An investigation on steady and transient passivation processes was carried out for Fe in H2SO4 and H2SO4 + HCl media. Results show that the passive film formed at the initial stage of passivation has a fractal dimension (Ds), and the Ds increases with passivation potential. Addition of Cl- into the H2SO4 solution can further increase the Ds and increase the rates of the formation and the active dissolution of the passive film as the Cl- changes the structure of the film.

A quantitative descriptive model was developed theoretically of the effects of overloading on corrosion fatigue crack growth (CFCG) of hydrogen embrittlement type. This is based on the comprehensive combination of the Willenborg' overloading residual stress field model with the following knowledge of the process of CFCG: crack tip surface deformation kinetics, the crack tip surface hydrogen evolution kinetics, and the hydrogen diffusion kinetics and thermodynamics in crack tip metal matriX interior. A series of criteria and equations were correspondingly deduced. Furthermore, this model has been used to predict the effects of overloading on CFCG. The results suggest that for ultra-high strength steel, if crack length, ad, grown during the interval of two overloading cycles, were smaller than the difference betwere the overloading plastic region size, rpOL,and the constant loading plastic region size, rpCLA,i. e. ad << rpOL-rpCLA,the retardation effect of overloading on CFCG would be more intensive in 3.5%NaCl solution than in air;on the contrary, if ap≥rpOL-rpCLA, the acceleration effect of overloading on CFCG would occur. It is note-worthy that the overloading acceleration effect does not occur for fatigue in air. The correctness and usefulness of these predicted results have been favourably proved by the macroscopic experimental laws and microscopic fractographic characteristics, the details of which will by presented in the next paper.

Experimental studies were conducted on the effects of single and periodic overloading oh corrosion fatigue crack growth (CFCG) in the two commonly used environments, air and 3.5% NaCl neutral solution,for GC-4 (40CrMnSiMoVA) steel. Test results show that the whole process of the effect of overloading on CFCG involves three phases in time sequence: the fast tearing, the retardation, and the recovery. For periodic overloading tests, if the crack length, ad, grown during the interval of two overloading cycles were far smaller than the difference between the overloading plastic region size, rpOL, and the constant plastic region size rpCLA, i.e. ad < References | Related Articles | Metrics

A steady corrosive wear was employed to study corrosive wear behavior of duplex stainless steel comparing to that for austenitic and ferritic stainless steels in 69%H3PO4. The microstructures of subsurface were examined by TEM and showed that the hardness of worn surface was largely enhanced due to the high density of dislocations induced by wear surface deformation, shich were blocked by precipitation perticles in austenitic phase with low content(6.4Wt%) of nickle. It indicates that surface deformation strengthening is an effective method to improve corrosive wear resistance of alloys.

An investigation on the transpassivation-repassivation of type 304 stainless steel in 0. 5 mol/ L H2SO4 was carried out, and a model concerned with electrochemical processes of the electrode was proposed With the aid of analyses of the steady process and the EIS of the electrode on the basis of the model, it is found that in the high potential range the transpassivation process is closely related with the electrochemical dissolution occurring on the interface between surface film and solution, and the repassivation is due to the sharp decrease in lower valence chides; while at the open circuit potential, the inverse electrochemical reaction plays an important role in maintaining the film formed in the high potential range, and makes the film more stable than normally passivated film.

The electrochemical behavior of 00Cr18Ni5Mo3Si2 (18-5) duplex stainless steel in chloride-containing alkaline solution at 90 ℃ was investigated. The results showed that general and pitting corrosion resistance of the 18-5 steel was superior to that of OCr18NigTi (18-8Ti) austenitic stainless steel. In alkaline solution,NH3 (free), CO32- and HCO3- acted as corrosion inhibitors for 18-5 steel, while S2- activated the steel. The surface films were analysed by EDAX, XPS and electron diffraction, The results obtained are summarized as follows. The surface films on both the steels were complex multi-Valence chide films, consisting mainly of CrOOH and Cr2O3. The Cr-enrichment in the film on the 18-5 steel was more than that on the 18-8Ti steel. The surface film on the 18-5 steel was more compact and steady. MoO3 in film effectively enhanced the pitting corrosion resistance of the 18-5 steel.

