Active-passive metals, definition

Definition of Important Electrochemical Parameters for Active-Passive Metals... 

The potentiostat is particularly useful in determining the behaviour of metals that show active-passive transition. Knowledge of the nature of passivity and the probable mechanisms involved has accumulated more rapidly since the introduction of the potentiostatic technique. Perhaps of more importance for the subject at hand are the practical implications of this method. We now have a tool which allows an operational definition of passivity and a means of determining the tendency of metals to become passive and resist corrosion under various conditions. 

The thermodynamic information is normally summarized in a Pourbaix diagram7. These diagrams are constructed from the relevant standard electrode potential values and equilibrium constants and show, for a given metal and as a function of pH, which is the most stable species at a particular potential and pH value. The ionic activity in solution affects the position of the boundaries between immunity, corrosion, and passivation zones. Normally ionic activity values of 10 6 are employed for boundary definition above this value corrosion is assumed to occur. Pourbaix diagrams for many metals are to be found in Ref. 7. 

Rassivation rpm. According to the first definition of passivation, a metal is passive when it behaves like a noble metal. In these iron EDTA solutions, a noble metal such as Rt measures the Fe +/Fe + ratio of the solution. From experience, the emf measured (Rt vs. SCE) in solutions containing predominantly Fe(lll)EDTA will be more positive than -100 mV. The active steel usually exhibits an OCR (Fe vs. SCE) value of approximately -800 mV. When the steel passivates, the OCR will increase by several hundred millivolts and will eventually reach . 100 mV. To conduct the experiment, the polished active RDE was lowered into the heated prepared Fe(lll)EDTA solution. Beginning from 0 rpm, the rotation rate was slowly increased until the steel emf sudden ly rose to approximately - 300 mV. The emf and rpm were recorded on the two-pen recorder. The passivation rpm was determined from the chart recorder traces. A typical trace is shown in Figure 5. The x axis reflects chart movement or time to passivation, but the time factor was not studied. The tests were repeated three to five times, and the rpm (pass) was averaged. A number of graphical relationships between [Fe +] rpm, free EDTA, and temperature were derived from these data. In addition, with the use of the values of D calculated above and Equation (2), /um at passivation was calculated. 

---