Performance Testing of Indium-Activated Aluminum Anodes in Seawater

Abstract

Indium-activated aluminum anodes have been selected as potential candidates to replace current military specification (MIL-A-18001J) zinc anodes for the cathodic protection of Navy ships. One concern regarding the use of indium-activated anodes is the anode performance in cold (40°F) seawater. The subject paper presents the results of a small-scale laboratory screening test investigating this possibility. Four indium-activated anodes were immersed in individual seawater-filled bare steel tanks for a period of 180 days. The seawater in three of the tanks was maintained nominally at 40°F. The fourth tank served as a warm water control. The control and two of the three test anodes were evaluated at a nominal anode current density of 500 mA/ft2. The third test anode was evaluated at 10 mA/ft2. Anode current output, anode potential, and tank potential were monitored throughout the test period. Seawater chemistry and temperature were also monitored. Over the test period, no signs of gross passivation were observed for any of the anodes. At times each of the anodes exposed in the cold seawater exhibited variations in current output as compared to the warm water control. However, these changes in current output were temporary and did not appear to indicate the onset of any passivation. The anodes exhibited efficiencies in the range of 67 to 79%. While this is lower than the values typically quoted by the manufacturers', the results agree quite well with other tests conducted. INTRODUCTION For the cathodic protection of ship hulls and floodable spaces, the U.S. Navy is considering the use of indium-activated aluminum anodes. These anodes are intended as replacements for presently used zinc anodes. The Navy cites potential cost and weight savings as the motivating factors for this change. One concern regarding the use of aluminum anodes is the anode performance in cold seawater. Representatives of the British Royal Navy during discussions with the U.S. Navy raised the possibility of anode passivation in cold seawater. The Royal Navy recalled results of a British Ministry of Defense study (circa 1965) that suggested aluminum anodes tend to passivate in cold seawater (40°F). The specific composition of aluminum alloys studied could not be determined. To resolve this matter and develop anode performance data in cold seawater, the Naval Sea Systems Command, Corrosion Control Division sponsored a small scale laboratory study. This study was designed as a screening test to detect any cold water passivation tendency. The tests evaluated the cathodic protection performance characteristics over a 6-month period. The following presents the results of this testing. TECHNICAL APPROACH For testing several indium-activated aluminum anodes were obtained from a qualified vendor. The chemical composition of these anodes was determined using atomic absorption spectroscopy. The anode manufacturer also provided an anode chemical composition in a mill report. The as-cast, as-received anodes used for this testing were cylindrical specimens nominally 1.5 inches in diameter by 5.6 inches in height. These dimensions field an exposed surface area of 175 cm2.