Time-dependent corrosion of AZ91D magnesium alloy in simulated body fluid for biodegradable implants

Magnesium alloys, particularly AZ91D, are promising materials for biodegradable implant applications due to their favourable strength-to-weight ratio, biocompatibility, and elastic modulus close to that of natural bone. However, their rapid degradation in physiological environments limits long-term clinical performance. This study systematically investigates the corrosion behaviour of AZ91D magnesium alloy through prolonged immersion in simulated body fluid (SBF) at 37 °C for 7, 30, 60, and 90 days. Surface degradation was evaluated through visual and morphological examination, while electrochemical characteristics were analyzed using Tafel polarization techniques. After 7 days, moderate corrosion activity was observed, with a corrosion potential near –1.75 V and microampere-level corrosion current density. Formation of Mg(OH)₂ and calcium–phosphate deposits was evident. At 30 and 60 days, progressive surface roughening, localized attack, and continued hydrogen evolution indicated sustained degradation. Although a slight reduction in corrosion current density suggested temporary surface film formation, the absence of a stable passive region confirmed ongoing active corrosion. After 90 days, severe structural deterioration and loss of integrity were observed, demonstrating breakdown of the protective surface layers.Most previous studies emphasize short-term immersion behaviour (≤30 days), with limited focus on long-term degradation and its correlation with electrochemical kinetics. This work addresses that gap by providing a comprehensive time-dependent analysis (7–90 days), linking immersion observations with polarization behaviour. The results clarify the transitional stages of corrosion and provide insight into the long-term bio-resorption performance of AZ91D under physiological conditions.