Abstract

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, which may cause hydrogen embrittlement. Aluminum can operate at less-negative voltages, therefore reducing hydrogen generation, but it forms a 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 UNS G11123 steel. It was found that the remelted anodes behaved more consistently and maintained stable corrosion behavior for longer times than the master heat. X-ray Photoelectron Spectroscopy analysis showed elevated concentrations of Ga inside 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.

Introduction

Aluminum is an attractive alternative to zinc for use as a sacrificial anode to protect steels in seawater. Aluminum has a large current capacity of 2974 A-hr/kg, and it can operate at higher potentials than zinc, -870 mV to -700 mVSCE.1,2 Steel corrodes severely at potentials above -600 mVSCE; the potential range necessary to protect steel in seawater is typically given as -1100 mVSCE to -850 mVSCE, though some sources give a higher maximum of -780 mVSCE.1-4 Pure Al forms a passive oxide layer, rendering it useless for cathodic protection, but Reding and Newport in 1966 published a landmark study that found a number of elements (Ga, In, Zn, Hg, etc) that could disrupt the passive layer and activate the aluminum.2 This study focused on understanding the behavior of Al-0.1 wt% Ga anodes in seawater.

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