EFFECT OF HEAT-TREATMENT ON THE BEHAVIOR OF ALLOY-600 IN LITHIATED WATER CONTAINING DISSOLVED HYDROGEN AT 25 TO 350-DEGREES-C

Measurement of the open-circuit potential, AC impedance, and anodic polarization curves were made for alloy 600 (UNS(1) N06600) in three different heat treatment conditions (mill annealed at 927-degrees-C, at 1024-degrees-C, and thermally treated by heating at 700-degrees-C for 28 h) on exposure at 25 to 350-degrees-C in lithiated water containing hydrogen. Values of the open-circuit potential (E(op)) measured at the same temperature for three differently heat-treated materials were identical (within the limits of the experimental scatter). In the 25 to 250-degrees-C range, the E(op) values were negative to the reversible hydrogen electrode (RHE) potential, hence they were mixed potentials. Values of the polarization resistance (R(p)) as determined from AC impedance data indicated superiority of the thermally treated material over the high-temperature annealed one. The low-temperature annealed material exhibited the lowest R(p). Anodic polarization curves showed smaller dissolution rates and greater tendencies to passivation for thermally treated and high-temperature annealed materials than to the low-temperature annealed material. The conclusion was made that carbides precipitated at the grain boundaries had a beneficial effect on the dissolution rate of the alloy and on its ability to assume a passive state. An opposite effect was produced by elemental carbon in solid solution.