Effect of biologically relevant ions on the corrosion products formed on alloy AZ31B: An improved understanding of magnesium corrosion

Abstract

Simulated physiological solutions mimicking human plasma have been utilized to study the in vitro corrosion of biodegradable metals. However, corrosion and corrosion product formation are different for different solutions with varied responses and, hence, the prediction of in vivo degradation behavior is not feasible based on these studies alone. This paper reports the role of physiologically relevant salts and their concentrations on the corrosion behavior of a magnesium alloy (AZ31B) and subsequent corrosion production formation. Immersion tests were performed for three different concentrations of Ca²⁺, ${\text{HPO}}_4^{2-}$, ${\text{HCO}}_3^{-}$ to identify the effect of each ion on the corrosion of AZ31B assessed at 1, 3 and 10 days. Time-lapse morphological characterization of the samples was performed using X-ray computed tomography and scanning electron microscopy. The chemical composition of the surface corrosion products was determined by electron dispersive X-ray spectroscopy and X-ray diffraction. The results show that: (1) calcium is not present in the corrosion product layer when only Cl⁻ and OH⁻ anions are available; (2) the presence of phosphate induces formation of a densely packed amorphous magnesium phosphate corrosion product layer when ${\text{HPO}}_4^{2-}$ and Cl⁻ are present in solution; (3) octacalcium phosphate and hydroxyapatite (HAp) are deposited on the surface of the magnesium alloy when ${\text{HPO}}_4^{2-}$ and Ca²⁺ are present together in NaCl solution (this coating limits localized corrosion and increases general corrosion resistance); (4) addition of ${\text{HCO}}_3^{-}$ accelerates the overall corrosion rate, which increases with increasing bicarbonate concentration; (5) the corrosion rate decreases due to the formation of insoluble HAp on the surface when ${\text{HCO}}_3^{-}$, Ca²⁺, and ${\text{HPO}}_4^{2-}$ are present together.

Introduction

Biodegradability and similar mechanical properties to human bone make magnesium a promising candidate for implantable materials used in medical devices. Recent efforts to develop more biocompatible metallic implants have focused not only on magnesium alloy design to match the engineering requirements demanded of medical devices [1], [2], [3], but also on understanding the degradation behavior in specific biomedical application environments [4], [5], [6], [7]. The biodegradation of metallic magnesium is fundamentally linked to studies of its “corrosion”, which is dependent on the interface dynamics between the material and its environment. Identifying and understanding the biological and material factors that govern the corrosion kinetics and mechanisms are of prime importance to the successful development of biodegradable implants.

Currently, applying in vitro test results generated using simulated body fluids (SBFs) to predict in vivo degradation behavior is unreliable because factors affecting the complex biomedical environment, such as the concentrations of salts, flow dynamics, protein absorption and active tissue formation, cannot be replicated solely by immersion in SBFs. Previous studies have used various solutions, such as NaCl solution [8], [9], Hank’s balanced salt solution [10], [11], SBF [12], [13], [14] and Dulbecco’s modified Eagle’s medium (DMEM) [15], [16] to study the corrosion behavior of magnesium alloys. Additionally, the effect of different inorganic ions, such as Cl^–[13], [17], [18], Ca²⁺[13], ${\text{HCO}}_3^{-}$[17], [18], [19], [20], and ${\text{HPO}}_4^{2-}$[13], [18], [21], have been investigated. However, the mechanism(s) of corrosion and corrosion product formation upon exposure to individual ions combined with salts and their concentrations is not well established.

This paper is an attempt to identify the effects of biologically relevant salts and their concentrations on the corrosion behavior and corrosion product formation using the standard immersion test. It is thought that this fundamental understanding of in vitro corrosion studies will ultimately provide a basis for comparison with results from in vivo studies while isolating and characterizing the corrosion products associated with the biodegradation process.