The oxidation behavior of the as-cast 925 superalloy were investigated in the temperature range of 1130~1190 ℃ by means of mass change measurement, X-ray diffractometer, scanning electron microscopy and energy dispersive X-ray spectroscopy. The results show that after homogenization treatment at 1160 ℃ for 20 h, the formed oxide-scale showed three-layered structure: an internal oxidation layer composed of Al₂O₃ and TiO₂, a continuous and dense middle layer of Cr₂O₃ and an outmost layer of the spinel oxidation of Cr and Fe. When the homogenization time increased, the layer of Cr₂O₃ became thinner and the spinel oxides of Cr and Fe begun to break down and the oxidation was aggravated. Taking the oxidation behavior of the alloy during homogenization into account, the appropriate homogenization of 925 superalloy should be conducted at 1160 ℃ for 20 h.

Stress corrosion crack growth rate (CGR) tests of nickel-based alloy Inconel 625, as a candidate material for 700 ℃ ultra-supercritical steam turbine, has been completed at 700~750 ℃ in environments of alternating air and water vapor, as well as in steam with 0~8000 μg/L dissolved oxygen. The applied load is constant stress with intensity factor (K) and the crack growth rate is detected online by measuring the direct current potential drop (DCPD). Results show that the CGR in water vapor is greater than in air and which increases with increasing temperature and dissolved oxygen content. The mechanisms concerning the influence of temperature, medium environment and dissolved oxygen content on stress corrosion cracking are discussed.

Ternary nitrate salts are widely used as thermal storage medium for solar thermal power generation system, but the corrosion damage of the heat storage pipe caused by the molten nitrate salts is significantly severe. In order to improve the corrosion resistance of the pipeline material, the corrosion behavior of 316 stainless steel in mixed molten nitrate-salts KNO₃-NaNO₂-NaNO₃ without and with addition of 1‰ (mass fraction) and 2‰Y₂O₃ was comparatively studied by means of electrochemical method and SEM/EDS. The results showed that the addition of Y₂O₃ can reduce the corrosion rate of stainless steel. The corrosion current density decreased from 9.47 mAcm^-2 to 7.13 and 3.73 mAcm^-2 respectively with the addition of 1‰ and 2‰Y₂O₃ and correspondingly, the transfer resistance of 316 stainless steel in mixed molten nitrate salts was enhanced. It can be concluded that the addition of trace of rare earth element is an effective way to improve the corrosion resistance of 316 stainless steel in the mixed molten nitrate salts.

The hot corrosion behavior of T91 steel in molten salts of 10%KCl+10%K₂SO₄+80%Na₂SO₄was studied at 600, 650 and 700 ℃ respectively. The microstructure and composition of the corrosion products were characterized by scanning electron microscope/energy dispersive spectroscope (SEM/EDS) and X-ray diffraction (XRD). Mass loss was observed at various temperatures during the tests, especially disastrous mass loss occurred at 650 and 700 ℃. The main corrosion product was identified as Fe₃O₄ after corrosion at 600 ℃. However, the corrosion products spalled off seriously during the hot corrosion at 650 and 700 ℃. The residual corrosion products were rich in S, which was the main reason for serious spallation of the corrosion products. Cl existed in the molten salts could react with Cr₂O₃ to form volatile CrO₂Cl₂, which accelerated the hot corrosion process.

An alternately multilayered coating Ti/TiAlN and a coating of single layer TiAlN were respectively deposited on Ti- alloy Ti-6Al-4V by arc ion plating. Corrosion resistance of the alloy Ti-6Al-4V with the two coatings covered with a solid NaCl film was comparatively investigated in flowing atmosphere of water vapor and oxygen at 600 ^oC. The results show that the coating of single layer TiAlN exhibits columnar structure, which may contains some intrinsic defects such as pinholes acting as short-circuit diffusion paths for the inwards migration of corrosive species. On the other hand, the direct connection of the above mentioned short-circuit diffusion paths located in any two nearest neighbour TiN layers may be disturbed due to the intercalation of Ti layers for the Ti/TiAlN multilayer coatings, thus being beneficial to the corrosion resistance of the coating. In fact, the coating of single layer TiN showed clearly characteristics of localized corrosion induced by NaCl in flowing atmosphere of H₂O and O₂, while the coating of multilayered Ti/TiAlN kept intact. In general, the corrosion resistance of the multilayered Ti/TiAlN coating was much better than that of the single layer TiAlN coating.

Double-ceramic-layered (DCL) TBCs of La₂Ce₂O₇(LC)/8YSZ were prepared by electron beam physical vapor deposition (EB-PVD) on Ni-base superalloy. The high temperature stability and corrosion-resistant property of TBCs was investigated in high temperature gas corrosion environment. The results show that no decomposition and phase transformation arises in the TBCs after 100 h exposure at 950 ℃ in the flowing air with aviation kerosene and artificial seawater. The TBCs display good high temperature gas corrosion-resistant property. A continuous and dense oxide scale forms on the bonding coat. No spallation of TBCs occurred after 100 h thermal cycling in the corrosion environment.

SrZrO₃ thermal barrier coating was prepared by solution precursor plasma spraying (SPPS). Corresponding to Taguchi method, the relevant processing parameters were optimized in terms of the deposition efficiency, microhardness, microstructure and phase stability of the prepared coatings. The phase constitutes, microstructure and microhardness of the coatings were characterized by XRD, SEM and a microhardness tester, respectively. The experimental results showed that, in the case that the spray distance, feedstock flow rate and atomization pressure are given, the optimized spray parameters were set as follows: arc current, 600 A; argon flow rate, 40 L/min; hydrogen flow rate, 10 L/min. The SrZrO₃ coating prepared with the optimized spray parameters had a single-pass coating thickness of 6.0 μm, porosity of 16.3%, and microhardness of 6.8 GPa. The results of phase stability analysis indicated that, the phase transition from t-ZrO₂ to m-ZrO₂ in the SrZrO₃ coating emerged gradually at 1450 ℃ with increasing time, while the SrZrO₃ phase did not change.

(Gd_0.7Sr_0.3)ZrO_3.35 coating was prepared on superalloy In718 by air plasma spray (APS). The corrosion behavior of (Gd_0.7Sr_0.3)ZrO_3.35 coated alloy beneath a deposit of 30 mg/cm² CMAS was examined in air at 1250 ℃for 1, 4, 8 and 12 h, respectively. While the reaction of the powder mixture of (Gd_0.7Sr_0.3)ZrO_3.35 and CMAS was also studied parallel. The phase constitutes and microstructures of the corrosion products were characterized by XRD and SEM, respectively. The results showed that the (Gd_0.7Sr_0.3)ZrO_3.35 coating had better resistance against CMAS corrosion. The corrosion products formed between CMAS and (Gd_0.7Sr_0.3)ZrO_3.35 coating during the corrosion process consists of apatite Ca₂Gd₈(SiO₄)₆O₂ and c-ZrO₂, which can effectively protect the (Gd_0.7Sr_0.3)ZrO_3.35 coating from further attack by CMAS.

SrZrO₃ powders were synthesized by solid-state reaction and then spray granulation. The SrZrO₃ coating on superalloy In718 was prepared by air plasma spray (APS). The corrosion behavior of the SrZrO₃ coated alloy beneath a thin deposit of CMAS (CaO-MgO-Al₂O₃-SiO₂) was examined in air at 1150 and 1250 ^oC for 1, 4 and 12 h respectively, while the reaction of powder mixture of SrZrO₃ and CMAS was investigated paralell. The corrosion products of SrZrO₃ powders and the microstructure of SrZrO₃ coating after corrosion were characterized by XRD and SEM, respectively. The reaction between the two powders of SrZrO₃ and CMAS did not occur at 1150 ^oC, whereas occurred at 1250 ^oC for 1 h, which resulted in the formation of corrosion products of ZrSiO₄, CaZrO₃, SrAl₂O₄ and t-ZrO₂, and then a new phase of m-ZrO₂ did additionally appear for 4 h corrosion. The corrosion product of t-ZrO₂ was formed on the SrZrO₃ coating surface after CMAS attack, and the phase transition from t-ZrO₂ to m-ZrO₂ occurred as the corrosion time increased, the formation of the corrosion products could suppress the further corrosion of the SrZrO₃ coating by CMAS.

