Corrosion in acid chloride

“Metals are attacked by acid” is a commonly‐heard statement that is reasonably correct but rather too sweeping. Some metals are “noble” in that they are not affected by ordinary, non‐oxidising acids such as hydrochloric acid, and the important noble metals in electronics are copper, silver, gold, palladium and platinum. Many metals are highly reactive, but cover themselves with an inert, self‐repairing layer of oxide when exposed to air. These include tantalum, titanium, stainless steel and aluminium. The oxide films vary widely in their resistance to the hydrochloric acid produced by burning PVC, with tantalum oxide remaining inert under all conditions and titanium oxide being attacked only by boiling, concentrated acid. The chromium oxide inert film on stainless steel, and the alumina film on aluminium react readily, giving the chloride salts of the metals and water: