HOW APPLIED

13.3 HOW APPLIED

Cathodic protection requires a source of direct current and an auxiliary electrode (anode) usually of iron or graphite located some distance away from the pro- tected structure. The direct current (dc) source is connected with its positive ter- minal to the auxiliary electrode and its negative terminal to the structure to be protected; in this way, current fl ows from the electrode through the electrolyte to the structure. The applied voltage is not critical — it need only be suffi cient to supply an adequate current density to all parts of the protected structure. In soils or waters of high resistivity, the applied voltage must be higher than in environ- ments of low resistivity. Or, when the extremities of a long pipeline are to be protected by a single anode, the applied voltage must be raised. A sketch of a cathodically protected buried pipeline is shown in Fig. 13.1 .

Figure 13.1. Sketch of cathodically protected pipe, auxiliary anode, and rectifi er.

C ATHODIC PROTEC TION

The source of current is usually a rectifi er supplying low - voltage dc of several amperes. Motor generators have been used, although maintenance is trouble- some. Windmill generators are employed in areas where prevailing winds are dependable. Even in periods of calm, some degree of protection of steel persists temporarily because of the inhibiting effect of alkaline electrolysis products at the cathode surface.

13.3.1 Sacrifi cial Anodes

If the auxiliary anode is composed of a metal more active in the Galvanic Series than the metal to be protected, a galvanic cell is set up with current direction exactly as described in the previous section. The impressed source of current (i.e., the rectifi er) can then be omitted, and the electrode is called a sacrifi cial anode , as shown in Fig. 13.2 . Sacrifi cial metals used for cathodic protection consist of magnesium - base and aluminum - base alloys and, to a lesser extent, zinc. Sacrifi cial anodes serve essentially as sources of portable electrical energy. They are useful particularly when electric power is not readily available, or in situations where it is not convenient or economical to install power lines for the purpose. The open -

circuit potential difference of magnesium with respect to steel is about 1 V ( φ H

for magnesium in seawater =

− 1.3 V), so that only a limited length of pipeline can

be protected by one anode, particularly in high - resistivity soils. This low voltage is sometimes an advantage over higher impressed voltages in that danger of overprotection to some portions of the system is less; and since the total current per anode is limited, the danger of stray - current damage (interference problems) to adjoining metal structures is reduced.

Figure 13.2. Cathodically protected pipe with sacrifi cial anode.

COMBINED USE WITH COATINGS

The potential of zinc is less than that of magnesium ( φ H in seawater = − 0.8 V); hence, current output per anode is also less. High - purity zinc is usually specifi ed in order to avoid signifi cant anodic polarization with resultant reduction of current output caused by accumulation of adherent insulating zinc reaction products on commercial zinc. This tendency is less pronounced in zinc of high purity.

Aluminum operates theoretically at a voltage between magnesium and zinc.

A disadvantage of aluminum as a sacrifi cial anode is that it tends to become passive in water or in soils with accompanying shift of potential to a value approaching that of steel. A special chemical environment high in chlorides sur- rounding the electrode can be provided in order to avoid passivity; however, such an environment, called backfi ll, is only a temporary measure. In seawater, passiv- ity can be avoided by alloying additions, such as tin, indium, antimony, or mercury. For example, alloying aluminum with 0.1% Sn followed by heat treatment at 620 ° C for 16 h and water quenching to retain the tin in solid solution very much decreases anodic polarization in chloride solutions [5] . The corrosion potential of the 0.1% Sn alloy in 0.1 N NaCl is − 1.2 V (S.H.E.) compared to − 0.5 V for pure

aluminum. Some sacrifi cial aluminum anodes contain about 0.1% Sn and 5% Zn [6, 7] . Another composition containing 0.6% Zn, 0.04% Hg, and 0.06% Fe, when tested in seawater for 254 days, operated at a current effi ciency of 94% (1270 A - h/lb); however, use of mercury has been banned in most locations because of pollution concerns [8] . For cathodic protection of offshore platforms, aluminum anodes, made from aluminum - zinc alloys, are the preferred material [8] .

For offshore applications, magnesium anodes have not been popular since the 1980s, because of improvements in aluminum and zinc anodes [8] . Magnesium anodes may be consumed before the structure has reached the end of its lifetime, whereas aluminum anodes are characterized by reliable long - term performance

[8] . Magnesium anodes are often alloyed with 6% Al and 3% Zn to reduce pitting - type attack and to increase current effi ciency. By using high - purity mag- nesium containing about 1% Mn, the advantage of a higher potential (with higher current output per anode) is obtained [9] . This alloy operates at a current effi - ciency in seawater similar to the fi rst - mentioned alloy, but at somewhat lower effi ciency in most soils. The observed effi ciency of magnesium anodes averages about 500 A - h/lb compared to a theoretical effi ciency of 1000 A - h/lb.

A sketch of a magnesium anode rod installed in a steel hot - water tank is shown in Fig. 13.3 . Such rods may increase the life of a steel tank by several years, particu- larly if the rod is renewed as required. The degree of protection is greater in waters of high conductivity, where the currents naturally set up by the magnesium – iron couple reach higher values than in waters of low conductivity (soft waters).