An ultrafine-grained surface layer with the average grain size of about 28 nm in the surface layer was fabricated on a pure Fe plate by the surface mechanical attrition treatment (SMAT). Lower temperature aluminizing treatments of the SMAT samples were investigated by scanning electron microscope and X-ray energy dispersive spectroscope. The electrochemical corrosion behavior of the aluminized SMAT sample was studied in 0.05 mol/L Na2SO4 + 0.05 mol/L H2SO4 solution, in comparison with the original SMAT and the coarse-grained sample. The results showed that SMAT had a negative effect on the corrosion resistance of Fe. An aluminized surface layer was formed on SMAT sample by aluminizing treatment at 400°C, which was much lower than that of the conventional aluminizing treatment. A successive lower temperature aluminizing process made the aluminized layer thicker and continuous. The SMAT sample treated by a successive lower temperature aluminizing had much higher corrosion resistance and exhibited passive behavior, which was due to the formation of a protective passive film.

Duplex stainless steel simultaneously comprises various phases, inclusions, precipitates and grain boundaries. It is full of academic and valuable significance for materials designing and processing optimization to ascertain the corrosion resistance of corresponding phases and various microstructures. However, up to date, efficient evaluation technique is still scare mainly due to difficulty in controlling of micropits. In this work, for the first time, optimized potentiostatic pulse technique (PPT) was established and applied to study the rule of initiation of pits of solution annealed and aged DSS in 1.0 mol/L NaCl solution. The results showed that the size of pits can be well-controlled by adjusting the condition of potential pulse. Combined with SEM/EDS system observation, it was found that for solution-annealed specimens, sulphide and oxide mixed inclusions were the preferential pit initiation sites, while for aged specimens, majority pits occurred at inclusions in area impoverished in Cr/Mo, which is vicinal to σ phase. The phenomena that growing pits is retarded by precipitation rich in Cr/Mo, i.e. σ phase, was testified and explained.

The present work aimed at defining optimal conditions using double loop electrochemical potentiokinetic reactivation (DL-EPR) method for evaluating intergranular corrosion (IGC) susceptibility of lean duplex stainless steel (LDX2101) aged at 700 °C between 3 min and 300 h. The results demonstrated that the modified DL-EPR measurement (solution of 33% H2SO4 + 0.1% HCl at 20 °C and scan rate of 2.5 mV/s) could successfully characterize the interactions between precipitation, chromium depletion and IGC of LDX2101 with high sensitivity and reproducibility. In addition, there was no indication of healing because the effect of formation of chromium-enriched precipitates was more dominative than that of redistribution of chromium in depleted zones.

The effect of annealing temperatures in the range 1000–1200 °C on the microstructure and corrosion behavior of 2101 lean duplex stainless steel was investigated. The results demonstrated that the volume fraction of austenite phase decreased with increasing annealing temperature. However, lower values for the pitting potential (Epit) and the critical pitting temperature (CPT) were obtained after annealing at higher temperatures. Pitting was initiated preferentially in the ferrite/austenite boundaries or inside the ferrite domains, indicating that the ferrite phase had inferior pitting corrosion resistance as compared to the austenite phase in as-received steel. Moreover, the PRE values of the ferrite phase fell with the annealing temperature, while the values for the austenite phase rose. No equal PRE values in the ferrite and austenite phase were found for this steel in the range of annealing temperatures. This was determined by evaluating the PRE values of both phases from EDS results and the Thermo-Calc database.

Effect of aging at 850 °C on pitting corrosion of UNS S31803 duplex stainless steel was examined in chloride solution by potentiostatic critical pitting temperature (CPT) measurements. The quantitative metallography coupled with X-ray diffraction technique was employed to follow the microstructure evolution. Moreover, the initiation and propagation of pitting corrosion had been imaged in relation to microstructure variations using scanning electron microscopy (SEM). The results demonstrated that the corrosion behavior is strongly dependent on the microstructure, namely the presence of sigma phase. A deterioration of pitting corrosion resistance is found after aging 4 min, resulting in a drop in CPT. In particular, the metastable current transients during CPT test can clearly reflect the initiation of pitting process. Pitting nucleates preferentially in the austenite phase for the solution-annealed specimen, while the initiation of pitting corrosion takes place around sigma phase, in the newly formed secondary austenite for the aged specimen.

