Protein Interactions with Corroding Metal Surfaces

Low-corrosion resistant metals, such as Mg or Fe based alloys, have been in the focus of intensive research activities in recent years to be used in biodegradable implants. A general challenge is to tailor the materials and surfaces to achieve the desired and controlled corrosion rate for the targeted applications. A further challenge is to elucidate the influence of the complex biological environment on the corrosion mechanisms of these materials. A vast majority of the studies in this field have been carried out on Mg based materials; far less attention has been given to Fe. A comparison of the behavior of Fe and Mg based alloys in simulated biological environments is on one hand side of practical interest in view of possible biomedical applications of these materials. Moreover, from mechanistic point of view, a direct comparison may aid in elucidating mechanisms of interactions between the constituents of the biological environment and the corroding metal surfaces in more detail. In the present study, we are investigating the influence of proteins on the corrosion behavior of Mg and Fe by electrochemical and surface analytical techniques. Proteins in solution can have multiple effects on corrosion reactions: on one hand side, protein adsorption layers can block electrochemically active surface sites and therefore slow down corrosion. Protein adsorption will be influenced by the surface chemistry of the metals (e.g., wettability, type of oxides/hydroxides formed on the surface, and the resulting surface charges) - therefore different interactions may be expected for Mg and Fe. On the other side, complexation of metal cations can take place by proteins (and amino acids), thereby reducing the free cation concentration and possibly accelerating corrosion. Also this effect depends on the nature of the metal cations; for Fe and Mg dissolution products different strength of interactions can be expected. Electrochemical studies indeed indicate very different interactions of proteins (bovine serum albumin, BSA) on Mg as compared with Fe.  For Mg, inhibition of corrosion in presence of albumin in solution can be observed for short-term experiments. For Fe, a slight increase of corrosion by addition of albumin in solution takes place. In general, the influence of BSA on the corrosion behaviour of Mg is much stronger than that observed for Fe. Surface analytical studies (XPS and ToF-SIMS) demonstrate stronger adsorption of albumin on Mg than on Fe. Moreover, protein adsorption on Mg and Fe varies for different proteins (for instance: albumin vs. lysozyme). Possible mechanisms and critical factors of protein interactions with corroding Fe and Mg surfaces will be discussed.