Niobium diffusion modified austenitic stainless steel bipolar plate for direct methanol fuel cell

An austenitic stainless steel with a niobium diffusion protective layer is evaluated for bipolar plate of direct methanol fuel cell (DMFC). Corrosion resistance of niobium diffusion modified AISI 304 stainless steel (niobized 304 SS) is investigated in simulated DMFC cathodic environment (0.5 M H₂SO₄ + 2 ppm HF + 0.1 M methanol solution at 50 °C) and anodic environment (0.5 M H₂SO₄ + 2 ppm HF + (1 M, 10 M and 20 M) methanol solution at 50 °C), respectively. Potentiodynamic, potentiostatic as well as electrochemical impedance spectroscopy tests show that, comparing with a bare 304 SS, the corrosion current density of niobized 304 SS is reduced greatly while the polarization resistance is raised in the simulated DMFC cathodic environment. Corrosion tests in the simulated anodic environment are applied to examine the effect of methanol on the corrosion behaviour of niobized 304 SS. It is interesting to find that the niobized 304 SS shows better corrosion resistance in the higher methanol concentration solutions.     

Fatigue crack growth behavior of JIS SCM440 steel near fatigue threshold in 9-MPa hydrogen gas environment

In this study, stress intensity factor range (ΔK) decreasing tests were conducted and the in-situ observations were used to investigate the fatigue crack growth behavior of JIS SCM440 steel near the fatigue threshold in a 9-MPa hydrogen gas environment. The fatigue crack growth rate reflected the threshold behavior of the material, although the crack propagation knee point immediately before the threshold stress intensity factor range (ΔK_th) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before ΔK_th. In contrast, the knee points in a helium gas environment and air were very distinct. Fractographic analysis further revealed the existence of intergranular facets, which were observed immediately before ΔK_th in the hydrogen gas environment. Conversely, no facet was observed immediately before ΔK_th in the helium gas environment and air. The formation of the facets was considered to be one of the causes of the uneven crack propagation immediately before ΔK_th in the hydrogen gas environment.     

Review of corrosion studies of metallic barrier in geological disposal conditions with respect to Belgian Supercontainer concept

Abstract

The Supercontainer design is the preferred option for the underground disposal of high level nuclear waste in Belgium. It consists of a carbon steel overpack surrounded by a thick concrete buffer. In this high alkaline environment and under normal conditions (without the ingress of aggressive species), the carbon steel overpack will be protected by a passive oxide film, which is believed to result in very low uniform corrosion rates. The backbone of the RD&D strategy, which aims to provide confidence that the integrity of the overpack will be maintained at least during the thermal phase, is based on demonstrating that each localised corrosion mechanism (e.g. pitting corrosion, crevice corrosion and stress corrosion cracking), other than uniform corrosion, cannot take place under the high pH conditions prevailing within the Supercontainer (the ‘exclusion principle’). This paper gives an overview of the status of the RD&D programme related to the anaerobic uniform corrosion of the carbon steel overpack. The outcome of the modelling efforts simulating the evolution of various parameters (temperature, pH, degree of saturation, corrosion potential and composition of aggressive species) that can potentially influence the corrosion processes, over geological timescales, is addressed.     

Time-dependent corrosion behavior of electroless Ni–P coating in H2S/Cl− environment

As a mature technology, electroless Ni–P alloy coating is widely applied in the protection of chemical equipment and pipelines owing to its excellent corrosion resistance, but its application and long-term service evaluation in the field of high-sulfur oil and gas are rare. Therefore, the time-dependent corrosion behavior of Ni–P coating, which was plated on the L360 steel surface, was investigated in a saturated H₂S medium by the method of surface analysis. The results indicate that Ni–P coating with a thickness of about 52.6 μm could significantly reduce the corrosion rate compared with uncoated pipeline steel. This is related to the structure of the dense, protective film on the surface. The uncoated pipeline steel suffered local corrosion during the immersion process, and then it developed into uniform corrosion with the formation of a large number of corrosion products. In comparison, Ni–P coatings corroded relatively mildly with only a thin corroded layer. However, during prolonged corrosion testing, the corrosive medium penetrated the coating/substrate interface at inherent defects, leading to severe local corrosion of the substrate.     

