Nano-scale chemical inhomogeneity in surface oxide films formed on a V- and N-containing martensite stainless steel and tempering heating induced changes are investigated by a combination of synchrotron- based hard X-ray Photoelectron emission spectroscopy (HAXPES) and microscopy (HAXPEEM) as well as microscopic X-ray absorption spectroscopy (μ-XAS) techniques. The results reveal the inhomogeneity in the oxide films on the micron-sized Cr2N- and VN-type particles, while the inhomogeneity on the martensite matrix phase exists due to localised formation of nano-sized tempering nitride particles at 600 °C. The oxide film formed on Cr2N-type particles is rich in Cr2O3 compared with that on the martensite matrix and VN-type particles. With the increase of tempering temperature, Cr2O3 formation is faster for the oxidation of Cr in the martensite matrix than the oxidation of Cr nitride-rich particles.

Corrosion and wear pose significant challenges to equipment operating in harsh environments. Thus, protective coatings are needed. Anti-corrosion and anti-wear coatings are traditionally fossil-based and often contain environmentally harmful additives. Achieving anti-corrosion and anti-wear coatings based on environmentally benign and sustainable materials is important and a significant challenge. This work focused on the development of organosolv lignin-based polyurethane (OS_lignin-PU) coatings. The coatings were synthesised and evaluated for corrosion protection using electrochemical impedance spectroscopy (EIS) and for wear properties using nanoindentation and nano scratch measurements. EIS revealed that the optimal lignin content for corrosion protection purposes in the OS_lignin-PU coatings was 15 wt%. Moreover, addition of 15 wt% lignin to the OS_lignin-PU coatings also enhanced their wear resistance, as evidenced by reduced thickness loss during tribometer tests. The nano scratch measurements revealed that OS_lignin-PU coatings containing 15 wt% lignin exhibited the lowest scratch depth and friction coefficient. It is found that the developed lignin-containing coating exhibits remarkable corrosion and wear resistance, making it a promising sustainable material in various applications for pursuing sustainable development.

The pitting potential, intrinsic surface acidity, point of zero charge of passive film on Al are studied using first-principles calculations to establish their relationships. Influences of alloying elements Zn, Cr, Nb, Si, Mo and Sc on adsorption of NH3 and NaCl, pHpzc of Al2O3 and pitting susceptibility of Al are investigated. The efficiency for enhancing pitting resistance of Al is evaluated, yielding the ratios Si: Zn: Cr: Mo: Nb: Sc = 1.8: − 0.3: 1: 1.9: 1.4: 0.2. A model for the dependence of pitting potential on the concentration of alloying elements in Al alloy matrix is developed, based on effects of alloying elements on the surface charge of passive film. The effects of Sc on pitting potential and pHpzc of Al oxide are predicted based on the calculated results, which are supported by electrochemical measurement, XPS analysis and contact angle titration.

Corrosion results in large costs and environmental impact but can be controlled by thin oxide films that passivate the metal surfaces and hinder further oxidation or dissolution in an aqueous environment. The structure, chemistry, and thickness of these oxide films play a significant role in determining their anti-corrosion properties and the early-stage oxidation dynamics affect the properties of the developed oxide. Here, we use in situ X-ray Photoelectron Spectroscopy (XPS) to study the early-stage oxidation of a Ni-Cr-Mo alloy at room temperature and up to 400 degrees C. Cr and Mo begin to oxidize immediately after exposure to O2, and Cr3+, Mo4+, and Mo6+ oxides are formed. In contrast, Ni does not contribute significantly to the oxide film. A self-limiting oxide thickness, which did not depend on temperature below 400 degrees C, is observed. This is attributed to the consumption of available Cr and Mo near the surface, which results in an enrichment of metallic Ni under the oxide. The self-limited oxide thickness is 6-8 angstrom, which corresponds to 3-4 atomic layers of cations in the oxide. At 400 degrees C, sublimation of Mo6+ oxide is observed, resulting in the formation of an almost pure layer of Cr2O3 on the alloy surface. Lastly, a mechanism is presented that explains the formation of the bi-layer oxide structure observed for Ni-Cr-Mo alloys, which involves the enhanced migration of hexavalent Mo ions in the electric field, which drives mass transport during oxidation according to both the Cabrera Mott model and the Point Defect Model.

Corrosion causes significant challenges in industrial settings, leading to economic losses and safety concerns. Previously, we developed a lignin-polydimethylsiloxane (lignin-PDMS) coating that exhibited high corrosion resistance. However, the adhesion of the developed lignin-PDMS coating to carbon steel was limited, affecting its overall performance. To address this, we incorporated dopamine (DOPA), known for its strong adhesive properties, as a pre-treatment before applying the coating. It was found that the adhesion and corrosion resistance of lignin-PDMS coated steel could be improved by adjusting the pH value of the DOPA solution. The steel treated with pH 4.5 DOPA solution showed two times higher adhesion strength to the coating than non-treated steel. After the DOPA treatment, the coating can maintain high barrier property for at least 3 months in 1 M NaCl solution, which is even better than commercial gelcoat, demonstrating super corrosion protection. Quartz Crystal Microbalance with Dissipation (QCM-D) and X-ray Photoelectron Spectroscopy (XPS) analyses confirmed the DOPA deposition on the steel surface. Our findings show that the DOPA-lignin-PDMS system is an environmentally friendly and efficient solution for enhancing the durability of steels in corrosive environments.

