Efeito da precipitação de fase secundária na resistência à corrosão da superliga UNS N26455
Alano, José Henrique
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The UNS N26455 is a superalloy of the Ni-Cr-Mo system used on offshore platforms as components of injection pumps. This alloy has low-carbon content and high levels of Cr and Mo. This composition gives the material high resistance to various forms of corrosion in different types of environments. However, in some aggressive conditions containing high concentrations of chloride, high temperatures and presence of corrosive gases, the alloy UNS N26455 may show localized corrosion. As an aggravating factor, prolonged exposure of this alloy at high temperatures may favor the preferential precipitation of intermetallic phase P. This precipitate is rich in Cr and Mo and changes the composition of the alloy in the matrix/precipitate interface, and in turn promotes localized corrosion of the material. In this work, the effect of P phase precipitation in localized corrosion resistance was studied using electrochemical techniques: anodic potentiodynamic polarization, cyclic polarization, potentiostatic technique, crevice repassivation potential (THE), electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS). The experiments were carried out in environments containing high concentrations of chloride ions (100.000 e 180.000 ppm), with and without CO2 saturation, and at temperatures of 25 °C, 40 °C and 75 °C. The potentiodynamic results indicated that the alloy UNS N26455 in as-cast condition is extremely resistant to pitting corrosion. On the other hand, increasing the concentration of precipitates in the microstructure of the alloy, active dissolution occurs in potentials above 250 mVAg/AgCl. After potentiostatic testing and analysis by SEM, it was found that the dissolution occurs at the interface matrix/precipitate. The repassivation potential is not affected by the concentration of P phase, however, the weight loss obtained in the crevice corrosion test increased with the addition of the second phase in the microstructure of the alloy. The EIS results showed that the polarization resistance decreases with increasing P precipitates, indicating changes in the characteristics of the passive viii film. MS experiments showed that the passive film formed on alloy UNS N26455 shows n-type semiconducting behavior and the donor density increases with increasing concentration of the second phase.