Efeito de tratamentos isotérmicos no comportamento em corrosão, tribologia e tribocorrosão do aço inoxidável duplex UNS S32205

Abstract

Duplex stainless steels (DSS) have the excellent combination of high mechanical and corrosion resistance. Yet, inadequate thermal cycling can induce the formation of intermetallic phases such as sigma, affecting the mechanical properties and the corrosion resistance. In this context, the present work aims to evaluate the effect of aging (850 and 950 ºC) and solubilization (1000 and 1150 ºC) heat treatments, and assess their performance in Intergranular Corrosion (CI), tribology, and tribocorrosion. The proportion of the phases was determined using the quantitative stereology technique. The results evinced the presence of sigma phase in samples treated below 1000 ºC, while the largest volumetric proportion was achieved at 850 ºC. Additionally, for samples with treatments commencing at the temperature of 1000 ºC, it was noted that the higher the temperature, the greater the equiaxiality of the grains and the bigger the ferrite fraction obtained. In the DL-EPR experiments conducted in H2SO4 solutions at the concentrations of 0,5; 1,5 and 2,5 mol/L with the addition of 1 mol/L of Hcl, it was found that the sensitization only occurred to samples submitted to 850 ºC in an electrolyte composed by H2SO4 2,5 mol/L and Hcl 1 mol/L. Even though the condition of 950 ºC presented the sigma phase, the superior corrosion resistance of it was associated with the morphology of the sigma phase, presenting smaller boundary areas. The morphology, sigma phase quantity and the concentrations of the sulphuric acid solution employed are determinant factors in the intergranular corrosion resistance of the UNS S32205 duplex stainless steel. Regarding the hardness evaluated by nanointentation, the α e γ grains varied between approximately 3,6 and 4,1 GPa, while for σ, between approximately 8 and 8,3 GPa. As for the tribological performance in microwear, it was observed a reduced wear profundity at the σ grains. In particular, the aged conditions presented lower friction coefficient values associated with higher hardness values due to the σ precipitation. Furthermore, the preliminary tribocorrosion results were conducted in the base material (MB) to evaluate the influence of the anodic potential (PA), cathodic potential (CP) and OCP in the parameters of the system in a 0,5 M NaCl solution. Additional experiments were conducted in distilled water. The conditions were divided in two categories in terms of friction and wear: I) higher COF and lesser wear volume for the conditions with PC smaller or equal to – 0,8 V (vs Ag/AgCl 3M), as well as in distilled water; II) smaller COF and higher wear volume for PA, OCP and 0,4 V. The behavior of each group is directly related with the chemical characterization inside the tracks by SEM-EDS and Raman spectroscopy. In all of the conditions evaluated in tribocorrosion it was observed the predominant presence of abrasion scratches in the direction of the sliding. In the materials submitted to the heat treatments, it was observed in the PA of 0,5 V vs Ag/AgCl 3M severe wear in the condition S850 due to a bigger percentage of the σ and its morphology. While for the other conditions ( -0,8 V, OCP and – 0,4 V)the condition of 850 ºC exhibited a bigger wear resistance and smaller COF. The synergy fraction related with the wear induced by corrosion (∆Wc) was superior to the one related with corrosion induced by wear (∆Cw) for all the heat treatments. Yet, the reason between them point to significant contributions and interactions among the mechanical and corrosive events in the tribosystems in study.