Surface and intergranular oxidation in simulated primary water of model Alloy 600 with aluminum and magnesium oxide inclusions.

Abstract

In 2011, intergranular stress corrosion cracking (IGSCC) was reported in Gravelines (France) nuclear power plant, in a bottom mounted instrumentation (BMI) nozzle manufactured with Alloy 600. Extended oxide inclusion bands were evidenced after destructive examinations, suggesting that these metallurgical defects probably promoted initiation of IGSCC. In this context, a study aimed at better understanding and quantifying the effect of oxide inclusions bands on intergranular oxidation and IGSCC initiation. As a first step, an emphasis was made on describing the oxidation behavior of the oxide inclusions. Model alloys that consist in a matrix of Alloy 600 containing bands of Al-rich and (Al,Mg)-rich oxide inclusions were casted. Fine characterizations of the as-received surface were conducted using scanning electron microscopy (SEM) coupled with energy-dispersive X-Ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) analyses, electron microprobe quantifications and transmission electron microscopy. Oxidation tests were performed in simulated hydrogenated primary water at 290°C, 325°C and 360°C using flat samples. The influence of oxide inclusions on the intergranular oxidation of A600 was investigated by comparing the as-received characterizations analyses with the ones following short (10 h), semi-short (304 h) and long (2015 to 7180 h) exposures to the simulated primary water. Hypotheses on the oxidation mechanism of such inclusions in simulated primary water conditions were proposed.