Metals

Chloride ions and aluminium and its alloys

The presence of chlorides in aluminium alloys containing copper is a problem because chloride ions:

• penetrate the protective oxide coating;

• cause aluminium pitting corrosion; and

• promote localised copper corrosion from within the alloy.

As the copper corrosion products move to the surface, they interact with the aluminium corrosion products and form a blue-green, copper-stained aluminium hydroxide corrosion matrix.

The real problem occurs when—as a result of electrochemical reduction by the parent metal—copper is redeposited in metallic form on the surface of the alloy. The redeposited copper acts as a cathode in a pitting corrosion cell.

The conservation problem is to remove a relatively unreactive metal deposit from the surface of a very reactive metal, while at the same time trying to remove the chloride ions.

One simple solution to the problem is to use a solution of ammonia and ammonium sulphate to wash away the chlorides and the metallic copper from the surface. This produces complex reactions, but is effective.

When a corroded sea plane float was treated this way, it took 12 months of steady soaking, scrubbing and cleaning to stabilise the corroded metal.