Influence of Zn injection on characteristics of oxide film on 304 stainless steel in borated and lithiated high temperature water

The oxide film on 304 stainless steel exposed to the hydrothermal environments at 573.15 K up to 20 days without/with 10 ppb Zn injection has been investigated ex situ by X-ray photoelectron spectroscopy (XPS). ZnFe₂O₄ and ZnCr₂O₄ were found to be formed in the oxide film at the initial stage of immersion by substitution reaction between Zn²⁺ and Fe²⁺, and ZnCr₂O₄ became dominant after long-term immersion. The calculations of potential-pH diagrams, solubilities and crystallographic features of spinels have been done to evaluate the oxide film structure and the inhibition mechanism caused by Zn injection.     

Effect of alternately changing the dissolved Ni ion concentration on the oxidation of 304 stainless steel in oxygenated high temperature water

Characteristics of oxide films formed on 304 stainless steel under alternately changing Ni²⁺ concentrations in oxygenated high temperature water were examined. Oxides preformed under low Ni²⁺ concentration evolve from hematite to spinel after subsequent immersion under high Ni²⁺ concentration. Meanwhile, Ni content in the surface layer of oxide rises up while Fe content drops. Oxide films preformed under high Ni²⁺ concentration show little change in phase composition after subsequent immersion under low Ni²⁺ concentration. Fe contents in surface layer rise up while the change of Ni contents depends on the original phase composition and whether residual Ni²⁺ is present.     

Electrochemical characteristics of zirconium oxide treated Type 304 stainless steels of different surface oxide structures in high temperature water

Electrochemical polarization analyses were conducted to investigate the impact of different oxide structures on corrosion resistance of ZrO₂-treated Type 304 stainless steel specimens in high temperature water. All specimens were pre-oxidized in high temperature water containing either 300 ppb dissolved oxygen or 50 ppb dissolved hydrogen, followed by a hydrothermal deposition treatment with ZrO₂ nanoparticles. Experimental results revealed that the corrosion potentials and corrosion current densities on the ZrO₂-treated specimens were lower than those on the untreated ones, and the decreases in these parameters were more distinct on the ZrO₂-treated specimens with oxides developed under dissolved oxygen condition.     

Corrosion behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and their corrosion behavior was compared with that of 304SS in 5 wt.% HCl solution. As compared to 304SS, E_corr, E_pit and E_rp values had shifted to a more negative region in 304SS containing TiC, indicating faster corrosion rate by TiC addition. The addition of TiC particles to 304SS resulted in no rapid pit propagation but maintained a high corrosion rate in the whole immersion time investigated.     

Corrosion of 304 stainless steel exposed to nitric acid-chloride environments

In an effort to examine the combined effect of HNO₃, NaCl, and temperature on the general corrosion behavior of 304 stainless steel (SS), electrochemical studies were performed. The corrosion response of 304 SS was bifurcated: materials were either continuously passive following immersion or spontaneously passivated following a period of active dissolution. Active dissolution was autocatalytic, with the corrosion rate increasing exponentially with time and potential. The period of active corrosion terminated following spontaneous passivation, resulting in a corrosion rate decrease of up to five orders of magnitude. The length of the active corrosion period was strongly dependent on the solution volume-to-surface area ratio. This finding, coupled with other results, suggested that spontaneous passivation arises solely from solution chemistry as opposed to changes in surface oxide composition. Increasing NaCl concentrations promoted pitting, active dissolution upon initial immersion, a smaller potential range for passivity, longer active corrosion periods, larger active anodic charge densities preceding spontaneous passivation, and larger corrosion current and peak current densities. In contrast, intermediate HNO₃ concentrations promoted active dissolution, with continuous passivity noted at HNO₃ concentration extremes. During active corrosion, increased HNO₃ concentrations increased the anodic charge density, corrosion current density, and peak current density. The time required for spontaneous passivation was greatest at intermediate HNO₃ concentrations. Susceptibility to pitting was also greatest at intermediate HNO₃ concentrations: the pit initiation and repassivation potentials decreased with increasing HNO₃ concentration until the HNO₃ concentration exceeded a critical concentration beyond which susceptibility to pitting was entirely eliminated. Increasing solution temperature increased the susceptibility to both pitting and active dissolution.     

