Study of corrosion pits in chloride solution.

摘要

A laser technique was used to initiate single corrosion pits as well as geometrically ordered arrays of pit on nickel (99.99%) immersed in 0.5 M NaCl solutions under potentiostatic condition. The current was measured during pit growth and the shape of each pit was recorded upon termination of each experiment. It was found that single pits had smooth surfaces and they were slightly shallower than hemispheres and for applied potentials between 0.5 and 0.6 V SCE, and for growth time greater than 20 s. Experimental measurements of pit radius and current density as a function of time at t ≤ 100 s agreed with calculations based on the hypothesis that the pit growth rate was limited by the diffusion controlled dissolution rate of a salt film. A transport model, which included both migration and diffusion, was used to interpret experimental results. The model predicted that the local environment inside the pit cavity was aggressive enough that a salt film might precipitate at the early stage of pit growth. Current interruption experiments demonstrated that pit stability was determined by the extent of relaxation of the pit environment. A critical concentration adjacent to the pit surface is required for the pit to maintain active. It was found to be independent of the pit size, but dependent on the applied potential. The critical applied potential above which pits continued to grow decreased with pit size. It was observed that the growth rate of pits at an applied potential of 0.5 V SCE decreased in the presence of flow, and pits repassivated at Pe number greater than 1000. A two-dimensional trench model predicted rinsing of cavity at Pe > 1000, which was in qualitative agreement with the experimental observations. Controlled multiple pits experiments demonstrated that pits do interact with each other within a critical distance. It was found that the neighboring pits enhanced the growth of the central pit in the lateral direction and the total current also increased under potentiostatic condition. The critical distance was affected by the bulk concentration of chloride.