The microstructure and properties of UNS S31803 duplex stainless steel had been investigated following isothermally aging in a temperature range of 450–1000 °C for 10 min, which simulates a fairly long time relative to a weld thermal cycle. The results showed that specimens aged between 600 and 950 °C are subjected to precipitation of secondary phases, such as Cr2N, σ and χ. And the most significant reduction in pitting corrosion resistance and impact energy occurs at 850 °C. Moreover, at aging temperatures below 600 °C, austenite is the preferred site for pitting initiation, which changes to precipitates modified ferrite phase at higher temperatures above 600 °C.

Lean duplex stainless steel 2101 (LDX2101) shows wide application potential due to its better corrosion performance and lower cost than traditional 304 austenite steel. This paper investigates the effects of thermal aging treatments at 700 °C for various aging times up to 100 h on the selective corrosion resistance of LDX2101 by two non-destructive electrochemical measurements: double-loop electrochemical potentiokinetic reactivation (DL-EPR) and electrochemical impedance spectroscopy (EIS). The evolution of microstructure was examined by optical microscopy, SEM microscopy and X-ray diffraction techniques (XRD). The results showed that the two applied electrochemical measurements agreed very well. Both methods were able to reveal the relationship between microstructure and selective corrosion resistance, which was related to the formation of chromium- and molybdenum-depleted zones around the precipitates, especially the σ phase, during aging. Nevertheless, more information could be obtained using EIS methods, including the interfacial charge transfer reaction and the corrosion product adsorption process. The results suggest that the susceptibility of the aged alloy to selective corrosion is presumably codetermined by the formation of chromium- and molybdenum-depleted areas, as well as by the replenishment of them, in these areas from the bulk during aging.

Under potentiostatic conditions on stainless steel electrodes in chloride solution, a novel electrochemical cell equipped with an ultrasonic transducer has been designed to evaluate the metastable and stable pitting corrosion behaviors. The advantage of the cell is that the rupture of pit cover can be controlled during the metastable or stable pitting growth periods, and in which the effect of the cover on pitting stability is affirmed. Some shallower stable pits, which were formed at higher anodic potential, were found to repassivate after ultrasonic cover rupture due to their shorter diffusional length. In terms of the relative susceptibility to localized corrosion, repassivation potential (Er) determined by the cyclic potentiodynamic polarization curve was found to describe more properties of the occluded pit cover than the others.

The oxidation behavior of Fe-16Cr steels in N2-12 vol pct H2O was studied at 850 °C. The oxide scale was compact and had excellent adhesion to the substrate; moreover, there were three layers of different compositions existing in the scale. To gain an insight into the transport mechanism, two-stage oxidation was carried out in N2-12 vol pct H216O and followed in N2-12 vol pct H218O gas mixtures. The oxygen isotope profiles in oxide scales were determined by secondary ion mass spectrometry. The results showed that oxidation in water vapor proceeded by outward chromium transport, especially, the oxidation involved inward transport of water molecules.

AbstractTwo new methods were developed for characterizing the kinetics of the complex and oxidation process of the thin films by using transmission spectrum and sheet resistance. Ag/TCNQ (7,7,8,8-tetracyanoquinodimethane) bilayer films and copper films were prepared on glass substrates by vacuum evaporation. The complexation kinetics of Ag/TCNQ bilayer thin films was studied via transmission spectrum. The oxidation kinetics of the copper thin films was studied via sheet resistance, which increased during the process. The results suggested that the two methods can be used to characterize the kinetics of the reaction process of thin films.