The high-temperature oxidation of bulk nanocrystalline 304 stainless steel in air

The high-temperature oxidation of bulk nanocrystalline 304 stainless steel (BN-SS304) and its conventional polycrystalline counterpart (CP-SS304) in air at 900 °C for 24 h were studied by thermogravimetric analysis, X-ray photoelectron spectroscopy and scanning electron microscope. We studied the valence electron configurations of BN-SS304, CP-SS304 and their oxide scales by ultra-violet photoelectron spectroscopy. The high-temperature oxidation resistance of BN-SS304 was enhanced in both initial and isothermal oxidation, which was attributed to its larger work function and more chemical stability, its more chemically stable and compact oxide scale, its weaker O₂ adsorption and diffusion, its weaker Cr and Mn atoms diffusions.     

Internal oxidation and carburisation of heat-resistant alloys

The commercial alloys Nicrofer-HT, Alloy 800 and Type 304 stainless steel have been exposed under thermal cycling conditions to CO-CO₂ gas mixtures at temperatures of 650-750 °C. Thermal cycling led to repeated scale spallation which accelerated chromium depletion from the alloy subsurface regions. Subsequent dissolution of carbon and oxygen into the alloys led to extensive internal precipitation of carbides and oxides. The large volume fractions of carbide and oxide left small quantities of iron-nickel-rich metal. The in situ oxidation of internal carbides in the stainless steel led to large volume expansions and the development of mechanical stress. This was increased during thermal cycling, leading to disintegration of the surface regions. Temperature and surface treatment were both found to be significant factors in the resistance of alloys to the CO-CO₂ atmosphere.     

Electrolytic pickling of the oxide layer on hot-rolled 304 stainless steel in sodium sulphate

Electrolytic pickling of hot-rolled 304 stainless steel in Na₂SO₄ electrolyte was investigated with electrochemical, weight loss and SEM-EDX measurements. Pickling took place upon both active and transpassive polarisations. Mechanism and kinetics of pickling during active and transpassive polarisations were unravelled. Metallic phase in oxide layer was dissolved during active polarisation while the oxide layer was significantly undercut. Chromium oxide in the oxide layer was oxidised to soluble anions during transpassive polarisation, while iron oxide and metallic phase could either be remained or removed. Pickling due to undercutting with active polarisation was highly pronounced.     

The mechanism of oxide film formation on Alloy 690 in oxygenated high temperature water

Oxide films formed on Alloy 690 exposed to 290 °C water containing 3 ppm O₂ were investigated. It was found that Cr rich oxides form initially through solid-state reactions. Ni–Fe spinels gradually develop on surface layer by precipitation with increasing immersion time. Initially formed Cr rich oxides react with outwards diffusing Ni and Fe to form small spinel particles which then vanish gradually. An inner layer develops from oxide/matrix interface through inward diffusion of oxidant. Cr is preferentially oxidized and tends to dissolve into solution. The resultant inner layer consists of predominant NiO which cannot serve as a protective barrier layer.     

Oxidation behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and exhibited the uniform distribution of TiC particles in the matrix. The oxidation behavior of 304SS-2TiC and 304SS-6TiC (all in weight percentage) was compared with that of 304SS at 850 °C in air for 96 h using thermogravimetry analysis. For 304SS, the rate of weight gain was very slow initially, but accelerated suddenly to a very high level, forming breakaway oxidation. The addition of TiC particles to 304SS resulted in no breakaway oxidation and maintained a low oxidation rate in the whole reaction time investigated. Examination of oxide scale morphology and cross-section analysis by scanning electron microscopy and optical microscopy showed a significant scale spallation and a deep oxide penetration in the case of 304SS, but a rather continuous, dense and adherent oxide layer formed on the surface of TiC particle-reinforced alloys. XRD analysis revealed the presence of Cr₂O₃ together with spinel-type oxides in the oxide scale. For TiC-containing alloys, fine TiO₂ was also found on the surface and the amount of this oxide increased with TiC addition. The TiC addition developed finer matrix structure before oxidation, which accelerates chromium diffusion. As a result, scale adherence was improved and oxidation resistance was increased.     

