing to the oxide films which are more stable than iodides considered. On the contrary, noble metals such as Au, Ag, and Pt are predicted to corrode in concentrated iodine-iodide solution. Thermodynamic prediction is in fairly good agreement with the experimental results reported before.

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The oxidative dissolution of NiFe₂O₄ in sulfuric acid-cerium (IV), SC, solution has been investigated in comparing with previous works on the reductive dissolution of nickel ferrites. Each dissolution rate of Ni and Fe components in the oxide of NiFe₂O₄ greatly depends on the redox potential of the SC solution. The magnitude of their rates is in the following order: Fe>Ni in 0.25M H₂SO₄ without Ce⁴⁺ or with the addition of ascorbic acid in the SC solution, and Ni>Fe in the SC solution, indicating that the rate of Ni increases in an oxidative solution. Both the rates mainly depend on temperrature (40-90°C), but are little affected by Ce⁴⁺ concentration (1.3-10×10^-3M) and H₂SO₄ concentration (0.25-0.5M). A little consumption of Ce⁴⁺ was observed only at the initial stage of the dissolution. This shows that the dissolution is not due to redox reactions, but is catalyzed by Ni³⁺ formed in the oxide by the oxidation of Ce⁴⁺ in the SC solution. The overall dissolution rate of NiFe₂O₄ is dominated by the rate of Fe in a reductive solution and by the rate of Ni in an oxidative solution.

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Field analyses of insulation in process plants and laboratory simulation tests have been done to clarify the External Stress Corrosion Cracking (ESCC) of austenitic stainless steel equipment and piping. It was shown that the about 70% of the insulation in Japanese process plants are so much contaminated with externally introduced chloride ions and unacceptable level of ASTM specification. These insulations are also contaminated with flouride ions which will be a potential detrimental ion for low temperature ESCC. Laboratory simulation tests on fluoride ions revealed that the heavily sensitized Type 304 stainless steel could easily suffer intergranular SCC even at ambient temperature within extremely short time. Weld gas during wet welding condition, weld scale, a kind of anti-sputter reagent are assumed to be possible fluoride sources. Fluoride SCC is, however, mitigated by the addition of some kinds of inhibitor.

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In order to further elucidate the mechanism of pitting corrosion of copper, laboratory reproduction of pitting was carried out in test loops simulating a hot-water supply system in buildings. In hot water containing 3mg/l residual chlorine and 35mg/l SiO₂, the electrode potential of copper exceeded the critical value for pitting (150mV vs SCE) in relatively early stages (in 15 days). In this case, the dissolved silicate concentration was adjusted by adding sodium metasilicate. Pits reproduced in the test loops showed the same features as those found in actual service line. The inner layer of the corrosion products consisted of cupric oxide (CuO) and cupric metasilicate (CuSiO₃·H₂O), while the outer layer was covered with porous amorphous copper silicate. The formation of silicate film is responsible for the acceleration of chlorine reduction on the cathode sites and consequently the nobler electrode potential. Experiments were carried out in hot water with residual chlorine concentration adjusted to 0.7mg/l, the level of normal Japanese drinking water. Even in this water the electrode potential of copper exceeded the critical value for pitting in 120 to 150 days and pits were visually observed to occur. Dissolved SiO₂ content was 15 to 20mg/l without the addition of sodium metasilicate. The addition of sodium hexametaphosphate of 15mg/l (P₂O₅) to hot water containing residual chlorine of 3mg/l suppressed both the potential ennoblement and pit formation on copper tubes. In this case, the inner layer of film consisted of cuprous oxide (Cu₂O) on which a layer was uniformly covered with the amorphous deposits consisting of copper, oxygen and phosphorus. Electrochemical polarization experiments showed poor catalytic property for the cathodic reduction of chlorine on surfaces covered with phosphate containing film.

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The stress corrosion cracking (SCC) of SUS 304 and SUS 316 austenitic stainless steels in 0.82kmol/m³ hydrochloric acid solution has been investigated as a function of temperature at various constant stresses by using constant load method. The logarithmic relationship between steady state creep rate (ε_ss) and time to failure (t_f) is found to show the linear function under constant stress irrespective of temperature and material (SUS 304 and SUS 316), that's, logε_ss=-logt_f+Const. In addition, it is also fund that the logarithmic relationship between ε_ss and t_f obtained as a function of stress for SUS 304 and SUS 316 shows the linear function under constant environmental condition, logε_ss=-2logt_f+C′. These imply that ε_ss becomes a parameter for prediction of t_f, since ε_ss is determined from corrosion creep curve at a time within 10 to 20% of t_f. The critical temperature (T_cri.), below which no SCC occurs, is estimated by using 1×10^-9m/s of ε_ss as an indicator,
T_cri.≈270K for SUS 304
T_cri.≈282K for SUS 316
On the basis of the results obtained, a mechanism of SCC is qualitatively discussed in terms of crack propagation in induction period and corrosion current at crack tip.

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For an open cooling water system with a zinc-polyphosphate inhibitor, a water quality monitoring system adopted Flow Injection Analysis was introduced. Concentrations of phosphate (inhibitor) and Fe(II) ions were measured automatically by the analytical flow system. Analytical curves for phosphat and Fe(II) ions have good linearity, and their correlation coefficients were 0.9995 and 0.9999, respectively.
Reproducibility of measured value for Fe(II) ions was very good and that for phosphate ions was not so good. But these results were satisfactory for practical purposes. When the analytical flow system was applied to on-line monitoring, measured value for phosphate ions have a little amount of scatter. The reason may be explained as follows: 1) the conversion rate of phosphorus forms, during water sample is running through the heated zone, has slightly different at each measurment, and 2) the capability of ascorbic acid as a reducing agent for antimony-phospho-molybdate complex becomes weak with lapse of time. However, this water quality monitoring system has been developed to the point of being usable in practical applications.

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