Characterization of oxides on FAC susceptible small-bore carbon steel piping of a power plant

Wall thinning, oxides phase(s) identification, and oxide morphology characterization of nine high-pressure turbine extraction pipes (as-found thickness for pipes A through F) received from Comanche Peak Nuclear Power Plant (CPNPP) located at Glen Rose, Texas, were studied in this investigation. Phase identification performed using Fourier Transform Infrared Spectrophotometry (FTIR) showed that maghemite (γ-Fe₂O₃), magnetite (Fe₃O₄), and trace amounts of Lepidocrocite (γ-FeOOH) were present in the oxide layer on the inside surface of these small-bore carbon steel pipes. Scallop formation and severe pitting were observed. The scallop pattern was observed utilizing an Environmental Scanning Electron Microscope (ESEM) and is attributed to removal of the protective film in a Flow-Accelerated Corrosion (FAC) process. The most severe wall thinning was observed in the vicinity of welds.     

Evaluation of Nb- or Mo-alloyed ferritic stainless steel as SOFC interconnect by using button cells

The Fe–22Cr–0.5Mn ferritic stainless steel alloyed with Nb or with Mo is evaluated in the button cell configuration at 750 °C in terms of degradation in ohmic resistance and cathodic polarization resistance. STS444 and Crofer22 APU are also evaluated for comparison. Each polarization element is separated by equivalent circuit analysis on the electrochemical impedance spectroscopy data. Cr deposition on the button cell cathode is also analyzed both qualitatively by transmission electron microscope and quantitatively by inductively coupled plasma. The Nb- or Mo-alloyed ferritic stainless steel shows comparable performance with Crofer22 APU in terms of the increase rate in ohmic resistance and Cr evaporation rate, even without the addition of reactive element such as La. When the same amount of Cr is deposited on the cathode, the cathode performance deteriorates more at the high Cr evaporation rate than at the low Cr evaporation rate.     

Corrosion resistance of stainless steel bipolar plates in a PEFC environment: A comprehensive study

Bipolar plate is a key component of Polymer Electrolyte Fuel Cell (PEFC) with several essential functions, and has to resist to harsh cell operating conditions. Usually made of graphite or carbon composite, this component is studied worldwide in order to develop a commercially viable alternative: different ways have been being investigated, and to date, stainless steel (SS) appears as a good candidate material, but corrosion issues hinder its general use. This paper offers a comprehensive study of parameters impacting SS plate material corrosion through ex situ electrochemical investigations. Impact of ageing under various conditions is reported and studied in terms of film structure, semiconductivity behaviour, and cation release, on as received 316L and 904L alloys, and also surface-treated by low-cost surface modifications.     

Carbon-coated stainless steel as PEFC bipolar plate material

Stainless steel is quite attractive as bipolar plate material for polymer electrolyte fuel cells (PEFCs). Passive film on stainless steel protects the bulk of it from corrosion. However, passive film is composed of mixed metal oxides and causes a decrease in the interfacial contact resistance (ICR) between the bipolar plate and gas diffusion layer. Low ICR and high corrosion resistance are both required. In order to impart low ICR to stainless steel (SUS304), carbon-coating was prepared by using plasma-assisted chemical vapor deposition. Carbon-coated SUS304 was characterized by Raman spectroscopy and atomic force microscopy. Anodic polarization behavior under PEFC operating conditions (H₂SO₄ solution bubbled with H₂ (anode)/O₂ (cathode) containing 2 ppm HF at 80 °C) was examined. Based on the results of the ICR evaluated before and after anodic polarization, the potential for using carbon-coated SUS304 as bipolar plate material for PEFC was discussed.     

