Role of alloyed copper on corrosion resistance of austenitic stainless steel in H2S–Cl− environment

Corrosion test, surface analysis and thermodynamic calculation were carried out in the H₂S–Cl⁻ environments to clarify the role of alloyed Cu on the corrosion resistance of austenitic alloys. The alloyed Cu improved pitting corrosion resistance in the H₂S–Cl⁻ environment. The surface film of Cu-containing alloy indicated double layer consists of copper sulfide and chromium oxide, and the copper sulfide was able to exist stably compared to iron sulfide and nickel sulfide. It is concluded that the copper sulfide would enhance the formation of chromium oxide film which improve the pitting corrosion resistance in the H₂S–Cl⁻ environment.     

High-temperature oxidation of duplex stainless steels S32101 and S32304 in air and simulated industrial reheating atmosphere

High-temperature oxidation of duplex stainless steels S32101 and S32304 was performed under three conditions: isothermal oxidation at 1050 °C in air and simulated industrial reheating atmosphere and in situ oxidation in air during the continuous heating to 1050 °C. Breakaway oxidation was found in S32101 but not in S32304, before which the weight gain per unit area was in parabolic relation to time. A triplex oxide layer of S32101 and a duplex-layer of S32304 oxidized for 120 min were observed, respectively. The initial oxidation phase was austenite for S32101 and ferrite for S32304 due to different manganese content.     

Discontinuous surface cracks during stress corrosion cracking of stainless steel single crystal

Single crystal 321 stainless steel stress corrosion cracking was studied in a 42 wt.% MgCl₂ solution. Cracks propagated macroscopically in the maximum tensile stress plane regardless of the notch orientation with respect to the applied tensile load direction. Some stress corrosion cracks nucleated discontinuously at the intersection of the two slip bands. Most cracks, however, were not related to the slip bands. Cleavage-like fracture was observed, and the river-markings exhibited microshear facets along the {1 1 1} plane. Interaction between the main crack and the discontinuous microcracks increased the calculated stress intensity factor by 17 times and promoted crack coalescence, resulting in mechanical fracture of the ligaments between the cracks.     

The influence of microstructure on the oxidation of duplex stainless steels in simulated propane combustion products at 1000 °C

A low nickel Type S32101 duplex stainless steel has been oxidised in simulated industrial reheating conditions. The surfaces have been studied using optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Observations show that local breakaway regions (LBRs) form on the austenitic regions whereas thinner oxides are observed on the ferritic regions of the substrate. The reason proposed for these differences is the formation of a continuous oxide layer on the ferrite region and a discontinuous layer on the austenitic region during the early stages of oxidation. The chemical composition of these LBRs have been shown to be oxide islands of iron and manganese and oxide craters of chromium rich oxides. The more protective regions consist of chromium and manganese rich oxides. A silica layer formed below the oxide which may be attributable to a slight enrichment of silicon in the ferritic regions or due to faster rates of diffusion in ferrite.     

Microstructural characteristics of the oxide scale formed on 304 stainless steel in oxygenated high temperature water

The microstructural characteristics of oxide scale formed on type 304 stainless steel in oxygenated high temperature water have been investigated. From outer to inner layer, the oxide scale consists of faceted spinel particles, irregularly shaped hematite particles and a compact layer of nano-sized spinels. Some outmost spinels formed on top of other particles are depleted in Cr, while the hematite particles tightly embedded into the inner layer contain more Cr in the inner than in the outer part. The inner nano-sized oxide grow inwards directly from the bottom of outer particles. The related oxidation mechanism is discussed.     

Quantitative analysis of Cr-depleted zone morphology in low carbon martensitic stainless steel using FE-(S)TEM

A new type of intergranular stress corrosion cracking (IGSCC) has been observed in low carbon martensitic stainless steel at heat affected zone (HAZ). Nano level microstructural analysis was carried out to investigate IGSCC factors. It was found that cracks propagate along prior austenite grain boundaries where a row of carbides had been formed. Cr-depleted zones at the grain boundaries were characterized by a STEM-EDX analysis and that morphology was obtained by deconvoluting this results. It is concluded that Cr-depleted zones only a few nanometers in width are enough to cause IGSCC at HAZ in this steel under certain circumstances.     

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.     

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.     

Application of the modified electrochemical potentiodynamic reactivation method to detect susceptibility to intergranular corrosion of a newly developed lean duplex stainless steel LDX2101

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.     

