Aluminum alloy for paints, anodes, and sacrificial coatings

An aluminum-tin-indium (Al-Sn-In) alloy for improved powder pigment protective coatings, bulk anodes, and sacrificial metallic coatings via cold spray

Navy diver

A Navy diver positions a 200-pound anodized zinc block on a pier. The Navy’s improved aluminum alloy weighs only a third as much as a zinc anode and is environmentally-friendly. (Christopher Dallaglio/Navy)

In search of effective alternatives to hexavalent chromium, such as improved powder pigment for active aluminum-rich coatings, the Naval Air Warfare Center’s Aircraft Division (NAWCAD) at Patuxent River, Maryland, has discovered an improved alloy based on aluminum, with small amounts of tin and indium.

As an active powder pigment, this alloy offers greater protection against corrosion (lower electrical potential and higher current output) as compared to previous Al-rich coatings, while eliminating the use of zinc, an aquatic toxin.

This same alloy, as a bulk sacrificial anode, addresses MIL DTL 24779 for ships, submarines, and marine structures, but is lighter than typical “zincs” and offers greater capacity. The density is 7% less than Al-Zn-In alloy making the current capacity that much higher for similar efficiency. The cost will also be lower by replacing (5%) zinc with aluminum. The combined cost and output improvements give 6.3 cents per Amp-hour versus 6.8 cents per Amp-hour.

Eliminating zinc and associated cadmium in this application also benefits the environment. A cruise ship with a 3,000-guest capacity releases an estimated 1.3 kilograms of eco-toxic heavy metals into the water per day as the ship’s anti-fouling paint degrades.

Markets for sacrificial anodes include pipelines, platforms, storage tanks, dock gates, wind turbine foundations, jetties, pontoons, and other partially or fully submerged metal parts and objects. The least mature application for the Navy’s new alloy is its use as a sacrificial coating on steel, as applied with a thermal spray or cold spray. Promising research is underway for this application.

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