Coatings

Mechanical impact during transportation and construction can locally damage protective coatings on steel bridge components and affect their subsequent barrier response. In this study, conventional baseline and modified multilayer coating systems were prepared on Q235B carbon steel and Q355NH weathering steel and subjected to drop-weight impacts of 0.98–2.94 J. Three-dimensional surface characterization, scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) were combined to evaluate impact-induced geometric damage, surface fracture morphology, and short-term post-impact electrochemical response. The projected damage area generally increased with impact energy, whereas indentation volume and maximum indentation depth exhibited different evolution trends. At 2.94 J, the modified systems exhibited projected damage areas of 22.24 and 22.63 mm², approximately half those of the corresponding baseline systems, while retaining comparatively high indentation volumes. Three-dimensional morphology and SEM observations showed that the baseline systems developed broader lateral deformation and more extensive peripheral surface damage, whereas the modified systems exhibited a smaller lateral extent of the overall impact-affected region. This damage localization therefore reflects reduced lateral spreading rather than a uniformly smaller central indentation. Short-term EIS responses were strongly system-dependent and showed no universal monotonic relationship with impact energy. These results indicate that reduced lateral damage extent does not necessarily imply reduced local deformation or improved electrochemical response, highlighting the need for multidimensional assessment of impact-damaged protective coatings.

Commercially pure titanium is widely used for load-bearing implants but its low surface hardness and high friction coefficient limit durability. In this work, calcium- and silicon-bearing coatings were deposited on CP-Ti (VT1-0/ASTM Grade 2) by micro-arc oxidation (MAO) in an aqueous electrolyte of 30 wt.% H₃PO₄, 60 g/L hydroxyapatite and 75 g/L wollastonite, at a fixed filling-parameter setting of 8% and two voltages, 200 and 300 V. The coatings were characterised by SEM, EDS, FIB cross-sectioning, progressive-load scratch testing, instrumented indentation (ISO 14577-1) and single-pass ball-on-flat sliding with 3D profilometry. Raising the voltage from 200 to 300 V thickened the layer from about 28 to 37 µm but rendered the cross-section coarsely porous and the coating–substrate interface markedly rougher. The first cohesive critical load fell from 6.15 N at 200 V to 3.73 N at 300 V, and the kinetic friction coefficient rose from 0.41 to 0.75. Instrumented indentation gave comparable properties for both coatings (Martens hardness 2254 versus 2169 N/mm²; modulus 103 versus 102 GPa), indicating that the higher-voltage penalty lies in coating architecture and interface quality rather than in intrinsic hardness. The lower voltage is therefore preferable for tribologically loaded coatings of this type.

The recovery of waste heat from post-combustion gases is often limited by sulfuric acid condensation, which promotes severe low-temperature corrosion. Hydrophobic and superhydrophobic coatings promote dropwise condensation rather than filmwise mode, potentially reducing the residence time of the condensed liquid on the surface. However, the dynamics of sulfuric acid dropwise condensation on such surfaces remain poorly understood. To address this gap, an individual-based model is developed to simulate the evolution of the condensing droplet population. Each droplet is tracked from nucleation to shear-driven sliding, accounting for growth by direct condensation and coalescence with other droplets. The heat and mass transfer model, used to evaluate the condensation rate of sulfuric acid for each droplet, includes latent heat transfer at the liquid–vapor interface and convection in the surrounding gas phase. Given its low mole fraction, all the sulfur trioxide is assumed to react with water to form sulfuric acid. Assuming that the wall temperature is below the acid dew point but above the water dew point, the composition of the condensed H₂SO₄-H₂O solution is determined from vapor–liquid equilibrium (VLE). The numerical model is validated against experimental data from the literature. The results show that, compared with hydrophilic surfaces, superhydrophobic surfaces substantially reduce the wet fraction of solid substrate and the residence time of condensed droplets, suggesting a lower risk of low-temperature corrosion.