Effect of Galvanic Coupling

7.2.4 Effect of Galvanic Coupling

The classic experiment of Whitman and Russell, which showed that weight loss of iron coupled to copper is the same as if the entire surface had been iron, also showed that the actual penetration of iron increases when iron is coupled to a more noble metal. This experiment, therefore, provides information about the effect of coupling on the corrosion rate of the less noble component of a couple. For the situation where diffusion of a depolarizer is controlling, the general rela- tion between penetration, p (proportional to corrosion rate), of a metal having

area A a coupled to a more noble metal of area A c , is given by

() A a

A p p 1 = c 0 + (7.13)

where p 0 is the normal penetration of the uncoupled metal. If the ratio of areas A c /A a is large, the increased corrosion caused by coupling can be considerable. Conductivity of the electrolyte and geometry of the system enter the problem because only that part of the cathode area is effective for which resistance between anode and cathode is not a controlling factor. In soft tap water, the critical distance between copper and iron may be 5 mm; in seawa- ter, it may be several decimeters. The critical distance is greater the larger the potential difference between anode and cathode. All more noble metals acceler- ate corrosion similarly, except when a surface fi lm (e.g., on lead) acts as a barrier to diffusion of oxygen or when the metal is a poor catalyst for reduction of oxygen.

In the case of coupled metals exposed to deaerated solution for which cor- rosion is accompanied by hydrogen evolution, increased area of the more noble metal also increases corrosion of the less noble metal. Figure 7.6 shows polariza- tion curves for an anode that polarizes little in comparison to a cathode at which hydrogen is evolved (cathodic control). Slope 1 represents polarization of a noble metal area having high hydrogen overpotential. Slopes 2 and 3 represent metals with lower hydrogen overpotential. The corresponding galvanic currents are given by projecting the intersection of anode – cathode polarization curves to the log I axis. In general, any metal on which hydrogen discharges acts as a hydrogen

electrode with an equilibrium potential at 1 atm hydrogen pressure of − 0.059pH volt. When a corroding metal is coupled to a more noble metal of vari- able area, the situation is shown in Fig. 7.7 , where log current density is plotted

instead of log total current. If the anode of area A a is coupled to the more noble

128 IRON AND STEEL

Figure 7.6. Effect of hydrogen overpotential of cathode on galvanic corrosion in deaerated nonoxidizing acids.

Figure 7.7. Effect of anode – cathode area ratio on corrosion of galvanic couples in deaerated nonoxidizing acids. ( a ) Large cathode coupled to small anode. ( b ) Large anode coupled to small cathode.

AQUEOUS ENVIRONMENTS

metal of area A c , then the galvanic current density at the anode produced by coupling can be shown to be

− φ galv − . 0 059 pH

log i

+ log , c A galv i 0 (7.14)

where φ galv is the corrosion potential (S.H.E.) of the galvanic couple (measured at a large distance compared to dimensions of the couple), β and i 0 are the Tafel constant and exchange current density, respectively, for hydrogen ion

I galv

discharge on the noble metal, and

= i , . A galv