The author previously proposed the speedy and easy electrochemical measurement of the depth-profile of Cr near the surface of Fe-Cr alloy, and so in this work, the validity of this measurement was evaluated by comparing the depth-profile of Cr measured by this method with that calculated from evapolation model. SUS 304 specimens were oxidized in air at 1, 273K, and after removing only oxide film by careful polishing, polarized at 1.40V (SCE) (transpassive region) in 10% H₃PO₄ (323K, air open) in order to measure the I_{t, p} (10^-3A·cm^-2)-Q(C·cm^-2) curves. From these curves, the experimental depth-profiles of Cr were obtained by converting I_{t, p} into Cr concentration and Q into depth. Comparing experimental and theoretical depth-profiles of Cr, the best fit values of the lattice interdiffusion coefficient, D, in the SUS 304 and the interface transfer coefficient, α, of Cr were estimated by a least squares method of computer simulation. The estimated values of D and α were in good agreement with those obtained in previous work.

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Crack growth and acoustic emission (AE) behavior during corrosion fatigue (CF) process has been studied in an (α+β) high strength Ti-6 Al-4V alloy solution treated at -1, 206K, and aged at -818K. Compact tension specimens were prepared so that the cracking direction was parallel (T-L) or normal (L-T) to the direction of the prior hot rolling at higher temperature (-1, 273K) above β/(α+β) transformation temperature. The tests were carried out in air and at several electrochemical potentials in a 3.5% NaCl solution under cyclic load. Macroscopic crack growth rate (da/dn) was shown to be strongly dependent upon specimen orientation. The T-L specimens exhibited more severe susceptibility than the L-T. This orientation dependence was attributed to the anisotropy of large prior β-phase grains and equiaxed (α+β) fine grains both elongated in the direction of the prior hot rolling. The difference of environments, on the other hand, did not influence da/dn under the present test condition. However, much greater AE activity in terms of detected AE event counts and peak amplitudes was observed in the NaCl solution than in air. The greater AE activity in the NaCl solution shows that AE can be very useful to monitor CF cracking process. Optical and SEM observations of cracking path and fracture surfaces found the fracture morphology to be a mixture of intergranular cracking along prior β-phase grain boundaries, α/β interface (interphase) cracking due to hydrides formed along the interfaces, and transgranular cleavage. The main AE sources during CF in NaCl solution were obviously crack jumps caused by the above cracking processes which occurred discontinuously in the material. Different carcking mechanisms are discussed in detail with respect to the energy levels of the emanated AE and the fracture morphology.

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In order to select the resistive metals to the environments corresponding to (1) the Interlinking process between 1st and 3rd reaction stage, (2) the transportating stage of Mg(IO₃)₂ from the 1st to the 2nd reaction stage, and (3) the transportating stage of MgO from the 3rd to the 1st reaction stage of Mg-I thermochemical hydrogen production process, several metals were examined in the corrosive and/or erosive environments. As for the environment (1), Ta, Nb, Ti, and Zr were tested in the solution of MgI₂-4.8 I₂-9.6 H₂O (molar ratio) at 200°C in glass ampoules. The results obtained showed that Ta and Nb are usable. To clarify the resistivity of metals in the erosive environments (2) and (3), the simulated experiments were conducted, that is, several metals were rotated at the speed of about 30cm/sec in quarts sands (20-30 mesh or 150-200 mesh) or alumina powder (4, 000mesh) with or without iodine at 120°C and 350°C. Generally, the smaller the size of particle is, the lower the rate of erosion-corrosion is. The preliminary erosion test in ethanol by sand-erosion tester using magnetostriction vibration suggested that erosion damage by MgO or Mg(IO₃)₂ powder does not exceed that by alumina powder. As the value of erosion-corrosion rate of Ta and Nb was less than 0.01mm/yr when they were rotated in alumina powder with iodine, it is concluded that Ta and Nb are applicable to the practical environments. Water vapor was effective to decrease the erosion-corrosion rate of Ta and Nb. Oxygen was also effective for Ta. The less the concentration of iodine is, the lower erosion-corrosion rate of Ti is, and it may be applicable if the concetration of iodine is less than 0.01mole/1kg-powder.

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A crack initiation time, T_i, and a propagation time, T_p, in a total failure time, T_f, for stress corrosion cracking of Type 304 stainless steel in a boiling magnesium chloride solution have been determined separately with a measurement of corrosion potential, and their probability distributions have been analyzed as a three parameter Weibull distribution. A location parameter, γ, is estimated by maximizing a correlation coefficient between Y=lnln 1/(1-F(t)) and X=ln(t-γ), and two other parameters, that is, a shape parameter, m, and a scale parameter, t₀, are estimated by using the Gumbels equation. The shape parameter thus determined is found to be nearly constant and about 1.3 for T_f, T_i and T_p. Thus, an exponential probability distribution is fitted especially for the crack initiation time, the shape parameter of which is close to unity. Change in the location parameter which corresponds to an incubation time, and a mean failure time, μ, with change in temperature and stress have been discussed with respect to the mechanism of stress corrosion cracking of the steel.

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Heater tubes, piping, heat exchanger tubes, and other facilities used at elevated temperatures in petroleum industries often suffer from carbon deposition. Such carbon deposition sometimes causes various kinds of failures in the materials. Some of these failures are brought about directly by carbon deposition while others are caused through carburization of the materials.
This paper deals with a number of such failures and describes the phenomena.

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