Double loop electrochemical potentiokinetic reactivation test of Nickel-base Alloy 600 surface-melted by a CO2 laser beam

Abstract

Ni-base Alloy 600 has been widely used as a steam generator (S/G) tubing material in nuclear power plants because of its good mechanical and corrosion properties at high temperatures. However, degradations of S/G tubes due to intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) during normal operation have been frequently reported. In particular, Alloy 600 can be very susceptible to IGA/IGSCC in some sulfur-bearing environments by sensitization. In this paper, the beneficial effects of laser surface melting (LSM) on intergranular corrosion of the sensitized Alloy 600 is presented from the results of the double loop electrochemical potentiokinetic reactivation (DL-EPR) test. The DL-EPR test was performed in de-aerated 0.01 M H 2 SO 4 +20 ppm KSCN at a scan rate of 0.5 mV/sec at room temperature. The degree of sensitization (DOS) of the sensitized Alloy 600 measured from the DL-EPR test was considerably reduced by LSM. The sensitized Alloy 600 after LSM also exhibited a relatively low DOS, compared with that of the sensitized but not laser treated Alloy 600. From the microscopic observation, it was found that the microstructural changes brought about by the LSM process, especially changes in the precipitation behavior of grain boundary Cr-rich carbides, caused the improvement of resistance to intergranular corrosion of the laser treated Alloy 600. The resistance to IGSCC of the laser treated Alloy 600 in sulfur-bearing environments was also discussed from the results of measured DOS and microstructural examination.

Electrochemical Potentiodynamic Reactivation: Development and Applications of the EPR Test

“…Double loop electrochemical potentiokinetic reactivation test of Alloy 600 & widely used for steam generator tubing of nuclear power plants because of its favorable mechanical and corrosion properties at high temperatures, in spite of numerous cases being reported of IGC and IGSCC during normal operation & brought evidence [50] of a beneficial effect of laser surface melting of the alloy on its resistance to both these kinds of attack.…”

“…These latter steps and contributions and, in particular, the most recent EPR implementation history, are also listed & albeit in a sketchy manner & in the 'history box' overleaf. o duplex stainless steels [28,43], ferritic stainless steels [29] o nickel base alloys [30,[44][45][46][47][48][49][50] o materials for nuclear applications [32,36,46,49,50,51] o alloy microstructure and precipitation [28,30,[33][34][35][36][40][41][42]46,[48][49][50] o improvements of technique, the understanding thereof and modifications thereto [26,29,30,37,38,46,[52][53][54][55] o coatings [56,57] Key Engineering Materials Vols. 261-263 857…”

“…*tefec and Franz (1975) is correctly assessed. The authors of paper[50] wrongly credit F Umemura et al (1980). with being the original proponents of the double loop EPR test.…”