In order to predict the service life of glass flake reinforced epoxy coating (GF; film thickness 1, 200μm), coated steels were immersed in sea water for 5 years and were also electrolyzed cathodically for several months. The following matters were assumed. (1) Coated steels weaken though two stages. During the 1st stage the paint film has no visible defect. Defects observed at the 2nd stage. The service life is (T₁+T₂), where T₁ and T₂ correspond to the period of each stage. (2) Under natural conditions the weakening rate of the film is expressed as dA/dT=α·I_corr. At the 1st stage the I_corr is expressed as I_corr=(E_c0-E_a0)/(R+R_rc+R_ra). (3) Unde the accelerated conditions the weakening rate is expressed as dA/dT=α′·I.
Logarithm of rust area vs. time plot and log I vs. time plot gave straight lines respectively. These results were successfully explained by equations above. At the 1st stage the value of α′ was independent of electrolysis voltages, but at the 2nd stage it depends. From experimental α′, 10.5 years was obtained as the period of T₁. T₂ could not be estimated by the electrolysis method.

View full abstract

Modulated reflection spectra of passive films on a series of Ni-Cr alloys (5 to 28 mass% Cr) have been measured in 1kmol·m^-3 Na₂SO₄ of pH 6.0. Quantitative relations between the characteristic of spectra and the composition of the films have been derived from spectra for thin composite films of NiO-Cr₂O₃ artificially prepared on Pt by MO-CVD technique. From these relations, cationic mass fractions of Cr³⁺ ions, X_Cr, of passive films on the alloys were determined.
It was found that passive films on the alloys were composed of Ni(II) and Cr(III) oxides, and that their cationic fractions of Cr³⁺ ions changed remarkably with Cr content of the alloy. That is, the values of X_Cr for the films formed at 0.10V increased from 0.10 for Ni-5 mass% Cr alloy to 0.88 for Ni-28 mass% Cr alloy with increasing Cr content of the alloy. This indicates that Cr³⁺ ions were substantially enriched in the films on the alloys containing more than 15 mass% Cr. The value of X_Cr for each alloy was almost independent of potential in the passivity region below 0.5V. However, in the transpassivity region above 0.6V, it decreased sharply with increasing potential to reach a value close to the mass fraction of Cr in the alloy at 0.70V.

View full abstract

Stress Corrosion Carcking (SCC) threshold stress level and SCC stress criteria of AISI Type 304 under plane stress condition were studied in three material-environment systems.
The results obtained are summarized as follows;
1) The SCC threshold stress of annealed Type 304 in boiling 42wt% MgCl₂ solution was measured to be 13.8kg/mm² which was well evaluated by the Total Strain Energy Theory. And it was very close to the elastic failure stress of the substrate metal which developed microscopic slip lines. The fracture mode in this system was changed depending on the loading modes. Microcracks stopped their propagation under the stress state of δ₂/δ₁≤-3.5.
2) The SCC threshold stress of sensitized Type 304 in boiling 22wt% NaCl solution was measured to be 21.3kg/mm² or 0.2% proof stress of substrate metal which was also well evaluated by the Total Strain Energy Theory. The fracture mode was changed from IGSCC under uniaxial tensile stress to TGSCC with the increase of shear stress component.
3) The threshold stress of IGSCC of sensitized Type 304 in acidified 1% tetrathionic solution at 25C was aslo found to be 13.8kg/mm² which was well explained by the Total Strain Energy Theory.
4) Although different SCC mechanisms might be operating in these systems, the test results clearly showed that the SCC threshold stress was well evaluated by the Total Strain energy Theory which was similar to the stress criteria (Shear Strain Energy Theory) for elastic failure of substrate metals.
5) In order to prevent SCC, both maximum and minimum principal stresses are required to be well controlled according to the proposed specific equation which has two different threshold stress levels depending on the systems.

View full abstract

In corrosion fatigue crack propagation, there are three important factors, one is crack closure, another is corrosion at the crack tip and the other is reverse slip. Paying attention to these factors, the corrosion fatigue crack propagation behavior of structural alloys, such as HT 80, SM 50B and SUS 304 steel, was studied under various frequencies (f=0.3, 10, 50Hz) in 3% NaCl aqueous solution.
The results show that the fatigue crack growth curves have constant acceleration rates against those in air except intergranular cracking ΔK regions when the crack closure effect is removed. In order to explain this, a model based on striation formation mechanism is proposed, which is that the corrosion of the bare surface produced in loading prevents the reverse slip in unloading to result the crack propagation, and the relation between the amount of corrosion on the bare surface and the crack propagation rate was studied by the scratching electrode method. As the result, it was found that there is a good correlation between them, and that almost all data obey this mechanism.

View full abstract

The plates of SUS 304L stainless steel were welded by using the welding wire of SUS Y 308L. The corrosion fatigue characteristics of the weld in artifitial seawater as well as in air have been investigated and compared with those of the base metal. As a result, it has been found that the fatigue life durations of the both metals in the seawater at any stress level are shorter than those in air at the same stress level. Thus, the effect of corrosion artificial seawater on the fatigue characteristics of these materials have been recognized. The life of corrosion fatigue of the weld metal was always shorter than that of the base metal under the same stress level. This can be attributed to welding. The degree of these effects depended on environmental temperature as well as on stress level: namely at stress levels higher than 180MPa, the life difference between the weld and the base metal was small at any temperature, but at stress levels lower than 180MPa, the difference increased with increasing temperature.

View full abstract

The damage of car body caused by de-icing salt is classified into cosmetic corrosion and perforation. The former is a paint de-adhesion and the latter is a crevice corrosion. Precoated steels can well suppress both types of damage. Hot dip galvanized- and galvannealed sheet, zinc alloy electroplated sheets, such as Zn-Ni and duplex Zn-Fe, and prepainted weldable sheets are now widely used for automotive bodies. Corrosion behaviors of these precoated steels in cyclic tests which simulate the actual car driving conditions are reviewed based on published technical data.

View full abstract

On the fatigue design of offshore structure was reviwed mainly from historical point. History of offshore structure and its design rules was tabulated.
Newly U. K. DOE “Guidance on design and construction” were explained briefly.

View full abstract