Metallic components used across the chemical, petrochemical, fertiliser, food-processing, and metal-finishing industries are routinely exposed to acidic process streams, cleaning solutions, and effluents. Copper, aluminium, and stainless steel are the three structural and functional metals most widely specified for such duty, yet they respond to acidic attack through fundamentally different mechanisms and at markedly different rates. This article reviews and compares the corrosion behaviour of copper, aluminium, and stainless steel in industrially relevant acidic environments, drawing on electrochemical, gravimetric, and surface-analytical studies reported in the corrosion-science literature. Copper corrodes chiefly through simple electrochemical dissolution that accelerates with oxidising acids, dissolved oxygen, and temperature, and lacks a self-healing passive layer once attacked. Aluminium relies on a thin, amphoteric oxide film that offers excellent resistance in mildly acidic and neutral media but breaks down rapidly once pH falls below about four or halide ions are present, producing severe localised pitting. Stainless steel possesses the most robust passive chromium-oxide film of the three, but this film is itself vulnerable to breakdown under chloride contamination, low pH, elevated temperature, and crevice geometries, sometimes yielding catastrophic localised failure despite a low overall corrosion rate. The comparative analysis shows that no single metal is universally superior; material selection in acidic industrial service must instead be matched to the specific acid, its concentration and temperature, the presence of chloride or other aggressive anions, and the geometry of the component. The article concludes with a discussion of practical mitigation strategies, including inhibitor use, cathodic protection, alloying, and protective coatings.