Abstract
The localized acidification mechanism for stable pitting growth was the result of the pitting research performed by many investigators, led by Dr. J.R. Galvele. The key point of the mechanism is that there is a pH drop on the anode surface, and that this pH drop is a necessary condition to sustain pitting.The analysis of the transport processes taking place inside a pit, taking into account the hydrolysis of the metal ions, allowed to explain the pitting potential significance and give a quantitative explanation of the influence of solution pH, reducible ions, inhibitors and aggressive ion concentration on the pitting potential. In the present work a brief description of the mechanism development and examples of the pitting and crevice systems that have confirmed its validity are going to be presented. Potential future lines of work, are also briefly suggested.
Development of the Mechanism
Transport processes inside a pit and pitting potential significance
Van Muylder et al.1 were the first to attempt an explanation for the existence of a pitting potential value with an acidification process. They used the E-pH equilibrium diagram for Cu-H2O to explain pitting of copper in chloride-containing solutions and concluded that the pitting potential is a thermodynamically based value, and is given by the equilibrium potential of equation 1.
Me = Mez+ + ze- (1)
This mechanism was successfully used to explain the pitting potential of copper in chloride-containing solutions. Nevertheless, it did not work with metals having Me/Mez+ equilibrium potential lower than that for hydrogen evolution. Neither could this model explain the fact that buffer solutions acted as pitting inhibitors, increasing the pitting potential of numerous metals in corrosive solutions. This mechanism was modified by Galvele and co-workers to explain the pitting potentials of such metals as zinc, cadmium, aluminium or iron, and the pitting potentials found in the presence of reducible anions or inhibitors.2-11.The model was based on the assumption that the aggressive ions were anions of strong acids, the only requirement being that they would not produce insoluble products when in contact with the metal and that the metal, while corroding in aqueous solutions, would react with water, producing localized depletion of HO- ions.The key point of the mechanism is that there is a pH drop on the anode surface, and that this pH drop is a necessary condition to sustain pitting. The rate at which the protons are produced will be given by equations 1 plus 2:
