Stress corrosion of zircaloy-4 in alcoholic iodine solutions

This work is part of a larger project, which aims to study the Stress Corrosion (ST) processes of compact hexagonal metals and alloys. At this time the susceptibility to intergranular attack and to the ST was studied in Zircaloy-4 in different alcohol solutions of iodine. The influence of variables such as water content in the solution, type of alcohol and temperature was evaluated for the above-mentioned phenomena. To determine the susceptibility to the ST the slow traction technique was used at a constant deformation velocity, with 1.2 mm diameter wire test pieces. In the absence of tensions, Intergranular attack tests were also carried out by submerging 2 cm long wires in the different solutions, with exposure times of 0.5 to 300 hours. The electrochemical behavior of the material was also evaluated in the mediums studied by plotting potentiodynamic polarization curves. Intergranular attack occurs in Zircaloy-4 exposed to alcoholic iodine solutions still in the absence of tensions and its velocity is controlled by the diffusion of the active species to the sites where the attack occurs. Susceptibility to intergranular attack is a necessary condition for ST to occur. Water inhibits this process, since in the presence of this solvent the rupture of the metal's protective film (that occurs with the iodine's chemical attack) competes with the repassivation of the surface. During the traction tests, there is a transition in the morphology of fissure propagation from intergranular to transgranular when the material reaches the KISCC. The transgranular propagation is the 'true' ST phenomena. When the molecular weight of the alcohol used as a solvent increases, the water content needed to inhibit the ST decreases. This occurs due to the repassivation process, favored by the presence of water, together with the steric impediment effect that hinders the arrival of the active specie at the end of the advancing fissure, and this effect increases with the size of the n-alcohol molecule. The susceptibility to ST also depends on the type of alcohol (primary, secondary or tertiary) used as a solvent, since the spatial configuration also produces a steric impediment effect that hinders the arrival of the active specie to the tip of the advancing fissure. The speed of fissure growth depends on the temperature, which is consistent with Arrhenius behavior, and the energy of activation follows a process controlled by diffusion. This result confirms that the ST of Zircaloy-4 is a phenomena controlled by the intergranular attack, which in turn depends on a diffusive process (CW)