A study of intergranular corrosion of austenitic stainless steel by electrochemical potentiodynamic reactivation, electron back-scattering diffraction and cellular automaton

The impact of solution and sensitization treatments on the intergranular corrosion (IGC) of austenitic stainless steel (316) was studied by electrochemical potentiodynamic reactivation (EPR) test, and the results showed the degree of sensitization (DOS) decreased as solution treatment temperature and time went up, but it increased as sensitization temperature prolonged. Factors that affected IGC were investigated by field emission scanning electron microscope (FE-SEM) and electron back-scattering diffraction (EBSD). Furthermore, the precipitation evolution of Cr-rich carbides and the distribution of chromium concentration were simulated by cellular automaton (CA), clearly showing the effects of solution and sensitization treatments on IGC.     

Characterization of microstructure and local deformation in 316NG weld heat-affected zone and stress corrosion cracking in high temperature water

Microstructure and local deformation in 316NG weld heat-affected zones were measured by electron-back scattering diffraction and hardness measurements. With increasing the distance from the fusion line, kernel average misorientation decreases and the fraction of Σ3 boundaries increases. Stress corrosion cracking growth rates in high temperature water were measured at different locations in the heat-affected zones that correspond to different levels of strain-hardening represented by kernel average misorientation and hardness distribution. Intergranular cracking along random boundaries as well as extensive intergranular crack branching is observed in the heat-affected zone near the weld fusion line.     

Carbon steel corrosion in clay-rich environment

We investigated the carbon steel corrosion in carbon dioxide clay-rich environment to understand its behavior under geological conditions. The results show the formation of magnetite as the main corrosion product in the first step of the corrosion process, followed by the formation of different corrosion products with complex mixtures of iron-oxide, hydroxycarbonate, hydroxychloride and sulfide phases. These results strongly contrast with similar experiments conducted under H₂ atmosphere where the major corrosion products consisted of iron sulfides. It appears then important to consider all the geochemical parameters including gas composition to better study corrosion of steel buried in geological formations.     

Acoustic emission monitoring of slow strain rate tensile tests of 304L stainless steel in supercritical water environment

An experimental set-up has been developed to perform slow strain rate tensile (SSRT) tests on tubular 304L stainless steel (SS) specimens in supercritical water (SCW) environment (550 °C, 250 bar). The supercritical water is circulated inside an internally pressurized tubular specimen mounted into a universal mechanical test rig and heated by a single loop resistance heating furnace. The set-up enables in situ monitoring of acoustic emission and electrochemical potential during the SSRT test. The SCW environment is found to significantly influence the mechanical performance of the material as a result of corrosion processes. A correlation between acoustic emission response and change of electrochemical potential is revealed. The findings are compared with preliminary results of tests performed on bulk SS specimens under the same condition in a commercial autoclave. The advantages and potentialities of both set-ups are discussed.     

Stress corrosion cracking under low stress: Continuous or discontinuous cracks?

Two-dimensional and three-dimensional crack morphologies of stress corrosion cracking (SCC) were studied by serial-sectioning and synchrotron-based X-ray computed tomography. Discontinuous surface cracks were actually continuous inside the specimen, which matched typical river-like fractographs and finite element simulations. A low stress SCC model was created, where a main crack continuously grew along the main propagation direction (MPD) due to anodic dissolution; then, discontinuous secondary microcracks emanated from MPD, angularly extending to the two sides of MPD. Finally, some of the secondary microcracks reached the sample surface, resulting in the formation of discontinuous surface cracks.     

Stress corrosion cracking of uni-directionally cold worked 316NG stainless steel in simulated PWR primary water with various dissolved hydrogen concentrations

Stress corrosion cracking growth rate of uni-directionally cold-rolled 316L stainless steel was monitored in simulated PWR primary water with different dissolved hydrogen (DH) concentrations at 320 °C. Crack growth rate at a DH of 0.16 cm³ (STP) H₂/kg H₂O is close to that at 5 cm³ (STP) H₂/kg H₂O. Crack growth rate at 30 cm³ (STP) H₂/kg H₂O is about one fourth of that at 5 cm³ (STP) H₂/kg H₂O or two times of that at 50 cm³ (STP) H₂/kg H₂O. Electron back scattering diffraction results show typical intergranular SCC along high angle boundaries with high levels of deformation.