A Comparative Study of the Electro-Assisted Grafting of Mono- and Bi-Phosphonic Acids on Nitinol

Abstract

Over the last few years, Nitinol (NiTi) has become one of the most attractive alloy materials for industrial applications. However, its implementation is still problematic due to its surface nickel content, making it sensitive to pitting corrosion. In applications, it is often necessary to modify NiTi surfaces by using organic coatings, which provides new physico-chemical properties as well as functionalities and often contributes to a reinforcement of the alloy corrosion resistance. In this work, we assess the differences between the molecular layers made out of methylphosphonic acid (C1P) and the bi-phosphonic acid derivatives: (methylimino)dimethylene-bisphophonic acid (MIP2) and 1-hydroxyethylidene-1,1-diphosphonic acid (HEP2) using conventional (CG) and electro-assisted (EG) graftings. The surface modifications with the bi-phosphonic derivatives (MIP2) and (HEP2) carried out with the EG process lead to denser layers and a reinforced NiTi corrosion resistance.