Aluminum coatings can greatly improve oxidation resistance of iron and steel and hot-corrosion resistance of superalloys. They are usually carried out industrially by pack cementation or hot dip process. In pack cementation, the articles to be coated are packed in a mixture of powdered ferro-aluminum and aluminum oxide and heated to 850-1000℃ for 5 to 24 hours. This process involves much labor and prolonged time. The hot dip process on the other hand, though simple and rapid, suffers from such drawbacks as surface roughness, dissolution of iron in aluminum and losses of aluminum through oxidation and evaporation. Fused salt electrolytic coating of aluminum is carried out as follows. Molten aluminum is connected as anode, and is covered by a molten salt mixture (1: 1 NaCl+KC1 with some cryolite) in which the work pieces are embedded. The work pieces are connected as cathode. Direct current is passed on, and Al passes into Al~(+++) ions in the fused salt, which upon discharge. deposit on the cathode, and diffusion takes place at the process temperature. The work pieces are not in contact with molten aluminum so that dissolution by the latter is out of the question. As molten aluminum is covered with salt, evaporation and oxidation losses are minimized. The surface is smooth and no post heat treatment is needed. The thickness of electrolytic Al coating increases with coating temperature, time and current density. The Al uptake depends, however, mainly on current density and time, and is independent of temperature. Faraday's law is obeyed, and the current efficiency amounts to about 85%. The structure of the coating as revealed by electron probe, X-ray diffraction and metallographic techniques shows the presence of a very thin film of pure Al at the outermost surface,and then the following phases in succession,namely: Al_3Fe Al_6Fe_2, AlFe and AlFe_3. Oxidation resistance of iron and steel is greatly enhanced with electrolytic Al coating, and hot-Corrosion resistance of Ni-base superalloys is also much improved by such coatings.

The dynamical processes of the nucleation and propagation of stress corrosion cracking(SCC) of four low alloy steels with a wide range of tensile strengths in aqueous media with various polarization conditions, in a K_2Cr_2O_7 inhibitor solution and in some organic solvent such as acetone and alcohol were traced with an optical microscope.Results show that if the tensile strength of the steel is higher than a critical value, which is different for different polarization conditions, and if K_1>K_(ISCC), the delayed plastic deformation occurs in all environments used, i.e.,the plastic zone in front of a loaded crack tip is enlarged with time. The nucleation and propagation of SCC will follow when this delayed plastic deformation develops to a critical condition. Neither anodic and cathodic polarization nor the addition of inhibitor can change the feature of the delayed plasticity and the nucleatioa and propagation of SCC in aqueous media. In all the environments used, K_(ISCC) increases and da/dt decreases with decreasing strength of the steel. Anodic polarization and the addition of the inhibitor make K_(ISCC) increase and da/dt decrease, while the effect of cathodic polarization is just the other way.

The rate of growth of silicide coatings on W, Nb and Ta in a fluidizedbed at 700-1050℃ increases with temperature as a simple function. The silicide coatings obtained in the fluidized-bed are similar to those prepared by other methods. However, Mo behaves differently as do its alloys such as Mo-Ti, Mo-Ti-C and Mo-Ti-Zr, etc. At relatively low temperatures, an abnormal peak of silicide growth rate appears, accompanied by a new phase unreported in the Mo-Si phase diagram. It has a hexagonal crystalline structure of C_(40) type, with lattice parameters a=4.580A,c=6.477A, and c/a=1.41. It is a metastable low temperature modification of the normal tetragonal MoSi_2.When treated in vacuum at high temperatures after siliciding, the hexagonal MoSi_2 converts rapidly and irreversibly into the tetragonal form. At abnormal peak temperature, the growth of the silicide coating on Mo-Ti obeys a linear law, and the silicide coating consists mainly of hexagonal MoSi_2. On the contrary, when pure Mo is silicided, the rate of growth obeys a linear law only at the very beginning, but then shifts approximately to a parabolic law. The silicide coating changes correspondingly from hexagonal to tetragonal MoSi_2. Both forms, however, have the same oxidation behavior at high temperatures.

