A Quantified Study of the Resistance of Duplex Stainless Steels to HISC: Part 2 - Significance of the hydrogen permeation properties and severity of stress raisers, on hydrogen transport and cracking

The quantified microstructural analysis carried out on a wrought and a hot isostatically-pressed (HIP) UNS S31803 duplex stainless steel (DSS) in the Part 1 publication of this study 1, established the significance of the three-dimensional (3D) distribution and morphology/geometry of the ferrite and austenite phases on hydrogen transport through two DSS product forms. This paper is a follow-on to Part 1, and focuses on the role of the other two key, interrelated components of hydrogen-induced stress cracking (HISC): stress/strain, and hydrogen. For this purpose, experimental hydrogen permeation measurements, and environmental fracture toughness testing (i.e. J R-curve testing) using conventional and non-standard single-edge notched bend test specimens were used. These particularly enabled interpretation of the hydrogen permeation and transport test data, and evaluation of suitability of environmental fracture toughness test methods for the assessment of resistance to HISC in DSSs. The latter is discussed, both from laboratory and component integrity perspectives, in the context of the findings from the 3D microstructural characterisation of the two phases, the role of stress raisers and their severity, and hydrogen transport through the bulk and from the surface.

A Quantified Study of the Resistance of Duplex Stainless Steels to HISC: Part 1 - Significance of the three-dimensional phase distributions and morphological properties on hydrogen transport

This paper is associated with a larger programme of research, studying the resistance to hydrogen-induced stress cracking (HISC) of a wrought and a hot isostatically-pressed (HIP) UNS S31803 duplex stainless steel (DSS), with respect to both the independent and interactive effects of the three key components of HISC: microstructure, stress/strain, and hydrogen. In the first part presented here, several material properties such as the three-dimensional (3D) microstructure, distribution and morphology/geometry of the two phases, i.e. ferrite and austenite, and their significance on hydrogen transport have been determined quantitatively, using X-ray computed tomography (CT) microstructural data analysis and modelling. This provided a foundation for the study to compare resistance to HISC initiation and propagation of the two DSSs with differing microstructures, using hydrogen permeation measurements, environmental fracture toughness testing of single-edge notched bend test specimens, in the Part 2 paper of this study [1].

Improvement of corrosion resistance of maraging steel manufactured by selective laser melting through Intercritical heat-treatment

Existing studies suggest that martensite-to-austenite reversion can increase the overall mechanical strength of maraging steel. Their effect on corrosion properties, however, is unclear. Selective laser melted (SLM) specimens were tempered near austenite finish temperatures to investigate the electrochemical effect of reversed austenite. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS) were used to characterize their microstructure. To define and test pitting performance, potentiodynamic polarization and open-circuit potential were performed in a 3.5 wt. % NaCl solution. The reversed austenite precipitated mainly along the martensite lath boundaries during the Intercritical heat treatment at 720°C. The nucleation of reversed austenite is allowed by the local Ni enrichment caused by the dissolution of intermetallic particles. As a result, the tempered 720°C specimens reported a higher pitting potential, lowest corrosion current density, and lowest corrosion rate than the as-printed, aged, and homogenized specimens. No investigations have been performed to date that demonstrate the impact of austenite reversion on the corrosion susceptibility of SLM maraging steel. Other than being nobler, austenite is lighter than martensite due to reduced precipitant density, accounting for fewer galvanic cells and a lower corrosion rate.

Role of chromium in anomalous behaviour of the passive layer in Ni-Cr-Mo alloys in 1M HCl solution

This work seeks to understand the underlying mechanism involved in passivity of Ni-Cr-Mo alloys in a less concentrated HCl solution (1M) by systematically varying contents of Cr and Mo solutes in model Ni-Cr-Mo alloys. Corrosion behaviour was evaluated based on potentiodynamic polarisation tests carried out in conjunction with electrochemical impedance and x-ray photoelectron spectroscopies of passive films that formed on alloys during their exposure to the HCl solution. Results have shown that an increase in Mo alone is not sufficient to improve the corrosion resistance of the alloys at lower concentrations of HCl. Optimum concentrations of Cr and Mo solutes have been found to be in the vicinity of ~17 wt.% Cr and ~19 wt.% Mo for superior corrosion resistance of the alloys. This was attributed to the protection of the Cr2O3 layer as a consequence of the enrichment of Mo6+ ions in the passive film in 1M HCl solution.

