pickle
A bath of dilute sulphuric or nitric acid, etc., to remove burnt sand, scale, rust, etc., from the surface of castings, or other articles of metal, or to brighten them or improve their colour.
pickle: acid bath for metal surface cleaning
A pickle is an acidic bath used to strip oxides, mill scale, and casting defects from metal surfaces. The most common formulation uses dilute sulphuric acid (typically 5 to 10 percent by volume) at temperatures between 50 and 70 degrees Celsius, though nitric acid, hydrochloric acid, or proprietary blends are used for specific alloys and results. The metal workpiece is immersed for minutes to hours depending on scale thickness and bath concentration.
In ferrous metalworking, pickling removes the black iron oxide layer (mill scale) that forms during hot rolling or forging. This scale, if left on the surface, prevents subsequent operations like plating, welding, or painting from bonding properly. Cast steel and iron castings are pickled after knockout to remove burnt-on sand and the rough oxide skin left by the mold. Non-ferrous metals require different chemistry: copper and brass typically use dilute sulphuric or nitric acid, while aluminium requires alkaline or specialized acidic solutions because strong mineral acids attack the base metal too aggressively.
The name derives from the pickling of food in vinegar, a much older practice, though the industrial analogy is imperfect. The metal is not preserved but rather cleaned and prepared. The bath works by acid dissolving metal oxides preferentially to the base metal, though timing matters critically: over-pickling in strong acid causes hydrogen embrittlement in high-strength steels and unnecessary material loss.
Practical considerations
Pickle tanks are typically mild steel or rubber-lined vessels equipped with heating and sometimes agitation. The acid grows weaker as it dissolves scale, so baths must be monitored and refreshed regularly. Spent pickle liquor is hazardous waste requiring neutralization before disposal. Workers handling pickle must wear acid-resistant clothing and eye protection; the fumes, especially nitric acid fumes, are corrosive and toxic.
Pickling is a gateway operation: it comes after casting or forging and before plating, painting, or precision machining. Some shops combine pickling with passivation, especially for stainless steel, to remove iron contamination and form a protective chromium oxide layer. Modern alternatives like electropolishing or mechanical abrasion exist but are slower or more costly for high-volume work, so pickling remains the industrial standard for bulk metal conditioning.