The corrosion resistance of stainless steel is mainly due to the addition of a higher content of Cr element (such as 316L W (Cr) = 16.00 to 18.00) in the steel. Cr element is easy to oxidize and can quickly form a dense Cr2O3 oxide film on the surface of the steel. The electrode potential of the steel and the corrosion resistance in the oxidizing medium are increased abruptly. The corrosion resistance of stainless steel mainly depends on the extremely thin (about 1nm) dense passivation film covered on the surface. This passivation film Isolation from corrosive media is the basic barrier for stainless steel protection. If the passivation film is incomplete or damaged by defects, the stainless steel will still be corroded.
The lifting, assembling, welding, weld inspection (such as color inspection, pressure test) and surface oil stains, scratches, rust, impurities, and impurities of the stainless steel chemical tanker Melting point metal contaminants, paint, welding slag, spatters, etc., these substances affect the surface quality of stainless steel, destroy the surface passivation film, reduce the surface corrosion resistance, and are easy to share with the corrosive medium in the chemicals carried in the future It can cause pitting corrosion, intergranular corrosion, and even stress corrosion cracking.
Stainless steel chemical tankers usually use seawater to wash the tanks in operation. The seawater is rich in chloride ions, which has a greater corrosive effect on the passivation film. The working conditions are harsh, so pickling passivation is indispensable.
The passive film of stainless steel has dynamic characteristics and should not be regarded as a complete stop of corrosion. Instead, it forms a protective layer that diffuses. Usually, it tends to destroy the passive film in the presence of reducing agents (such as chloride ions). Air) can protect and repair the passivation film when it exists.
Stainless steel will form an oxide film when placed in the air, but the protection of this film is not perfect. An average layer of 10μm thick on the surface of the stainless steel is corroded by pickling. The chemical activity of the acid makes the dissolution rate of the defect site. It is higher than other parts on the surface, so pickling can make the entire surface tend to be evenly balanced. More importantly, through pickling passivation, iron and iron oxides are dissolved preferentially than chromium and chromium oxides, and the poor are removed. The chromium layer enriches the surface of stainless steel with chromium, and produces a complete and stable passivation film on the surface under the passivation of an oxidant. The potential of this chromium-rich passivation film can reach +1.0V (SCE), which is close to the potential of noble metals and increases Stability against corrosion.