Electrochemical Tests in Stainless Steel Surgical Implants

In the present work the corrosion resistance of stainless steels was evaluated following the specifications of ASTM F-745 standards. The specimens were obtained by two methods, the conventional process of investment casting and countergravity low pressure casting. Cyclic polarization tests were performed according to ASTM F 2129, using a conventional three-electrode cell, with Pt as a counter electrode and saturated calomel as a reference electrode. The experiments were performed at (37 ± 1) °C and, before starting the test, the electrolyte was purged with N2 gas for 15 minutes. As electrolyte, a solution of 0.9% NaCl was used (pH≈7), the open circuit potential (OCP) was monitored for 1 hour and then the potentiodynamic scanning started in the anodic direction, at a speed of 0.167 mV/s, from a potential of 100 mV below the OCP. The sweep was reversed after passing the threshold of current density at a value hundred times greater than the current density of pitting. Measurements of electrochemical impedance spectroscopy were performed after one hour of exposure of the specimens to OCPs performed in a frequency range from 100 kHz to 1 mHz and potential amplitude of ± 10 mV. The microstructure of the samples was analyzed before and after testing using optical microscopy. The results indicated that the samples in general exhibit crack corrosion, and in some cases the occurrence of pitting corrosion was observed. Also, the capacity values were within the range corresponding to the electric double layer and the charge transfer resistance increased on the specimens obtained by conventional casting in ceramic mold.