The oxidation of a Ni-20Cr alloy was studied at temperatures between 1273 and 1423K under various oxygen partial pressures by mass gain measurements, microstructure observations with optical and scanning electron microscopies, X-ray diffraction and electron probe microanalysis. Oxygen pressures were obtained using air and CO-CO₂ mixtures with various ratios, ranging from 2.1×10⁴Pa to the pressures below the dissociation pressure of NiO. Above the dissociation pressure of NiO, the oxidation rate was affected by oxygen pressure in the early stage. The amount of NiO initially formed on the alloy decreased with lowering oxygen pressure. In the later stage, the growth of the oxide scale obeyed the parabolic rate law, and optical micrographs showed the formation of the compact oxide layers. These two results suggested that the oxidation was controlled by the diffusion of Cr ions through the Cr₂O₃ scale. The parabolic rate constants were independent of oxygen pressure, which was explained by the n-type or intrinsic defect structure of Cr₂O₃.

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Oxidation behaviours of titanium-vanadium alloys containing 5 and 10at.% titanium have been investigated at temperatures ranging from 800 to 993K and oxygen pressures of 1.33 and 133Pa by means of a thermo-electrobalance, an X-ray diffractometer, an electron probe microanalyzer as well as metallographic techniques. Mass gain by oxidation mostly followed parabolic rate law. Oxidation resistance of vanadium has been highly improved, especially at low temperatures and high titanium concentration. Both alloys exhibited similar oxidation kinetics and scale structure. The activation energy for the parabolic oxidation was about 173kJ/mol, higher than that of vanadium. The oxide scale was a monolayer of VO₂ or (V, Ti) O₂ except for 993K and 133Pa. A very thin layer of TiO was formed at the oxide scale-alloy substrate interface, inside which was formed a very hard internal oxidation zone. The former appeared to retard the diffusion of oxygen into the substrate.

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Interaction between a Ni-base heat-resistant alloy and simulated HTR primary coolant environment, which contained controlled concentrations of both oxidizing and carburizing impurities, was examined with emphasis placed on the reactions inside narrow crevice gaps. A test method of providing artificial crevice using a Mo container was developed to obtain quantitative results with reasonable reproducibility. The test environment was helium of low oxidizing potential with trace amount of gaseous impurities: H₂, H₂O, CO₂ and CH₄. The state of oxidation on the inner surfaces of the crevice was examined by measuring the change in concentration of Cr on the specimen surface using EPMA. Significant difference in the chenge of the surface was seen along with the distance from the crevice entrance. There was a preferential consumption of oxidizing species in the outer part of the crevice due to the formation of continuous oxide film, leading to porous or discontinuous oxide film formation, and the resultant extensive carburizations in the inner part of the crevice. Vaporization was noted far inside the crevice where oxide film was scarecely formed. The zone of continuous oxide film formation was found to extend into the crevice as a function of the 1/4 power of exposure time, and its reaching distance in a given time was proportional to the square root of opening of crevice gap. These relations were consistent with a model with an assumption that the diffusion of reactants through the gas was a rate determining step. The model based on the results obtained will be applied to the crevices with the similar geometrical effects such as the internals of fatigue and creep cracks and also actual crevice formed on the surface of heat exchanger tubes.

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The corrosion of some Fe-Al alloys containing up to 7 mass% Al at 1073K under SO₂ partial pressures below 10³Pa was studied by thermogravimetry, X-ray diffraction, SEM, and EPMA. The effect of pre-oxidation on the corrosion of the alloys was also clarified. The addition of aluminum yielded a significant improvement in the corrosion resistance of the alloys to SO₂ gas. The parabolic rate constants obtained at given SO₂ partial pressures were exponentially decreased with the aluminum content, which was due to the increasing depression of the sulfide formation in the external scale. The increased corrosion resistance was interconnected to the depressed formation of sulfide for the alloys with high aluminum content, because of the reduced transport rate of rations through the oxide scale. The pre-oxidation treatment for the alloys yielded the remarkable decrease of the corrosion rate and the sulfide in the scales was scarcely found during the subsequent corrosion in the SO₂ atmospheres.

