Comportamento frente a corrosão do aço inoxidavél duplex UNS S32205 nitretado e soldado

Abstract

This work presents a study on the corrosion behavior of duplex stainless steel UNS S32205 treated with low-temperature thermochemical whit plasma nitriding and welded using the TIG process according to industry-standard parameters. The main topics addressed in this study are localized corrosion (pitting and crevice), wear, and tribocorrosion. The results of these studies were presented in three publications in scientific journals: “The effect of plasma nitriding on the synergism between wear and corrosion of SAF2205 duplex stainless steel”, “Effect of lowtemperature plasma nitriding on corrosion and surface properties of duplex stainless steel UNS S32205” and “Pitting and crevice corrosion behavior of the duplex stainless steel UNS S32205 welded using the GTAW process”. The main conclusions of this study are as follows: Lowtemperature plasma nitriding (380 °C) in duplex stainless steel UNS S32205 promoted the formation of a continuous layer of expanded austenite with an approximate thickness of 3,5 µm and a hardness of which is three times the hardness of the substrate. Cyclic polarization tests revealed an improvement in the resistance to localized corrosion of the nitrided steel, with higher pit potentials, repassivation potentials, and a smaller hysteresis loop compared to the untreated condition. The presence of an expanded austenite layer resulted in a reduction in pure wear and wear rate due to the effect of tribocorrosion. The TIG welding process employed in duplex stainless steel UNS S32205 maintained the electrochemical behavior of the material close to that of the base metal. This is due to several factors, such as the balanced 50/50 ratio of austenite-ferrite phases throughout the weld metal, the absence of detrimental precipitates (σ, χ), and the proper distribution of alloying elements between the base metal and the weld metal. Despite the microstructural and chemical composition changes resulting from both evaluated processes (nitriding and welding), they did not compromise the main characteristic of duplex stainless steel, which is its resistance to corrosion in chloridecontaining environments.