Transport Processes and the Mechanism of Pitting of Metals

A pit model was developed on the assumption that the metal ions hydrolyze inside the pits and that the corrosion products are transported by diffusion. Concentrations of Me e+, Me(OH)+, and H + ions, as a function of pit depth and current density, for Zn, Fe, Ni, Co, AI, and Cr were calculated. The main reason for passivity breakdown at the initial stages of pit growth, was found to be the localized acidification due to metal ions hydrolysis. Assuming a critical pH value for pit initiation, the following experimental facts could be explained: (i) the effect of the external pH on the pitting potential of Fe and stainless steel; (it) the effect of sodium borate concentration on the pitting potential of Zn; (it{) the effect of weak acid salts on the pitting potential of AI; (iv) the oscillations of the electrode potential of stainless steel and nickel in solutions of Cl+ SO4 = ions; (v) the existence of a pitting inhibition potential; and (vi) the existence of a pitting protection potential. Through analysis of the transport processes inside a pit it was also concluded that the pitting potential of a metal should change with the CIion concentration according to the equation E~ = E/ -B 9 log [CI-] B = 0.059V being the slope of the curve at room temperature. It is a genera l ly accepted fact that p i t t ing star ts at a cer ta in cri t ical potential , known as p i t t ing potent ia l (1). Y e t t h e nature of such a potent ia l st i l l remains uncertain. Severa l explanat ions have been given, such as zero charge potential , po ten t i a l induced ion mig ra tion, e lect r ica l ly produced mechanical breakdown, competitive adsorption, salt-formation equilibrium potential, etc. (I). Nevertheless, none of these mechanisms could account for the way in which variables such as pH, ionic concentration, and inhibitor concentration affect the pitting potential. In recent publications, Wexler and Galvele (2) and Alvarez and GaIvele (3) reported that the pitting potential was the minimum potential at which localized acidity could be mainta ined inside a pit. The present paper , based on simple t ranspor t equations shows tha t such acidification c a n be obta ined even at the very ear ly stages of p i t ting, and that, as was recent ly repor ted (4), t r anspor t calculat ions inside the pi t exp la in why var iables such as an aggressive anion concentration, nonaggressive anion concentrat ion, ex te rna l pH, presence of weak acid salts, and alloying elements modify the pitting potent ia l o'f a metal . I t is concluded tha t most of the observat ions made so far on the p i t t ing potent ia l can be expla ined s imply by t ranspor t phenomena, and tha t processes l ike compet i t ive adsorption, sal t formation, film contaminat ion, * Electrochemical Society Act ive Member . 1 On leave f r o m the Comi~ion Nacional de Energ ia Atomica, Depa r t amento de Metalurgia , Buenos Aires, Argent ina .