Efeito de tratamentos isotérmicos na microestrutura e resistência à corrosão do aço inoxidável duplex UNS S32205 (SAF 2205)

Abstract

Duplex stainless steels (AID’s) have excellent mechanical properties combined with high corrosion resistance. However, inadequate thermal cycling can induce precipitation of intermetallic phases such as sigma and chi, as well as alter the phase ratio. With the objective of adding data for an adequate thermal cycle of the UNS S32205 standard duplex steel, this work evaluated the effect of the isothermal treatment temperatures of 850°C, 950°C, 1000°C, 1150°C with soaking time of 30 minutes followed by cooling in water and in the state of supply (EF). By means of images obtained by optical microscopy in the transverse section of the samples and using etching with Behara reagent (solubilized – 1000 and 1150°C) and modified Behara (aged – 850 and 950°C), it was possible to obtain by quantitative stereology the volumetric fractions in each condition. The precipitation of the sigma phase (σ) occurred at temperatures of 850 and 950°C, with proportions of 7.9 ± 1.6% and 3.3 ± 1.3%, respectively. While, for the EF conditions, 1000 and 1150°C, there is a biphasic microstructure, with ferrite (δ) and austenite (γ), obtaining higher percentages of δ for the highest heat treatment temperature (1150°C). Higher % σ induced higher hardness values. In the cyclic polarization tests, the samples with a more balanced microstructure of δ/γ (50/50), EF and 1000°C, obtained lower generalized corrosion rate values, as well as more protective passive layers and better repassivation capacity. At the temperature of 1150°C, worse performance was observed, associated with the reduction of %Cr in the ferrite, observed by the EDS measurements. Among the conditions with δ, γ and σ, at 950°C, the small percentage of sigma did not impact the generalized and localized corrosion parameters, as well as the stability of the passive film, when compared to the solubilized conditions. While at 850°C, there was a reduction in localized corrosion performance, associated with the galvanic effect promoted by the difference in potentials and alloying elements between phases. Finally, the susceptibility to crevice corrosion was evaluated using an adaptation of the ASTM G192-08 standard. The EF and 1000°C conditions exhibited less crevice corrosion, with lower current densities in the potentiostatic test in the transpassive region (1.1 and 1.2 V), followed by the 1150°C and 950°C conditions. In the 850°C condition, as emphasized in the cyclic polarization, it exhibited a high crevice corrosion rate, due to the presence of the sigma phase and the possible formation of galvanic piles. Given the above, the results show that sigma phase precipitation at levels greater than 8% reduces localized corrosion of the UNS S32205 duplex steel, therefor precipitation of the sigma phase must be controlled and avoided in the industry.