Corrosion and wear remain significant challenges for materials, causing substantial economic losses and safety risks. Anti-corrosion and anti-wear coatings provide an effective solution. However, traditional coatings are often fossil-based and contain heavy metals, posing environmental concerns. The drive for eco-friendly coatings has led to the exploration of green materials. This study combined lignin, an abundant organic material, and polydimethylsiloxane (PDMS), a known hydrophobic material, to address the challenges. Organosolv lignin was functionalised with (3-Aminopropyl)triethoxysilane (APTES), then chemically grafted on PDMS for the final coating synthesis. The optimised coating achieved through an eco-friendly process, exhibiting enhanced hydrophobicity and barrier properties, showing excellent long-term corrosion resistance in NaCl solution. The optimal coating formulation contained 15 wt% lignin and 40 wt% PDMS, demonstrating a high corrosion resistance (measured impedance of 1010 Ω·cm2), which remains effective even after 3 weeks of immersion in 1 M NaCl solution. This coating also showed good wear resistance, with a low friction coefficient evident from nano scratch tests.

Effect of tempering temperature on the composition of the passive film of a martensitic tool alloy was studied by synchrotron-based hard/soft X-ray photoelectron spectroscopy and electrochemical analyses. The contents of Cr and Mo in the passive film are affected by precipitation of tempering carbides. Increase of tempering temperature from 200 to 525°C leads to enhanced formation of Cr/Mo-rich tempering carbides and Cr depletion. Tempering at 525°C results in a Cr content < 11 at% in the underlying metallic layer and formation of a Cr-deficient defective passive film, and thus loss of passivity for the tool alloy in corrosive conditions.

Potential drop at the oxide film/solution interface plays a critical part in numerous electrochemical processes, especially for the passivated metal-electrolyte system, affecting the passive film breakdown and the pitting initiation. There are still some controversies over the dependence of potential drop at the passive film/solution interface (φf/s) on potential and pH of electrolyte. Herein, we develop a model presenting the linear dependence of φf/s on both potential and pH, as represented by φf/s=φf/s(V=0,pH=0)0+αV+βpH. By analyzing the surface charge (i.e., point of zero charge, pHpzc) and performing first-principles calculations, we provide the insights into the effect of potential and pH on the φf/s, and into the linear relation between φf/s with pH beyond the Nernst relation that is attributed to the role of point defects in pHpzc of passive film on iron. In addition, two methods, e.g., a combination of Mott-Schottky measurement with first-principles, contact angle titration, are suggested to determine the values of α, β and φf/s(V=0,pH=0)0, respectively. The study of passivity of iron in borate buffer solutions validates our model.

Traditional methods to study electrochemical (EC) processes, although successful, are based on current/voltage measurements, providing information about performances rather than offering a direct observation of chemical and structural changes occurring at the electrode surface. These processes are localized at the electrode-electrolyte interface, the structure of which is the main determinant of their behavior, but most surface sensitive experimental techniques are limited to ex situ conditions, owing to the need for an ultra-high vacuum environment. In this contribution, we report operando X-ray absorption spectroscopy in total external reflection geometry (Refle-XAFS) at P64 beamline (DESY, Hamburg), using a simple and versatile EC flow cell designed for multimodal surface sensitive studies with hard X-ray scattering and spectroscopy techniques. We show that the Refle-XAFS method can be used to study chemical surface changes of industrial alloys and model electrodes in harsh electrochemical environments, without being limited to thin film samples. The surface passive film development and breakdown of a corrosion-resistant Ni-Cr-Mo alloy and the electro-oxidation of polycrystalline gold (poly-Au), relevant for fundamental studies on water electrolysis, were investigated. Despite the strong attenuation of the beam by the electrolyte and the PEEK walls of the EC cell, nanoscale surface oxide films were detected using beam energies down to 8 keV. The passivity breakdown region of Ni alloy 59 in 1 M NaCl at pH 7 and pH 12 was identified, showing differences in the composition of the surface oxides during anodic polarization. The electro-oxidation of poly-Au in 0.05 M H2SO4 was observed, showing a progression from two-dimensional Au1+/3+ to three-dimensional thick Au3+ surface oxide/hydroxide during OER.

Surface films formed on pre-oxidized copper in anoxic simulated groundwater with sulphide were characterized by field emission gun scanning electron microscopy (FEG-SEM), Fourier transform infrared spectroscopy (FT-IR), open circuit potential (OCP) measurements, and via analysing the water chemistry and weight changes in the specimens. Additionally, films developed under identical conditions on pre-oxidized and ground copper specimens were characterized by glow discharge optical emission spectroscopy (GDOES). The results revealed that the sulphide content in the groundwater significantly influences the morphology, composition and thickness of the surface film. The build-up of Cu2S was evidenced at the sulphide contents of 32 mg/L and 320 mg/L. GDOES depth profiling revealed that sulphur and oxygen coexisted in the film all through its thickness, yet the surface was essentially rich in sulphur. The results from characterization are presented in detail in this paper and discussed from the perspective of capabilities of the used methods.