Galvanic couple hydrogen evolution

A nonuniform distribution of the reactions may arise when the metal s surface is inhomogeneous, particularly when it contains inclusions of other metals. In many cases (e.g., zinc with iron inclusions), the polarization of hydrogen evolution is much lower at the inclusions than at the base metal hence, hydrogen evolution at the inclusions will be faster (Fig. 22.3). Accordingly, the rate of the coupled anodic reaction (dissolution of the base metal) will also be faster. The electrode s OCP will become more positive under these conditions. At such surfaces, the cathodic reaction is concentrated at the inclusions, while the anodic reaction occurs at the base metal. This mechanism is reminiscent of the operation of shorted galvanic couples with spatially separated reactions Metal dissolves from one electrode hydrogen evolves at the other. Hence, such inclusions have been named local cells or microcells. 

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 estimates galvanic couple corrosion for metals P and Q (Fig. 6.8). Metal P has a more positive corrosion potential and lower corrosion due to lower hydrogen evolution current density, (° Metal Q... 

Some of the factors that control corrosion rate in galvanic couples are the corrosion potential difference between dissimilar metals, hydrogen evolution exchange current... 

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