Stress corrosion cracking of En 24 steel has been systematically investigated using constant displacement cantilever beam tests, slow strain rate tests,potentiodynamic polarization and etching technique. The experiments were performed in natural sea water at various applied potentials and a range of heat treatments was used to obtain hardness from 250 to 550 kgfmm~(-2). The results show that the trend of environment sensitive fracture of En 24 steel decreases with the decrease of strength level. There is a critical strength level, below which no SCC is observed. The stress corrosion susceptibility is dependent upon applied potential, being least at an applied potential of-900 mV(SCE). The experimental data, including crack growth rates, fractographs, physico-metallurgical and electrochemical parameters etc, suggest that the mechanism of SCC of En 24 steel is closely related to the applied potential. At potentials more negative than-900 mV, hydrogen induced cracking occurs. From-570 to-900 mV, environmentally assisted cracking results from an anodic dissolution mechanism while both mechanisms may be operative at potentials near to-900 mV.

The corrosion fatigue behaviour of 55SiMnMo steel in aqueous media were studied. The fatigue fracture was retarded by the medium in such a manner that the crack propagation was retarded with increasing oxygen concentration in the medium. The retardation was attributed to the early crack closure brought about by both the corrosion products wedging into the cracks and the rough fracture surfaces. The crack propagation rates (da/dN)in the aqueous medium were estimated in terms of the calculated oxygen concentrations at the crack tip.

The corrosion mechanism of Zn-Ni alloy coating has been investigated. The composition, distribution and chemical states of the alloy coating and corrosion products have been measured by means of X-ray diffraction, Auger Electro Spectroscopy, X-ray photoelectron spectroscopy and Ion Microanalysis. The Zn-Ni alloy plating containing about 13％(wt) Ni has the best corrosion resistance in salt spray test. The structure of the alloy with about 13％ Ni is γ phase and has the composition of NiZn_3 or Ni_5Zn_(21).The main corrosion product of 13％ Ni alloy deposit is ZnCl_2·4Zn(OH)_2 which can restrain the corrosion process.

The effects of chemistry of molten salts on hot-corrosion resistance of aluminized and chromized coatings on superalloys have been investigated. The results showed that in molten sulphate, chromized coating had good hot-corrosion resistance, but the resistance of aluminized coating was dependent upon the substrate alloy. The higher the chromium content in substrate, the longer the corrosion incubation of the aluminized coating. It was found that NaC1 severely decreased the hot-corrosion resistance of chromized coating but had little influence on aluminized coating In molten salts consisting mainly of NaCl the two coatings could not provide satisfactory protection against hot corrosion for Ni-base superalloys.

The modified metalliding cell from molten salt film consists of three parts namely, cathode, anode and salt film. The sample is cathode, the graphite powder or metal powder is anode, and the salt film is composed of Al_2O_3 powder, salt and an inorganic adhesion agent. They are all fixed in a stainless steel container. Metal!iding was then processed in a furnace. In this way the electrolytic cell can he made smaller, so that some disadvantageous aspects of ordinary metalliding from fused salt bath can be overcome. There are three types of aluminium impregation by the present technique: (a)the anode is not soluble, (b)the anode can supply aluminium, (e)electroless. The disproportionatio reaction: 3AlCl AlCl_3+2Al went after the electrode reaction supplies aluminium atoms. The film is a dispersal system composed of solid and liquid phases. The mass transport in the film depends on diffusion and ionic mobility. Cathodi polarization makes codeposition easier. Coatings of Al, Al-Cr, Al-Si, Al-Mn, Al-Ce and Al-Y have been attainedby this method.

Surface sensitive techniques, such as SEM and ESCA, were used for comparing and analyzing the metallurgical structure, surface morphology and composition of Al-Zn-In-Si alloy before and after discharging. ESCA combined with Ar~+ ion sputtering technique was employed for profile analysis of the composition of the oxide film which was formed at the surface of aluminum anode. The measurement results show: (1)The metallurgical structure of Al-Zn-In-Si alloy: There are three kinds of segregative phase which exist in Al-Zn solid solution matrix, One is rich in In. The other two are Al-Fe-Si and Al-Fe-In-Si precipitates which are situated in interdendritic region. (2) The surface film of Al alloy anode formed in the air, is mainly composed of aluminum and zinc oxide. A thin layer of indium was found between the oxide film and the matrix. (3)When discharging, the surface of aluminum anode was activated and dissolved. The so-formed pits expanded rapidly and joined together. The In-riched precipitates bear no relation to the initiation of pits. After long term discharglng,the surface was covered with thick aluminum oxide, upon which there were some deposits of metal zinc and indium, which seem to be responsible for keeping continuous activation and dissolution of alumminum alloy anode.

