Preparation And Properties Of Gradient Degradation Coating On Magnesium Alloy

Magnesium (Mg) alloys have drawn wide attention as a new type of potential degradable vascular stent biomaterials due to their good mechanical properties, excellent biocompatibility, as well as degradability. However, the rapid corrosion rate of Mg alloys in body fluids inhibits its future clinical application. The rapid biodegradation of Mg alloy stents could cause the loss of its biofunctionability, and the corrosion products with high concentration could also lead to some toxic side effects on the body or block the vessels. So it is significant to control the degradation rate of Mg alloys as the coming stent materials.In this research, PA-HF, ORMOSIL, ORMOSIL-PTMC and ORMOSIL-PTDLA coatings have been prepared on AZ31 alloy. The corrosion-proof and degradability of the coated samples have been measured through electrochemical corrosion and immersion tests. The main results are as follows:(1) The micro-cracks in the PA coating gradually healed up during the post-treatment in HF, which was determined by the post-treatment period. The 24-h post-treatment can lead to the best surface integrity of PA-HF coating. It is found that the Ecorr of 24-h PA-HF coating was -1.29 VSCE, which is about 210 mV greater than that of PA coating. Its corrosion current density in Hank’s solution was about 3.4×10-7 A/cm2, which was reduced more than an order of magnitude than that of PA coating. The film resistance of PA-HF coating was about 4540 Ω·cm2,15 times higher than PA coating, and the transfer resistance was approximately 1.91×105 Ω·cm2,40 times higher than PA coating. However, this PA-HF composite coating suffered serious corrosion attack after 5-day exposure in Hank’s solution.(2) In this section, KH570 and two kinds of ORMOSIL coatings have been prepared by sol-gel method on AZ31 alloy. It is found that the Ecorr of the ORMOSIL-2 coating was about-1.46 V, followed by KH570 and ORMOSIL-1 coating, which is approximately 280 mV greater than AZ31 alloy. Its corrosion current density in Hank’s solution was about 2.48×10-7 A/cm2, which was reduced more than an order of magnitude than that of KH570 and ORMOSIL-1 coating. The ORMOSIL-2 coating suffered slight corrosion attack after 5-day exposure in Hank’s solution, but after 10-day exposure in Hank’s solution led to serious localized corrosion, which indicates that the corrosion-proof property of ORMOSIL-2 coating is better than PA-HF coating.(3) The existence of ORMOSIL intermedia layer is beneficial to improving the adhesion strength between the upper polymer coating and AZ31 alloy. The corrosion-proof property of the composite layer, which is composed of the ORMOSIL intermedia layer and polymer coating, was determined by its integrity, composition and thickness. Compared to the ORMOSIL-PTMC5 coating, the ORMOSIL-PTDLA5 coating showed better corrosion-proof property after 10-day exposure in Hank’s solution. Then the ORMOSIL-PTDLA5 coating showed gradual corrosion attack after 10-day immersion, and the coated AZ31 alloy appeared serious corrosion after immersing for 30 days. The corrosion morphology indicates that the failure of the composite coating was controlled by the gradient degradation to some extent.