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Strain generation behavior during the oxidation of mild steels at 400°C in pressurized CO₂, 1.45MN.m^-2, was studied by X-ray diffraction technique. The observed strains in both the steel and the surface oxide are explained qualitatively with a model derived from the combined effects of local elastic stress and lattice parameter change due to compositional change. The material with low silicon content, SS 41 A (0.003wt%), showed breakaway oxidation with high post-breakaway rate, while the other with high silicon content, SS 41 D (0.27wt%), showed very low post-breakaway rate. The onset of the breakaway in the material, SS 41 A, correlated well with the occurence of the lattice expansion. In contrast, slight contraction of the lattice in the material, SS 41 D, was observed. It is concluded that the change in lattice parameteres results from local compositional change in carbon and/or silicon at the oxide-metal interface. Metallographic observation suggests that the oxidation reaction proceeds at the inner side interface of the porous scale, and carbon deposition occurs due to the Boudouard reaction. AES analysis reveals that the carbon deposit in the breakaway scale is in a graphite state. Critical discussion is given to the previous model for the scale stress, which is based on the observed creep of mild steel foil. The obtained results indicate rather negative possibility of the strain measurement in predicting the onset of breakaway.

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Oxidation behaviors of Fe, Co, Ni, Cu, and four Ni-Cu alloys (81, 64, 42, and 21% Ni) have been studied in 10 Torr (exceptionally, 1×10^-3 Torr) of NO at the elevated temperatures from 580-780°C. The oxidation behaviors of such metals and alloys have been examined also in O₂ at the same pressures and temperatures for comparison. Various methods such as microgravimetry, X-ray diffractometry, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and Ion micro analysis were adopted for this investigation. The results obtained are summarized as follows: (1) The oxidation of Fe and Co in NO obeyed the cubic and the parabolic rate laws, respectively; while those of Ni and Cu obeyed the linear rate law. The oxidation characteristics (rate, rate law, or activation energy) of these metals in NO were much different from those in O₂. (2) Nickel was rapidly oxidized in NO accompanied with the expansion of samples which was caused by the preferential oxidation of grain boundary. (3) The oxidation of Cu in NO was rate-determined by the dissociation of NO on the oxide surface. (4) The oxide formed on Fe, Co, Ni, and Cu in NO were Fe₃O₄, CoO, NiO, and Cu₂O, respectively; while in O₂ were Fe₃O₄ and Fe₂O₃, CoO, NiO, and Cu₂O, respectively. (5) In the oxidation of Ni-Cu alloy system in NO, an accelerating oxidation was observed for Ni-rich alloys. The total amount of oxygen uptake of the Ni-rich alloy was greater than those of the Cu-rich alloys after the accelerating oxidation in NO occurred. (6) The selective oxidation characteristics for Ni-Cu alloy system during the oxidation in NO was also different from those in O₂. In the former case, preferential oxidation of Ni occurred, resulting in the formation of NiO-rich oxide layer on the alloy. For the Ni-rich alloy and copper was scarcely oxidized forming only a small amount of Cu₂O in the outer layer. As has generally been reported, the oxide film on Ni-Cu alloy formed in O₂ consisted of two distinct layers; i. e. the component of the outer layer was copper oxide (mainly CuO) and the inner was NiO.

