Knowledge of stainless steel pickling and passivation

First, the principle of pickling passivation

The corrosion resistance of stainless steel is mainly due to the higher content of Cr added to the steel (such as 316L W (Cr) = 16.00 ～ 18.00). Cr is easily oxidized and can 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 suddenly improved. The corrosion resistance of stainless steel mainly depends on the extremely thin (about 1 nm) dense passivation film covering the surface. This layer of passivation film Isolation from the corrosive medium is the basic barrier for stainless steel protection. If the passivation film is incomplete or defective, the stainless steel will still be corroded.

The stainless steel chemical tanker lifts, assembles, welds, inspects welds (such as color inspection, pressure test) and the surface oil contamination, scratches, rust, impurities, low Melting point metal contaminants, paints, welding slags, spatters, etc. These materials affect the surface quality of stainless steel, destroy the surface passivation film, reduce the surface corrosion resistance, and are easy to work with the corrosive media in chemicals carried later Effect, causing pitting corrosion, intergranular corrosion, and even stress corrosion cracking.

Stainless steel chemical tankers usually use seawater to wash the tank during operation. The seawater is rich in chloride ions, which has a large corrosive effect on the passivation film, and the working conditions are harsh. Pickling and passivation is indispensable.

The stainless steel passivation film has dynamic characteristics and should not be considered as a complete stop of corrosion. Instead, it forms a protective layer of diffusion. Generally, it tends to destroy the passivation film in the presence of reducing agents (such as chloride ions). Air) protects and repairs the passivation film in the presence.

The stainless steel will form an oxide film when placed in the air, but the protection of this film is not perfect. The surface of the stainless steel has an average thickness of 10 μm. The surface is corroded by acid washing. The chemical activity of the acid solution causes 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 uniform and balanced. More importantly, passivation makes the iron and iron oxides preferentially dissolve over chromium and chromium oxides, eliminating the poor The chromium layer makes the stainless steel surface rich in chromium. Under the effect of oxidant passivation, a complete and stable passivation film is produced on the surface. The potential of this chromium-rich passivation film can reach + 1.0V (SCE), which is close to the potential of precious metals, which increases the potential of Stability against corrosion.

Second, pickling passivation

According to different operation methods, stainless steel acid pickling passivation treatment mainly includes 6 methods such as dipping method, paste method, brushing method, spray method, circulation method, electrochemical method, etc. Among them, dipping method, paste method and spray method are compared. Suitable for pickling and passivation of stainless steel chemical tankers and equipment.

1. Dipping method:

Stainless steel pipelines, elbows, small parts, etc. are most suitable for this method, and the processing effect is the best. Because the treated part can be fully immersed in the pickling passivation solution, the surface reaction is complete, and the passivation film is dense and uniform. Chuandong Shipyard has used our pickling workshop to pickle "Ninghua 417" stainless steel pipes in batches, and achieved good results. This method is suitable for continuous batch operations, but it is necessary to continuously replenish new liquid as the reaction concentration of the solution decreases. The disadvantage is that due to the limitation of the shape and capacity of the acid tank, it is not suitable for large-capacity equipment and pipelines that are too long and wide in shape; long-term use will reduce the effect due to solution volatilization and other reasons. Special sites, acid pools and heating equipment are required.

2. Paste method:

The stainless steel pickling passivation paste has been widely used in China and has a series of products. The main ingredients are composed of nitric acid, hydrofluoric acid, corrosion inhibitor, thickener, etc. in a certain proportion. CB / T3595-94 has specific inspection rules, manual operation, suitable for on-site construction. It is applicable to stainless steel chemical boat welding seam treatment, welding discoloration, top of the deck, corner dead corner, back of the escalator and large area of ​​the tank. .

The advantages of the paste method are that no special equipment and site are needed, no heating equipment is required, and the site is flexible in operation. The pickling and passivation are completed at once, and the independence is strong; the passivation paste has a long shelf life, and a new passivation paste is used for each application. In general, the reaction stops after the passivation of the surface layer is finished, it is not easy to be over-corroded, and it is not limited by the subsequent flushing time. Weakness such as welds can also strengthen passivation. Disadvantages are poor operating environment for workers, high labor intensity, high cost, and poor treatment of the inner wall of stainless steel pipelines, which need to be combined with other methods.