Pit Simulation Study of Zirconium Under Simple Immersion Conditions in Oxidizing Chloride Solutions

Pitting corrosion behavior of zirconium was studied in an oxidizing chloride solution of 1 N hydrochloric acid (HCl) + 1,000 ppm Fe³⁺ under simple immersion conditions. Different surface treatments led to different potential transients at the onset of pitting corrosion. The pit morphology varied under different surface treatment conditions. Minimum exposed areas of 0.5 cm² for the dry, mechanically abraded surface and 15 cm² for the pickled surface were required for pitting corrosion to occur under simple immersion conditions. During pit simulation studies, when a zirconium wire anode containing 4,000 ppm tin was galvanically coupled to a cathode strip of zirconium containing 25 ppm tin in 1 N HCl + 1,000 ppm Fe³⁺ solution, pit initiation, early pit growth, and stable pit growth were observed. Since the cathode remained passive and unattacked because of low tin content in the pit simulation studies, the generated galvanic current truly represented the dissolution current at the pit anode of zirconium. The maximum active dissolution current density with a pit anode of zirconium containing 4,000 ppm tin varied exponentially with the exposed cathode area when the cathode surface was in the dry, mechanically abraded condition. Pit density was 3 pits/cm² for wet-ground samples and 2 pits/cm² for dry-ground samples when 8 cm² of the sample surface was exposed for a day in each experiment. The pit density was 1 pit/cm² when 25 cm² of pickled surface was exposed for 4 months. The pit simulations study rationalized this observation.