51316-7710-Understanding the Corrosion of Low-Voltage Al-Ga Anodes

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Product Number: 51316-7710-SG

ISBN: 7710 2016 CP

Author: Devon Baker

Publication Date: 2016

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Aluminum is an attractive metal for use as an anode in the cathodic protection of steels in seawater due to its low cost and high current capacity. Zinc is often used for its ability to readily corrode but it has a low current capacity and it operates at very negative voltages leading to hydrogen generation at the steel cathode causing hydrogen embrittlement. Aluminum can operate at less-negative voltages greatly reducing hydrogen generation but it forms a very passive oxide film preventing the anode from corroding. Ga is added to aluminum in small amounts (0.1 wt%) to destabilize this oxide film and allow for active corrosion. The mechanism of how Ga activates Al is still not well-known though there are prevailing proposals. A previous study noted a difference in behavior between Al-Ga master heats and the alloys that were later produced by re-melting them. This study is focused on characterizing the corrosion behavior of Al-0.1 wt% Ga in synthetic seawater with samples from a master heat and two subsequent remelts. Galvanostatic potentiostatic and open-circuit tests were run as well as galvanic coupling with 1123 steel. It was found that the remelted anodes behaved more consistently and maintained stable corrosion behavior for longer times than the master heat. XPS analysis showed elevated surface concentrations of Ga just beneath the oxide layer. The findings support the mechanism in the literature of discrete particles of Ga forming under the oxide film but do not support the mechanism of an amalgam layer formation.