Galvanic coupling parameters

Instruments providing simultaneous measurement of a number of parameters on multi-element probes have been developed, including potential noise , galvanic coupling, potential monitoring, and a.c. impedance . 

Many other parameters tend to influence the corrosion of metals immersed in sea water. When two metals of different potentials are galvanically coupled, the acceleration of the attack on the less noble metal of the two is observed frequently. A small area of an anodic metal coupled to a large area of a second metal that is cathodic can be particularly dangerous. A useful guide to help predict unfavorable combinations is the galvanic series of metals in sea water (0). The reverse situation—namely, a small cathode coupled to an anode that is large in area—often proves satisfactory in service. 

To capture this evolution in a model requires that relationships between propagation rate (Rcc or /c) and 02 concentration and temperature be determined. This can be achieved in the galvanic coupling experiment by activating a crevice and then changing the 02 concentration or temperature and recording Ic as a function of these two parameters. The results of such experiments have been published elsewhere (49), and the relationship for 02 concentration is shown in Fig. 33. Once 02 reaches a sufficiently low concentration, Ic drops to zero i.e., repassivation occurs. This enables us to specify a repassivation criterion based on a critical 02 concentration ([02]P) as noted in the figure. A similar dependence of 7C (and hence Rcc) on temperature was determined. 

What is clear from these analyses is that the avoidance of crevice corrosion will delay eventual container failure significantly, irrespective of whether it occurs by wall penetration or by HIC. With this is mind, the galvanic coupling technique (along with the associated analytical methods outlined above) can be used to compare qualitatively the crevice corrosion performance of a series of titanium alloys. Figs. 36A and B compare the parameter (/c, Ec, Ep) values ob-... 

Figure 36 Values of the parameters, Ic (crevice current), Ec (crevice potential), and Ep (planar potential) recorded in galvanic coupling experiments at 125°C in 0.27 mol dm-3 NaCl (A) Ti-2 (commercially pure alloy) (B) Ti-12 (0.8 wt% Ni + 0.3 wt% Mo).

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

Case I Galvanically Coupled Metals with Similar Electrochemical Parameters... 

Fig. 4.19 Schematic representation of polarization curves for the analysis of galvanic coupling when the coupled metals have similar electrochemical parameters. Tafel polarization is represented.

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 anode and cathode corrosion currents, fcorr.A and fcorr,B. respectively, are estimated at the intersection of the cathode and anode polarization of uncoupled metals A and B. Conventional electrochemical cells as well as the polarization systems described in Chapter 5 are used to measure electrochemical kinetic parameters in galvanic couples. Galvanic corrosion rates are determined from galvanic currents at the anode. The rates are controlled by electrochemical kinetic parameters like hydrogen evolution exchange current density on the noble and active metal, exchange current density of the corroding metal, Tafel slopes, relative electroactive area, electrolyte composition, and temperature. 

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... 

E6.6. Sn and Pt are immersed in an acidic solution with unit hydrogen ion activity. Using the electrochemical parameters listed below, construct the Evans diagram and evaluate the effect of the cathode-sacrificial anode electrode surface area ratio on galvanic corrosion of a tin-platinum galvanic couple (see Case Study 6.1). 

E6.9. Construct the Evans diagram for a nickel and iron galvanic couple immersed in 1 M acidic solution. Using the electrochemical parameters Hsted in Exercise E6.8, estimate graphically the following parameters and tabulate the results ... 

This latter chemical point illustrates the complexity of the mi-croseopic description of the galvanic coupling. The chemical gradient is consequently an important parameter which can be also at the origin of the eharaeteristie length of the time evolution of the... 

Figure 3.5 (a) A typical potentiodynamic polarization plot for a Co electrode, graphically analyzed to determine E on and icon- (b) and (c) display coupled Tafel plots recorded in selected slurry solutions for the Co—A1 and Cu—Ta bimetallic systems, respectively. The juncture point (p) of the two plots in (h) corresponds to the galvanic parameters g and ig for the Co—A1 galvanic system. The plots in (c) represent a reversal of conventional galvanic polarities, where there are two crossover points (pi and P2). In all cases of (a), (b), and (c), the potential was scanned at a speed of 5 mV/s. 

It should be pointed out that the corrosion potential in the galvanic series (Table 2.3) and that in the emf series (Table 2.4) are not the same. The former may be measured as a coupled potential at different temperatures and ionic concentrations used in the latter series. Therefore, the galvanic series must be used with caution since it is ten )erature and concentration dependent kinetic parameter. 

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