Electrochemical parameters active-passive metals

E4.1. Calctilate and construct, using mixed potential theory, the critical passivation current density (a) potentiostatic and (b) galvanostatic polarization curve for anodic dissolution for active-passive metal that has the following electrochemical parameters Ecorr= 0-5 V vs. SCE, Epp= — 0.4 V vs. SCE, 4orr= 10 A/cm, K = 0.05,. , = 10 A/cm andEtr= + l-0V vs. SCE. 

Table E4.2 Electrochemical Parameters of an Active-Passive Metal...

Mixed potential theory is used to estimate the galvanic current and the galvanic potential in an active-passive metal that passivates at potentials less noble than the reversible hydrogen potential. A galvanic couple between titanium and platinum of equal area of 1 cm is exposed to 1 M HCl. The electrochemical parameters for the active-passive alloy are eeq xi = —163 V vs. SHE anodic Tafel, b Ti = 0.1 exchange current density, ixi= 10 A/cm passivation potential, pp= —0.73 V passivation current, 7pass= 10 A/cm transpassive potential, = 0.4 V vs. SHE and activity of dissolved species [Ti ] = 1 M. The exchange current densities, i°, on platinum and titanium... 

Fig. E4.3 Polarization curve for anodic dissolution for active-passive metal. Table E4.2 Electrochemical Parameters of an Active-Passive Metal...

E4.5. Plot the polarization curve for anodic dissolution of the active passive metal that has the foUowing electrochemical parameters given in Table E4.3 ... 

Fig. E4.4 Evans diagram for active-passive metal constructed using electrochemical parameters given in Table E4.2.

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

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

The amount of metal dissolved per cycle depends on the electrochemical parameters of the system, such as the passivation current density and the repassivation rate. On the other hand, the rate of deformation determines the frequency of activation. In Figure 11.42, crack propagation rates are plotted as a function of the repassivation... 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

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