Corrosion behaviour of low-density steels for biomedical applications: a critical review of composition and microstructural effects

Low-density steels (LDS) are promising candidates for next-generation biomaterials. This is attributable to reduced density, tunable mechanical and chemical properties, and intentional elimination of cytotoxic alloying elements commonly found in conventional implants. The corrosion behaviour of LDS in physiological environments is evaluated, while highlighting the mechanistic interplay between composition, processing, microstructure, and in vitro/in vivo degradation assessment. The incorporation of minor amounts of passivating elements (Cr, Mo, and Ti) enables lean alloy designs that promote the formation of adherent and tenacious oxide films. However, excessive alloying can lead to compositional saturation, solute instability, and the precipitation of deleterious intermetallic phases, thereby compromising structural integrity and accelerating corrosion. Microstructural effects are significantly pronounced: solution heat treatment of Fe-Mn-Al-C systems reduces micro-galvanic coupling, while grain refinement has been shown to substantially low corrosion rates (∼50% compared to coarse grain sizes). Furthermore, enhanced biostable surface films and tailored surface morphologies are achievable by surface engineering techniques such as gas nitriding. For optimised composition and processing parameters, better corrosion resistance are observed for LDS in simulated body fluids than established implant materials including 316L stainless steel, Co-Cr alloys, and Ti-6Al-4V. Strategic knowledge gaps and future research directions for the design of robust, biocompatible LDS for biomedical applications are highlighted.