Sachin A. Mali

In vivo oxide‐induced stress corrosion cracking of Ti‐6Al‐4V in a neck–stem modular taper: Emergent behavior in a new mechanism of in vivo corrosion

In vivo modular taper corrosion in orthopedic total joint replacements has been documented to occur for head-neck tapers, modular-body tapers, and neck-stem tapers. While the fretting corrosion mechanism by which this corrosion occurs has been described in the literature, this report shows new and as yet unreported mechanisms at play. A retrieved Ti-6Al-4V/Ti-6Al-4V neck-stem taper interface, implanted for 6 years is subjected to failure analysis to document taper corrosion processes that lead to oxide driven crack formation on the medial side of the taper. Metallurgical sectioning techniques and scanning electron microscopy analysis are used to document the taper corrosion processes. The results show large penetrating pitting attack of both sides of the taper interface where corrosion selectively attacks the beta phase of the microstructure and eventually consumes the alpha phase. The pitting attack evolves into plunging pits that ultimately develop into cracks where the crack propagation process is one of corrosion resulting in oxide formation and subsequent reorganization. This process drives open the crack and advances the front by a combination of oxide-driven crack opening stresses and corrosion attack at the tip. The oxide that forms has a complex evolving structure including a network of transport channels that provide access of fluid to the crack tip. This emergent behavior does not appear to require continued fretting corrosion to propagate the pitting and cracking. This new mechanism is similar to stress corrosion cracking where the crack tip stresses arise from the oxide formation in the crack and not externally applied tensile stresses.

Medical Implant Corrosion: Electrochemistry at Metallic Biomaterial Surfaces

Metallic biomaterials represent the class of materials with the largest use in medical devices in humans today. This fact will likely continue for decades to come because of the unique combination of strength, wear resistance, and corrosion resistance. However, metallic biomaterials also pose unique and specific concerns related to electrochemical behavior in the body. This chapter will focus on the elements of most importance in understanding the complex interactions present in the human body during corrosion of metallic implants. The concepts associated with oxide films and their interaction with the biological, mechanical, and electrochemical environments are discussed to provide insight into why corrosion is a critically important factor in the long-term performance of devices. Mechanically assisted corrosion in the biological system is discussed in terms of the structural, electrochemical, and biological interactions, and the idea of electrochemical history is presented to explain why such severe evidence of corrosion is observed in vivo. Finally, specific examples of mechanically assisted corrosion in vivo (or biotribocorrosion) are presented, and recent observations concerning the important role the reduction half-cell plays in the biological response to corrosion are discussed.

Area-dependent impedance-based voltage shifts during tribocorrosion of Ti-6Al-4V biomaterials: theory and experiment

Tribocorrosion of medical devices causes the electrode potential across the device-solution interface to become more negative. This study provides a theoretical impedance-based understanding of voltage versus time changes that arise. It combines tribocorrosion with the voltage-dependent impedance characteristics of the surface, the relative anodic and cathodic areas and the mechanics and electrochemistry of oxide abrasion. An area-dependent Randles circuit is used with the tribocorrosion current equation to show the time-dependent voltage change with disruption and repassivation of the oxide. Heredity integrals are used to predict voltage over time for any arbitrary current-time path. Experiments using titanium pin-on-disk fretting corrosion are used to assess the theoretical model and to demonstrate its behavior.

Biomimetic nanostructured hydroxyapatite coatings on metallic implant materials

Metallic implant materials used in load-bearing applications are inert in nature in their native state. The surface properties of the material and its interaction with the surrounding physiological fluid determine the success of the biomedical implant. In this regard, bioactive nanostructured coatings are being recognised as potential approach to enhance the biological and corrosion properties of the conventional inert materials. In this review, recent advances in biomedical applications of nanostructured hydroxyapatite coatings on stainless steel implant materials are highlighted with special focus on the electrochemical deposition of hydroxyapatite and their consequent biological activity. Furthermore, osteoblasts functions and cellular activity on the nanostructured hydroxyapatite coatings processed by other techniques is also discussed. The potential application of such next generation materials as biomedical implants is also addressed.

Influence of the annealing treatment on the tribocorrosion properties of Ca and P containing TiO2 produced by plasma electrolytic oxidation

Titanium is widely used as implant material; however, regardless of the excellent properties such as low density, corrosion resistance and biocompatibility, it usually presents poor tribological behaviour. The plasma electrolytic oxidation (PEO) technique produces a high hardness ceramic layer on the titanium surface, by the interaction of anodic oxide growth and plasma channel shock caused by the dielectric breakdown at high voltages taking place in an aqueous electrolyte. The characteristics of the oxide layer, such as the roughness, thickness, porosity, crystallinity and chemical composition, can be tailored changing the PEO parameters or by a posterior annealing treatment at temperatures above 400 °C. This work aims to evaluate the influence of the PEO voltage and annealing treatment on the tribocorrosion properties. Cp–titanium were submitted to PEO treatment at 300 and 400 V for 1 min in an electrolyte containing Ca and P. The annealing treatment was carried out at 600 °C for 1 h following slow cooling at furnace. Surface properties were evaluated by X-ray diffraction, scanning electron microscopy (SEM/EDS) and profilometry. Tribocorrosion was evaluated using a linear reciprocating ball on flat tribometer with a three-electrode electrochemical cell coupled to potentiostat/galvanostat in Phosphate Buffered Saline solution. Results show that PEO layer increases the tribocorrosion resistance of bare titanium significantly. The tribocorrosion resistance is also increased by the presence of rutile phase through voltage or posterior heat treatment.

Mechanically assisted crevice corrosion in metallic biomaterials: a review

Mechanically assisted crevice corrosion (MACC) of metallic biomaterials continues to be a concern for highly loaded medical devices for spinal, dental, cardiovascular and orthopaedic applications. Increasing usage of modularity (multiple-component system) and mixed-alloy metal–metal junctions in orthopaedic surgery gives the surgeon increased intraoperative flexibility for choosing optimal components; however, these design changes can accelerate corrosion reactions and significantly impact local biological processes and mechanical integrity of the implants. The goal of this review is to discuss the MACC processes observed in modular implants especially in orthopaedic hip prosthesis. The concept associated with MACC is described to provide insight into why corrosion is a critically important factor in the long-term performance of devices. Specific examples of MACC in vivo followed by summary of in vitro testing and recent biological assessments of orthopaedic implants are discussed.

Corrosion characterisation of medical alloys modified by forming titanium nanotubes via anodic oxidation and annealing process

Abstract Three medical alloys Ti–6Al–4V, Ti–10Co and Ti–20Co were treated to form Ti nanotubes (TNTs) by anodic oxidation process. The average diameter of TNTs for Ti–6Al–4V and Ti–10Co alloys was 35 and 100 nm respectively. Owing to the α and β phases of Ti–6Al–4V alloy, TNTs had different morphologies. Porous and dendrite structures were formed on the surface of Ti–20Co alloy. X-ray diffraction (XRD) patterns, optical micrograph images and scanning electron microscope images were used to analyse the produced structures. The XRD profiles of the treated Ti–6Al–4V alloys illustrate the amorphous, anatase and rutile structures of as anodised, annealed samples respectively. The TNTs over β phase areas dissolved after anodic process from the surfaces of Ti–6Al–4V and Ti–10Co alloys. Rutile phase was the dominant phase in all XRD patterns of all annealed alloys at high temperatures. The effects of anodising process and annealing treatment on the corrosion parameters of the used alloy surfaces inside the simulated body fluid are studied. The annealing process enhanced the corrosion parameters for all samples due to the formation of a thick oxide layer.

Effect of mixed alloy combinations on fretting corrosion performance of spinal screw and rod implants.

Spinal implants are made from a variety of materials to meet the unique mechanical demands of each application. However, the medical device community has raised concern about mixing dissimilar metals in an implant because of fear of inducing corrosion. There is a lack of systematic studies on the effects of mixing metals on performance of spinal implants, especially in fretting corrosion conditions. Hence, the goal was to determine whether mixing stainless steel (SS316L), titanium alloy (Ti6Al4V) and cobalt chromium (CoCrMo) alloy components in a spinal implant leads to any increased risk of corrosion degradation. Spinal constructs consisting of single assembly screw-connector-rod components were tested using a novel short-term cyclic fretting corrosion test method. A total of 17 alloy component combinations (comprised of SS316L, Ti6Al4V-anodized and CoCrMo alloy for rod, screws and connectors) were tested under three anatomic orientations. Spinal constructs having all SS316L were most susceptible to fretting-initiated crevice corrosion attack and showed higher average fretting currents (∼25 - 30 µA), whereas constructs containing all Ti6Al4V components were less susceptible to fretting corrosion with average fretting currents in the range of 1 - 6 µA. Mixed groups showed evidence of fretting corrosion but they were not as severe as all SS316L group. SEM results showed evidence of severe corrosion attack in constructs having SS316L components. There also did not appear to be any galvanic effects of combining alloys together.