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Although oxidation rate of metals and alloys is often accelerated by the presence of water vapor, no clear explanation is given of the acceleration mechanism. The object of this investigation is to evaluate the magnitude of increase in oxidation rate and to elucidate the acceleration mechanism by the presence of water vapor. Fe-Cr alloys containing 5-30%Cr were oxidized in a stream of O₂ and O₂-10vol% H₂O at 1023 and 1173K. Sulfur decoration method was applied to detect defects in the scale which allow gas penetration to the scale/alloy interface. A scale on alloys containing 5-15%Cr cracks sometimes to allow gas penetration to an alloy surface, developing an inner scale layer of a spinel type. In dry O₂, however, the cracks are filled with a newly formed oxide in them and gas penetration stops. On the other hand, gas penetration was observed throughout oxidation in wet O₂ and the mass gain was greater an order of magnitude than that in dry O₂. Fe-20%Cr alloy is fairly oxidation resistant in dry O₂. It is covered with a Cr rich scale mainly consisting of Cr₂O₃ and readily healed though the cracks are sometimes generated in the scale. In wet O₂ this alloy, though covered with a Cr rich scale at first, produced sooner or later an inner scale layer of a spinel type similarly to lower Cr alloys because of significant gas penetration. Fe-30Cr was also covered with a Cr rich oxide scale. This scale was maintained even in wet O₂ in spite of considerable gas penetration. These results lead to the conclusion that the accelerated oxidation in wet atmosphere is caused by oxidant gas penetration through defects in a scale to an alloy surface, and that 20%Cr is not sufficient for Fe-Cr alloy to be oxidation resistant in wet O₂. The dissociation mechanism is considered to have, if any, only minor significance in accelerated oxidation.

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It is widely known that the presence of water vapor in an oxidizing atmosphere changes the oxidation behaviour of stainless steels. In the present work, the influence of water vapor on the oxidation behaviour of SUS430 (17%Cr) steel and the effect of Si addition to 11%Cr steels were studied. Especially, an accelerated oxidation accompanied with the formation of reddishbrown scale (α-Fe₂O₃) was investigated in a temperature range of 500 to 700°C in which ferritic stainless steels usually suffer a slight attack.
Such scale was observed on SUS430 steel in a high H₂O atmosphere (in Ar+0.3%O₂+20%H₂O) and not in low H₂O atmospheres (in Ar+0.3%O₂+2%H₂O and in air). In the former atmosphere more Cr-rich initial scale is formed than in the latter. As a result, the substrate becomes more Cr-depleted, which permits the growth of Fe-rich scale after the breakdown of the initial Cr-rich scale. According to the oxidation study of various Fe-Cr binary alloys, more than 18%Cr is necessary to minimize the occurrence of such scale. Addition of more than 1% Si to 11%Cr ferritic steel improved its oxidation resistance in the high H₂O atmosphere. 11%Cr steel with 2%Si showed protective oxidation over the temperature range of 500 to 900°C and was insensitive to the concentration of water vapor. The addition of silicon promoted the formation of Cr-rich scale probably due to the oxygen gettering effect suggested by Wagner.

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Thermogravimetric measurement and oxide scale analysis have been done to study the oxidation characteristics of type 430 stainless steel. In atmospheres of low P_O2 with high content of water vapor, thick scale forms on specimens with commercially annealed and pickled surface (2D surface)at around 600°C. The appearance of the scale is Velvet-like and consists mainly of three layers: α-Fe₂O₃ whisker, Fe₃O₄ layer and FeO+(Fe, Cr)₃O₄ layer. The Velvet-like scale grows rapidly according to the parabolic rate law in the early stage of oxidation process. Deformation layer caused by surface grinding or atmosphere of high P_O2 prevents the formation of this scale by promoting the formation of protective (Fe, Cr) oxide. And also protective Cr₂O₃ type oxide forms at 800°C on the 2D surface of specimens.
During heating up to 800°C, nonprotective Fe oxide formed at around 600°C changes to protective (Fe, Cr, Mn) oxide. Therefore, temperature dependence of weight gain shows maximum at about 600°C and minimum at about 800°C in the atmospheres of low P_O2 with high content of water vapor, in contrast to the monotonic change in dry atmospheres.

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A new type of austenitic stainless steel has been developed containing 5pct Al and forming a protective Al₂O₃ film. This steel is free from oxide film spalling and exhibits a greater oxidation resistance than any other conventional austenitic stainless steel. Formation of the Al₂O₃ film requires the removal of the internal oxidation zone, caused by hot-working, prior to being subjected to high temperatures. The application of a special Al paint is also effective in eliminating the internal oxidation zone. Abnormal oxidation also takes place in this steel, but has less effect than in conventional ferritic Cr-Al steels. The oxidation behavior of this steel can be explained by the low diffusion rates of Al and N in the austenitic phase and also by the reduction in the oxidation rate by Ni.

