The necessity of stainless steel pickling passivation

Austenitic stainless steel has good corrosion resistance and high temperature oxidation resistance properties, good low temperature performance and excellent mechanical and processing properties. Therefore, it is widely used in chemical, petroleum, power, nuclear engineering, aerospace, marine, pharmaceutical, light industry, textile and other sectors. Its main purpose is to prevent corrosion and rust. The corrosion resistance of stainless steel mainly depends on the surface passivation film. If the film is incomplete or defective, the stainless steel will still be corroded.

In engineering, pickling and passivation treatment is usually carried out to make the corrosion resistance potential of stainless steel play a greater role. In the process of forming, assembling, welding, welding seam inspection (such as flaw detection, pressure test) and construction marking of stainless steel equipment and components, surface oil stains, rust, non-metallic dirt, low melting point metal contaminants, paint, and welding Slag and spatter, etc., these substances affect the surface quality of stainless steel equipment and parts, destroy the oxide film on the surface, reduce the overall corrosion resistance of steel and local corrosion resistance (including pitting corrosion, crevice corrosion), and even cause Stress corrosion cracking.

The surface cleaning, pickling and passivation of stainless steel seamless pipes not only maximize corrosion resistance, but also prevent product contamination and obtain beautiful appearance. According to ASME “Steel Pressure Vessels”, “the surface of vessels made of stainless steel and clad steel plates with anti-corrosion requirements should be pickled and passivated”. This provision is for pressure vessels used in petrochemical industries, because these equipment are used in direct contact with corrosive media. From the perspective of ensuring corrosion resistance, pickling passivation is necessary. For other industrial sectors, if it is not for the purpose of anti-corrosion, it is only based on the requirements of cleanliness and beauty, and the use of stainless steel materials does not require pickling passivation. However, the welds of stainless steel equipment need to be pickled and passivated. For nuclear engineering, certain chemical equipment and other applications with strict requirements, in addition to pickling passivation, high-purity media must be used for final fine cleaning or mechanical, chemical and electrolytic polishing and other finishing treatments.

Stainless steel pickling passivation principle: the corrosion resistance of stainless steel is mainly due to the surface is covered with a very thin (about 1nm) dense passivation film, which is isolated by 1n corrosive medium and is the basic barrier for stainless steel protection. Stainless steel passivation has dynamic characteristics and should not be regarded as a complete stop of corrosion, but as a barrier to diffusion, which greatly reduces the anode reaction speed. Usually in the presence of reducing agents (such as chloride ions) tend to damage the membrane, and in the presence of oxidants (such as air) can maintain or repair the membrane.

Stainless steel workpieces will form an oxide film when placed in the air, but the protection of this film is not perfect. Usually, thorough cleaning, including alkali cleaning and acid cleaning, and then passivation with an oxidant is required to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation treatment and ensure the formation of high-quality passivation films. Because the surface of the stainless steel surface is corroded by an average of 10μm thick by pickling, the chemical activity of the acid makes the dissolution rate of defect parts higher than other parts on the surface, so pickling can make the entire surface tend to be evenly balanced. The hidden dangers that could easily cause corrosion have been removed. But more importantly, through pickling passivation, the iron and iron oxides are dissolved preferentially than the chromium and chromium oxides, and the chromium-poor layer is removed, resulting in the enrichment of chromium on the surface of the stainless steel. This chromium-rich passivation film The potential can reach +1.0V (SCE), which is close to the potential of precious metals, which improves the stability of corrosion resistance. Different passivation treatments will also affect the composition and structure of the film, thereby affecting the rust resistance. For example, the electrochemical modification treatment can make the passivation film have a multilayer structure, forming CrO3 or Cr2O3 on the barrier layer, or forming a glass state The oxide film of stainless steel can exert maximum corrosion resistance.

The necessity of stainless steel pickling passivation

The necessity of stainless steel pickling passivation

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