Acid-Induced Surface Degradation of Metallic Biomaterials: Alloy-Dependent Behavior and Implications for Surface Functionality

Metallic biomaterials are frequently exposed to chemically aggressive environments that may compromise surface integrity and corrosion resistance. Acidic media containing organic acids represent a relevant challenge for metallic systems, as they can destabilize passive oxide layers and promote surface degradation processes. The present in vitro study investigated acid-induced surface alterations in four commercially relevant orthodontic alloys—nickel–titanium (NiTi), copper–nickel–titanium (CuNiTi), titanium–molybdenum alloy (TMA), and stainless steel—as representative metallic biomaterials. Specimens were exposed to two commercially available acidic beverages with distinct pH conditions, followed by analysis of surface morphology, roughness, and elemental composition using atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The results demonstrated pronounced alloy-dependent differences in degradation behavior. Stainless steel and TMAs exhibited significant increases in surface roughness and morphological alterations, whereas NiTi-based alloys showed comparatively stable surface characteristics. Elemental analysis revealed material-specific compositional variations, suggesting selective surface modification processes under acidic exposure. These differences can be attributed to variations in alloy composition, microstructure, and the stability of passive oxide layers, which collectively govern corrosion resistance in metallic systems. The findings provide insight into acid-induced degradation mechanisms in metallic biomaterials and highlight the importance of material-dependent corrosion behavior under chemically aggressive conditions. These observations may have implications for surface-mediated biological responses and long-term functional performance of metallic biomaterials.