Active-passive alloys

FIG. 28-9 Typical electrochemical polarization curve for an active/passive alloy (with cathodic trace) showing active, passive, and transpassive regions and other important features. (NOTE Epp = primary passive potential, Ecaa- — freely corroding potential). 

FIG. 28-10 Six possible types of behavior for an active/passive alloy in a corrosive environment. 

Fig. 3.3 Anodic polarization curve representative of active/passive alloys.

Fie. 4.20 Galvanic series of various metals in flowing seawater at 2.4 to 4.0 m/sat 5 " to 30 °C (volts vs. saturated calomel reference electrode). Note Dark boxes indicate active behavior of active-passive alloys. Source Ref 12 and 1 3... 

Anodic Polarization of Several Active-Passive Alloy Systems... 

Fig. 5.35 Schematic polarization curve for an active-passive alloy having susceptibility to localized corrosion (pitting) due to chloride ions. Pitting initiates at Eb,pit- Small-dashed section is observed when chloride ion concentration initiates penetration of the passive film.

Fig. 4.11 Effect of solution velocity on corrosion of active-passive alloy in deaerated and aerated alkaline solution (pH = 11).

Mixed potential theory and the Levich equation are used to construct the anodic and cathodic polarization curve of the active-passive alloy and to estimate the value of the oxygen limiting diffusion current when a spontaneous passive film is formed on the surface. Figure 4.11 correlates the relationship between potential and current for the... 

E4.2. For active-passive alloys A and B with the following prameters given in Table E4.1, calculate the critical passivation current density and determine which is more easily passivated. 

Table E4.1 Electrochemical Parameters for Active-Passive Alloys A and B...

E4.3. Construct an anodic polarization curve and calculate the critical passivation current density of an active-passive alloy using mixed potential theory with the following electrochemical parameters Econ = 0-b5V vs. SCE /corr= 10 " A/cm fca = 0.1, Epp= —0.3 V vs. SCE and /pass = 10 A/cm, Etr = + 0.9 V. 

E4.6. Hypothetical active-passive alloys A and B have the electrochemical parameters given in Table E4.4 ... 

E4.8. An active-passive alloy corrodes in the active state in an acidic solution in the presence of the redox couple Fe Fe. The corrosion potential and corrosion current are Ecorr= 0.35 V vs. SHE and Jcorr=10 iA/cm. If the reduction occurs under activation control with bc = — 0.1 V/decade, concentration of Fe of 10 M, and the exchange current density of the redox couple p 2+ = 0.01 pA/cw , calculate the concentration of Fe " oxidizer necessary to passivate the alloy. Recalculate the oxidizer concentration for an exchange current density of 0.1 and 1 ihJcrr for the redox couple. 

E4.9. Table E4.6 lists the parameters of the active-passive alloys A, B, C, and D. Calculate critical current densities of the active-passive alloys and determine ... 

Fig. 6.1 Galvanic series in sea water. The dark boxes indicate the active behavior for listed active-passive alloys [4]. Reprinted with permission of ASM international. Materials Park, OH. www. asminternationai.org.

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. 7.23 Polarization curve of active-passive alloy in the presence of chloride ions.

E7.8. Electrokinetic parameters of an active-passive alloy are given in Table E7.5 and the anodic polarization curve of the active-passive alloy in the presence of chloride ions is shown in Fig. 7.23. Pitting corrosion initiation occurs at 0.045 V vs. SHE. 

Table E7.5 Electrokinetic Parameters of an Active-Passive Alloy...

---