Avaliação da resistência à corrosão de aço maraging 300 comercial e ligas modificadas com titânio e nióbio em solução de água do mar sintética

Abstract

Maraging steels belong to the class of low-carbon alloys, presenting a martensitic microstructure, giving the material useful properties such as high mechanical strength, toughness and, mainly, high corrosion resistance, in addition to diverse applications in the aerospace and military sectors, the chemical industry, parts for automobile competitions, uranium enrichment plants, among others. Given their versatility, such alloys can serve as an alternative for oil extraction pipelines on offshore platforms, where the solution used to fill the explored region has a composition similar to seawater. Seeking to improve their properties, including corrosion resistance, new alloying elements, titanium and niobium, were added. To evaluate the influence of titanium and niobium contents on the corrosion resistance of maraging steels, this study analyzed the behavior of commercial 300 maraging steel and alloys modified by the amounts of titanium and niobium, in synthetic seawater, standardized according to current standards. For this purpose, electrochemical tests were performed to monitor open circuit potential and linear potentiodynamic polarization, microstructural characterization via X-ray diffractometer (XRD), optical microscope and scanning electron microscope (SEM), in addition to the analysis of mechanical properties by Rockwell C hardness test. The samples were produced using the Vacuum Induction Melting (VIM) process, with the commercial 300 maraging steel also undergoing the Vacuum Arc Remelting (VAR) process. Upon receipt, the samples were homogenized at 1200 °C for 6 hours, followed by hot rolling at 1060 °C, followed by solution and aging heat treatments. The results indicate slightly greater corrosion resistance for all modified alloys, resulting in more noble corrosion potentials (Ecorr) and lower current density (icorr) compared to the commercial alloy. Sample A, modified only in titanium content, achieved the best performance, with values of - 0.36 V and 1.02 x 10-5 A/cm2, respectively. However, all samples suffered uniform and widespread corrosion. The material with the best hardness yield was the sample with Nb addition, with an average value of 53.72 HRC. Microstructure and planes consistent with the martensite phase were identified in all modified samples, according to the results obtained by microstructural characterization.