A new boiler to work with highly concentrated water was designed to study the effect of dissolved oxygen (DO) on corrosion behavior of carbon steel in high temperature and high pressured boiler water system. The operational characteristics of the new test boiler were examined and it was confirmed that the new test boiler exemplified actual boilers in terms of strength factors such as heat transfer rate, blow rate, recirculating rate and linear velocity rate. With maintaining the temperature of boiler water at 150-200°C, the corrosion behavior of carbon steel with a heat flux was examined under the different concentration of DO and/or cycles of concentration of boiler water. It was found that there was a critical value of DO (DOcri), which was defined that no ferrous ion was detected below DOcri and the amount of dissolved ferrous ions (WFe) increased remarkably with increasing DO above DOcri, and DOcri was dependent on the cycles of concentration of boiler water. The results showed that a small amount of DO caused serious corrosion of carbon steel in high temperature and highly concentrated boiler water.
The cyclic-oxidation resistance of a TiAl coupon at 1300K was significantly improved by the preoxidation at 1200K for 100ks in a pack of a mixture of 70mass% Cr2O3 and 30mass% metallic Cr powders. This is attributable to the formation of a thin dense scale of about 1μm thickness consisting mainly of α-Al2O3. The scale contains small amounts of TiO2 and Cr2O3, and a very small amount of metallic Cr on its surface. The scale became thicker very slightly after ten-cycle oxidation (720ks) with no significant structural changes. Other combinations of temperature (1100, 1200, and 1300K) and pack composition (100, 70, and 30% Cr2O3 with balance of metallic Cr) lead to the formation of poorly protective scales. There is a general trend; when temperature is low or the amount of Cr2O3 in the pack is small, thin defective scales very rich in Al are formed, while when temperature is high or the Cr2O3 content is large, thick porous scales consisting of TiO2 and Al2O3 are developed. The above can be explained in terms of the rate of oxygen supply.
In order to clarify the reason why the caustic IGSCC resistance of nickel base alloy 600 is improved by the precipitation of chromium carbides at the grain boundaries, the author investigated the role of grain boundary chromium carbides on the caustic IGSCC resistance of nickel base alloy 600 in high temperature NaOH solution. The results obtained were as follows: The author made clear that the dominant contribution factor of grain boundary chromium carbides to the caustic IGSCC resistance is to be attributable to the cleansing of the grain boundaries by the trapping of grain boundary segregated elements such as C, B, P by the chromium carbides. The formation of corrosion resistant film due to CrO2- ions caused by the chromium carbides dissolution at the crack tip may be also contributable to the caustic IGSCC resistance apart from the extent of contribution. However, inhabitant effect of IGSCC propagation by the insolubility of carbide itseif, improving effect of the caustic IGSCC resistance by the formation of grain boundary chromium depleted zones, electro-chemical protection effect by the precipitation of chromium carbides from the interface boundaries of alloy 600 which have large difference of the corrosion potential and grain boundary strengthening due to the alternative coherent precipitation of chromium carbides along the grain boundaries seem to be not contributed to the caustic IGSCC resistance of alloy 600.
In order to produce aluminide coatings with an excellent oxidation resistance, aluminizing of titanium substrate was conducted utilizing a high power infrared laser fusion reaction. Titanium substrate painted with aluminum powder was laser fused in argon gas atmosphere. Three types of titanium aluminide coatings of 150μm thickness, i.e., single phase of Ti3Al, TiAl and composite phase of TiAl3+Al, could easily be produced with a high reproducibility. The chemical composition of these coatings could be controlled by choosing both the laser irradiation conditions and the amount of painted aluminum powder. Corrosion resistance of these coatings was evaluated at 1073, 1173 and 1273K in the stagnant air. Changes of the composition and the stracture of the laser produced coatings during oxidation were carefully examined by SEM-EPMA and micro-area X-ray diffraction every 6 hours. The Ti3Al coating was slightly oxidized already at 1073K according to a parabolic law. The TiAl coating was highly resistant to oxidation at 1073K, however, was oxidized at temperatures above 1173K. On the contrary, the composite coating of TiAl3+Al showed an excellent performance against high temperature oxidation even at 1273K. Aluminum in this composite coating was oxidized during early short exposure and produced alumina of 160μm thickness which contributed as a diffusion barrier of oxygen. The high oxidation resistance of this coating was due to the formation of alumina by the oxidation of Al and also to the thermal diffusion of Al into the titanium substrate, resulting in a protective thick TiAl3 layer.
The aspects of surface analytical techniques by detecting outcoming ions and particles from samples concerned were summarized. Typical ion beam analyses by the use of high resolution mass spectrometric methods coupled to various ionization techniques such as spark source, glow discharge, ion bombardment, laser ionization. Elemental distribution in depth of surface layers by the ion scattering spectrometry using low, medium, high energy, elastic recoil detection analysis, and nuclear spectrometry were described. Several crystallographic defect analyses by the combination of channeling effect of incident ions and Rutherford back scattering spectrometry were introduced. The possibility of surface analysis using atomic, molecular and neutron beam was also mentioned.
Advance in the research of anodic film structure formed on aluminum which has been done mainly by authors was reviewed. Furthermore, some information about the application of the films developed recently in Japan as a functional material using their novel property of minute and regular porous structure was given.