Investigation on the corrosion resistance of laser cladding Fe-based alloy coating against molten zinc

In this paper, laser cladding technology was used to prepare a Fe-based coating on H13 steel substrate and its corrosion behavior in molten zinc was studied. The results show that laser-cladding Fe-based coating can effectively protect the substrate from the corrosion of molten zinc, which is mainly related to its microstructure. The typical microstructure of the coating is composed of α-(Fe, Cr) solid solution matrix and CrFeB eutectic phases continuously distribute around the matrix. When molten zinc contacts with the surface of the coating, it corrodes the α phase matrix preferentially and CrFeB eutectic phases with better corrosion resistance interweave with each other to form a three-dimensional skeletal structure, which can play the role of diffusion barrier and slow down the diffusion rate of liquid zinc. The corrosion by molten zinc leads to the formation of a transition layer and an outer corrosion layer above the coatings. With the prolongation of the corrosion time, a large number of micro cracks are generated inside the transition layer and fracture gradually occurs under the action of thermal stress. The partial spalling of the transition layer and the corrosion of α phase matrix occur at the same time, making the corrosion depth of the coating increase continuously. However, the dense corrosion layer above the coating and the dispersed boride fragments can still function as a barrier to the inward diffusion of molten zinc.

Sour Environmental Severity for HIC Susceptibility

The severity of sour environments has been determined in accordance with the European Federation of Corrosion 16 and NACE MR0175/ISO 15156-2:2015 standards for carbon and low-alloy steels, based on the experimental results of sulfide stress cracking (SSC). However, the severity map obtained from SSC test results cannot be applicable to the hydrogen-induced cracking (HIC) susceptibility. In this study, the hydrogen permeability and crack area ratio (CAR) of HIC under various pH and H2S partial pressures (pH2S) were measured to establish the link between the sour environmental severity and HIC susceptibility using grades X65 to X80 linepipe steels. In addition, the hydrogen concentration at the location of the HIC was calculated by the finite element analysis. The results showed that the sour environmental severity map obtained from hydrogen permeation tests changes with time, because the hydrogen permeability reached maximum values in the early stage and steady-state values in the later stage. Then, the HIC susceptibility did not correspond to the maximum permeability, but to the steady-state hydrogen permeability. In addition, the hydrogen content at the location of the HIC did not correspond to the maximum hydrogen permeability but corresponded to the steady-state hydrogen permeability, because HIC occurred in the center segregation part and the hydrogen atoms required a certain time to diffuse from the metal surface to the mid-thickness. These results suggest that the HIC susceptibility is dominated by the severity map obtained from the steady-state hydrogen permeability.

Effect of High-Current Pulsed Electron Beam Processing of Zr-1%Nb Alloy on Its Oxidation Kinetics at 1200C in Air and Steam

The paper reports the effect of high-current pulsed electron beam (HCPEB) processing of the Zr-1%Nb alloy, as one of the most widely used in water-cooled nuclear reactors, on the kinetics of its oxidation at 1200 °C in air and steam (these conditions are typical for potential loss-of-coolant accidents). It was shown that HCPEB processing caused a change in the surface morphology of the samples. In particular, craters with diameters of about 100 μm were found on the modified surfaces. They had initiated at an energy density of 5 J/cm2 and were characterized by relevant reliefs with microcracks. After HCPEB processing at 10 J/cm2, the craters were deeper with fractured surface layers. In addition, a pronounced surface relief corresponding to quenched martensitic microstructures was observed on the modified sample surfaces that had formed due to high heating and cooling rates. Due to sufficient degradation of the sample surfaces after HCPEB processing at 10 J/cm2, the kinetics of high-temperature oxidation was estimated only for the as-received samples and ones treated at 5 J/cm2. It was found that the as-received samples showed slightly greater weight gain levels in both air and steam environments, which fully correlated with the thickness ratio of the oxide, α-Zr(O) and prior-β layers. These phenomena and further research directions were discussed.

