Passivation and Pickling of Stainless Steel

The main purpose of austenitic stainless steel is to rust and rust. Stainless steel corrosion depends mainly on the surface passivation film, if the passivation film is incomplete or defective, stainless steel will still be corroded. Engineering is usually carried out pickling passivation treatment, so that the corrosion resistance of stainless steel to play even greater. In the process of forming, assembling, welding, weld inspection (such as flaw detection, pressure test) and construction marking, stainless steel equipment and components bring surface oil, rust, non-metallic dirt, low melting metal contaminants, paint, slag and spatter, etc. These materials affect the surface quality of stainless steel equipment and components, destroying the oxide film on its surface, reducing the overall corrosion resistance and corrosion resistance of the steel (including pitting, crevice corrosion), and even lead to stress corrosion cracking.

Stainless steel tube surface cleaning, pickling and passivation, in addition to maximize the increase in corrosion resistance, but also to prevent product contamination and get a beautiful role. In GBl50 a 1998 "steel pressure vessel" provides that "the requirements of corrosion-resistant stainless steel and composite steel plate made of the surface of the container should be pickled passivation." This requirement is for the use of pressure vessels in petrochemical industry, because these devices are used in direct contact with the corrosive medium, from the corrosion and corrosion resistance to ensure that pickling passivation is necessary. For other industrial sectors, if not for corrosion purposes, only based on clean and beautiful requirements, and the use of stainless steel is no need for pickling passivation. But the weld of stainless steel equipment also need to pickling passivation. On nuclear engineering, some chemical equipment and other requirements of strict use, in addition to pickling passivation, but also the use of high purity media for the final fine cleaning or mechanical, chemical and electrolytic polishing and other finishing.

2. Stainless steel pickling passivation principle

The corrosion resistance of stainless steel is mainly due to the surface covered with a very thin layer (about 1nm) dense passivation film, this film 1n corrosive media isolation, stainless steel protection is the basic barrier. Stainless steel passivation with dynamic characteristics, should not be seen as corrosion completely stopped, but the formation of diffusion barrier, so that the anode reaction rate greatly reduced. Usually in the presence of a reducing agent such as chloride ion, destroying the membrane while maintaining or repairing the membrane in the presence of an oxidizing agent such as air.

Stainless steel workpiece placed in the air will form an oxide film, but the protection of this film is not perfect. Usually must be thoroughly cleaned, including alkaline washing and pickling, and then passivation of oxidants in order to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation treatment to ensure the formation of high quality passivation film. Because the surface of the stainless steel surface by an average of 10μm thick layer of surface corrosion, acid chemical activity makes the defect site of the dissolution rate higher than other parts of the surface, so pickling can make the whole surface tends to even balance, some of the original Easy to cause corrosion hazards were cleared. But more importantly, through the pickling passivation, so that iron and iron oxides than chromium and chromium oxide given priority to dissolve, remove the chromium layer, resulting in chromium in the stainless steel surface enrichment, this chromium-rich passivation film Of the potential up to + 1.0 V (SCE), close to the potential of precious metals, improve the stability of corrosion resistance. Different passivating treatments also affect the composition and structure of the film and thus affect the rust, such as by electrochemical modification, allowing the passivation film to have a multilayer structure, forming CrO3 or Cr2O3 in the barrier layer, or forming a glassy Of the oxide film, so that stainless steel can play the greatest corrosion resistance.