Photogenerated cathode protection properties of nano-sized TiO2/WO3 coating

Nano-sized TiO₂/WO₃ bilayer coatings were prepared on type 304 stainless steel substrate by sol-gel method. The performance of photo-electrochemical and photogenerated cathode protection of the coating was investigated by the electrochemical method. The results show that the bilayer coating with four TiO₂ layers and three WO₃ layers exhibits the highest photo-electrochemical efficiency and the best corrosion resistance property. Type 304 stainless steel with the coating can maintain cathode protection for 6 h in the dark after irradiation by UV illumination for 1 h. In addition, the mechanism of the photogenerated cathode protection for the bilayer coating was also explored.     

The oxidation behaviour of 304 stainless steel in oxygenated high temperature water

Characteristics of the oxide films formed on 304 stainless steel immersed in 290 °C oxygenated water for different duration time were examined. The results show that the oxide film is mainly composed of outer irregularly shaped α-(Fe, Cr)₂O₃ and minor spinel in the inner layer. The morphology and chemical composition of the oxide film evolve with increasing immersion time. The surface layer is first enriched in Cr and then enriched in Fe. It is proposed that the oxide nucleated by solid-state reactions with the selective dissolution of Fe and Ni, and grew up through precipitation of extraneous metallic ions from solution.     

XPS analysis of the passive film formed on austenitic stainless steel coated with conductive polymer

Improvement of the passivation behavior of Type 304 austenitic stainless steel (SS) by coating with conductive polymers (CPs), like polyaniline (PANI) and poly(o-phenylenediamine) (PoPD), followed by exposure in an acid solution has been demonstrated. The passive films formed on SSs (after peeling off the polymer layer) are compared with those formed during anodic polarization under the same exposure condition. The passive films beneath the CPs are thicker and less hydrated than those formed on uncoated stainless steel. The polymer layer enhances the enrichment of chromium and nickel in the entire passive oxide, forming a more protective film than that formed during anodic polarization. The elemental distribution within the passive film is different in the two modes of passivation. The type of the polymer influences on the composition of the passive film. The best passivation is obtained by PoPD, with the passive film resulting in significant resistance of the SS to pitting corrosion in the 3% NaCl solution. The oxide film of this steel is characterized, in its inner and outer layers, by the highest ratio of Cr(OH)₃/Cr₂O₃ and the lowest content of iron species.     

Effect of hydrochloric acid on pickling of hot-rolled 304 stainless steel in iron chloride-based electrolytes

Weight loss, corrosion potential and EIS measurements were performed to understand the role of HCl in the pickling of oxidised 304 stainless steel in iron chloride-based electrolytes. The surface finish was analysed with SEM-EDX. The oxidised 304 stainless steel is active on immersion, resulting in a low corrosion potential and a high weight loss. After certain duration the stainless steel either remains active or becomes passive depending on HCl content. At low HCl contents, an ongoing active-to-passive transition results in localised corrosion at pits, grain boundaries and honeycombed recesses. The corrosion potential becomes high and the weight loss is suppressed. The weight loss decreases in the initial stage and rises on extended pickling with adding HCl. Because of anodic brightening, the 304 stainless steel is always active as HCl is concentrated. In contrast with the material that is passivated, the charge transfer resistance is considerably low and the double layer capacitance is large during that brightening.     