Composite coatings of Fe40Al-50%Cr₃C₂ with the thickness of 30 μm were prepared by high velocity oxygen-fuel (HVOF) technology on 304 steel. The change of its microstructure by heat treatment at different temperatures was studied by means of XRD and SEM/EDS. The surface of the coating is very rough and the element distribution is inhomogeneous before the heat treatment. The composition of the coating is mainly composed of Fe, Al, intermetallics plus a little oxide of Fe and Al. A large amount of particles of Fe with big size are present in the outer layer, while a few particles of Al are present in the inner layer. The homogeneity of the coating has great change after vacuum annealing, though some holes still exist in the coating. With the increase of the annealing temperature from 700 ℃ to 900 ℃, the pure Fe and Al particles of big size disappear gradually due to their full diffusion and thereby formation of more intermetallics.

TiN coating was prepared on the surface of 304 stainless steel by plasma spraying technology. The microstructure and phase composition of the coating was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion performance of the 304 stainless steel without and with TiN coating was assessed by the potentiodynamic polarization and electrochemical impedance in the simulated proton exchange membrane fuel cell solutions containing 0.3 mol/L H₂SO₄ plus 2 mg/L HF. Moreover, electrical conductivity of them was also compared. The results indicated that TiN coating significantly increased the free corrosion potential of the steel and induced a decrease of the corresponding corrosion current density by one order of magnitude, which is attributed to the presence of the continuous and compact coating of about 20 μm in thickness. During the whole period of 360 h immersion, the open circuit potential of the coating was obviously higher than that of the bare substrate, suggesting the excellent stability of coating. The impedance of the coating decreased slightly with immersion time but still remained in high level, suggesting that the TiN coating could provide the effective protection for the substrate. The interface contact resistance of the TiN coating was about 50 mΩcm^-2 by an applied load of 138 Ncm^-2, obviously smaller than that of the bare 304 stainless steel, exhibiting better conductivity.

The static and cyclic oxidation behavior of Fe-65Cu-15Ni-5Al and Fe-45Cu-15Ni-5Al alloyshas been investigated in air at 900 ℃ respectively. In the case of static oxidation, the oxidation rate of Fe-65Cu-15Ni-5Al is much lower than that of Fe-45Cu-15Ni-5Al. The oxidation kinetics of Fe-65Cu-15Ni-5Al obeys parabolic law during the first oxidation stage, followed by a linear law, while that of Fe-45Cu-15Ni-5Al obeys parabolic law in the whole oxidation period. However, In the case of cyclic oxidation, Fe-65Cu-15Ni-5Al shows a much faster oxidation rate than Fe-45Cu-15Ni-5Al. For the above two cases, the formed scales are both composed of an outermost thick CuO layer, a middle layer is mainly composed of mixture of oxides of Fe, Ni and Al, together with their compounds. Internal oxidation only occurs for the Fe-65Cu-15Ni-5Al alloy statically oxidized. While in other cases, discontinuous inner layer of Al₂O₃ is present, which is connected with the internal oxidation region of Al. In the case of cyclic oxidation, the scale of inner Al₂O₃ layer of Cu-rich Fe-65Cu-15Ni-5Al alloy has undergone rupture easily due to the presence of high stress, resulting in high corrosion rate. On the contrary, in the case of static oxidation at 900 ℃ the critical Al content needed to form a protective outer Al₂O₃ layer in the α phase has been reached, which can fairly well explain its better oxidation resistant than Fe-45Cu-15Ni-5Al.

Oxidation tests of Cu and Q235 steel were conducted in air alone and a combustion atmosphere of kerosene-air mixture at 600, 700 and 800 ℃ respectively, the later aims to simulate the fire scene environment with the presence of fuel accelerant. Cracking and spallation of the formed oxide scales were observed by using optical microscopy. The results revealed that the existence of kerosene accelerated the oxidation of Cu and Q235, and changed the surface morphologies of the oxide scales. With the increasing temperature and time, serious spallation of the oxide scales was observed. Especially, due to the spallation of the oxide scale formed on Cu, the mass loss of Cu did occurr. Based on the features of the oxide scales formed on Cu and Q235, it is expected to offer complementary insight on determining the fire characteristics, such as exposure temperature, time period and whether liquid accelerant is involved.