COMBINED USE WITH COATINGS

13.4 COMBINED USE WITH COATINGS

The distribution of current in a cathodically protected steel water tank is not ideal — too much current may fl ow to the sides and not enough to the top and

C ATHODIC PROTEC TION

Figure 13.3. Cathodically protected hot - water tank with magnesium anode.

bottom. Better distribution is accomplished by using an insulating coating (e.g., an organic coating at ordinary temperatures or a glass coating at elevated tem- peratures). The insulating coating need not be pore - free, since the protective current fl ows preferably to exposed metal areas, wherever located, which are precisely the areas needing protection. Also, since the total required current is less than that for an uncoated tank, the magnesium anode lasts longer.

In hard waters, a partially protective coating may form on steel that consists largely of CaCO 3 precipitated by alkalies generated as reaction products at the cathode surface. A similar coating forms gradually on cathodically protected surfaces exposed to seawater (more rapidly at high current densities). Such coat- ings, if adherent, are also useful in distributing the protective current and in reducing total current requirements.

In the general application of cathodic protection using either impressed current or sacrifi cial anodes, it is expedient to use an insulating coating, and this

MAGNITUDE OF CURRENT REQUIRED

combination is the accepted practice today. For example, the distribution of current to a coated pipeline is much improved over that to a bare pipeline; the total current and required number of anodes are less; and the total length of pipeline protected by one anode is much greater. Since the earth, taken as a whole, is a good electrical conductor, and the resistivity of the soil is localized only within the region of the pipeline or the electrodes, one magnesium anode can protect as much as 8 km (5 miles) of a coated pipeline. For a bare pipeline, the corresponding distance might be only 30 m (100 ft). Using an impressed current at higher applied voltages, one anode might protect as much as 80 km (50 miles) of a coated pipeline. The limiting length of pipe protected per anode is imposed not by resistance of the soil, but by the metallic resistance of the pipeline itself.

The potential decay, E x , along an infi nite pipeline measured from the point of attachment to the dc source having potential E A is expressed as an exponential relation with respect to distance, x , along the pipeline in accord with

⎣⎢ ( kz ) ⎦⎥

Both E x and E A represent differences between polarized potential with current fl owing and corrosion potential in absence of current, R L is the resistance of pipe of radius r per unit length, k is a constant, and z is the resistance of pipe coating per unit area (for derivation, see Appendix, Section 29.4 ). This equation is derived by assuming that polarization of the cathodically protected surface is a linear

function of current density. Note that E x becomes zero at x = ∞. Considering a fi nite pipeline for which a /2 is half the distance to the next

point of bonding, and potential E x at a /2 = E B ,

12 ⎡ 2 π rR

⎣⎢ ( kz ) () 2 ⎦⎥

x = B cosh

Cathodic protection is optimum within a specifi c potential range (see Section

13.7 ), so that the length of pipeline protected by one anode increases as the metallic pipe resistance, R L , decreases, and coating resistance, z , increases.