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Creep rupture tests of molybdenum-free and molybdenum-bearing Fe-42 Ni-15 Cr alloys were carried out at 800°C in static air for specimens, with or without a synthetic ash mixture coating composed of 90%Na₂SO₄ plus 10%NaCl, and the effect of hot corrosion on creep rupture properties was investigated in comparison with the case of a nickel-base superalloy Inconel 751. In comparison with Inconel 751, a decrease in the rupture strength of both Fe-42 Ni-15 Cr alloys, caused by hot corrosion, was noticeably suppressed. In particular, the Fe-42 Ni-15 Cr-3 Mo alloy exhibited the highest rupture strength in a hot corrosive environment due to the presence of molybdenum as a solid-solution strengthener which has no detrimental effect on hot corrosion resistance. In air, the creep rupture in all specimens occurred as a result of the growth and coalescence of a large number of grain boundary cracks formed in the interior of the specimens. In a hot corrosive environment, on the other hand, the rupture behavior differed completely between Incone 751 and the Fe-42 Ni-15 Cr alloys. In the former, a premature fracture occurred in a brittle mode by only a few aggressive intergranular penetrations of sulfides which propagated rapidly under applied stress. In the latter, however, fairly ductile fracture took place based on essentially the same behavior as in air, in which many interior cracks resulting from grain boundary sliding were responsible for rupture, as well as surface cracks stimulated somewhat by intergranular penetration of sulfides. These results emphasize that the creep rupture properties of these alloys subjected to hot corrosion is chiefly controlled by the behavior of aggressive intergranular penetrations. From the metallographic observation of the aggressive intergranular penetrations, it is also suggested that the superior resistance of the Fe-42 Ni-15 Cr alloys to a rapid propagation of such a penetration should be attributed to not only their lower nickel content but also to the decreased tendency of these alloys to form continuous chromium-depletion due to the precipitation of chromium-rich carbides at grain boundaries.

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In order to predict the survice life of boiler supperheater tubes under high temperature corrosion and creep conditions, two models for describing the damage accumulation with crack and without crack respectively were costructed. Life time was calculated numerically for combinations of various corrosion conditions and creep conditions. Basing on the models proposed here, many situations notionally assumed were analysed. Results obtained suggested that the improvement of corrosion resistance was the most effective countermeasure for survice life improvement. Material degradation in creep resistance shorten a survice life although the strengthing in creep resistance does not effectively prolong the survice life. Finally, a possibility that synergestic reaction of corrosion and creep may considerably reduce the survice life is suggested from the creep test result performed under corrosive environment.

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U-520 and X-45 coated with CoCrAlY alloy by the ion plating method were investigated on there structure, adhesion and corrosion resistance, etc. The results obtained are summarized as follows: I) The structure of coated layer had no defect. The layer showed excellent adhesion against cyclic thermal shock [1100°C_??_Water (15°C)]. II) Cross sectional element distribution of the coated layers on U-520, X-45 were similar to each other, and Cr concentration was higher in the inner layer than in the outer layer and Al and Co concentrations were higher in the outer layer. This phenomenon seems to be resulted from difference of element's vapor pressure and the oxidation of Al. III) Oxide scale mainly composed of Al₂O₃ was formed on the CoCrAlY coated layer at elevated temperature. Consequently, the layer showed excellent corrosion resistance against V₂O₅-Na₂SO₄, NaCl-Na₂SO₄ synthetic ash corrosion test.

