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| KINETIC ANALYSIS ON PITTING GROWTH |
| TANG Zi-long (Dept. Materials; Tianjing University; Tianjin 300072) |
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Abstract On the basis of law of conservation of mass and charge and electrochemical kinetics of corrosion, a kinetic analysis has been performed on self-propagation process of pits under four typical conditions, i.e. hemispherical and cylindrical pits in the presence and absence of precipitation layer inside the pit. The time dependence of pitting current, depth and radius for the four kinds of pit was obtained. The summarized results demonstrate that the dependence of pitting current on time comply with four functions of time, that are t1/2, t, t2 and t*ln(t), and each of them corresponds to a specific pattern of pitting growth. Furthermore, a general equation describing pitting current increasing with time has been put forward by linearly combining these extracted time functions. The resistance of participation layer plays an important role in pitting growth more obviously on hemispherical pits than on cylindrical pits. The possible ways to retard pitting growth by inhibitor and the necessary requirements for inhibitor were discussed. The kinetic equation established was validated from three aspects. The variation of pitting current with time for 304 stainless steel in NaCl solution under potential control can be explained well according to the equation. Second, the maximum increase of pitting current is the square of time and the minimum is square root. This result is confirmed by a great amount of previous studies. Finally, pit depth varies with time only as a function of power or logarithm. This is consistent with statistical analysis of pit depth data.
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Received: 25 August 1998
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