Biodegradation behaviour of Fe-based alloys in Hanks’ Balanced Salt Solutions: Part II. The evolution of local pH and dissolved oxygen concentration at metal interface

Abstract

Commercially pure Fe, Fe35Mn, and (Fe35Mn)5Ag alloys were prepared by uniaxial pressing of the mixture of individual powders, followed by sintering. The influence of the alloying elements Mn and Ag on the corrosion behaviour of these Fe-based alloys was investigated in Hanks’ Balanced Salt Solution (HBSS). Furthermore, the role of the components in HBSS, particularly Ca 2+ ions during alloys degradation was studied. Distribution of local pH and dissolved oxygen concentration was measured 50 μm above the interface of the degrading alloys. The results revealed that 5 wt% Ag addition to Fe35Mn alloy triggered micro-galvanic corrosion, while uniform corrosion dominated in pure Fe and Fe35Mn. Fast precipitation of Ca–P-containing products on the surface of these Fe-based alloys buffered local pH at the metal interface, and blocked oxygen diffusion at the initial stages of immersion. In the (Fe35Mn)5Ag, the detachment or structural changes of Ca–P-containing products gradually diminished their barrier property. These findings provided valuable insights into the degradation mechanism of promising biodegradable Fe-based alloys.

FeMn with Phases of a Degradable Ag Alloy for Residue-Free and Adapted Bioresorbability

“…For the presented results, the influence of the gaps is too small, and the weight of the remaining Ag phases counteracts an effective increase in degradation. Furthermore, the effectiveness of Ag and AgCaLa at low pH values is in accordance with the blocking effect, since the solubility of degradation products increases at low pH values, and the absence of inhibiting layers at low pH values is conclusive [ 44 , 72 , 73 ].…”

“…However, further aspects have to be considered to design such material. The various elements and chemical compounds in the human body result in the formation of various corrosion products, forming a multilayer structure [ 36 , 43 , 44 ]. These structures might affect further degradation.…”

“…The released ions can form phosphates which deposit on the surface. Compounds with Fe and Mn are present on the surface, but the degradation can proceed further since exchange with the electrolyte is still possible [ 16 , 19 , 30 , 44 ]. In contrast, Ca-Phosphate (P), originating from the electrolyte, creates an inhibiting layer deteriorating the degradation rate [ 13 , 19 , 28 , 30 , 36 , 45 ].…”

“…Therefore, the formation of such inhibiting layers must be suppressed to enable further degradation. Wang et al [ 44 ] suggest that noble phases can minimize the blocking effect, as a detachment of inhibiting layers and a changed structure can be observed. This suggestion is confirmed by the increased degradation rate (in vivo) of materials modified with Ag [ 35 , 38 ].…”

“…Achieving a reliable and suited degradation under all possible conditions is challenging, since the degradation of Fe-based alloys strongly depends on the ambient conditions. The degradation is affected by the ingredients of the solution, the pH value, which determines the solubility of the corrosion products, and the amount of dissolved gases, such as oxygen (O), that is required for the cathodic reaction [ 14 , 33 , 35 , 37 , 44 ]. For implant design, the slowest degradation must guarantee a sufficient supporting effect for healing, while the fastest degradation must enable the dissolution in a defined period.…”