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For the purpose of improving the corrosion resistance of superalloys for gas turbine, the single coating of Al and Cr, and the composite coating of Cr-Al, Cr-Si and Al-Si by diffusion coating method were studied on Ni-base superalloy. In this paper, the coating conditions and the structure of coating layers are reported. The formation rate of Al coating layers was measured as a function of temperature, time and powder composition to find out the optimum coating condition. For the Cr coating, the uniform coating layers were obtained by using the Fe-free Cr powder, rather than Fe-containg powder. To achive the more improved corrosion resistance, the composite coating of each two elements of Cr, Al and Si were tried by means of diffusion coating and the combination of diffusion coating and ion plating. The most uniform and adherent coating layer was obtained by the combination of ion plating of Si and the subsequent diffusion coating of Cr in this study. From the results of EPMA and X-ray diffraction studies, it was found that the Cr rich, coating layer cosists of α-Cr phase and the Al rich coating layer is of NiAl intermetallic phase, each layer containing the additional elements from the alloy substrate and coating materials, respectively.

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The corrosion tests of the Ni-base superalloy with Al and Cr single coating, and Cr-Al, Cr-Si and Al-Si composite coating, the structure of which was reported in the previous paper, were carried out by cyclic oxidation at 1100°C and cyclic sulfidation at 900°C. The corrosion resistance of each coating specimen was evaluated by the measurement of weight change during cyclic corrosion test, the observation of the cross section by micro- and macrophotography and the analysis by EPMA. From the results of above measurements, the following corrosion resistance behavior was found. The Al rich coating layers of NiAl phase formed in Al and Cr-Al coating showed good resistance for oxidation test, although these layers showed poor resistance for the sulfidation test. On the other hand, the Cr rich coating layers of α-Cr phase formed in Cr and Cr-Si coating showed good resistance for sulfidation test. Among the coating specimens, the Si-Cr composite coating formed by combination of ion plating of Si and diffusion coating of Cr showed very good performance for severe sulfidation test. These Cr rich coating layers, however, showed not so good oxidation resistance at high temperature.

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Oxidation of iron has been studied in air at temperatures from 450 to 600°C by in situ measurements of Mössbauer absorption spectra combined with thermogravimetry and scanning electron microscopy. The oxidation proceeded in two succesive stages from α-Fe to Fe₃O₄ and to α-Fe₂O₃. The first stage of oxidation was well expressed by a first order rate law and the second stage by a parabolic rate law. The first stage of oxidation was attributed to a two-dimensional, heterogeneous growth of Fe₃O₄ on the iron surface and the second stage to a diffusion controlled process. The activation energies were 46.6 for the first stage and 48.4kcal/mol for the second stage. Thermogravimetric results were shown to follow the same rate laws obtained by Mössbauer spectroscopy. Observations by scanning electron microscope supported the interpretations. Mössbauer spectroscopy showed that the total iron contained in the detected constituents decreased with the increase in measuring temperature. This was determined to be due to the temperature dependence of the Mössbauer absorption intensity being higher for the oxidized Fe₃O₄ than for bulk Fe₃O₄. These observations are ascribed to the marked difference in the force constsnt, or the Debye temperature, between both kinds of Fe₃O₄

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In order to evaluate the survice life of supper heater tubes of oil fired boiler, a new high temperature corrosion test technique was developed where specimens were surrounded by double layer corrosives. Inner layer is 85% V₂O₅+10%Na₂SO₄+5%Fe₂O₃ (melted once at 900°C to form compounds with low melting point such as Na₂O·V₂O₄·5V₂O₅ and NaO·V₂O₄ and clushed to powder) corrosive and outer 10%V₂O₅+85%Na₂SO₄+5%Fe₂O₃ corrosive, simulating the situation commonly observed on the S/H tubes extracted from the actual boiler. This test environment reproduced local concentration of Ni and S at the interface between scale and metal as commonly observed on damaged tubes during survice operation. Corrosion resistance of some S/H tube materials are examined using this newly developed test technique. Furthermore, the effects of rare earth metal addition to 316HTB stainless steel named as 316-X on high temperature corrosion rate was examined by this technique. Results obtained showed that the 347 s. s. has higher resistance than 316 and 321 s. s. and also the addition of rare earth metal to 316 s. s. markedly improved the corrosion resistance. Some future problems which have to be solved for life prediction of boiler S/H tubes are discussed.

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