The performance of magnesium galvanic anodes in underground service is reviewed in the light of the past five years of field testing in which upwards of 2500 anodes have been tested in some 40 different backfills and in various soils. The pertinent indices of galvanic anode performance are cited and the various factors influencing the performance of magnesium anodes are enumerated. A summary of the most outstanding findings relative to anode current efficiency, anode current output and stability of current flow is presented. The effects of such factors as anode composition and impurities, current density, time, backfill composition, installation practice, soil characteristics, and weather on anode performance are discussed. The high purity Mg-6Al-3Zn alloy continues to provide the most consistently efficient performance. At current flows equivalent to ten year life, efficiencies corresponding to 500 ampere-hours per pound are usually realized except in high chloride soils where higher current densities are needed to get comparable efficiencies. Impurities, notably nickel, iron, and copper must be held at low concentration levels in order to realize good anode efficiency. Pure commercial (cell) magnesium anodes occasionally exhibit high efficiencies but statistical experience shows their performance to be decidedly inferior to that of the high purity alloy. Field tests show the efficiency of the alloy anodes to increase with current density as previously indicated by laboratory data. Likewise, efficiency tends to increase with time for periods up to at least two years. With the exception of the magnesium sulfite and chromic acid backfills, the composition of the backfills used appears to have no very significant effect on anode efficiency.
Anode current output is essentially a function of soil resistivity and cathode potential although it is influenced to some extent by backfill composition and backfill installation practice. Statistical data indicates that backfills installed dry provide a higher level of current output than is obtained with wet (mud) backfills. Likewise, high conductivity backfills appear to provide significantly higher current flow.
Excellent maintenance of current flow has been observed for periods of three to five years on relatively large numbers of anodes. The most stable current flows are associated with deep installations and poor to fair soil drainage. Well drained installations on slopes or elevations tend to exhibit poor current maintenance. Depletion of soluble backfill ingredients (gypsum, sodium sulfate) is evident after three to four years of service but current flow is not adversely affected so long as the moisture supply is adequate. Bentonite rich back-fills are highly moisture sensitive and tend to expand or shrink excessively with fluctuating moisture levels. If installed wet they sometime shrink enough to adversely affect anode-to-earth resistance. Stability of current flow is also affected by rainfall and temperature, the temperature effect being most pronounced where winters are coldest. Current flows are usually well maintained until the anode is 85-95 percent exhausted.
