Changing the timing of fluoride ion addition, passivation behavior at constant potentials and anodic polarization one for SUS 304 stainless steel were investigated in sodium sulfate aqueous solutions of pH 2-13, along with ICP analysis of solutions and XPS one of passive film. No pitting corrosion occurred for SUS 304 in fluoride solutions, but accelerated dissolution in the transpassive region and slight increase of passive current were observed in lower pHs. It was verified that fluoride ion dissolved preferentially iron component of SUS 304. This iron dissolution occurred immediately after polarization start in originally F- mixed solutions and after F- addition when being prepassivated in F- free solutions. XPS analysis revealed fluoride incorporation into the passive film formed in originally F- mixed solutions. It should be noted that no fluoride incorporation was observed in the film even though F- was added in the course of passivation. At the same time, chloride incorporation was not also detected in the film, independent of the timing of Cl- addition.
The passive current noise of SUS 304 observed in H2SO4 aq and in ethanol solutions with various H2O contents was simulated together with the PSD∼frequency relation by the Monte Carlo method. To simulate the PSDs, two probability distribution functions determined from statistical analysis of real current noise data were used. One concerns the current change per unit period and the other the time interval in which the current noise continues to evolve either in anodic or catholic direction. The PSD level and its time change were found to be simulated by mixing each parameter in both distribution functions. Comparing the experimental data with the simulated results, the PSD level and its time change were interpreted in terms of the nature of two kinds of bound water in the passive film.
Fluctuation of the corrosion potential during immersion of solution treated or sensitized SUS 304 steels in 0.05M H2SO4 was analyzed by FFT method to estimate the extents of sensitization non-perturbatively. Differences were readily noted, between the solution treated and sensitized specimens, in the power spectral density (PSD) vs. frequency curves at frequencies lower than about 0.3Hz. The PSDs below 0.3Hz increased markedly with increase in the sensitization time. At a frequency of 0.05Hz, for instance, the PSD was 4×10-12V2·Hz-1 for the solution treated SUS 304. While, it increased to 4×10-11 and 1×10-10V2·Hz-1 after sensitization for 1 and 10h, respectively. An increase in the slope of the PSD vs. frequency curve was also noted with increase in the sensitization time. Thus, it appears that the FFT analysis of the fluctuation of the corrosion potential in H2SO4 can be utilized as a non-perturbative, diagnostic tool for the determination of the extent of sensitization. Additionally, detailed observations of the waveforms of potential fluctuation and optical microscopy of the corresponding specimens have shown that the fluctuation is mainly due to the local anodic dissolution of iron component of the steel and subsequent formation of chromium oxide and/or hydroxide at these local dissolution sites.
Effects of some factors on the anodic polarization curves of TiAl-base alloys were studied in 2.5kmol·m-3 sulfuric acid at 20°C. The factors were the duration of catholic treatment (TRc) at a fixed potential of -900mV vs. SCE, alloying elements such as Cr or Mn, the content of Ti or Al, and a heat treatment at 1300°C for 1h. The results were compared with those of Ti and Al. The catholic current density (Ic) for the alloys increased, as the time of TRc increased, and attained a maximum after 80min, while the Ic of Ti and Al did not increase. The polarization curves of the alloys starting from the potential of -900mV were similar to that of Ti, that is, they had a peak (Icrit) and a passive range. The values of Icrit for the alloys were higher than that for Ti and the current density of Al at the same potential. It increased linearly on a log scale with increasing the initial Ic (Ic in) at the potential starting to sweep. And, it also decreased as Ti content increased. The effects of alloying elements and heat treatment were observed only when the time of TRc was short. It is concluded that the alloys absorb more hydrogen than Ti and Al at the condition investigated in this study, consequently the number of active sites in the gamma phase increase, and this gives rise to the difference in the polarization behavior mentioned above.
Fracture strength and dynamics of three titanium aluminide coatings (Ti3Al, TiAl and Al+TiAl3) at ambient temperature were examined by AE source inversion processing during cantilever bending. Mode I cracking (vertical crack) occurred prior to the Mode II cracking (exfoliation). Threshold stress to cause a first Mode I micro-cracking decreases with the increase of aluminum concentration in the aluminide. Vertical cracking through the coating was observed for Ti3Al and TiAl coatings, whereas microcracking was found in the TiAl3 layer for TiAl3+Al composite coating. Very few exfoliation was observed at higher stresses. The cracks through the coating was found to be filled with alumina and titania after the subsequent exposure at high temperature, while the cracks locked in the composite coating was filled with TiN. Oxidation performance of these coatings was kept at high level by the self healing capability of cracks.
Principle, apparatus and application of a scanning vibrating electrode technique are described as in situ measurement technique for localized corrosion. Advantages and limitations are described with detailed operation instruction.