Tratamento termoquímico assistido a plasma para incremento da resistência à corrosão de aço-carbono

Abstract

This paper presents a study to facilitate the application of duplex treatment of nitriding and post oxidation plasma as a way of enhancing the corrosion resistance of carbon steel. The influence of microstructure, phase distribution and chemical composition of crystalline corrosion behavior of the material were evaluated, and the role of inclusions content was analyzed. The study of the crystalline phases present was determined by means of X-ray diffraction (XRD). The morphology and thickness of the layers was evaluated by scanning electron microscopy (SEM). Analyzes of chemical composition on the surface and cross section were determined by spectroscopy of glow discharge optical emission (GD-OES) spectroscopy and X-ray energy dispersive (EDS). Two complementary techniques for evaluation of corrosion resistance, cyclic potentiodynamic polarization (CPP) and salt spray were performed, being the second is the technique used by the industry for reference. The treatment of post-oxidation was formed only the stage magnetite desirable to have improved mechanical properties compared to hematite. Parameters for 90 minutes at 480°C of post-oxidation provided the best characteristics of the outer layer in terms of oxide microstructural uniformity and high due to the absence of defects. PPC Tests corroborate this type of microstructure has the highest corrosion resistance. In salt spray there was no reproducible results, and further analysis indicated the presence of inclusions of manganese sulfide (MnS) damaging the oxidized layer. We calculated the severity level of the samples according to ASTM E-45 and this value was related with the time of corrosion resistance in salt spray test. If the level of severity of the 1045 steel is less than 2, one can replace the application of the industrial hard chromium by nitriding and post-oxidizing plasma process proposed, in regard to corrosion resistance.