Previously reported investigations into the characteristics of zinc galvanic anodes in sea water have been confined to studies of the effects of composition under conditions of uncontrolled or a single controlled anode current density. Since the preferred galvanic anode alloy is one that will show little anodic polarization with time over a wide range of anode current densities, it is necessary to include current density as a factor when comparing alloy performance.
This paper presents data on the anodic polarization of various zinc alloys in natural sea water. The experimental technique and procedures employed are described. The combined effects of iron,, as an impurity, over the range of 0.0003 percent to 0.0320 percent, and aluminum additions over the range 0 to 22 percent, were investigated at five controlled anode current densities ranging from 0.5 to 20 ma/in2, for times up to 100 days. The interrelated effects of iron, aluminum, current density and time are presented by means of graphs and charts.
The test results confirm the major conclusions of earlier investigators with respect to the detrimental effect of iron and the beneficial effect of small aluminum additions. The results show that some of the anomalous results of the earlier work may be explained on the basis of the interrelated effects of composition, current density and time. Within the range of compositions tested, the preferred zinc anode alloy is shown to be one containing 0.3 to 0.6 percent Al with the iron content controlled in the range 0.001 to 0.005 percent. 5.2.2