Identification by photoelectrochemistry of oxide phases grown during the initial stages of thermal oxidation of AISI 441 ferritic stainless steel in air or in water vapour

Room temperature photoelectrochemistry was used to characterise oxide phases grown during the initial stages of oxidation of the ferritic stainless steel AISI441 at 650°C and 850°C in synthetic air or in water vapour. Grazing incidence X-ray diffraction and Raman spectroscopy were additionally used to discuss PEC results. Haematite Fe₂O₃ (∼2.0 eV), chromia Cr₂O₃ (3.0 and 3.5 eV) and their mutual solid solution (∼ 2.5 eV) were detected by their respective bandgap values determined from photocurrent vs. energy curves. The Cr/Fe ratio of the films increased with time/temperature and was higher in air-grown than in H₂O-grown oxides. Observation of photocurrent vs. potential curves indicated that chromia was N-type in all specimens, resulting from thermodynamic equilibrium with the metallic substrate and not with the gas phase.     

Determination of threshold stress intensity for chloride stress corrosion cracking of solution-annealed and sensitized austenitic stainless steel by circumferential notch tensile technique

This paper discusses a new approach to determination of threshold stress intensity factor for stress corrosion cracking (K_ISCC) of stainless steel in 42% MgCl₂ environment at 154 °C. K_ISCC of solution-annealed and sensitized AISI 304 stainless steel in chloride environment has been determined using circumferential notch tensile (CNT) technique. K_ISCC data generated using CNT technique have been compared with those generated using traditional techniques such as compact tension and double cantilever beam (reported in the literature). The results presented here validate the ability of CNT technique for determination of K_ISCC of sensitized as well as solution-annealed austenitic stainless steel. This paper also discusses the mechanistic aspects of the difference in fractographic features of the sensitized and solution-annealed stainless steel.     

Acoustic emission during pitting corrosion of 304 stainless steel

Acoustic emission (AE) during pitting corrosion of 304 stainless steel (304 SS) in H₂SO₄ solutions with different pH values and Cl⁻ concentrations was studied. Two types of AE signals are detected in all solutions. Each type of signals is characterized by AE parameters (rise time, counts number, duration and amplitude) and waveform carefully. It is believed that the hydrogen bubbles evolution inside the pits is the AE source.     

Photocurrent and capacitance investigations into the nature of the passive films on austenitic stainless steels

Photocurrent and capacitance measurements of semiconductor passive films formed on metals and alloys can be used to study the electronic properties and reveal indirect information about structure and composition. The current work used these techniques to investigate the electronic properties of the passive films formed on three austenitic stainless steels, types 304L, 316L and 254SMO, in borate. Evidence was found for the existence of a large number of localised mid bandgap states, consistent with amorphous oxides. However, the flat-band potentials of the austenitic stainless steel passive films were found to be independent of both composition and measuring frequency. The most credible explanation for the bandgap values determined from photocurrent measurements is that the passive films are formed as dual layers, iron oxide outer layer and chromium oxide inner layer. This model does not need to evoke the potential dependent bandgaps used by previous authors.     

Influence of nitridation on the oxidation of a 304 steel at 800 °C

This paper presents a study on the oxide growth on a AISI 304 chromia-forming alloy, in air at 800 °C. After the nitridation treatment was performed on the steel surface, a γ_N solid solution is detected. In this case, no nitride formation in the alloy surface could be observed. In situ X-ray diffraction has been used to follow the oxides evolution at testing temperature. At the beginning of the oxidation test, CrN is formed together with Fe₂O₃. Nevertheless, Cr₂O₃ quickly appears and leads to a protective oxide scale formation growing according to a parabolic rate law. During oxidation in situ X-ray diffraction also shows that Fe₂O₃ is transformed into FeCr₂O₄. Our results show that nitridation increases the high temperature oxidation resistance of 304 steels at 800 °C.     

Corrosion behaviour of AISI type 304L stainless steel in nitric acid media containing oxidizing species

The corrosion behaviour of AISI type 304L stainless steel (SS) in different concentration of 0.01 M, 1 M and 5 M HNO₃ in presence of oxidizing ions at different temperatures has been evaluated. The main objective of this study is to assess the corrosion resistance of type 304L SS in non-radioactive conditions encountered during storage of liquid nuclear waste. Electrochemical impedance spectroscopy (EIS) and laser Raman spectroscopy (LRS) has clearly brought out the deleterious effect of oxidizing species on the passive film leading to increased corrosion along with increase in HNO₃ concentration and higher temperature.