High-temperature Corrosion Behaviors Of Alloy 690 Heat Exchange Tube For Steam Generators

Uniform corrosion rateand the oxide film structure of Alloy 690 in the simulated pressurized water reactor(PWR) primary and secondary water conditions were investigated by autoclave exposure tests, SEM observation and XPS analysis. The effects of temperature and dissolved oxygen(DO) on the electrochemical behaviors of Alloy 690 were studied in PWR primary and secondary water. The resistance to stress corrosion cracking of alloy 690 was evaluated using C-ring and U-bending specimens in autoclave simulating PWR primary water at 360℃, and exposure to 10%NaOH solutionat 325℃, and by slow strain rate test in 10%NaOH solution at 325℃, respectively.The results of uniform corrosion tests reveals that the uniform corrosion rates of three Alloy 690 tubesin PWR primary and secondary waters are quite low, while it is higher in primary water than in secondary water. The passive films formed in primay and secondary water show a duplex structure including an inner layer of Cr2O3 or/and Nix,Fe1-xCr2O4 and an ourter layer of Fe- and Ni-hydroxides and NiFe2O4.The results of electrochemical measurements show that with theincreasing solution temperature, the corrosion potential(Ecorr) shifted to negativeside,and corrosion current density(Icorr) increased while the passive potential region became shorter and parallel to this shortening of the passive region the breakdown(pitting) potential(Eb) shifted toward negative values; With increasing DO concentration, Ecorr shifted to positive potentials,and Icorr increased, while the passive potential region became shorter. Alloy 690 in secondary water exhibited superior corrosion resistant characteristics than that in primary water.All of the Alloy 690 tubes tested show excellent SCC resistance in the long time exposure tests to simulated PWR primary water and concentrated caustic environments. The SSRTstress strain curves and afterwards surface morphology analysis show that all of Alloy 690 tubes showed obvious SCC susceptibility in 10%NaOH solution. J09 showed the lowest ultimate tensile strength(UTS) and yield strength(YS) but the highest elongation, whileG05 exhibited the highest UTS and YS although it is most susceptible to SCC in high temperature caustic solution.