Propriedades de corrosão e caracterização química – metalográfica de próteses em aços inoxidáveis ISO 5832-9 e F138, removidas de pacientes

Abstract

This work has studied the chemical and metallographic composition (microstructure, grain size, inclusion rate) of austenitic stainless steels which were developed to be applied as biomaterials, and used in the production of orthopedical implants, removed from patients which had been affected by inflammation; it has also been compared the sample surfaces, by means of scanning electronic microscopy (SEM) after electrochemical attack. The results of chemical analysis, performed by optical emission spectroscopy and Energy Dispersive Spectroscopy (EDS) have shown that all samples are in agreement with the ASTM F138-92 e ABNT NBR ISO 5832-9:2008 regulations, whereas the grain size has been determined by optical microscopy and SEM. The surfaces of the samples were analyzed by SEM and EDS techniques; samples were submitted to potentiodynamic cyclic polarization tests in 50 mV potential after the passivation region in Ringer lactate and NaCl 0,9 mol.L-1 solutions at 36,5C; such SEM measurements were done to identify the type of corrosion over the samples. It has been concluded that all implants showed average grain sizes above the recommended by the regulations; also the presence of delta ferrite in eight of the twelve investigated implants has been seen, which according to the ASTM 138-92 regulation this would not been possible even with a 100-fold increase at the optical microscope. It has also been verified that in NaCl medium the ISO 5832-9 steel did not present any kind of localized corrosion, instead of F 138 steel; in Ringer lactate solution, after 15 minutes attack at 1000mV/SCE, has been noted the presence of pits for ISO 5832-9 steel. The cyclic polarization measurements have revealed that one of the investigated steels, named ISO 5832-9, presented a very high resistance to localized corrosion when compared to the F 138 samples, besides they did not release metallic ions to the electrolytic solutions. This characteristic can be mainly attributed to the increasing of the passivation film stability, which is also favored by the presence of nitrogen atoms in the solid solution composing the structure of ISO 5832-9 steel. The pit corrosion tests have shown very useful to indicate the low resistance to pit corrosion, present “in-vitro” in all the studied samples. The EDS mapping analysis of F 138 samples, submitted to electrochemical attack in Ringer lactate medium, have shown a loss of 63% of nickel and 26% of iron, when compared to the initial composition; such metals were electrochemically solubilized to the Ringer lactate solution, where they can probably precipitate from the solution, being a strong indication that the reactions taking place in the patients have been motivated by these metal ions, which can be incorporated by the human body. The obtained results of this work are the basis of a tentative proposition to the governmental organisms which are the responsible for the regulation of the technical specifications for the prosthesis producers, requiring from them technical reports assuring the quality for the implant pieces, i.e., for each manufacturing lot a description or a special certificate containing the chemical composition and the metallographic characteristics specified by the Brazilian laws.