The high temperature performance of the aluminium-silicon coated nickelbase superalloy K3 has been studied by static high-temperature oxidation test, hot-corrosion test and many physical analysis methods. The form of silicon distribution in the coating and its change during high-temperature exposure are revealed. The protective properties of the Al-Si coatings are evidently superior to aluminized coating and increase with silicon content for the following reasons. (a) The occurrence of "MC carbide notch" is diminished or prevented in Al-Si coatings. (b) The silicon-containing γ'-phases have a superior oxidation resistance. (c) A continuous Si-rich M(?)C "partition" formed at the interface between Al-Si coating and substrate after a short-time exposure at 1100℃ acts as a diffusion barrier.

The polarization and stress corrosion cracking behavior of 12 MnMoVNbTi steels in H_2S saturated solution was studied and comparison with hydrogen stress cracking behavior was made. The results show that the variation of Mo, Nb contents and difference in microstructure of steels have no important effect on corrosion rate but influence their stress corrosion cracking behavior considerably. The similarity between stress cracking behavior of these steels in H_2S solution and in hydrogen charging condition implies that the nature of sulfide stress cracking of steels is hydrogen embrittlement. The experimental results of stress relaxation and constant load tests in hydrogen charging condition prove that hydrogen has dual action of softening and hardening steel.It is found that the entrance of hydrogen induces plastic deformation of 12MnMoVNbTi steels below their yield stress level. The plastic deformation rate slows down with time and will not lead to fracture. From the relationship between hydrogen stress cracking and plastic deformation behavior, it is suggested that the dislocation movement in this deformation process would help the trasport of hydrogen to plastic zone and crack tip and promote hydrogen embrittlement.

The hot corrosion behavior of seven superalloys of Udimet 500 system with different Co amounts was studied by burner-rig test at 750℃,800℃,850℃ in an atmosphere containing 100 ppm salt. Total time of test was 62,5 hours. Experimental results showed that the corrosion resistance of these alloys were markedly improved by cobalt. The microstructures were studied with optical microscope, X-ray diffraction,electron probe and glow discharge spectro scopy analysis. It was observed that Co was beneficial to form a cotinuous protective oxide film on the surfaces of alloys with improved adhesion during the incubation period of hot corrosion. Moreover, Co in the superalloys can effectively retard the diffusion of sulphur in these alloys.

The hot corrosion-fatigue resistance of three heat-resisting steels, namely: 4CrlOSi2Mo, 18SR, 20-14 and two alloy coatings Co-50, Ni-81 are studied in this paper. For the sake of contrast, the "HCF" ( hot corrosionfatingue test) method was used under an automatic immediate comparative condition with the spraying of NaCl-Na_2SO_4 salt solution. The degree of corrosion failure is measured by depth of corrosion of samples. The hot corrosionfatigue failure process of various steels and the veasons for effective resisting hot corrosion-fatigue of two alloy coatings are investigated by means of metallography, X-ray diffraction and electron probe analysis techniques. In the "HCF" testing condition, it is found that:(1) The loose corrosion products on heat-resisting steels crack and flake away layer after layer. Sulphur and oxygen penetrate immediately into the interior of steels, with oxygen penetrateation deeper than that of sulphur. (2) The degree of hot corrosion-fatigue failure of the heat-resisting steels depends mainly on the type of their internal erosion zone. (3)A compact protective film is formed on the surface of alloy coatings, no matter it is Ni-base or Co-base alloy coatings. This test can be successfully applied to the study of coatings used for extending the service life of internal combustion engine exhaust valves.

The electrochemical behavior of several Cr-Mn-Nstainless steels in dilute sulphuric acid solution has been investigated. Aconsiderable increase in corrosion resistance is shown when these steels are alloyed with Cu and Mo. However, optimum efficiency for raising the corrosion resistance is achieved by alloying with Cu and Mo or Ni and Mo. By Comparison of anodic polarization curves, the effect of alloying elements on corrosion behavior of Cr-Mn-N stainless steels has been examined.