Thermally-sprayed aluminum (TSA) is known to provide satisfactory and long-time corrosion protection to steel structures in seawater. Information about its performance under high velocity flow conditions is limited. The purpose of the present work is to investigate the self-corrosion rate of TSA on steel relative to a similar dense Al alloy and whether TSA can provide the necessary cathodic protection to the steel substrate under high flow rates up to 3.6 m/s in cross flow around cylindrical specimens. Both TSA and the dense alloy were based on an aluminum alloy containing 5% Mg (AlMg5). Corrosion rate of the TSA-coated steel samples, determined by weight loss measurements, was about one order of magnitude larger than the dense samples. The difference could not be rationalized by surface-roughness considerations or galvanic corrosion alone. When exposed to flowing seawater, the corrosion potential of the TSA decreased to -1.1 VSCE, negative enough for successful cathodic protection of the underlying steel. Its corrosion rate was attributed to the higher anodic activity of the TSA relative to the AlMg5 alloy, possibly by use of an activating trace element. Another possible contributing factor was cavitation erosion on a rough surface.