Improved corrosion resistance and interfacial contact resistance of 316L stainless-steel for proton exchange membrane fuel cell bipolar plates by chromizing surface treatment

The electrochemical performance and electrical contact resistance of chromized 316 stainless-steel (SS) are investigated under simulated operating condition in a proton-exchange membrane fuel cell (PEMFC). The corrosion resistance of the chromized stainless steel is assessed by potentiodynamic and potentiostatic tests and the interfacial contact resistance (ICR) is examined by measuring the electrical contact resistance as a function of the compaction force. The results show that the chromizing surface treatment improves the corrosion resistance of the stainless steel due to the high-chromium concentration in the diffuse coating layer. On the other hand, the excess Chromium content on the surface increases the contact resistance of the steel plate to a level that is excessively high for commercial applications. This study examines the root cause of the high-contact resistance after chromizing and reports the optimum process to improve the corrosion resistance without sacrificing the ICR by obtaining a chrome carbide on the outer layer.     

Can electrochemically active biofilm protect stainless steel used as electrodes in bioelectrochemical systems in a similar way as galvanic corrosion protection?

Stainless steel (SS) has been reported as a suitable electrode material for the growth of electrochemically active biofilm whether it is for a microbial fuel cell (MFC) or microbial electrolysis cell (MEC) in the bioelectrochemical system. Although the flame oxidation technique could improve SS property as electrodes, it comes with an increased risk of corrosion. The undesirable corrosion may cause the release of a toxic element such as chromium. At present, mitigation actions have been identified such as connecting iron to a sacrificial metal in a mechanism known as galvanic corrosion protection (GCP). An external power source could be used as an alternative to supply current similar to the sacrificial metal, which technique applied in MEC. In this review, the electron flow mechanisms between microbiologically influenced corrosion (MIC) and MEC biocathode will be addressed. Thus, it proposes a hypothesis of SS protection from corrosion in a similar way as in the GCP.     

Microstructure and properties of Nb-bearing high-strength low-alloy surfacing layers

Based on Q345 steel, high-strength low-alloy surfacing layers with Nb content ranging from 0.0041 to 0.26?wt-% were prepared by adding Nb element into electrode coating and manual electric arc welding, the microstructure as well as mechanical properties of surfacing layers were evaluated and analysed. The experimental results show that with the increasing of Nb content, the grain size becomes smaller meanwhile the microstructure distribution is more uniform, the size of hard phase martensite/austenite gradually decreases and the NbC precipitates increases. The yield and tensile strength greatly increase for the fine-grain strengthening of Nb and the precipitation strengthening of NbC, moreover, the impact toughness is significantly improved due to the microstructure variation and obvious grain refinement.     

Effects of niobium and titanium addition and surface treatment on electrical conductivity of 316 stainless steel as bipolar plates for proton-exchange membrane fuel cells

Niobium and titanium are added to 316 stainless steel, and then heat treatment and surface treatment are performed on the 316 stainless steel and the Nb- and Ti-added alloys. All samples exhibit enhanced electrical conductivity after surface treatment but have low electrical conductivity before surface treatment due to the existence of non-conductive passive films on the alloy surfaces. In particular, the Nb- and Ti-added alloys experience a remarkable enhancement of electrical conductivity and cell performance compared with the original 316 stainless steel. Surface characterization reveals the presence of small carbide particles on the alloy surface after treatment, whereas the untreated alloys have a flat surface structure. Cr₂₃C₆ forms on the 316 stainless steel, and NbC and TiC forms on the Nb- and Ti-added alloys, respectively. The enhanced electrical conductivity after surface treatment is attributed to the formation of these carbide particles, which possibly act as electro-conductive channels through the passive film. Furthermore, NbC and TiC are considered to be more effective carbides than Cr₂₃C₆ as electro-conductive channels for stainless steel.     

