Influence of ECO coating thickness and microstructural defects on corrosion, corrosion fatigue, and stress corrosion cracking in biomedical Mg alloys

Abstract

Resorbable magnesium (Mg) alloys are attractive for orthopaedic and cardiovascular implants but can degrade rapidly in physiological electrolytes, and combined corrosion–mechanical loading may trigger premature failure. Electrochemical oxidation (ECO) coatings can reduce corrosion by forming a ceramic-like surface enriched with fluoride and phosphate species, yet their brittleness raises concerns regarding integrity under concurrent corrosion and loading. Here, we investigate how ECO coating thickness influences corrosion behaviour and mechanical response under physiologically relevant conditions. AZ31 and X0 alloys were coated with 5 μm and 15 μm ECO layers and tested under static and cyclic three-point bending (3PB) in Hank's balanced salt solution (HBSS). 3PB fatigue tests revealed that cracks in the alloy initiate at the tensile surface and are associated with coating defects such as pits and cracks in the coating, or localised Ca-P deposits. Under non-corroded conditions and after 1-week of corrosion, uncoated alloy rods demonstrated slightly higher fatigue resistance and delayed crack initiation compared to coated counterparts. After 3 weeks of immersion, only the coated rods retained load-bearing capacity, with the 15 μm coating showing the longest fatigue life. Under static sustained loading, all coatings reduced crack initiation under elastic stress, whereas the 15 μm coating suppressed through-thickness crack propagation under plastic loading and thereby prevented catastrophic stress corrosion cracking (SCC)-style failure over the test duration. Overall, ECO coatings do not universally eliminate corrosion–mechanical interactions; rather, their protective efficacy is strongly condition-dependent and increases for the thicker coating under the environment–load combination examined.