Efeito do duplo envelhecimento e nitretação por plasma em baixa temperatura na tribocorrosão do inconel 718

Abstract

The nickel-based superalloy Inconel 718, employed in the petrochemical industry due to its high mechanical strength and corrosion resistance, may have its applicability limited under specific operating conditions because it exhibits low wear resistance in the single-aged condition. This limitation is particularly relevant in offshore oil and gas production, where the alloy is subjected to tribocorrosion. Consequently, treatments such as double aging or plasma nitriding have emerged as effective approaches to overcome these drawbacks. However, the literature still shows a scarcity of studies addressing the tribocorrosion behavior of plasma-nitrided Inconel 718. In this context, the present work aimed to analyze the effect of double aging and low-temperature plasma nitriding on the tribocorrosion resistance of the Inconel 718 alloy. The material was subjected to microstructural, topographical, mechanical, and triboelectrochemical characterizations. Optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were employed to analyze the microstructure, as well as the corrosion and wear mechanisms, after the triboelectrochemical tests. Optical interferometry was used to evaluate surface roughness, topography, and the volume of material removed during the triboelectrochemical experiments. Vickers microhardness measurements were conducted to quantify the hardness increase induced by double aging and plasma nitriding. In addition, cathodic polarization, open-circuit potential (OCP), and linear polarization resistance (LPR) tests were performed in a reciprocating ball-on-flat sliding system, all in a saline medium (3.56% NaCl), to determine material loss rates due to pure corrosion, pure wear, tribocorrosion, and the synergistic effect. Furthermore, the predominant factor governing the material loss rate under tribocorrosion conditions was identified. The main results indicated that plasma nitriding led to the formation of nitrided layers composed of γN and CrN phases, with similar thicknesses and hardness values, regardless of the thermal treatment applied prior to nitriding. Low-temperature plasma-nitrided conditions exhibited similar pure corrosion rates, higher hardness, higher corrosion potentials, and improved behavior under cathodic polarization compared with the non-nitrided material. Conversely, the condition nitrided from a single-aged substrate tended to result in higher material loss due to tribocorrosion, which was attributed to layer delamination and a pronounced synergistic effect, where corrosion was identified as the predominant factor accelerating alloy degradation. Therefore, the combined double-aging and plasma-nitriding treatment proved to be the most effective in protecting Inconel 718, as it did not exhibit cracking or delamination on the wear tracks after tribocorrosion testing, showed greater recovery in polarization resistance after tribocorrosion, and demonstrated a tendency toward lower pure wear under cathodic protection conditions.