Comportamento em corrosão do aço inoxidável austenítico ASTM F138 na presença de fluido corporal simulado

Abstract

The austenitic stainless steel ASTM F138 is widely used in the medical orthopedic field in the form of pins, fixation plates, and rods, primarily due to its high corrosion resistance, biocompatibility, and good surface finish. This steel is a 316LVM-type alloy, vacuum melted to ensure a microstructure free from delta ferrite and deleterious intermetallic phases such as sigma and chi. This process is responsible for ensuring purity and enhancing pitting corrosion resistance—essential characteristics for the aforementioned applications. Generally, ASTM F138 steel is marketed in the solution-annealed condition, achieved through a heat treatment process that dissolves undesirable carbides, such as chromium carbide, which is known to reduce resistance to intergranular corrosion caused by sensitization. Potentiodynamic polarization tests can be performed to characterize the corrosion behavior of the material in a given medium, providing information such as corrosion potential and susceptibility to pitting. In such tests, a potentiostat is used to continuously sweep the electrode potential at a controlled rate while measuring the current. In the present study, the corrosion behavior of this steel was analyzed under three different conditions: cold-rolled samples, solution-treated samples at 1050°C for 1 hour—which aims to produce the optimal microstructure—and sensitized samples at 675°C for 6 hours—a condition that exposes the steel to a higher tendency for intergranular corrosion. The corrosive medium used was phosphate-buffered saline (PBS), a simulated body fluid that contains the main corrosive components present in the human body, as well as osmolarity and ion concentrations similar to those of human fluids. In this context, the aim was to compare the performance of the steel in the three conditions using potentiodynamic polarization tests, which provide corrosion curves that relate potential and electrical current, in the presence of PBS. From the results, it was observed that the pitting potential was very similar for the cold-rolled and solution-treated samples, and their corrosion behavior was distinguished through analysis of metastable pit nucleation and repassivation characteristics. The sensitized samples, in contrast, showed a lower pitting potential and, consequently, breakdown of the passive film at much lower potentials when compared to the other conditions, indicating that the sensitization process was responsible for the reduced corrosion resistance of the steel under study. Thus, it can be concluded that heat treatment plays a fundamental role in the corrosion performance of ASTM F138 steel, with the solution-treated condition being the most suitable as it ensures greater stability of the passive layer, while sensitization significantly compromises this protection, reducing the steel’s corrosion resistance in a simulated physiological environment.