Pickling (Preparation for Metal Coating)

Articles which are intended for coating with metal must first be cleaned free from heavy and dirt; oxide scale which may have been formed during hot fabrication operation such as annealing, casting and welding must also accomplished by solvent vapor degreasing or by immersion in a hot alkaline metal cleaning solution. These processes are describe n the section dealing with preparation for electroplating because they must, in any case, be repeated at that stage, especially if the article has been polished with greasy polishing compounds.

Oxide scale on iron and steel is removed by pickling, that is by immersion in hot dilute (about 10%) sulfuric acid solution or in hydrochloric acid solution. Similar acid solution are used for scaled copper and its alloys, for brass and for nickel-silver. The dissolution of the oxide copper of the oxide in the acid is a purely chemical reaction and is rather slow, taking many minutes to complete if the scale is thick. As the clean metal becomes exposed, it too would become attacked, which is undesirable, if small quantities of complex organic pickling inhibitors were not added. Unlike the dissolution of oxide, the acid attack on the metal is electrochemical and anodic, as already explained, and hydrogen gas is evolved in cathode places. Inhibitors are chosen to block either the anodic or the cathodic part of this process. Apart from the loss of sound metal caused by attack of the acid, the hydrogen evolved may be absorbed in part into the metal itself. With hardened high tensile steel this may lead to dangerous hydrogen embitterment, to such an extent that section which are already stressed may crack during pickling. Pickling inhibitors, however, reduce the hydrogen absorption. The absorbed hydrogen can be driven out subsequently by a low temperature baking treatment.

Pickling also removes the casting skin and burnt sand residue from castings. Scales and sand can alternatively be removed from rigid objective by sand or shot blasting. This process is rather costly and leaves a rough and pitted surface which is unfavorable for electroplated or hot dipped coatings, but which provides a useful mechanical key for sprayed metal coatings and for paints.