We investigated the unique passive behaviour of Hybrid steel in de-aerated sulfuric acid and aqueous sodium chloride solutions through corrosion tests, surface analysis, and thermochemical modelling. Electrochemical measurements confirmed that Hybrid steel possesses stainless steel characteristics, including passivity, breakdown, and pitting, akin to low-alloyed stainless steel. Synchrotron hard X-ray photoelectron spectroscopy revealed a dynamically protective nanoscale passive film composed of Fe, Cr, Ni, and Al oxides, contributing to its stainless nature. The presence of Al and Ni enhances Cr’s role in forming a spontaneously passive and protective surface, resulting in exceptional corrosion resistance in acidic and chloride-containing solutions. Hybrid steel’s surface oxides remain robust even beyond the Cr(III)-to-Cr(VI) redox potential, distinguishing it from other stainless steels. This work demonstrates the potential for designing sustainable stainless steel with high-strength properties without requiring the conventional Cr threshold concentration of 10.5 per cent.

The effect of platinum (Pt) loadings of air-cathodes in the 0-0.5 mg cm-2 range on single chamber microbial fuel cell (MFC) performance and cathode impedance was evaluated. In MFC tests, reducing benchmarking Pt loading of 0.5 mg cm-2 to 0.1-0 mg cm-2 decreased maximum power density by between 38% and 84%. The decrease in cathode open circuit potential with reduced loadings was small down to a catalyst loading of 0.03 mg cm-2, but was significant when the loading was further reduced to 0.01 or 0 mg cm-2. Impedance measurements of cathodes revealed that both charge-transfer and diffusion resistance increase with decreasing catalyst loadings on cathodes. Charge-transfer resistance of benchmarking cathode increased to a small extent when loadings were reduced to 0.1-0.03 mg cm-2. Below 0.03 mg cm-2, dramatic increase of charge-transfer resistance suggested that 0.03 mg cm-2 can be considered as the minimum Pt loading for which kinetic limitations are not of great concern and can be overcome to a large extent compared to lower loadings. In comparison to charge-transfer resistance, diffusion resistance differed more significantly between the loadings of 0.03 and 0.5 mg cm-2; and it was therefore the main component that changed the internal resistance of these cathodes.

This study has been conducted for investigating the contribution of polyaniline (PANI) to the electroactivity of nickel hydroxide (NH) by using a combination of electrochemical, structural and morphological characterization techniques. NH, PANI and nickel hydroxide/PANI composite (NHP) electrodes were produced on nickel foam substrates. Electrodeposition and chemical bath deposition methods were used for the preparation of NH and PANI, respectively. All the electrochemical experiments were conducted in alkaline solutions. NH and PANI were used as reference materials and exhibited properties in accordance with the literature. Namely, for NH electrode capacity decayed by cycling because of the phase transformation from α to β‐Ni(OH)₂, and particles growth from 350 to 850 nm. Also, flower‐like structure of the as prepared Ni(OH)₂ faded after 2000 cycles. On the other hand, PANI electrode although exhibited a decrease in the conductivity because of its degradation retained its capacity over cycling because of swelling and shrinking that led to an increased surface area. Composite electrode consisting of PANI and NH resulted in an improvement of capacity retention. At the beginning of cycling capacitance of the composite electrode was 0.64 F/cm², capacity decreased to 0.47 F/cm² after 500 cycles then, continuously increased and finally reached to 0.54 F/cm² after 2000 cycles. Presence of PANI in combination with NH, limited the particle growth and contributed to the preservation of flower like structure of NH. Contrary to both NH and PANI electrodes, charge transfer resistance of NHP exhibited a decrease with cycling indicating a synergy between NH and PANI in addition to morphological changes.

We examined the unusual passive character of Hybrid steel in de-aerated sulfuric acid and aqueous sodium chloride solutions by benchmark corrosion and surface analytical tests in combination with CalPhaD-based thermochemical modelling. The electrochemical potentiodynamic polarization measurements have shown that Hybrid steel possesses all characteristic features of what makes steel stainless, such as passivity, breakdown and pitting, similar to standard low-alloyed stainless steel. Synchrotron hard X-ray photoelectron spectroscopy (HAXPES) analysis revealed that the stainless nature of Hybrid steel is achieved by a dynamically protective nanometre-sized passive film consisting of Fe, Cr, Ni, and Al oxides. The thermodynamic calculations showed that the surface oxide composition is Fe₂O₃•FeCr₂O₄•NiO•Al₂O₃, which changes structure, fraction and existence over electrochemical polarization. It has become understood that the presence of Al and Ni supports Cr in forming a spontaneously passive and hence protective surface, yielding exceptional corrosion resistance in acidic and chloride-containing aqueous solutions. The surface oxide could withstand breakdown and remain passive/repassivated even after transpassing the Cr(III)-to-Cr(VI) redox potential. While Cr is the prime passivating agent, an adverse effect of Cr was seen on grade 304 and 420 stainless steel when Cr(VI) species were released, savaging the passive film due to extensive interfacial pH reduction. However, among all tested stainless steels, Hybrid steel could repassivate due to the remaining and enriching Al and Ni oxides providing superior anodic passivation. Our work demonstrates that delicate alloying and microstructure engineering can design sustainable stainless steel with optimum high-strength properties without needing the well-known Cr threshold concentration of 10.5 per cent.

Repurposing natural gas pipelines for hydrogen transport is a cost-efficient strategy to accelerate the hydrogen economy. However, beyond the well-established risk of hydrogen embrittlement, hydrogen permeation may increase the intrinsic corrosion susceptibility of steel and create subsurface conditions that favour microbiologically influenced corrosion by enriching hydrogen-utilising microbial communities along buried pipelines. We exposed hydrogen-charged steel in a soil-inoculated medium containing a field-relevant microbial community and observed severe corrosion. The combined effects of hydrogen effusing and microbial activity led to excessive rust, carbonate, and carbonaceous compounds, intensifying localised attack. Pronounced microbial shifts were observed in response to hydrogen-charged specimens, favouring hydrogen-metabolising bacteria, indicating that hydrogen permeation stimulates metabolic activity and corrosion, revealing a corrosion mechanism currently overlooked in pipeline risk assessments.