The present paper describes a study on corrosion mechanism of A12(SO4)3 evaporator tubes made of titanium by means of polarization curve measurements, weight loss tests, hot wall tabe testing and hydrogen embrittlement studies. Results show that the reasons of their failure include active dissolution and hydrogen embrittlement caused by the break down of passive film. The critical potential of hydrogen embrittlement of titanium in A12(SO4)3 is -270mV (SCE). The formation of titanium hydride was proved by XPS analysis.

The effeCts of overlap-implanting rare-earth erbium and chromuim ions with high-medium energy on stress corrosion cracking (SCC) have been studied for generator retaining ring steel 50Mn18Cr4. The results of comparstive SCC tests show that: (1) The time to SCC appearance if in the nitrate solution prepared according to QHJ-79 standard for implanted specimens is postponed averagely over 6 times, and the critical current densities required to cause and maintain passivation decrease by one order of magnitude. (2) In the medium for cathodally charging of hydrogen, the steel is insensitive to hydrogeninduced corrosion (HIC); the HIC resistance can be improved by Er++Cr+ implantation remarkably. The mechanism of SCC for the steel in QHJ-79 standard solution and of improvement of resistance to SCC and HIC by ion-implantation is discussed on the basis of the results of AES-PRO, RBS, XPS-PRO, EDAX, metallography and electrochemistry tests.

In this srudy, the accelerated weathering test consisting of cyclic immersion-wet-dry steps was applied to simulate the process of atmospheric corrosion. The solutions of NaCl, Na2SO4, NaNO3, NaCl+Na2SO4 and NaNO3+Na2SO4 were selected as immersion media. The corrosion behaviour of mild steel in these artificially polluted environments was examined. The relative corrosiveness of the pollutants was ranked in the order: SO42- >Cl- >NO3- with the NO3 acting as a passivating agent. The corrosion performance of the steel in mixed media was different from that in single medium. Every component played its own role in different period of corrosion process. Protective character and composition of rust layer on mild steel after the accelerated test were evaluated and analysed by electrochemical measurements and XRD.The corrosion mechanism involved is briefly discussed.

The surface morphology, structure and high temperature stability of multi-arc plated Ti-N film as a diffusion barrier on nickel-based alloy GH220 have been studied. A lot of palticales which are found on surface of the Ti-N film have great influence on the structure and properties of the magnetic-controlled sputtered MCrAIX coating overlaying the Ti-N film. X-ray diffraction analysis indicates that the Ti-N film consists of δ-TiN, ε-Ti2N, α-Ti, etc. The δ-TiN in the Ti-N film has a strongly preferred {111} orientation,　but the preference disappears after the film is diffusion-annealed. Experimental results of aging at 1000℃　demonstrate that the Ti-N film is of good high temperature stability, and the elemental Ti in Ti-N film does not inter-diffuse towards either GH220 alloy or MCrAIX coating. Thus it could be concluded that Ti-N film can be used as a diffusion barrier to prevent the alloying elements from inter-diffusion or slow down the rate of inter-diffusion of alloying elements between the substrate and MCrAIX coating.

The iron-bed alloy with high contents of chronium, nickle and molybdenum under certain conditions could generate intermetallics distributed homogeneously in the alloy and result in composites which are resistant to wear and corrosion as well. The corrosion resistance of the composites in a solution simulated to that in phosphoric acid process was evaluated with electrochemical polarization measurements. With the increasing volume fraction of intermetallics, the passivity of the composite materials degrades slightly. As a whole, however, the composites under study with various amount of intermetallics exhibit excellent corrosion resistance.