Polarization conditions can be improved with WIM
Depolarized pickling is not a new idea. For many years it has been known that a polarized condition exists at the point of acid attack when using pure acid or acid plus conventional inhibitors. It has also been known that this condition can be improved, providing a depolarizing agent can be introduced into the pickle.
The depolarizing pickle accelerates the rate of scale removal which is nothing more than a surface conditioning effect. The accelerated rate of scale removal, being the most obvious effect of the surface conditioning opens the door to; lower acid concentrations, lower pickling temperatures, better pickling efficiency, faster line speed, longer tank life, improved acid recovery, improved recovered acid efficiency and better dumping points.
Controlled depolarization and its many benefits have been used since 1946 for pickling a broad spectrum of steel: rod and wire, pipe, conduit, sheet, bars, strip and parts for plating, enameling, and hot dip and electro-galvanizing.
Controlled depolarization does not rely on wetters or on chelating agents. Controlled depolarization is a very efficient method to pickle, particularly with the current requirement to gain greater control over pickle waste and high energy costs.
We view the reaction of a pickling acid on steel as electrolytic corrosion cells set up all over the surface. When the hydrogen ions of an acid attack steel they do so at the anode of the electrolytic cell and produce a hydrogen gas. This gas forms a barrier film between the steel surface and the acid solution. This film will retard the action of further hydrogen ions until this gas develops enough buoyant energy to rise from the surface of the corrosion cell. Until it obtains its buoyancy, it “polarizes” or retards the cell’s activity.
The controlled depolarizers react with the nascent hydrogen forming a chemical compound that aids in faster removal of the barrier film from the cell. The action of the acid on the scale, undercutting and gaseous removal of the scale, is accelerated. The cell is then “depolarized” and prepared to repeat the process over and over again. The depolarizing action will cease once the soluble portion of the scale is removed.
The attack centers, the anodic centers of the electrolytic cells shift rapidly as this barrier film is formed and removed leaving a fine uniform etch on the pickled surface. The nascent hydrogen, because it is removed from the steel surface, has less time to react with the steel. Utilizing the controlled depolarizer attack, less acid is required to remove the scale and get a clean metal surface than would be needed using normal inhibitor practice.
The controlled depolarizers provide the kind of reaction at the point of acid attack that can overcome the inhibiting effect of iron in solution and allows pickling to higher iron concentrations. The iron in solution builds at a lower rate as the acid attack is not forced to dissolve as much iron in the scale removal process as in conventionally inhibited baths.