The results of research on hot corrosion mechanisms are reviewed ,with emphasis on the progress in studies on fluxing mechanism which is still widely accepted today. The progress includes Rapp-Goto criterion and the establishment of solubilities of metal oxides in fused Na_2SO_4. In the meanwhile, the limitation of fluxing mechanism is discussed on the basis of experimental evidences. A tentative proposal is brought forth in this paper that the development of hot corrosion follows electrochemical mechanism. A attempt is made to illustrate with examples hot corrosion processes in terms of electrochemical mechanism.

The susceptibility of 70/30 brass with various Sn contents in the range from 0.1% to 2% to stress corrosion cracking (SCC) in distilled water (applied potential of OV, SCE) and Mattsson solution(pH7. 2,open circuit condition) subjected to slow strain-rate test(SSRT)was investigated. The results showed that 70/30 brass containing 0.4％ Sn exhibited optimum resistance to SCC in distilled water, while in Mattsson solution the resistance to SCC increased with the increases of Sn content. The addition of Sn was in favour of the transition to transgranular cracking from intergranular cracking in the brass. The analysis of the test solution indicated that the primary effect of Sn upon the inhibition of the SCC of 70/30 brass was due to its effect upon the surpression of dezincification. It was unable to evaluate the susceptibility of the brass to SCC in Mattsson solution by the conventional dezincification coefficient as the inherent copper ions in the solution were deposited by reduction during the SCC process. In this case,a reasonable agreement was found between its zinc dissolution rate per unit area and the average crack propagation rate.

Thermodynamic study of the formation of low melting-point Na_2SO_4-Fe_2 (SO_4)_3 was made by equilibrating Fe_2O_3-Na_2SO_4 mixtures with SO_3 of different pressures in the temperature range of 943-1003K. The molar fractions of Fe_2(SO_4)_3 in the molten Na_2SO_4Fe_2(SO_4)_3 were determined by chemical analysis. With the data obtained, the free enthalpy change for the reaction Fe_2O_3 (s)+3SO_3 (g)=Fe_2(SO_4)_3 (1, dissolved in liquid Na_2SO_4)was calculated by means of regular solution approximation as well as Temkin mixing model. The calculated △G°is given as △G°=-116830+141.15T,cal/mol (943～1003K) This was used to estimate the critical partial pressures of SO_3 required for liquid sulfate formation, with the help of the Na_2SO_4-Fe_2(SO_4)_3 phase diagram. For lack of necessary thermodynamic data, the critical partial pressures of SO_3 required for the formation of molten K_2SO_4-Na_2SO_4-Fe_2(SO_4)_3 were determined experimentally.

The effect of CeCl_3 additon to bulk melt 75wt% Na_2SO_4+25wt% NaCl on the hot corrosion behavior of alloy HK40 at 800℃ was investigated by electrochemical methods. The free corrosion potential, the polarization resistance and the corrosion current density were measured and calculated. Potentionstatie polarization tests at EAg=812mV and at EAg=-588mV were carried out. The corrosion product mophology was examined. The results show that the addition of 5wt% CeC1_3 greatly decreased the corrosion rate, changed the morphology of corrosion products,and significantly improved hot corrosion resistance of alloy HK40,due to precipitation of cerium oxide on the surface as a promotor for the formation of protective Cr-rich oxide.

It has been observed that the Knoop microhardness value of single crystals of calcite can be reduced 2 to 8 percents and fine microerack or micro-region break would occur on the polished surfaces of samples after exposure to 0.1 to 0.4 MPa hydrogen gas for 2 hours. The dropping amplitude would increase and traces of microcracks expanding automatically and cleavage fractures occur on the polished surfaces due to extension of exposure time and growth of hydrogen pressure,and crystal parameters, a and c, would decrease slightly at the same time. Similar phenomena were observer for dolomite ,antigorite, LiF and SlO_2 single crystals. Reduction in strength and stress cracking, caused by hydrogen diffusion,imply that hydrogen embrittlement exists in nonmetals as in metals. The changing of crystal structure may provide a clue to the understanding of hydrogen embrittlement in nonmetals,but more detailed study about its causes and mechanism are needed.

There are two possible concepts on mechanism of inhibition of thin layer rust preventive oils (RPO). The first explanation is the chemisorption of inhibitors present in RPO on metallic substrate. Another is the lower permeability of water and oxygen through RPO. The main purpose of this work is to evaluate the contribution due to the second mechanism. For experimental investigation, the oil films were prepared by soaking filter paper with the oil under study. The permeability of the oil film to water was determined in terms of the weight loss of water from a container covered with oiled filter paper. Oxygen diffusion tests were conducted on the basis of the change of oxygen concentration in a chamber as a result of passage of oxygen through oil film measured by coulometric oxygen sensor. Two RPOs showing satisfactory rust prevention performance were tested. No difference is observed in water and oxygen permeation behaviour between the two RPOs and their uninhibited base oil. It means that the presence of thinhibitors in RPO has little effect on permeability. The data on water and oxygen permeation rate obtained also show that RPO films are so permeable to water and oxygen that they cannot inhibit corrosion by impeding diffusion of these two corrodents. It could be concluded that the inhibitive action of RPO is not dependent on the prevention of water and oxygen permeation.