Material dependent fretting corrosion in spinal fusion devices: Evaluation of onset and long-term response: MATERIAL DEPENDENT FRETTING CORROSION

Posterior spinal fusion implants include number of interconnecting components, which are subjected to micromotion under physiological loading conditions inducing a potential for fretting corrosion. There is very little known about the fretting corrosion in these devices in terms of the minimum angular displacement (threshold) necessary to induce fretting corrosion or the amount of fretting corrosion that can arise during the life of the implant. Therefore, the first goal was to evaluate the threshold fretting corrosion in three anatomical orientations and second the long-term fretting corrosion for the three different material types of spinal implants under physiological loading conditions. In threshold test, axial rotation exhibited highest changes in open circuit potential (VOCP in mV) and induced fretting currents (Ifrett in µA) for cobalt chrome (ΔVOCP : 24.71 ± 5.53; ΔIfrett : 4.03 ± 0.51) and stainless steel (ΔVOCP : 28.21 ± 6.97; ΔIfrett : 2.98 ± 0.42) constructs whereas it was flexion-extension for titanium constructs (ΔVOCP : 4.51 ± 2.48; ΔIfrett : 0.38 ± 0.12). Long-term test indicated that the titanium (VOCP :101 ± 0.06; Ifrett : 0.07 ± 0.02) and cobalt chrome (VOCP : 140.67 ± 0.04; Ifrett : 0.12 ± 0.05) constructs were more resistant to the fretting corrosion compared to stainless steel (VOCP : -135.33 ± 0.31; Ifrett : 2.63 ± 1.06).

Electrochemical and mechanical behavior of implantable materials

Most implantable materials used in medical implants are vulnerable to degradation due to corrosion during their service life. The purpose of this special issue is to review the current understanding of the electrochemical, mechanical and biological processes that are responsible for the degradation of a variety of implantable biomaterials. The specific focus is to establish a forum for the dissemination and discussion of recent advances in understanding degradation mechanisms in most of the metallic implantable materials including stainless steel, titanium and its alloys, cobalt chromium, magnesium alloys, nitinol and other biomaterials used in medical devices (orthopaedic, dental, spinal, and other applications). There is abundance of literature on retrievals and in vitro studies elucidating corrosion-related concerns in metallic medical devices. Despite these findings, corrosion in medical devices is not well understood. This speaks to the need for extending research efforts to gain insights into fundamental understanding of the corrosion processes associated with implantable materials used in the medical devices. The electrochemical and mechanical aspect of corrosion processes (pitting, crevice, intergranular, stress corrosion cracking, galvanic, fretting, corrosion fatigue and wear) and related issues in different types of metallic biomaterials is comprehensively discussed by Sridhar et al.1 A complex combination of the above corrosion processes, so-called ‘Mechanically assisted Crevice Corrosion (MACC)’ is described in the review article by Mali.2 MACC continues to be a serious concern for highly loaded medical devices especially modular hip prosthesis. MACC is a multifactorial process that includes mechanical, geometric, chemical and electrochemical interactions which can lead to accelerated increase in corrosion rates, thereby decreasing structural and mechanical integrity of the implant.2 The review article also provides discussion on retrieval analysis, in vitro testing and recent biological evaluations on orthopaedic implants and materials. The corrosion of biomaterials used for dental implants also has a significant clinical relevance. In a review article, Chaturvedi3 has provided an overview of various aspects of dental implant materials and its interaction with oral environment. Metallic dental implant materials are susceptible to corrosion processes in the presence of hostile electrolytic oral environment (low pH, high fluoride ion concentration).3 Magnesium (Mg) alloys are emerging as important candidates for biodegradable temporary implants applications (e.g. plates, screws, wires, etc.) given their strength-to-weight ratio and biocompatibility. However, in spite of significant research and improvement, problems of fast degradation rates for Mg-based alloys continue to be a challenge for their successful implementation for a variety of applications in the body. Current research now focuses on developing improved Mg alloys with lower biodegradation rates and without any potential toxicity problems of the alloying elements. The biodegradation mechanism of Mg alloy and methods to improve their corrosion resistance by alloying and surface treatment is described by Chen et al.4 Similarly, Trivedi et al.5 have described the state-of-art knowledge on new class of Mg alloy with rare earth metals for use as biodegradable implant with improved mechanical and biodegradation properties. Titanium and its alloy are one of the most used metals for biomedical applications owing to their excellent mechanical properties, corrosion resistance and biocompatibility. Despite its excellent properties, titanium and its alloy are susceptible to tribocorrosion processes, stress shielding and possible toxicity issues related to alloying elements (vanadium, aluminium). Significant advancement have been made in the field towards development of new titanium alloys with low modulus,6 surface treatment methods and coatings to improve surface-related properties including hardness, wear, corrosion and tribocorrosion.6, 7 An interesting surface modification technique, Plasma Electrolytic Oxidation (PEO) for titanium is described by Laurindo et al.8 In this study, the porosity, chemical composition, crystalline structure and oxide layer thickness were modified due to increase in PEO voltage and annealing treatment, leading to improved wear resistance of the titanium. I have been greatly privileged to act as Guest Editor for this Special Issue of Materials Technology: Advance Performance Materials on such an important and interesting subject. I hope that the material presented in this collection of eight original Review and Research papers will provide a glimpse of the various research and development activities already recorded in this field while discussing further topics which needs additional assessments. I am grateful to the Editor, Professor Devesh Misra, Editorin-Chief, for constant encouragement and support. I would also like to thank and congratulate the authors for their timely and greatly appreciated contributions to this Special Issue.