Effects of solution pH and impurity additives on the intergranular stress corrosion cracking (IGSCC) of carbon steel in monoethanolamine (MEA) solutions have been studied using slow strain rate testing (SSRT) technique under potentiostatic polarization. Experiments were conducted in 20mass% MEA solutions with and without impurity additives in the pH range of 9-11 at 333K. It was found that the IGSCC was observed on the steel in active/passive transition potential regions in 20mass% MEA solutions without carbon dioxide (or carbonate), which has been thought to be a dominant cause for the IGSCC in amine solutions. The susceptibility to the IGSCC increased with decreasing pH value and with increasing concentration of impurity additives such as tiosulfate and hydroxyethyleethylenediamine (HEED). The simultaneous addition of these additives to MEA solutions caused the synergistic acceleration of IGSCC on the steel.
Decarburization behavior of 21/4Cr-1Mo steel used for the stam generator tubes of the fast breeder reactor in high temperature sodium was studied by following tow tests. (1) Long term test to grasp outline of decarburization behavior of 21/4Cr-1Mo steel in the temperature range of 380-600°C included operating temperature of steam generator. (2) Accelerated test in high temperature range of 500-700°C to study more detail decarburization behavior of 21/4Cr-1Mo steel. In long term test, it was found that logarithm of diffusion rate constant calculated from decarburization rate of 21/4Cr-1Mo steel was proportional with reciprocal sodium temperature. In accelerated test, however, the relation between them was not the same to the relation of long term test but changed with sodium temperature. From the results, it was clarified that decarburization mechanism was different with temperature and was affected by formation, decomposition and transformation of carbides in 21/4Cr-1Mo steel under the tests. Decrease of carbon concentration of 21/4Cr-1Mo steel for 40 years was estimated by the use of diffusion rate constant obtained in the tests.
Hot-corrosion susceptibility was investigated for the alloys whose compositions laid on a tie-line of a γ/γ′-two-phase Ni-Cr-Al-based alloy by a burner-rig-test (BR-test) in a combustion gas containing Na2SO4 and dip-tests in a Na2SO4/NaCl-salt mixture. Severity of corrosion differed for both tests in regions of γ′-phase content of I: <50%, II: 50-100% and III: >100% (γ′-phase). Relative severity among the regions was not identical between the BR-test and the dip tests. In the case of the BR-test, the corrosion was moderate in the regions I and III and the most severe in the region II, especially at a γ′-phase content of 85%. This is presumably because chromia and alumina are effective in retarding the corrosion respectively in the regions I and III, while both are less so in the region II. In the case of dip tests, the corrosion was very mild in the region I, considerable in the region II and cataststrophic in the region III. This is considered to be caused by loss of protective effect of alumina in the region III due to the basic fluxing owing to low oxygen content in the salt. The above information suggests that unfairly pessimistic results than would be materialized in gas turbines might follow, if hot corrosion susceptibility of nickel-base alloys or nickel-aluminide-based materials is assessed by dip tests.
Corrosion states of stainless steels in chloride solutions were observed by a voltammetry using random potential pulses, which is called the random pulse method. Step signals of 512 random potential were applied to a specimen, and the current responses to these potentials were arranged in a 8 by 8 matrix. Thus a three-dimensional voltammogram was obtained. Voltammograms showed characteristic shapes depending on the corrosion states of steel surfaces. The slope of the voltammogram and the polarization resistance reflected the corrosion susceptibility of materials. The variation of the voltammograms with time corresponds to the change of corrosion state of steels. The random pulse method could be recognized as a useful tool to monitor the corrosion states of stainless steels.
Both the scanning tunneling microscope (STM) and the atomic force microscope (AFM) were developed by Binnig group. These microscopes have already become very popular to every field of scientists. Because these microscope can be used not only in vacuum but also in air and in aqueous solution. These microscopes enable the real time observation of the reaction in atomic scale. In this paper, the principle of these microscopes and the examples of measurement are introduced. The various functions of these microscopes, such as nano-fabrication, nano-processing, tunneling spectroscopy and etc., are also shown. The techniques of STM measurement in aqueous solution are shown in detail based on author's experience.
A study has been made of surface modification of materials by ion implantation. Concepts are explained briefly of ion implantation and surface finishing. Use of direct ion implantation is introduced for surface modification of metals, ceramics, polymers and carbon. Ion beam mixing, especially dynamic mixing, related to ion implantation is indicated to be useful for practical applications. Some opinions are proposed for practical use of ion beam technology.