November 26, 2008

Chemicals for Pickling

Sulfuric acid is the workhorse of the industry and finds wide application for pickling steel, copper, and brass. It is used with chromic acid and the dichromates for deoxidizing and desmutting aluminum. It is used with hydrofluoric or nitric acid or both for descaling stainless steel. The anodic sulfuric acid etch is an effective way of removing the last traces of smut and scale from steel before plating. The Bullard-Dunn process using sulfuric acid with tin or lead salts and cathodic current is still used for picklng parts where dimensional change must be minimized.

Hydrochloric acid has the advantage of being an effective pickle for many metals at room temperature. Its one disadvantage is the fumes of HCl that may cause secondary problems.

Nitric acid is an ingredient of many bright dips. It finds wide use in combination with hydrofluoric acid for removing heat scale from aluminum, stainless steels, nickel- and iron-base alloys, titanium, zirconium, and certain cobalt base alloys.

Phosphoric acid is widely used for removing rust from steel and in special baths for stainless steel, aluminum, brass and copper. The phosphoric-nitric-acetic acid bath is often used to prepare aluminum for bright anodizing. Flouboric acid has proved to be most effective pickle for lead base alloys or soldered copper and brass parts that are to plated. It seems to work when other acids fail.

Sodium bisulfate (NaHSO4) is the basis for most dry acid pickling salts. It is a convenient way of handling sulfuric acid in dry powder form and eliminates the handling of carboys. Proprietary acid salts also incorporate compatible surface active agents and activating chemicals that make them more effective for certain plating lines than raw acids.

Post by Aluminum Anodizing.

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