Stainless steel can corrode in service if there is contamination of the surface. Both pickling and passivation are chemical treatments applied to the surface of stainless steel to remove these contaminants and assist the formation of a continuous chromium-oxide, passive film.
Pickling and Passivation are both acid treatments and neither will remove grease or oil. If the fabricated part is dirty, it may be necessary to use a detergent or alkaline cleaner before pickling or passivation. At Control Electropolishing, we offer a full line of pickling and passivation with either Nitric or Citric Acid. Our line has two independent cleaning tanks used to prepare your parts prior to processing.
Pickling is the removal of any high temperature scale and any adjacent low chromium layer of metal from the surface of stainless steel by chemical means.
An example of good post-weld practice would be where the steel has been heated by welding, heat treatments or other means (see photo above), to the point where the surface has changed color, a chromium depleted layer forms on the surface of the steel underneath the oxide layer. The lower chromium content causes less corrosion resistance. To restore the metal to the best corrosion resistant performance, the damaged metal layer must be removed, exposing a fully alloyed stainless steel surface. Mechanical removal may leave abrasive or other particles embedded (interfering with corrosion performance) or may be impractical, so chemical means are usually employed.
Procedures incorporating pickling solutions remove the scale and the underlying chromium depleted layer and restore the corrosion resistance. Pickling solutions also remove contaminants such as ferrous and ferric oxide particles. After processing, parts are rinsed in a deionized water tank to accelerate the drying process and leave your parts free of stains, fingerprints, giving you a clean passive surface.