Surface Preparation – Acid Pickling

Removal of rusts, tarnishes, heavy scales, heat-treat scales and oxides requires stringent treatment in acid before the workpiece enter the plating bath. Treatment in acidic solution may be identified by various terms based on the intended application. Common acid treatment terms are pickling, descaling, deoxidizing, desmutting, neutralizing, acid dipping, acid activation etc. Often one step serves several purposes.

Pickling refers to the process of removal of metals from a surface by acids. Pickling can be made more effective by the employment of current. Cathodic pickling is used on stainless steel. Anodic pickling is effective for removing the last traces of smut and scale from high-carbon steels.

The common acids for pickling are mentioned below.

• Sulfuric acid is the most widely used as it is cheap, non-fuming, strongly acidic and forms soluble salts with most metals. It is used for pickling steel, copper and brass; with chromic acid or dichromates for desmutting and deoxidizing aluminum; and with hydrochloric and nitric acid for descaling stainless steel.
• Hydrochloric acid is somewhat more vigorous in action than sulfuric and can be used at room temperature. It gives off noxious fumes, but less noticeable in dilute solutions. It forms salt with most metals that are more soluble than sulfates.
• Mixed pickles offer many advantages over the component acids.
• Sulfuric-nitric pickles of various ratios are used to treat heat-treatable steels, magnesium and copper.
• Sulfuric-hydrochloric-nitric pickles are useful for stainless steel and sulfuric-hydroflouric-chromic pickles for aluminum.
• Nitric acid in combination with hydrofluoric acid is used for removing heat scale from aluminum, stainless steel, nickel-iron alloys, titanium alloys and cobalt alloys.
• Phosphoric acid in combination is useful for removing rust from stainless steel, aluminum, copper and brass.
• Fluoroboric acid is an effective pickle for lead alloys or soldered copper and brass parts.
• Sodium bisulfate (NaHSO4) acid salt is a convenient way of handling sulfuric acid in dry form, thus eliminating the hazards of handling this corrosive and dangerous material. It is the basis of many proprietary pickling compounds.

Corrosion inhibitors like thiourea or sulfonated quinides are widely used in descaling and pickling at the mill. However, in a plating shop, inhibitors can cause trouble and are generally avoided.

Smuts are residues of carbides or graphites remaining after acid treatment; they are difficult to remove. Substantial amounts of smut should be removed by an anodic cyanide or alkaline treatment; anodic acid treatment is capable of removing light smut formed by the preceding pickling operation.

Md. Nazmul Hasan Rasel

Materials Engineer

Email: hasanrasel.mse@gmail.com

Cell: +8801724512021