Influence of the alloying elements and the crystal structure of the stainless steels on the corrosion behavior in molten carbonate was investigated by the immersion test at 650°C for 1000h. Weight loss and amount of dissolved alloying elements decreased with increasing the contents of Cr and Ni in stainless steel. Corrosion resistance of austenitic stainless steels was superior than of ferritic steels because Ni forming a solid solution with LiFeO2 layer suppressed the outward diffusion of Fe with the formation of Cr2O3. In addition, it was revealed on the basis of the diffusion data that Cr2O3 formed on the austenitic steel has a tendency to inhibit the outward diffusion of Fe compared with that of the ferritic steels. Attempt made to use the austenitic stability index of stainless steel was pointed out the importance of the effect of the crystal structure for evaluating the corrosion behavior in the molten carbonate.
Influence of the alloying elements of Fe-Cr-Ni-Al alloys and Al diffusion coated stainless steels on the corrosion behavior in molten carbonate was investigated by the immersion test at 650°C for 1000h. Weight loss and dissolution of the alloying elements decreased with increasing the contents of Cr and Al in the alloy. Corrosion resistance was improved by a increase of a corrosion index (Cr+3Al). The concentration of Fe in the corrosion product decreased with increasing Al content in the alloy because of the formation of Al2O3 and Cr2O3 on the material. Corrosion resistance of the Al diffused stainless steels was improved by increasing Al and Cr concentration in the diffusion layer. Al diffusion treatment on stainless steels was found to be effective for the molten carbonate environment because the thickness and Al concentration in the layer remained unchanged at least up to 2, 000h by the corrosion test at 650°C.
Anodic polarization curves of a carbon steel (JIS G 3106 SM 400B, 0.12 mass% C) have been measured in a boric-borate solutions (pH 8.45), dilute NaOH solutions (pH 8, 9, 10), water in contact with bentonite (pH 8.2-8.3), and mixed solutions of 5.72mol⋅m-3 Na2SO4 +7.99mol⋅m-3 NaHCO3 (pH 7, 8, 9, 10), which simulate the water in contact with bentonite. The formation and breakdown of passive films on the steel have been examined by in-situ ellipsometry. It was found that in the boric-borate solution of pH 8.45 the steel readily passivates with the formation of passive films. The thiclsness of the passive films increases almost linearly with increasing potential, and at the same time the real and imaginary parts of optical constants of the films tend to increase. In the water of pH 8.2-8.3, which had been in contact with bentonite for 10 and 60 days, the steel dissolved actively from the corrosion potential and did not passivate. Similar polarization characteristics were observed in 5.72mol⋅m-3 Na2SO4+7.99mol⋅m-3 NaHCO3 solutions of pH 7-9 and 5.72mol⋅m-3 Na2SO4 solution of pH 9, while the active-passive transition took place at 0.1 V in 7.99mol⋅m-3 NaHCO3 solution of pH 9. The anodic dissolution rate at potentials ranging from -0.4 to 0.1V in the Na2SO4+NaHCO3 solution of pH 9 was higher than that at correspon ding potentials in the NaHCO3 solution having the same pH value. These results suggest that SO42- ions promote active dissolution and impede the formation of passive films. Passivation occurred more easily when pH value was increased up to 10. In solutions containing SO42- ions, as in Na2SO4+NaHCO3 solutions, however, the steel suffered from pitting immediately after passivation was completed. The depassivation of the steel prepassivated in the boric-borate solution of pH 8.45 occurred by SO42- ions when the steel was immersed in aerated Na2SO4+NaHCO3 solutions of pH 7-10. No apparent change in the thickness and optical constants of the films was observed during the depassivation process, indicating that the depassivation occurs through local breakdown of the films.
Anodic polarization curves of a commercial carbon steel with 0.12 mass% C and high purity carbon steels with 0.12 and 1.16 mass% C have been measured in solutions of 5.72 mol⋅m-3 Na2SO4+7.99mol⋅m-3 NaHCO3 (pH 8.3-10), which simulate the water in contact with bentonite, 5.72mol⋅m-3 Na2SO4 (pH 8-10), and 7.99mol⋅m-3 NaHCO3 (pH 8.3-10). The thickness and optical constants of passive films formed on the steels in a boric-borate solution of pH 8.45 have been determined by using in-situ ellipsometry. The results were compared with those obtained for pure Fe and bulk Fe3C, which simulate two constituent phases of carbon steel, i.e., ferrite and cementite, respectively. It was found that in 5.72 mol⋅m-3 Na2SO4+7.99mol⋅m-3 NaHCO3 solutions of pH 8.3 and 9, complete passivation took place on pure Fe and Fe3C but did not on the three carbon steels used. The current densities at potentials ranging from 0.3 to 1.1V (vs. SHE), where both pure Fe and Fe3C undergo passivation, increased in the following order: pure Fe<Fe3C<high purity 1.16% C steel<high purity 0.12%C steel<commercial 0.12%C steel. In the Na2SO4+NaHCO3 solution of pH 10, pitting occurred on the commercial 0.12%C steel after the passivation was completed. These results suggest that the formation of uniform and stable passive films is difficult on the 0.12%C steels having ferrite-pearlite structure, and impurities in carbon steel have a detrimental effect on the passivation and depassivation process. The passive current density in 7.99mol⋅m-3 NaHCO3 solutions of pH 8.3-10 and a boric-borate solution of pH 8.45 increased with increasing C content of specimens, indicating that carbon increases the dissolution rate of passive films. The results of ellipsometric analysis showed that the optical constants, N2=n2-k2i, of passive films formed in the boric-borate solution are strongly dependent on the C content of specimens; the valueof n2 increased from 2.45 for pure Fe to 2.75 for Fe3C with increasing C content, and at the same time the value of k2 decreased from 0.35 to 0.15.
This paper reviews the stress corrosion cracking (SCC) of austenitic stainless steels type 304, type 316 and type 310, and ferritic stainless steel type 430, which has been investigated as functions of applied stress, potential and environmental factors (temperature, concentration, pH, anion species) in acidic solutions by using a constant load method. The constant load provides a corrosion elongation curve from which three parameters such as steady state elongation rate, time to failure and transition time are obtained. The results show that the steady state elongation rate in the SCC-dominated region becomes a useful parameter both for predicting time to failure at a time within 10 to 20% of time to failure and for the assessment of SCC susceptibility irrespective of the above factors. Furthermore a SCC mechanism is discussed in terms of corrosion current density at crack tips, the length of crack propagation and so on.
Marine structure models were constructed in 1974 to study new corrosion protection sysytems over a long term. The results based on 12 years exposure indicate that excellent sysytems for splash and tidal zones were polyethylene, rubber, epoxy mastic and metal wrappings with catholic protection. The laboratory tests showed that the minimum rupture energy of a sprayed urethane coating was proportional to the square of coating thickness and gave minimum coating thickness required. This paper also reviewes new corrosion control measures applied later to the gigantic maritime structures, which are desired to posess 50 to 100 years durability. The protective coatings using in atmospheric zone are fluoro-resin and acrylic silicone resin paints with excellent weatherability. In the splash and tidal zones super heavy duty epoxy mastic and corrosion resistant metal (SUS 316 L, Ti) wrapping are preferably listed in the specification in conbination with catholic protection.