抄録
The pit growth of austenitic stainless steels in 5% NaCl aqueous solution at 70°C was investigated electrochemically by an artificial pit method.
Since the pit growth depends mainly on the macrocell formed inside and outside a pit and an artificial pit method can separate the macro-anode from the cathode, this method is the most suitable for the quantitative study of this process.
The artificial pit specimen used in this experiment consists of two disks of the stainless steel to be tested sandwiching a sheet of Teflon in a Teflon holder, and has advantages of easiness to make and less appearing of the crevice corrosion. The inside surface of the artificial pit was activated galvanostatically with anodic current prior to electrochemical measurements.
Measurements conducted were the potentiostatic anodic polarization of the pit, dissolution current in natural pit growth, and so on. The corrosive medium formed during the pit growth was assumed from comparison of the anodic polarization curve of the pit with those in the model solutions made from. HCl and NaCl.
Pit growth rate due to the macro-cell mechanism was shown to be 102-104 (-105)μA/cm2 in this environment and when the cell circuit was open the rest potential of the pit was about -0.25V vs. SCE. During the pit growth the concentration of hydrogen ion in the pit changed to at least N/100 and that of chloride ion to about 5N.
Comparison of anodic polarization curves suggested that the dissolution of the pit took place through a kind of unstable film in the highly concentrated chloride solution of low pH.
