Aluminides and silicides are receiving much attention as high-temperature structural materials having high oxidation resistance and consequently the oxidation resistance of these intermetallics has been investigated extensively. This review focuses on the oxidation behavior of Ni-aluminides, Ti-aluminides and Mo-silicides. The topics in Nialuminides are the time-depending change in the scale structure and the void formation at the scale/substrate interface. In Ti-aluminides, the modification of the scale structure by alloying additions and the effects of atmospheres on the oxidation behavior are described. The topic in Mo-suicides is the effects of defects and H2O vapor on the accelerated oxidation.
Carbon/carbon (C/C) composites are known to posses low density and excellent mechanical properties even at high temperature. For this reason, C/C composites are attractive material systems for high temperature structural applications. However, C/C composites are easily oxidized in air above 500°C. In order to overcome this serious defect, SiC coating has been currently studied for use as a oxidation protection system. However, it is well known that a significant mismatch in thermal expansion between the C/C substrate and SiC coating causes a lot of coating cracks during the cooling process after the coating treatment. These cracks allow the oxygen diffusion toward the C/C substrate and degrade the oxidation resistance. In this paper, oxidation behavior of non-coated C/C composite (Bare C/C) and SiC-coated C/C composite is explained.
Silicon-based ceramics have an excellent oxidation resistance at high temperatures due to a protective SiO2 layer which prevents further oxidation. At low oxygen partial pressures, on the other hand, the surface of these ceramics would be directly attacked by an oxidant forming SiO vapor, which may significantly degrade their mechanical or protective performance. These two kinds of distinct behavior are termed passive and active oxidation. Another characteristic oxidation feature of the silicon-based ceramics is bubble formation which probably determines the highest temperature for usage. Therefore, it is important to investigate the oxidation behavior of these ceramics in a wide range of oxygen partial pressures and temperatures. In the present paper, the transition mechanism from the active to passive oxidation and several issues on the passive oxidation are reviewed.
Various manners of degradation behavior for different kinds of heat-resistant coating systems were demonstrated on the basis of failure analysis for specimen subjected to the corrosive damage both through a practical service as gas turbine hot section component and in a laboratory experimental. Coating systems include Al diffusion coating, plasma spraying mainly for the thermal barrier coating (TBC). Main causes to enhance the coating failure can be classified into four types in relation with the thermal-mechanical-chemical aspects of coating properties. In particular, an importance of considering the simultaneous effect both of thermomechanical loading such as creep or fatigue, and of thermochemical loading such as hot corrosion was emphasized mainly for TBC degradation. Technical subjects for developing more advanced coating system also were briefly pointed out.
For the purpose of non-destructive evaluation of the degree of temper-embrittlement in 2.25wt%Cr-1wt%Mo steels, anodic and single loop electrochemical potentiokinetic reactivation (SL-EPR) polarization curves of the embrittled specimens which had been serviced for oil refinery reactor for the maximum 25 years have been measured in 55wt% Ca (NO3)2 solution at 30°C. The characteristic of electrochemical polarization is influenced by fracture appearance transition temperature shift (ΔFATT) of Charpy impact test, P content and X-ray diffraction intensity ratio of M23C6/M6C in temper-embrittled specimens. Difference in current density (IP2-E-Ipass-E) between active second peak and passive in an anodic polarization curve of temper-embrittled specimens increases linearly with increase in ΔFATT in the range of mode transfer over transgranular cleavage to intergranular of Charpy impact fracture. On the other hand, difference in current density (IPA1-E-Ipass-E) between active first peak and passive in a SL-EPR polarization curve of largely temper-embrittled specimens turned up in region of ΔFATT of above 69°C, P content of above 0.01wt% and X-ray diffraction intensity ratio of M23C6/M6C of above 1.38 in the range of intergranular mode of Charpy impact fracture. Therefore, it is possible to evaluate nondestructively the degree of temper-embrittlement in the range of mode transfer over transgranular cleavage to intergranular, in addition to judge transgranular or intergranular mode of Charpy impact fracture which indicate a criterion of intergranular bond deteriorate by measuring IP2-E-Ipass-E and IPAl-E-Ipass-E of temper-embrittled specimens.
A two-stage HNO3 passivation treatment was introduced in order to improve the pitting corrosion resistance of type 304 stainless steel. In the 1st stage, the steel was immersed in 1.5kmol·m-3 HNO3 solution containing 0 to 5×10-2kmol·m-3 NaF at 313K for 3.6ks. The following 2nd stage was immersion in 1.5 to 10.5kmol·m-3 HNO3 solutions at 323K for 3.6ks. Pitting potentials for the steel subjected to the above mentioned treatments were measured by potentio-dynamic polarization in 1.0kmol·m-3 NaCl solution at 313K to reveal that the combination of 1.5kmol·m-3 HNO3+5×10.3 NaF solution for the 1st stage and 7.5kmol·m-3 HNO3 solution for the 2nd stage was the most preferable condition for the most noble pitting potential of about 1.0V (SCE). XPS analysis of treated steels revealed that Cr content in the passive film extensively increased and F- ions still remained in the film after the 2nd treatment. It is considered that F- ions penetrated in the passive film accelerated a preferential dissolution of iron in the film in the 1st stage, then Cr enrichment in the films was fairly enhanced by the high corrosion potential provided by HNO3 in the 2nd stage. Furthermore, F- ions located in the film and at the film/electrolyte interface seemed to act as an inhibitor to prevent the harmful attack by Cl- ions, resulting in the highly improved pitting corrosion resistance of the steel in chloride containing aqueous solution.
One point of technological importance in the use of stainless steels for natural water environments such as sea and fresh waters lies with their liability to stress-corrosion cracking (SCC). In this paper, we intend to discuss (1) the critical condition for initiation of IGSCC via corrosion crevice, or (2) the minimum initiation lifetime using exponential distribution model of SCC fracture, of sensitized stainless steel especially effects of electrode potential, stress, temperature, chloride concentration, crevice and degree of sensitization as placed in neutral chloride environments. We describe the life prediction model from the results of lower-limit lifetime of stress-corrosion cracking distribution in the corrodible domain.
The effect of nitrogen alloying on the initiation, propagation and repassivation of crevice corrosion of SUS 304 and SUS 316 austenitic stainless steels was investigated through potentiostatic polarizations, cyclic polarizations and in-situ measurements of depth profiles by the moiré method. The results showed that nitrogen alloying in austenitic stainless steels increased to more noble values both the critical potential for steady crevice corrosion, VC, CREV, and the critical potential for repassivation of crevice corrosion EC, CREV. Concerning the in-situ measurements results, the dissolution rate just above the repassivation potential, VII*, decreased with nitrogen alloying for the SUS 304 alloys, and did not change in the SUS 316 alloys. The ratio of repassivating picture elements increased in both SUS 304 and SUS 316 with nitrogen alloying.