The experimental potential-pH diagrams for three groups of ferritic stainless steels(17Cr-xMo,25Cr-xMo and 30Cr-xMo,where x=0,2,4,6)in 1N C1~- solutions at 25℃ have been established through measuring the anodic polarization curves of these alloys in solutions with various pH values.The chromium contents in the alloys show significant effect on the initiation of pitting,while the effect of molybdenum contents on pitting breakdown potentials strongly depends on the chromium levels in the alloys.Small additions of molybdenum to Fe-Cr alloys result in remarkable shrinkage of the active corrosion region at lower pH and lower potentials,and thus exert significant inhibition effect on the propagation of localized comosion.Chromium also shows some influence on this active region,but this effect is much less signifi- cant than that of molybdenum.The synergic role of chromium and molybdenum at proper levels would be beneficial for the super-performance alloys to effect- ively resist the initiation and propagation of localized corrosion.

The process of crevice corrosion of 2Cr13 stainless steel freely exposed in 3.5% NaCl solution has been examined by means of mieroelectrode technique and ellipsometry.Experimental results show that there are three stages in crevice corrosion of stainless steels,the incubation period,the development period and the stablization period.The decrease of pH values and the increase of Cl~- concentration within a crevice can promote the dissolution of the passive film and accelerate crevice corrosion.In transition from the incubation period to the development period,there exists a critcal crevice solution(CCS),under the effect of which the alloys passive film breaks down.In the present work, experimental results indicated that the composition of the CCS was about 1.2 N C1~- with pH 3.5. In the incubation period,the passive film thickness within a crevice gradually decreases with time until the permanent breakdown of the passive film,Then, the development period of crevice corrosion ensues. Results also show that mieroelectrode technique and ellipsometry can be applied to in-situ measurement of the process in crevice corrosion of stainless steels.

The influence of 0.15 or 0.70wt% Ce addition on the oxidation of Fe-15Cr and Fe-30Cr alloys in air at the temperature range between 800 and 1300℃ up to 500hr was studied.The oxidation kinetics indicated that the oxidation rates of Fe-15Cr alloy at 1000℃ and 1100℃ and that of Fe-30Cr alloy at the tem peratures from 1000 to 1200℃ reduced remarkablely owing to the 0.7wt%Ce added.However,the effect of 0.15wt%Ce was much less. The morphology observation,X-ray diffraction analyses and EPMA were carried out for the oxide layers on test alloys.The studies showed that Cr_2O_3 films could be formed rapidly on the surfaces of all test alloys at an early stage of oxidation,and then the Fe-ion concentration in Cr_2O_3 remained somewhere about 1wt% for a long time.However,the 0.7wt%Ce addition decreased the growth rate of Cr_2O_3 layer slightly.The most obvious effect of Ce was to inhibit Cr_2O_3 layer degeneracy in which Cr_2O_3 was replaced gra- dually by rapidly growing Fe-rich oxide,and to delay the breakaway of oxide film. The concentration of oxygen diffused into alloy from Cr_2O_3 layer is reduced by a factor of 10~7 owing to internal oxidation of Ce.With the reduction of oxygen concentration in alloy,internal oxidation of Si in Fe-15Cr alloys which contained about 0.2wt%Si took place in the vicinity of the Cr_2O_3 layer/alloy interface,so that a SiO_2 interface layer was formed in the Fe-15Cr-0.7Ce alloy.

The hot corrosion behavior of four Ni-based alloys—Ni-16Cr,Ni-16Cr-2Nb, IN738 and 537—between 900～1000℃ were studied by the use of Na_2SO_4 deposit. The kinetics curves of hot corrosion were obtained by using a thermobalance. Corrosion products were examined with metallography,SEM,electronic microprobe,X-rays diffraction and ~(36)S tracer.It was found that Nb was enriched in surface oxide scale of Ni-16Cr-2Nb.Nb could decrease the O~(2-) ion activity of Na_2SO_4,melts by the formation of NaNbO_3,and then reaction between Cr_2O_3 scale and the melts was retarded.For multi-component alloys like IN738 and 537,Nb might promote acidic fluxing of the oxide scale.Nb also promoted continuous segregation of sulfur at grain boundaries of Ni-16Cr- 2Nb alloy and speed up the process of internal sulfidation.

ESCA and AES combined with electrochemical methods were used to study the chemical composition and depth profile of anodic passive film on 18-8 stainless steel in Cl~- containing solution.The effect of Cl~- on chemical composition of passive film was examined emphatically.The results show that the anodic passive film on stainless steel surface consists mainly of Cr(Ⅲ) and Fe(Ⅲ)(Ⅱ)oxides and hydroxides,and bound water is rich in surface of passive film.The extent of oxidation of metal in passive film decreases with mcreasing depth.Cl~- can incorporate in the outer layer of passive film. However.the Cl~- incorporation in passive film is not the only factor causing the breakdown of passivity ot stainless steel.

Model 4110(phase sensitive detector)and 4114(phase control unit)are integrated circuit modules(Evans Associates,Berkeley,CA,U.S.A.) which can be coupled to form a lock-in amplifier originally designed for use at narrow frequency ranges(±33% around the chosen central frequency f_0). We improved the modules and constructed the Model A lock-in amplifier useful for a wide band of frequencies(0.2 Hz to 40K Hz).The Model A lock-in amplifier has been applied to corrosion systems Fe/acid and proved satisfaetory for impedance spectra measurements of these systems.

An instrument for potential distribution determination has been made by combination of ordinary X-Y recorder,sweep apparatus,etc.The potential distribution curves for the welded specimens were determined.There are good agreements between measurements and field expousure results of corrosion at sea.

It is found that there is a Cr and Mo‘rich zone’in AZ-2aluminide coating. It is located at about 12μm under the surface.In the outlayer of the coating two different γ'-phases appear after diffusion treatment and after static oxidation. The former is retained in the matrix during the coating process,while the latter is formed by the resolution of β-NiAl during static oxidation. σ-Phase is found in the Cr and Mo‘rich zone’,and is similar in composi- tion to that found in the inner layer of the coating.

The corrosion rates of iron in 0.5～2N sulfamic acid of 25～70℃ are investigated by the conventional gravimetric method and by chemical analysis, Based on the experimental data,the several kinetic parameters of iron in sulfamic acid have been calculated,such as rate constant(K),activation energy(E),frequency factor(A)and another constant(B),with the empericat equation put forward by Mathur.Comparing them with the results, of rion in H_2SO_4, H_3PO_4,and HCl,it is found that energy barrier of corrosion reactions in all four acids is almost the same and so the effect of temperature on these reactions is similar.On the other hand,the values of K, B and A change markedly with type and concentration of acid.The corrosion rate of iron is influenced by the multiple effect of the magnitudes of various kinetic parameters.Corrosion rates of iron in sulfamic acid are smaller than those in H_2SO_4,but larger than those in H_3PO_4 or HCl.