It is difficult to estimate all the electrochemical kinetic parameters of corrosion process if the cathodic process is under mixed activation and mass transport control. The best way to deal with this problem is to estimate the values of kinetic parameters by fitting the weak polarization curves measured on corroding metals. It is shown that the Guauss-Newton curve-fitting method can be applied for this purpose. A computing program in BASIC language is worked out and the problems encountered in compiling the program are discussed.

The impedance behaviour of aluminium supporting air-formed and anodie films has been examined during immersion in neut al molar Na_2CrO_4/Na_2Cr_2O_7 solution. High frequency data were largely associated with dielectric properties of the oxide films whilst low frequency data, below 1 Hz, gave information on faradaic processes proceeding at flaws. With air-formed films the faradaic processes decreased progressively to a constant rate consistant with the increase in potential, indicating the healing of surface flaws. The processes occurring on immersed anodic films were more complex. During the first 10 hours the initial film dissolution was accompanied by a decrease in potential with an apparent plugging of flaws. However, after about 20 hours, the anodic film thickness decreased sufficiently to allow the faradaic processes toinerease to, the final steady state value.

The structure and chemical composition of the oxide film on 316 and 321 stainless steels corroded in high temperature and high pressure water, containing 2000 ppm Cl~- and 8 ppm dissolved oxygen, was studied by conversion electron Mossbauer spectroscopy(CEMS), AES and XPS before and after SCC. A correlation between the structure arid chemical composition of the oxide film and SCC resistance was established. Experimental results show that the alloying element Mo reacted with corrosive medium to form Fe_2(MoO_4)_3 phase. Mo in 316 alloy affected the composition and growth rate of iron oxides, decreased the dissolution rate of Cr at transpassive region and inhibited Cl~- in its penetration through the oxide film, playing therefore a role to prolong the incubation period of SCC. Results obtained showed that Mo-bearing 316 alloy has better SCC resistance than 321 alloy with different SCC fractographs. The role of Mo on SCC resistance was also discussed based oa alloy thermody namics.

Severe corrosion failure of HP-40 superalloy tube bends in ethylene pyrolysis unit has been observed in a petrochemical plant nearby Shanghai. Preliminary survey indicates that the attack is probably associated with the sulfur content in gas oil. Scales formed on tube bends with various amount of sulfur contamination in gas oil are investigated by means of metallography, x-ray diffraction and electron microprobe analyses. Results indicate that scales formed in gas oil with various amount of sulfur contamination show distinguished difference in corrosion products. When sulfur content of gas oil reaches 0.06％ (wt), there is a large amount of pits covered by about 10mm thick stratified scale on the inside of bend. Every layer in the scale is a multiphase mixture consisting mainly of oxides of chromium or iron with sulfides of nickel. Besides, some metallic nickel as well as deposited carbon are found in the scale. Elementary sulfur in crystalling state exists in outer layer of scale. Enrichment of sulfur is found in the interface between scale and grain-boundary of alloy matrix. The alloy is attacked intergranularly and the corresponding corrosion products are sulfides and oxides of chromium. The nature of corrosion is high-temperature sulfidationoxidation. Corrosion by penetrated sulfur leads to localized breakdown of protective scale, promoting further oxidation of the substrate alloy, causing chromium depletion. The overall result is sulfidation and oxidation along grainboundaries. When sulfur content of gas oil reaches 0.02％ (wt), only a thin scale of oxide of chromium is formed on the inside of bend. For a better understanding of the corrosion processes in this study, thermodynamic stability of corrosion products and the role of sulfur in breakdown of protective oxide and chromium depletion are discussed.

An on-line measurement system of corrosion electrochemistry with a usual microcomputer was established. It consists of a computer controlled potentiostat. a series of softwares for electrochemical measurement and applied programs for the determinations of corrosion parameters. By the arrangement of computer program, generation of a successive step signal, electrochemical measurement, and the acquisition, analysis and calculation of data have been performed automatically. Calculation of metal corrosion rates from polarisation data were taken with the aid of the kinetic equation of corrosion system. By solving the residual equations through iteration, the corrosion rates and Tafel constants can be obtained. Examples of this method are shown and experimental technique is introduced. The reliability and the field of application are also analysed and discussed.

Five superalloys(GH-30, GH-128, GH-37. GH-49 and K-38)were subjected to screening tests in liquid lithium at temperatures of 800°, 850° and 900℃ respectively. The corrosion resistance was evaluated by gravimetric method and by measuring the depth of grain boundary penetration. The results show that the weight losses for all these five alloys increase with temperature, GH-49 and K-38 having the least loss and GH-30 having the biggest loss. For the γ′strengthered alloys (GH-37, GH-49 and K-38), it is found that lithium penetrates along the grain boundaries. For the solid solution strengthered alloys, on the other hand, nickel is preferentially leached into lithiumand Kirkendall effect is observed with voids in the base alloy, thus causing the formation of a loose and porous α-Cr(Mo. W) skeleton layer. In the γ′t strengthered alloys which are composed of (γ+γ′) phases, the grain boundary mainly consists of carbide enveloped by a γ or γ′ film. The penetration of lithium along the grain boundaries, results in the formation of compounds such as Li_2C_2 which are insoluble in Li. Preferential leaching of the γ or γ′ film took place, thereby forming deep corrosion channels around Li_2C_2. When such channels connect each other, the isolated particles would fall off, which accounts for the weight loss of such alloys.

The effect of a non-ionic surfactant on the corrosion behaviour of carbon steel and the interaction between nonionic surfactant and main inhibitors was investigated by measurement of weight loss and AC impedance in 1N HCl and 1N H_2SO_4 solutions. It has been shown that the nonionic surfactant has marked inhibiting effect on the corrosion of carbon steel. The same adsorption model is obtained both in IN HCl and 1N H_2SO_4 when concentration of thenonionic surfactant is not less than 0.001％. It seems that the surfactant is loosely adsorbed on carbon steel. At low concentrations of these main inhibitors, simultaneous adsorption of nonionic surfactant and main inhibitor occurs. At higher concentrations of the main inhibitors, there is a competetive adsorption effect between the main inhibitor and non-ionic surfactant.

In this paper both principle and technique of determinating δscc are presented. In accordance with the specifications of the National Standard Method of δ_c determination and in view of the characteristics of constant displacement testing for stress corrosion cracking, a resistance curve method with constant displacement specimens is used. Test results of this method are comparable with those of different methods obtained by other authors.

The effects of N~+ ion beam bombardment on the aqueous corrosion bebaviour of Fe/Al and Fe/Cr system,have been investigated by means of electrochemical and electron-optical techniques. Fe deposited with Al (800(?)) and Cr (400(?)) were bombarded by a N~+ ion beam with an energy of 150 KeV at the dose of 5x10~(6)N~+/cm~2. Anodic current Ic and Im on anodic polarization curves for both Fe/Al and Fe/Cr systems were decreased by one order of magnitude, it is mainly related with the chemical effect of N~+ ion implantation,and presumably associated with formation of a metastable phase of nitride as a barrier layer reducing the anodic dissolution.