The pit growth of the austenitic stainless steel, SUS 33, and its prevention in 5% NaCl aqueous solution has been investigated electrochemically by an artificial pit method. The inside surface of the artificial pit was activated galvanostatically. In this procedure, pH in the pit, measured with a small glass electrode, was shown to be changed to nearly zero, and the chloride ion concentration, analyzed with Volhard's method, to about 2.7N.
The temperature dependence of the pit dissolution current at the constant potential of -0.20V vs. S. C. E. was measured. Activation energy values were 3-9kcal/mol in high temperature range and about 15kcal/mol in low temperature one. This indicates that the dissolution mechanism of the pit depends on the dissolution rate, hereupon the temperature, and the diffusion process is dominant when this rate is large. These also suggest that the large dissolution rate of the pit consists of the balance between dissolution and diffusion processes, therefore the breakdown of the balance, for example the inhibition of the anodic dissolution, is effective to prevent the pit growth.
The cathodic polarization measurement showed that the pit dissolution was prevented by polarizing to the cathodic potential region less noble than -0.4 V vs. S. C. E. in this solution at 70°C.

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Transient anodic polarization characteristics of the electropolished zirconium were measured in ammonium nitrate solution containing various amounts of ammonium halides. In a solution free from halides the polarization characteristics, either potentiostatically or galvanostatically, obeyed the so-called logarithmic law which indicated the formation of compact passive layer on the surface. However, an addition of halide brought the zirconium anode into an activated state by accompanying the formation of corrosion pits. From the observation of the pitting potential, it was concluded that the activating ability of halide weakened in the sequence of chloride, bromide and iodide. In comparison with these three halides, the fluoride was more destructive even at relatively small concentration. An oscillation of current or potential was observed in a solution containing fluorides.

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