The stress corrosion cracking (SCC) behavior of the AI-Li 8090 alloy sheet has been evaluated by using the slow strain rate technique(SSRT). The 8090 sheet was exposed to 3.5％NaCI solution under potentiostatic conditions and tested at different tensile strain rate. The most susceptible strain rate (1×10~(-6)·s~(-1)) and a critical potential (—800mV, SCE)have been observed. Aging conditions have great influence on the susceptibility to SCC, with underaged temper being the most susceptible, peak-aged temper being less susceptible and overaged temper being non-susceptible. By synthesizing the results and the fractographie characteristics, a conclusion can be drawn that anodic dissolution is generally the main mechanism of SCC. However hydrogen embrittlement may play a dominant role in the cracking mechanism, when hydrogen heavily permeats into the materials.

The inhibition effect of aluminium ion in combination with C_2O~(2-)_4 ion on corrosion of mild steel in water has been investigated. The state of Al~(3+)after adding C_2O_(2-)_4 ion has also been studied by Ion Chromatograph(IC)method. It was found that aluminium ion complexed with C_2O(2-)_4 ion,thus changed the condition and mechanism of film forming on the surface of mild steel,and thereby resulted in synergistic effect of the inhibitors. With a result,the inhibitive effect of aluminium ion considerably increased.

The experimental approach for carrying out electrochemical impedance measuremerits for coated metals using probe cells is described. Equivalent circuit for the dualprobe cell is given, and the experimental impedance diagrams are discussed in terms ofcoating and interface properties and their changes during exposure to corrosive environmerits. The systems studied include alkyd coating and electrophoretic paints with or without inhibitor added. It is found that the potential difference △E_se for dual-probe cell taken each time before making impedance measurement has good correlation with the deterioration degree of the coating measured hereafter.

A new method of measuring electrode/electrolyte interfacial capacitance has been introduced. By making use of the Laplace transformation and convolution theorem, it can be deduced from simple equivalent circuit of electrode/solution interface that there is a linear dependence of the product of peak response current(ira)flowing through the electrode, which is polarized by sine-wave voltage with various angular frequence but the same range,and angular frequence(ω)on the square of angular frequence: im·ω=A·Cd·ω~2+B The interfacial capacitance (Cd)of electrode/solution can be calculated from the derived linear equation. The effects of solution resistance and charge transfer resistance on the suitability of solution resistance and charge transfer resistance on the suitability of the method have also been discussed. The method has proven experimentally superior to those available in textbooks and literatures, and it is shown that the method is applicable to most actual electrochemical systems in spite of its inability for application to ideally nonpolarizable electrode.

An investigation of the effects of A. V. modulated passivation and post-treatment on the composition and stability of passive film on 302 and 316 s.s. was carried out by means of AES analysis,ellipsometry and electrochemical measurements. The experimental results showed that the concentration and profile distribution of elements in the passive film was changed by A. V. passivation, the relative concentrations of O and Cr and the Fe~(3+)/Fe~(2+) in the A. V. passivated film were higher than the corresponding ones in the D.V. passivated film. During the process of post-treatment molybdenun-containing ions were adsorbed onto the surface of passive film. The causes of increase of the stability of passive film by A.V. passivation and post-treatment were discussed.

The effects of temperature, chloride content and dissolved oxygen content on the susceptibility to stress corrosion cracking of type 304 stainless steel in high temperature water have been investigated. Both U bend and slow strain rate stress corrosion tests were performed,complemented by electrochemical polarization curve measurements and extensive oxide film analyses by Auger electron spectroscopy(AES). The U bend data showed decreasing susceptibility to cracking with increasing temperature, which correlated with thicker oxide films and greater enrichment of chromium in the films. In contrast,the slow strain rate tests indicated maximum susceptibility to cracking at 250℃ in the temperature range 200 to 300℃ and the susceptibility to cracking increased with increasing oxide thickness, The extent of chromium enhancements in the oxide layers was different for the two specimen types tested in equivalent environments. It is concluded that the best choice of test procedure for acquiring accelerated test data depnds upon the details of the application. U bend data will be more relevant for truly static load conditions that do not induce dynamic plastic strain, whereas slow strain rate data are more relevant for components that may experience plastic deformation during service.

If the product of Young's modulus of the oxide scale and its thickness can not be neglected as compared with that of the same two factors for substrate ,the stress released due to bending of specimen is high ,so the measured stress is different from the real stress in the oxide scale on non-bending substrate. And if the solubility of oxygen is high in substrate,the gradient of oxygen concentration will result in tensile stress in oxide scale. However, the direction of stress and that of bending are not identical. Under these conditions, the generally used stoney's formula for calculating the stress of oxide scale in the single-face oxidation induced bending test must be modified. The analysis is significant for further study of the oxide stress.

The corrosion inhibition and adsorption of biearboxylate on carbon steel in pH=9 carbonate-bicarbonate buffer solution at 25℃ has been investigated by weight loss method and electrochemical technique. Results show that the corrosion inhibition and adsorption of bicarboxylates decrease according to the following order: azelaic acid>sebacic acid>pimelic acid>suberic acid>adipic acid There is a zigzag relationship between the inhibition efficiency and the number of carbon atoms in bicarboxylate molecules, i. e. the bicarboxylie acid which contains odd number of carbon atoms is more efficient than the neighbouring acid whiates on carbon steel follows modified Bockris-Swinkels isotherm. The effects of the molecular structure of bicarboxylates on the corrosion inhibition and adsorption are also discussed.