Solid State Phenomena Vols. 124-126

Abstract: Bulk Al-8Fe-2Mo-2V-1Zr (wt.%) alloys were produced by melt spinning which can give rise to develope a nano crystalline structure in terms of rapid cooling and subsequent hot extrusion. The bulk alloys exhibited multi-phase microstructures consisting of ultra fine equiaxed grains with the average grain size of 100nm and a fine intermetallic Al-Fe, Al-V and Al-Zr based phase having less than 50nm in particle size. From compression test, it was revealed that the bulk alloys have very high yield strength at both room temperature (942MPa) and elevated temperatures (651MPa at 473K, 500MPa at 573K, respectively).

Abstract: Hydrolysis and dispersing behavior of aqueous ZrB2 slurry were investigated. The isoelectric point (IEP) of ZrB2 slurry was measured to be pH 6.3. The acid and base slurry underwent rather rapid change of pH after fabrication for one and three hours, respectively. The oxygen content of the boride powder increased slowly with static ageing time in acid and neutral slurries, while the value decreased in base slurry. Application of poly(ethyleneimine) changed IEP of the boride slurry to pH 10.4. Sedimentation test informed that stability of the slurry was improved when increasing the molecular weight of poly(ethyleneimine) to 70,000 and in acid slurry (pH 3).

Abstract: Growth model of a circumferential outer diameter stress corrosion crack (ODSCC) in a retired steam generator tube of the Kori 1 nuclear power plant was proposed based on extensive destructive examinations of the pulled tubes of Alloy 600 from the Kori 1 plant. A small ODSCC grows in a lateral direction as well as a forward direction until it meets a neighboring ODSCC which also grows in a lateral direction as well as a forward direction. And then, the two ODSCCs which meet on the same circumferential plane are consolidated into a single ODSCC. By repeating such a consolidation process with time, it seems that the apparent growth rate of an ODSCC in the lateral direction is much faster than that in the forward direction. Growth model of a circumferential ODSCC from a retired steam generator tube of the Kori 1 plant reveals that many ODSCCs are initiated and grow in both directions independently until they meet and finally they are consolidated.

Abstract: More than 2% δ-ferrite phase is presented in type 316L stainless steels due to improvement of hot workability or welding process. In order to clarify the mechanism on how the δ-ferrite affects the corrosion resistance, electrochemical tests in a micro-droplet cell were conducted and microstructures were analyzed by TEM-EDS and SEM-EDS. Especially, the micro-droplet cell was modified at our laboratory to observe the polarization behavior of δ-ferrite phase in an acidic chloride solution. The anodic polarization behavior in both active zone and passive zone was affected by δ-ferrite. The degradation of corrosion resistance in the low potential zone with δ-ferrite phase is due to the preferential dissolution of a matrix, γ phase. It is explained with the fact that the formation of δ-ferrite lowers the concentrations of chromium and molybdenum in γ phase. It is also confirmed by potentiostatic test, EDS analyses and phase-equilibrium prediction. In addition, the micro-droplet cell and EDS analysis reveled that the pitting potential in the neutral pH solution was affected by Ca containing inclusion rather than δ-ferrite phase.

Abstract: The aim of this work is to evaluate the difference in the corrosion resistance between unleaded containing bismuth automotive epoxy primers and leaded one. It was also discussed the effect of zinc phosphate pretreatment. The specimen coated unleaded epoxy primer showed 0.5V higher corrosion potential than that of bare steel. The result of salt spray test did not indicate remarkable difference of corrosion resistance in all specimens above 10
thickness up to 1200 hours. In the cyclic corrosion test, unleaded epoxy primers on phosphated substrate performed good corrosion properties until 800 hours. The lead free primer performed the equivalent corrosion resistance as leaded coating on phosphated steel, but slightly inferior to that of leaded on bare steel. These results show that the pre-treatment of zinc phosphating is effective as well as pigment changing in performing anti-corrosion properties in automotive bodies.

Abstract: The aim of this study was to investigate the effects of air plasma spraying process parameters on the tribological behavior and surface characteristics of quasicrystal coatings. For this purpose, the response surface method (RSM), one of the designs of experiment (DOE) was utilized to systematically investigate the combined effects of each process parameters. The RSM analysis revealed that the superior tribological performance of quasicrystal coatings was obtained by the optimized condition of specific micro-hardness and porosity level. It was specially quoted that the relatively low hardness of quasicrystal coatings caused the improvement of abrasive wear resistance during the current investigation.

Abstract: Since the PbSCC(Lead stress corrosion cracking) of alloy 600 tubing materials was reported by Copson and Dean in 1965, the effect of lead on a corrosion film and cracking morphology have been continually debated. An electrochemical interaction of lead with the alloying elements of SG tubings was studied and the corrosion products were analyzed. It was found that lead enhanced the anodic dissolution of alloy 600 and alloy 690 in the electrochemical test. The lead preferentially dissolved the Cr from the corrosion film of alloy 600 and alloy 690 in alkaline water. The lead ion seemed to penetrate into the TG crack tip and react with the corrosion film. A selective Cr depletion was observed to weaken the stability of the passive film on the alloys. Whereas passivity of Ni became stable in lead containing solution, Cr and Fe passivity became unstable.

Abstract: Bond coatings, CoNiCrAlY, are prepared on Inconel 738 substrate by vacuum plasma spray (VPS). Mechanical properties of VPSed CoNiCrAlY coatings are investigated via Taguchi method and L18(21×37)
orthogonal arrays to determine the optimal setting and the relationship of experimental variables. Seven parameters were considered as follows: (A) arc current; (B) primary gas flow rate; (C) secondary gas flow rate; (D) stand-off distance; (E) working pressure; (F) carrier gas flow rate (Ar); and (G) powder feeding rate.
The effect of carrier gas flow rate on the porosity is determined to be the highest among the parameters investigated. Higher microhardness values are observed for the VPSed coatings as compared to the coatings prepared by conventional high velocity oxygen fuel probably due to low amount of the porosity.

Abstract: In order to prevent pitting corrosions in aluminum thin film, zirconium incorporated chemical conversion coating on sputtered aluminum film was studied. Several pretreatment methods for conversion coating on the thin film were examined. There existed the optimum pretreatment and coating condition for successful growing of anticorrosive conversion coating layer in a given aggressive environment. Pit densities of aluminum thin films were significantly decreased with conversion coating. Electrochemical behaviors of conversion coated aluminum thin film were also discussed.

Abstract: Effects of Si, W and Cu on the localized and stress corrosion of 18 % Cr austenitic stainless steel were investigated using experimetal strategy of full factorial design. The resistance to localized and stress corrosion of the alloy was improved with increasing the contents of Si, W and Cu in the range of 0.5 ~ 1.5 % for Si, 0.1 ~ 0.5 % for W and 0.2 ~ 2.0 % for Cu. In addition, we found an interaction effect between Si and Cu on the reistance to localized and stress corrosion of the alloy.