Effect of Accelerated Cooling on Linepipe Steel Mill Scale and Resulting Localized Corrosion Susceptibility

The structure and composition of mill scale on linepipe steel formed with and without accelerated cooling conditions (ACC) was investigated and correlated to localized corrosion susceptibility. The mill scale structure/composition was investigated using scanning electron microscopy equipped with X-ray energy dispersive spectroscopy and electron back scatter diffraction, as well as X-ray diffraction. Localized dissolution of the mill scale was investigated using electrochemical techniques including open circuit potential measurements, electrochemical impedance spectroscopy, and electrochemical noise measurements in a corrosive phase solution. The various surface analytical and electrochemical techniques indicated that the mill scale formed without ACC consists of a relatively crack-free, thick inner wüstite layer with a thinner magnetite outer layer. However, the mill scale formed with ACC comprised a magnetite layer containing islands of retained wüstite, with some evidence of magnetite/iron eutectoid formation and which exhibited a relatively high density of through-scale cracks. These cracks can provide direct paths that connect the corrosive solution to the steel substrate, leading to more rapid breakdown of the mill scale. Additionally, the cracks can form a crevice between the mill scale and the steel surface, providing sites for pit initiation and growth. Coefficient of thermal expansion mismatch thermal stress calculations indicate that a magnetite-based scale is more susceptible to cracking/spalling than a wüstite-based scale, resulting in the ACC plate being more susceptible to localized corrosion.

An Inhibitive Effect of Aeration on the Pitting Corrosion of Steels in Ethanolic Environments

This work is aimed at improving the understanding of the localized corrosion of carbon steel in ethanolic solutions. The role of ethanol dehydration, chloride, and oxygen level in the pitting behaviour of carbon steel in ethanolic environments in the presence of supporting electrolytes was investigated. Open Circuit Potential measurement, Cyclic Potentiodynamic Polarization and Potentiostatic testing were conducted on specimens exposed to ethanolic environments prepared from pure dehydrated ethanol to study the pitting behaviour of carbon steel. Corrosion and passivation potentials significantly reduce due to the change in the cathodic reaction and the decrease in passivation kinetics under de-aerated conditions. SEM and EDX examination indicated that no pitting corrosion is observed without chlorides, and chloride significantly destabilizes the surface film resulting in decreases of both corrosion potential and passivation potential. A decrease in the dissolved oxygen in the solution reduces but does not eliminate the pitting susceptibility. Iron oxide is identified as the significant corrosion product at different water and oxygen content. Therefore, ethanol aeration can be a proper method to increase pitting corrosion resistance in ethanolic solutions.

Electrochemical Corrosion Behavior of Heat Treated HVOF Coatings on ASTM SA213-T22 steel

Herein we report the electrochemical corrosion behavior of pre and post heat-treated composite coatings of NiCrMoFeCoAl-30%SiO2 and NiCrMoFeCoAl-30%Cr2O3 on ASTM SA213-T22 boiler tube steel by high velocity oxygen fuel (HVOF) spraying technique. The samples were subjected to hot molten salt (Na2SO4–60%V2O5) corrosion environment in a tubular furnace at 7000C under thermocyclic conditions. The microscopic, structural and electrochemical investigations of post-heat treated specimens reveal NiCrMoFeCoAl-30%Cr2O3 composite HVOF coating exhibits a superior corrosion resistance compared to NiCrMoFeCoAl-30%SiO2 composite coating and bare ASTM SA213-T22 steel boiler tube steel in neutral electrolyte. The room temperature potentiodynamic and impedance investigations of heat-treated samples suggest high interfacial charge transfer resistance for HVOF coatings over a wide anodic potential window. This could be ascribed to the protective nature of the chromium oxide containing coatings on high temperature treatment. AC impedance analysis reveals NiCrMoFeCoAl-30%Cr2O3 coating exhibits very high resistive behaviour with very high charge transfer resistance, in the order of 106 Ohm higher than the NiCrMoFeCoAl-30%SiO2 coating and uncoated ASTM SA213-T22 steel boiler tube steel. Furthermore, the high temperature induced formation of metal chromates/chromites along with the presence of Cr2O3 provides good resistance towards corrosion.