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

An interesting variation of the effect of galvanic coupling occurs with metals that exhibit active-passive transitions. When noble metals such as platinum, which are good catalysts for hydrogen reduction, are coupled to a metal with an active-passive transition below the reversible proton-hydrogen potential, spontaneous passivation may ensue (Fig. 7). Thus, a porous coating of noble metal on titanium, chromium, or stainless steels will result in anodic protection of the substrate. 

RG. 7—Spontaneous passivation of an active-passive metal, such as titanium, by galvanically coupling to a noble metal such as platinum. The noble metal has a high rale constant for the proton-hydrogen reaction thus, the corrosion potential of the system Is near to the reversible potential for this reaction [7]. 

Note that Reference" draws attention to the possibility of an increase of anodic polarisation of the more negative member of a couple leading to a decrease in galvanic corrosion rate. There can also be a risk of increased corrosion of the more positive member of a couple. Both these features can arise as a result of active/passive transition effects on certain metals in certain environments. 

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. 

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. 

Another factor that alters the galvanic position of some metals is the tendency, especially in oxidizing environments, to form specific surface films. These films shift the measured potential in the noble direction. In this state, the metal is said to be passive (see Chapter 6). Hence, chromium, although normally near zinc in the EMF Series, behaves galvanically more like silver in many air-saturated aqueous solutions because of a passive film that forms over its surface. The metal acts like an oxygen electrode instead of like chromium hence, when coupled with iron, chromium becomes the cathode and current flow accelerates the corrosion of iron. In the active state (e.g., in hydrochloric acid), the reverse polarity occurs that is, chromium becomes anodic to iron. Many metals, especially the transition metals of the periodic table, commonly exhibit passivity in aerated aqueous solutions. 

Chromium = -0.74 V) is more active in the Emf Series than is iron (< )° = -0.44V), but chromium has a strong tendency to become passive (< )p = 0.2V). Hence, the potential of chromium in aqueous media is usually noble to that of steel. However, in galvanic couples of the two metals, especially in acid media, chromium is polarized below its Flade potential, so that it erfiibits an active potential. Hence, the corrosion potential of chromium-plated steel, which is always porous to some degree, is more active than that of either passive... 

The first is a passive protection that consists in connecting electrically the metal to a less noble material that will result in a galvanic coupling of the two materials, which leads to the anodic dissolution of the sacrificial anode. The second method is an active protection that consists in using an impressed current power supply in order to polarize cathodically the workpiece versus a nonconsumable or inert anode. 

When in contact with other metals, titanium alloys are not subject to galvanic corrosion in seawater. However titanium may accelerate attack on active metals such as steel, aluminum, and copper alloys. The extent of galvanic corrosion will depend on many factors such as anode-to-cathode ratio, seawater velocity, and seawater chemistry. The most successful strategies eliminate this galvanic couple by using more resistant, compatible, and passive metals with titanium, alltitanium construction, or dielectric (insulating) joints. 

As shown in Figure 1.25b, the coupling between active and passive parts of the same metal surface forms a galvanic cell. It has been shown that the electrochemical coupling of these active and passive sites determine the delamination rate of the polymer [70]. 

---