Chemical Passivation of Titanium and Ti-6Al-4V Alloy and Impact on Biocompatibility: A Comprehensive 50-Year Review and Best Practice Guidelines

Abstract

Chemical passivation is a standard surface finishing treatment applied to commercially pure Titanium (cp-Ti) and Titanium-6Aluminum-4Vanadium (Ti-6Al-4V) when preparing them for use in medical devices. Although this treatment has been widely used for over 50 years, there remains a lack of consensus regarding its precise impact on surface characteristics and subsequent biocompatibility. This article comprehensively reviews the literature on this topic and sheds light on the reasons behind the conflicting research findings even after decades of practice. First, the nitric acid-based passivation protocol, originally developed for stainless steel, is often directly applied to cp-Ti and Ti-6Al-4V without specific optimization studies. Second, the pre-treatment processes, pre-cleaning procedures, and passivation protocols are significantly varied among the studies in the literature to provide vastly different outcomes regarding surface properties and subsequent material performance. Third, the ASTM F86 standard, commonly used in many studies, has several limitations. It provides open-ended recommendations for certain parameters (such as acid treatment time) without specifying maximum limits, lacks clearly defined validation processes, and relies on inadequate inspection methods (like visual examination). These limitations result in significant variations in outcomes and incomplete characterizations. Fourth, the surface characteristics subjected to chemical passivation differ due to the diverse manufacturing processes used in the studies. This results in varying outcomes, even when the same passivation protocol is employed. Considering the various shortcomings in the literature, a set of best practice recommendations is provided to enhance the reliability and consistency of research outcomes in the chemical passivation of cp-Ti and Ti-6Al-4V.