A new strategy for corrosion protection of porous stainless steel using polypyrrole films

In this work, a method to improve the protection against corrosion of porous sintered stainless steel is presented. It is based on the electrodeposition of polypyrrole (PPy) layers doped with a large size counterion such as dodecylbenzenesulphonic acid (DBSA), a conducting polymer with high corrosion resistance and good biocompatibility. The efficacy of PPy coating depends on the adequate adhesion between the metal substrate and the coating layer. The protection against corrosion has been tested using 316 L stainless steel (SS) powder sintered at different conditions to evaluate the effect of the sintering atmosphere (nitrogen and vacuum) and cooling rates (furnace and water) on corrosion resistance, while wrought 316 L SS has been used as reference material. In addition, two electrochemical deposition techniques have been tested to select the most adequate. Open circuit potential evolution, anodic polarization measurements and electrochemical impedance spectra have been used to evaluate corrosion protection in phosphate buffer saline medium. It has been evidenced that a more homogeneous and stable coating was obtained in the case of porous stainless steel. The corrosion potential shifted to nobler values and the anodic polarization branch became more stable. Coated porous samples have a good passivation performance with a lower stable passive current density and a higher breakdown potential. The transfer electronic resistance and the impedance module increase more than one order of magnitude. Therefore, the porosity of sintered stainless steel is seen as an advantage for the improvement of the adherence of the PPy coatings. The best corrosion protection is found for samples sintered in nitrogen and water-cooled.