Novel high stable electrocatalyst based on non-stoichiometric nanocrystalline niobium carbide toward effective hydrogen evolution

In this work, we have developed a method for the synthesis of non-stoichiometric nanocrystalline niobium carbide (NbC_y) using special heat treatment of niobium citrate in a vacuum. The powder synthesized was investigated by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), and low-temperature nitrogen sorption-desorption technique. The PXRD results showed that the synthesized niobium carbide nanocrystals had a cubic structure (space group Fm-3m), isometric morphology, and average crystallite size of about 12 nm. The Rietveld method was used to refine the unit cell parameters: a = b = c = 446.8 pm; R_wp = 5.48%. The specific surface area about 212 m²/g (BET) and the porosity about 0.02 cm³/g (BJH) of the sample were determined by adsorption-structural analysis; it was found that niobium carbide had a weakly pronounced microporous structure associated with the presence of interparticle porosity, which was also confirmed by the HRSEM results. The catalytic activity of non-stoichiometric niobium carbide in the process of electrolytic reforming of an aqueous ethanol solution was analyzed. The electrocatalyst has a low hydrogen overpotential value (?245 mV), a Taffel slope (90 mV/dec), and high operational stability: the absolute value of the overvoltage increases by 21 mV after 500 voltammetry cycles, and the current density decreases by 5% after 20 h of chronoamperometry. The results obtained make it possible to consider non-stoichiometric niobium carbide as a promising electrode base for electrocatalytic production of hydrogen from renewable aqueous-alcoholic solutions.     

Electrochromism of sol–gel derived niobium oxide films

Niobium oxide films are promising cathodic electrochromics that in many aspects can compete with the more frequently studied WO₃ films. The films reported herein were prepared using the sol–gel route from a NbCl₅ precursor. The electrochromic properties were pronounced for crystalline films heat-treated at 500°C and exhibited transmittance changes between coloured and bleached states of 60% in the ultraviolet (UV) and 80% in the visible (VIS) and near-infrared (NIR) regions. Improved bleaching and more reversible electrochromism of thick niobium oxide films (thickness (d)>250 nm) were obtained by lithiation.Electrochromic (EC) devices were also prepared by assembling niobium oxide and lithiated niobium oxide films of different thicknesses with a hybrid inorganic/organic Li⁺ ionic conductor (organically modified electrolyte-ormolyte) and a molybdenum and antimony doped tin oxide (SnO₂ : Sb(7%) : Mo(10%) counter electrode films. The EC devices exhibited adequate colouring/bleaching kinetics (<2 min) and colouring/bleaching changes up to 40–50%.     

Hydrogen permeation and distribution at a high-strength X80 steel weld under stressing conditions and the implication on pipeline failure

Hydrogen permeation and distribution at pipeline welds is critical to integrity maintenance of the pipelines, especially for those made of high-strength steels. The situation becomes even more important under stressing conditions. In this work, metallographic characterization and micro-hardness measurements were conducted at an X80 steel weld. Potentiodynamic polarization and electrochemical hydrogen permeation testing were performance at various zones at the weld, along with numerical modeling of hydrogen distribution at the zones. The X80 steel contains a microstructure of bainite bundles and polygonal ferrite. There are more polygonal ferrite, fewer bainite and some segregated cementite at heat-affected zone (HAZ). The weld metal is featured with acicular ferrite and some grain boundary ferrite. HAZ softening occurs at the weld. The hardness of the weld metal, HAZ and base steel is about 290, 248 and 261 HV_0.2, respectively. There is the greatest corrosion current density, i.e., corrosion rate, at HAZ under both elastic and plastic stresses. An applied stress further increases the corrosion current density. Under the plastic stress of 1.1σ_ys (σ_ys is yield strength), the corrosion current densities of HAZ, base steel and weld metal are 41.04, 17.03 and 25.49 μA/cm², respectively. There are always the greatest hydrogen trapping density and the smallest hydrogen diffusivity at HAZ. Hydrogen, once penetrating the welded steel, tends to accumulate at the HAZ, compared with other two zones. When the welded steel is under stresses, especially a plastic stress (i.e., 1.1σ_ys), the hydrogen diffusivity and permeability decrease, while the subsurface hydrogen concentration and hydrogen trapping density increase remarkably. Plastic deformation favors the hydrogen permeation and trapping at weld, especially the HAZ, to elevate the susceptibility to hydrogen damage. The hydrogen distribution at different welding zones can be evaluated and determined by a developed modeling method.