Corrosion protection and repassivation after the deformation of magnesium alloys coated with a protective magnesium fluoride layer

摘要

The development of a biodegradable, cardiovascular implant (stent) made from a resorbable magnesium alloy demands an accurately defined degradation profile. The corrosion protection of the stent for the first 4-6 weeks after implantation is only obtainable by usage of a protective surface coating. During this time the implant is able to grow into the vessel. After the steady solution of the coating the base material is resorbed by a normal corrosion process in chloride media. During the implantation the material has to tolerate partial deformations. The influence of the induced micro-cracks during the deformation process has been analyzed in a 4-point bending test combined with an electrochemical corrosion test. The material systems consist of a Mg(99,8%) or ZM21-alloy and a dense MgF_2-coating. The coating process is based on the conversion of the natural layer by a treatment with hydrofluoric acid. The specimens show micro-cracks after the deformation but no increased corrosion activity. The measurement of the corrosion current indicates a repassivation of the surface. Detailed EDX and REM analyses prove the regeneration of Mg(OH)_2/MgO on the crack ground.