Impact-Induced Damage Localization and Post-Damage Electrochemical Response of Multilayer Protective Coatings on Carbon and Weathering Steels

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.