ANODE MATERIALS AND BACKFILL

13.6 ANODE MATERIALS AND BACKFILL

Auxiliary anodes for use with impressed current are usually composed of scrap iron or graphite. Scrap iron is consumed at the rate of 15 – 20 lb/A - year and must

be renewed periodically. Graphite anodes are consumed at a lower rate, not exceeding perhaps 2 lb/A - year. Graphite costs more than scrap iron, both initially and in subsequent higher electrical power costs because of the noble potential

and accompanying high overpotential of oxygen (or Cl 2 ) evolution on graphite

compared to an active potential and lower overvoltage for the reaction Fe → Fe 2+ + 2e − . Graphite is also fragile compared to scrap iron and must be installed with greater care. The advantages and disadvantages of graphite apply in similar measure to 14% Si − Fe alloy anodes and magnetite anodes, which have also been

recommended. For protection of structures in seawater, platinum - clad copper, 2% Ag − Pb, platinized titanium, or platinized niobium have been recommended as corrosion - resistant anodes using impressed current [10 – 12] . Whereas sacrifi cial magnesium anodes require replacement approximately every 2 years, the 2% Ag − Pb anodes are estimated to last more than 10 years, and the 90% Pt – 10% Ir anodes still

ANODE MATERIALS AND BACKFILL

longer [11] . In fresh waters, aluminum anodes are sometimes used for impressed current systems.

Because the effective resistivity of soil surrounding an anode is confi ned to the immediate region of the electrode, it is common practice to reduce local resistance by backfi ll. For impressed current systems, this consists of surrounding the anode with a thick bed of coke and adding a mixture of perhaps 3 or 4 parts

gypsum (CaSO 4 ·H 2 O) to 1 part NaCl. The coke backfi ll, being a conductor, carries part of the current, reducing in some measure consumption of the anode itself. Backfi ll may not be required if the anode is immersed in a river bed, lake, or the ocean.

Whereas auxiliary anodes need not be consumed in order to fulfi ll their purpose, sacrifi cial anodes are consumed not less than is required by Faraday ’ s law in order to supply an equivalent electric current. In general, the observed rate of consumption is greater than the theoretical. For zinc the difference is not large, but for magnesium it is appreciable, with the cause being ascribed to local -

action currents on the metal surface, to formation of colloidal metal particles [13, 14] or, perhaps more important, to initial formation of univalent magnesium ions [15] . The latter ions are unstable and react in part with water in accord with

2 Mg + + 2 HO 2 → Mg OH ( ) + Mg 2 2 + + H 2 Hence, in dilute sodium chloride, about half the magnesium corroding anodically

appears as Mg(OH) 2 and half as MgCl 2 , accompanied by hydrogen evolution in about the amount expected according to this reaction [15] . Additional lesser side reactions may also take place at the same time. Accordingly, the observed yield of a magnesium anode is only about one - half the 1000 A - h/lb calculated on the basis of Mg 2+ formation.

For magnesium anodes, backfi ll has the advantage of reducing resistance of insulating corrosion - product fi lms, such as Mg(OH) 2 , as well as increasing conductivity of the immediate environment. A suitable backfi ll may consist of approximately 20% bentonite (an inorganic colloid used for retention of mois-

ture), 75% gypsum, and 5% Na 2 SO 4 . Sometimes, the backfi ll is packaged before- hand in a bag surrounding the anode, so that anode and backfi ll can be placed simultaneously into position in the soil.

13.6.1 Overprotection

Moderate overprotection of steel structures usually does no harm. The main dis- advantages are waste of electric power and increased consumption of auxiliary anodes. In the extreme, additional disadvantages result if so much hydrogen is generated at the protected structure that blistering or disbonding of organic coat- ings, hydrogen embrittlement of the steel (loss of ductility through absorption of hydrogen), or hydrogen cracking (see Section 8.4 ) is caused. Damage to steel by hydrogen absorption is particularly apt to occur in environments containing sul- fi des [16] for reasons discussed in Section 5.5 .

C ATHODIC PROTEC TION

In the case of amphoteric metals, such as aluminum, zinc, lead, and tin, excess alkalies generated at the surface of overprotected systems damage the metals by causing increased attack rather than reduction of corrosion. It was shown that cathodic protection of lead continues into the alkaline range of pH, but the criti- cal potential for complete protection (see below) shifts to more active values [17] . Aluminum can be cathodically protected against pitting by coupling it to zinc

[18] used as a sacrifi cial anode; but if coupled to magnesium, overprotection may result with consequent damage to the aluminum.