The dissolution behavior of chromium-rich oxides in sulfuric acid-cerium (IV), SC, solution has been investigated in connection with developing a chemical decontamination process for nuclear facilities. Each dissolution rate of Cr, Ni, and Fe components in the oxide, Cr0.6Ni0.6Fe1.8O4, greatly depends on the redox potential and pH of the SC solution. The magnitude of their rates is in the following order: Fe>>Ni>Cr=0 in 0.25M H2SO4 without Ce4+ and Cr>>Ni>Fe in the SC solution, indicating that the rates of Cr and Ni increase in an oxidative solution. The rate of Cr is a first order dependence on Ce4+ concentration. The amount of Ce4+ required to dissolve the oxides can be calculated if the amount of the oxides and their components were known. Namely, the dissolution is governed by redox reactions, in which Fe (II) and Cr (III) in the oxides are completely oxidized to Fe (III) and Cr (VI), but Ni (II) and Fe (III) unchanged. The difference in the total dissolution fractions between in the SC and HNO3-Ce4+ solutions suggests that the ability of a decontamination solution to dissolve the oxides should be evaluated not only by their initial dissolution rates but also the total amount of the oxides dissolved.
Degradation of coated steels were studied with ac impedance method for the frequency rage of 1mHz to 100kHz. The impedances at a non-deteriorated area, an anodic blistering area, and cathodic blistering areas with film delaminations were measured separately, then compared with those at the areas where the films were artificially deteriorated by anodic or cathodic polarization. The results revealed that the impedance measured for the whole area was nearly equal to that for cathode area. It was found that the cathode area was about one hundred times larger than the anodic blister. Using model blisters with free films and acidic solution as anolyte and basic solution as catholyte, the film resistances in unit area for anodic and cathodic blisters showed a similar value and they decreased in an almost similar rate by the polarization. From discussions on the equivalent circuits of these three different types of area, the impedance for the whole area is expressed by a parallel connection of these circuits so it will correspond to the lowest impedance of these in the measured frequency range. In practice, large area of cathodic deterioration is mostly responsible for the impedance characteristics measured for the whole area. For the evaluation of deteriorated coating films, it was proposed to use the break point frequency appeared in the high frequency range since this should be proportional to the area of cathodic delaminations and blisters.
Sodium molybdate gives an excellent corrosion inhibition for carbon steel in the presence of lithium hydroxide in a solution of 12.45mol/l lithium bromide at 160°C which is the operating condition of a two stage absorption refrigerator. However, in a low concentration, 0.001mol/l, of sodium molybdate the effect decreases enormously. We have revealed that the low concentration of sodium molybdate mixed with benzotriazole gives an excellent corrosion inhibition for carbon steel in the same manner as the high concentration of sodium molybdate. In the presence of 0.15mol/l lithium hydroxide, the most effective solution is the 0.001mol/l sodium molybdate one added with 0.0015 to 0.0075mol/l benzotriazole. In addition, this mixed inhibitor is also effective for galvanic and crevice corrosion inhibition. It is suggested that this corrosion suppression effect by the above mixed inhibitor is resulted from the complemental effect of the compact inner layer film composed of molybdate oxides mainly such as MoO2 and Fe3O4, and the outer layer film composed of molybdate compounds, Fe-benzotriazole and α-Fe2O3 which are formed on the surface of carbon steel.
In order to find out the corrosion behaviour of practical sheet pile for sea wall, the U-type sheet pile (YSP-II) which was used for half a century was investigated. Summary of the results was as follows; (1) the corrosion occurred in coast-site mainly and was observed a little in onshore-site and under sea-bottom, (2) the inside of joint was not corroded severely because of the packing effect of the initial corrosion products and soil, and (3) the tensile strength of the web location decreased in proportion to the decrease of its thickness, but the joint strength has been kept according to the initial design. From the above-mentioned summary, the maintenance-free system of the sheet pile in a long service period can be established by the protection of the coast-site web and flange, for example, polyethylene.
Repassivation potentials, ER, CREV, for metal/metal-crevices of 54 kinds of stainless steels were measured in 3% NaCl solution at 80°C. Obtained ER, CREV values were analysed in relations to chemical compositions of the 52 steels into multiple linear regressions in the form of Y=b0+∑bjXj for every one of steel groups, which are α (ferritic, 15 steels), α+γ (duplex, 11 steels), γ (austenitic, 18 steels with Cu<0.4%) and γCu (austenitic, 8 steels with 0.9-3.5% Cu). Y is the predicted value for ER, CREV and Xj denotes the content of alloying element j in wt% except j=Ni where Xj=logNi. Alloyed Cu more than 2% to austenitic steels changes the value of blogNi from -97mV/decade for γ to +45mV/decade for γCu-group steels. Well known beneficial effects of Mo were confirmed for all groups of steels except the γCu-group steels containing 2-3% Cu, of which ER, CREV values are kept constant independently on Mo contents. Addition of N up to 0.2% were found to be beneficial for the duplex steels, but not significant for the austenitic steels in spite of beneficial effects of N reported on pitting potentials.
The evaluation method for corrosion of steel in soil has not been firmly established yet. The soil aggressivity has been evaluated, so far, by using soil conductivity, redox potential and pH value as main factors. However, corrosion mechanism of steel in soil are so complicated that further approach for evaluation having the introduction of some other factors concerning argological condition are required. In this paper, the contributions of argological factors such as soil kind, soil structure, balance of water and gas phase and chemical composition in soil solution, were taken up and discussed.