High temperature Pourbaix-type thermodynamic phase stability diagrams for Na-M-S-O systems are established. The solubility of metal oxides in fused Na_2SO_4 measured in the past few years are reviewed and explained in terms of thermodynamic phase stability diagrams. The activity coefficients of soluble metal species in fused Na_2SO_4 are also given. The fluxing mechanism of hot corrosion induced by the deposit of molten Na_2SO_4 are discussed in the light of the dissolution behavior of metal oxides in fused Na_2SO_4.

The application of conventional impressed current cathodic protection is restricted to metals which are exposed to aqueous environments. In order to protect cathodically metals against atmospheric corrosion, a special paint is formulated, which contains solid electrolyte sodium-β″(β)-alumina as electrolytically conducting component. The metal object to be protected is coated with the paint and sequentially covercd with auxiliary anode coating, which is an electronic conductor. The sandwich structure of the coating helps to carryout impressed current cathodic protection in atmospheric environments. The inner coating of high ionic conductivity serves as electrical path for protective current. A graphite reference electrode is installed to control the metal potential within a predetermined protective potential range. The additional advantage of the new arrangement is uniform potential distribution due to close proximity of cathodically protected surface to anode coating. The above considerations have been verified by laboratory testsThe practical effectiveness of the new technique has also been demonstrated by the significant suppression of atmospheric corrosion of an overhead pipe.

The localized a. c. impedance scanning technique can be used to detect in-situ distributions of local a. c. impedance on electrode surface and to study electrochemical behavior of the small area under investigation. A scanning probe which scanned over electrode surface was designed. Measuring procedure was not influenced by solution conductivity The impedante distributions of metal galvanic couple surface was measured. It was shown that, under cathodic protection, localized corrosion rate decreased to various extent. The optimum potential of cathodic protection could be determined.

Four-point-bending corrosion fatigue tests were used to obtain conditional thresholdst ress intensity range (ΔK_(th)) of 4340 steel in artificial seawater under different initial loading conaition. The morphology of crack propagation near threshold area was examined by scanning electron microscope Results showed that the AKth measured by decrease loading method was lower than that by increase loading method, In stage I of da/dN～ΔK1 curves, the dominant driving force of transgranular crack near the threshold is mechaniealo A slightly increased ΔK_(th) was observed as a consequence of the cathodic protection potential, It is very important to choose an initial. stress intensity factor range(ΔK_(th))in order to determine ΔK_(th) accurately.

When the corrosion initiation rate is low and the number of samples is not great enough, the reasonable corrosion initiation rate cannot be obtained by traditional calculation. According to some logic inferences, if the samples had been corroded in a shorter test time, they should also oorroded when tested for a longer period. On the other hand, if the sampies had not been corroded in a longer test time, they should also survive when tested for a shorter period. Then a method for calculating the corrosion initiation rate approximatelly can be developed. It not only gives the result which agrees better with that obtained by traditional calculation when the corrosion initiation rate is high and the number of samples is great, but also gives a raasonable result in case where the corrosion initiation rate is low and the number of samples is not great enough.

Impedance measurements in wide frequency range(65kHz-lmHz)using 1250 FRA have been applied to 3C, 16Mn and 09MnNb steels with and without oxide film, and to 09MnNb steel painted with epoxy resin in 3 NaCl solution at different potentials with a polarization range of-300 mV. The optimum cathodic protection potentials(E_p)of different steels were obtained. The efficiency of cathodic protection can be calculated from Faradaic impedance measured at E_(corr) and E_p under various conditions.

The scale morphology, constitution phases and elements distribution were examined by EPMA et on type N(Ni-14Cr-1.4Si/Ni-4.4Si)and K(Ni-10Cr-1 Co/Ni-2.5Si)thermocouple alloys after test in air at 1300℃ 483h. In comparing the results, it is obvious that type N alloy has higher oxidation resistance.