Passivation is the process whereby chrome oxide is regenerated in an oxidising environment , Chrome oxide is depleted by grinding, sandblasting, polishing, pickling, surface damage and contact with corrosive substances. Failure of stainless steel to become or remain passive leads to corrosion, Corrosion can occur due to the presence of embedded iron oxides and iron chlorides, other surface contamination, or following Mechanical surface treatment methods such as grinding, polishing etc. Surface passivation treatment restores the corrosion resistant properties of stainless steel, Auto-passivation in air may not occur and does not Always ensure corrosion resistance Chemical passivation with a nitric acid based passivator, enhances the restoration of the chrome oxide passive film. 

Ferrous materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate. By using phosphoric acid and further protected by surface coating. As the uncoated surface is water-soluble , a preferred method is to form manganese or Zinc compounds by a process commonly known as Parkerizing or phosphate conversion. Older, less-effective but chemically-similar electrochemical Conversion coatings included black oxiding, historically known as bluing or browning. Ordinary steel form a passivating layer in alkali environments, As rebar does in concrete. A typical passivation process of cleaning stainless steel tanks involves cleaning with sodium hydroxide and citric acid followed by nitric acid (up to 20% at 120 °F) and a complete water rinse. This process will restore the film, remove metal particles, dirt, and welding-generated compounds (e.g. Oxides). 

Pickling

Chemical treatment of stainless steel is a vital component of the fabrication process, Pickling of stainless steel following welding leaves a visually and chemically clean Surface, which allows a greater chance of the stainless steel forming a  passive chrome Oxide film.
For the removal of weld scale, heat tint,ferric oxides, chlorides and other surface contamination Leaves the weld and heat affected zone chemically And visually clean, Aids the auto-passivation process Confers greater corrosion protection,than grinding or sandblasting its Non residual, and cost effective.

Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, rust or scale, from ferrous metals, copper, And aluminum alloys.[1] A solution called pickle liquor, which contains strong acids, is used to remove the surface impurities.  It is commonly used To descale or clean steel in various steel making processes. Many hot working processes and other processes that occur at high temperatures leave a discoloring oxide layer or scale on the surface. In order to remove the scale the workpiece is dipped into a vat of pickle Liquor. The primary acid used is hydrochloric acid, although sulfuric acid was previously more common. Hydrochloric acid is more expensive than sulfuric acid, But it pickles much faster while minimizing base metal loss. The speed is a requirement for integration in automatic steel mills that run production At high speed; speeds as high as 800 ft/min (~243 metres/min) have been reported. [2] Carbon steels, with an alloy content less than or equal to 6%, are often pickled in hydrochloric or sulfuric acid. Steels with an alloy content  Greater than 6% must be pickled in two steps and other acids are used, such as phosphoric, nitric and hydrofluoric acid. Rust and acid resistant Chromium-nickel steels are pickled in a bath of hydrochloric and nitric acid. Most copper alloys are pickled in dilute sulfuric acid, but brass is Pickled in concentrated sulfuric and nitric acid mixed with sodium chloride and soot. [1]Sheet steel that undergoes acid pickling will oxidize (rust) when exposed to atmospheric conditions of moderately high humidity. For this reason, A thin film of oil or similar waterproof coating is applied to create a barrier to moisture in the air. This oil film must later be removed for many Fabrication, plating or Painting processes.