Pickling Operations and Material Selection

Pickling, or the chemical removal of oxide scale by the action of aqueous solutions of inorganic acids, is one of the most economical methods for cleaning metal. Two acids are commonly used by the ferrous industry to clean their product. They are Hydrochloric Acid (HCl) and Sulfuric Acid (H₂SO₄). While design and fabrication of the equipment used is similar, material selection is the key to cost effective operations. The corrosive effects of these two acids on materials used for pickling equipment are considerably different.

Sulfuric Acid, commonly used in concentrations up to 20% and temperatures to 180⁰F, will attack most alloys used for pickling equipment, over time dissolving them and uniformly thinning the cross-section. This attack will not be particularly noticeable and does not present a problem to the operation until the reduced cross-section upsets the structural integrity of the equipment.

The chloride ions present in Hydrochloric Acid operating at temperatures up to 125⁰F and concentrations up to 10% HCl will cause chemical pitting or cavities in most alloys. This also distresses the structural integrity of the equipment.

In addition to the type of acid being used, other factors such as temperature, aeration, agitation and inhibitors must all be considered when selecting the proper alloy for pickling equipment. Temperature increases tend to accelerate corrosion rates. Introduction of air into the pickling tank, which is a common practice, will increase the aggressive action on materials. This occurs in Sulfuric Acid where no air is bubbled into the tank but at the acid/air interface. The hook shows an accelerated thinning compared to the surfaces generally submerged. Ferric and Cupric salts which are present in the solution will also considerably increase the corrosion rate. These salts can be introduced from accelerators used in pickling houses or can develop as the acid is being spent. Many alloys which are recommended for use in handling pure acids are not recommended when these salts or ions are present in the solution.

Controls for the pickling operation are important if the equipment being used is to provide cost effective life. This all begins with the proper material selection.

For more information regarding material selection, please fill out the contact us section. Alloy Engineering is very experienced in material engineering and can assist you in the selection of optimum materials.