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 unattached 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