Passivity galvanic couples

Note also that a galvanic couple can be established between passive regions and active regions of the same stainless steel component. For... 

The presence of galvanic couples can affect passivity in three ways (two of which are bad), as demonstrated in Fig. 14 (1) increasing the potential of the active-... 

Figure 30 The general form of the crevice current (7C), crevice potential (Ec), and planar electrode potential (E ) measured using a galvanic coupling technique. (A) Range of planar electrode (corrosion) potentials (Ef) measured for passive corrosion under oxidizing conditions (B) range of Ef. values measured for passive corrosion under anoxic conditions (C) range of crevice potentials (Ec) measured during active crevice propagation.

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. 

Figure 6. Spontaneous passivation of titanium by galvanically coupling to platinum...

Structures immersed in seawater. Macrocells may form between rebars reached by chlorides and passive rebars on which, for any reason, oxygen is available. Macrocell current is then controlled by the amount of oxygen that can be reduced on the passive rebars. The galvanic coupling lowers the potential on these rebars and produces alkalinity on their surface. Therefore the macrocell contributes to maintaining the steel passive. 

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... 

In summary, when a metal with a more negative corrosion potential (such as Ti) is galvanically coupled with a more positive metal (such as Pt), the corrosion rate of the more negative metal is accelerated. However, the anomalous behavior observed in Fig. 6.9 is explained by titanium passivation in the absence of oxidizers at more active critical potentials (negative) than the reversible hydrogen potential. 

Fig. 6.10 Galvanic couple between platinum and active-passive metal Fe in air-free acidic...

The series of standard reversible potentials of the various metals (Section 3.6) are now and then used to explain and estimate the risk of galvanic corrosion. This can be very misleading, because 1) these potentials express thermodynamic properties, which do not tell us anything about the reaction rate (e.g. passivation tendencies are not taken into account), and 2) if the potential difference between the two metals in a galvanic couple is large, the more noble metal does not take part in the corrosion process with its own ions. Thus, under this condition, the reversible potential of the... 

---