The Emergence and Propagation of Filiform Corrosion on the Surface of ZX10 Alloy under the Influence of Chlorides

Abstract

Self-dissolving medical implants, such as screws for bone fracture fixation or vascular stents, represent a promising application of magnesium alloys. Magnesium-based bioresorbable materials are currently not only the subject of research by scientific groups worldwide but also the raw material for producing commercial products—medical metallic implants that are actively used in patient treatment. Nevertheless, many technological issues remain unresolved. Chloride-containing fluxes are widely used in casting magnesium alloys. It is unclear whether the presence of flux particles in materials for bioresorbable implants poses a risk of corrosion damage to the surface of the product. This study investigates the processes of initiation and development of filiform corrosion caused by the presence of a chloride-containing particle on the metal surface. Energy-dispersive spectroscopy was used to determine the composition of corrosion products, and Kelvin probe atomic force microscopy was employed to measure their electrode potential relative to the magnesium matrix. It was shown that, under ambient temperature of 25°C and 30% humidity, filiform corrosion is initiated near the chloride-containing particle. Despite the shallow depth of damage (2–3 µm), corrosion spreads over a large area and is characterized by a high propagation rate (tens of microns per day). Analysis of the chemical composition of the corrosion products revealed that the process involves reactions leading to formation of hydroxide and its breakdown under the influence of chloride and CO₂. The corrosion products exhibit a positive potential relative to the metal, enabling the activation of anodic dissolution of the matrix. Placing the material in a vacuum completely halts progression of corrosion, which resumes upon exposure to air. This demonstrates the necessity of avoiding the use of chloride-containing fluxes in the production of bioresorbable alloys and storing finished products in a moisture-free environment whenever possible.