Abstract
During the course of development of cathodic protection systems suitable for the protection of active ships, a number of different designs of anodes have been tried and considerable differences in their behavior have been noted in ship trials. The designs tested have included sand cast cylindrical anodes with cast-in steel conductor rods, which corroded in a very uniform pattern, and chill cast rectangular anodes with cast-in retaining straps, which corroded preferentially at the cast-instraps. The magnesium alloy used in all cases was AZ-63. Chemical analysis of the metal indicates that the only major difference in the two anodes is inverse segregation of the major alloying constituents in the chill cast anodes. The inverse segregation found produces an anode with the most actively anodic metal on the inside of the casting. Any puncture of the anode surface (in the vicinity of a cast-in strap, for example) will lead to preferential attack at the point of puncture. Laboratory tests of the anode metal from chill cast ships' anodes and unsegregated anodes, indicates that there is sufficient difference between the outside metal and inside metal to account for the corrosion pattern observed in ship trials of chill cast anodes. 5.2.2
Redesign of sand-cast magnesium alloy baseplate for medium mortar