Influência da concentração de hidrogênio na suscetibilidade à corrosão sob tensão de junta soldada com metais dissimilares em ambiente do circuito primário de reator nuclear

Abstract

The stress corrosion cracking (SCC) is a phenomenon that occurs in nuclear power plants metallic components where susceptibility materials are subjected to the simultaneously effect of mechanical stress and an aggressive media with different compositions. SCC is one of degradationprocess that gradually introduces damage of components, change their characteristics with the operation time. The nickel alloy 600, and their weld metals (nickel alloys 82 and 182), originally selected due to its high corrosion resistance, it exhibit after long operation period (20 years),susceptibility to the SCC. The actual alternatives to mitigate nickel alloy SCC damage are weld overlay, changes in the PWR primary water chemical environment by zinc addition and by optimizing dissolved hydrogen levels. In the last years, experimental studies have indicated that the dissolved hydrogen added to prevent radiolysis of the water might be affected with regard to nickel based alloys SCC susceptibility. Changes in the hydrogen contents might do that the corrosion potential reaches the Ni/NiO transition line, and have some influence in the stresscorrosion cracking behavior. In the present study, the influence of dissolved hydrogen contents on the susceptibility to SCC of nickel alloy 182, used as weld metal in a dissimilar weld between the steel ASTM A-508 G3 and stainless steel AISI 316L, similar of the weld exist in a pressurizer nozzle of Angra 1 nuclear power plant was evaluated. In this study was used a simulated PWR primary coolant water chemistry at 325ºC and pressure of 12,5 MPa containing dissolved hydrogen of 2, 10, 25, 50 cm3 H2/kg H2O at standard temperature and pressure (STP). Slow strain rate tensile test (SSRT) was used to evaluate the nickel alloy 182 SCC susceptibility. Open circuit potential was measured in different hydrogen concentrations to evaluate their effect in the material electrochemical corrosion. The results indicated that at 325ºC nickel alloy 182 is less susceptible to SCC at 50 cm3 H2 (STP)/kg H2O, showed the benefic effect in maintain hydrogen concentration in a high level in the PWR primary coolant water