Stainless steel pickling passivation knowledge introduction

1. The principle of pickling passivation

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 steel. Cr element is easy to oxidize and can quickly form a dense Cr2O3 oxide film on the surface of 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, stainless steel will still be corroded.

The lifting, assembling, welding, weld inspection (such as staining flaw detection, pressure test) and the surface oil stains, scratches, rust, impurities, and impurities of the stainless steel chemical tanker Melting point metal contaminants, paint, welding slag, spatter, 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 tanks in operation. The seawater is rich in chloride ions, which has a great corrosive effect on the passivation film. The working conditions are bad, 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 in the presence of a reducing agent (such as chloride ions), it tends to destroy the passive film. 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 defects 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 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.

2. Pickling passivation method

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

1. Dipping method:

This method is most suitable for stainless steel pipelines, elbows, and small parts, and the treatment effect is the best. Because the treated parts can be fully immersed in the pickling passivation solution, the surface reaction is complete and the passivation film is dense and uniform. Chuandong Shipyard used its pickling workshop to pickle "Ninghua 417" stainless steel pipelines in batches and achieved good results. This method is suitable for continuous batch operation, but it needs to be continuously replenished with new liquid as the reaction concentration of the solution decreases. The disadvantage is that it is limited by the shape and capacity of the acid tank, and it is not suitable for large-capacity equipment and pipelines that are too long or too wide; if it is not used for a long time, the effect will be reduced due to the volatilization of the solution. It requires a dedicated site, acid pool and heating equipment.

2. Ointment method:

Stainless steel pickling passivation paste has been widely used in China and a series of products are currently available. The main components are nitric acid, hydrofluoric acid, corrosion inhibitors, viscous agents, etc. in a certain proportion. The ship is labeled "Stainless steel pickling passivation paste" CB/T3595-94 has specific inspection rules, manual operation, suitable for on-site construction. It is suitable for welding seam treatment, welding discoloration, deck top, corner blind corner, back of escalator and large area in the tank of stainless steel chemical tanker .

The advantage of the paste method is that it does not require special equipment and space, no heating equipment, flexible on-site operation, pickling passivation is completed at one time, and strong independence; the passivation paste has a long shelf life, and a new passivation paste is used for each application. For flexible use, the reaction will stop after the passivation of the surface layer is completed, it is not easy to be over-corrosive, and is not limited by the subsequent washing time, and weak links such as welds can also strengthen passivation. The disadvantage is that workers have poor operating environment, high labor intensity, and high cost. The treatment effect on the inner wall of the stainless steel pipeline is slightly poor, and other methods need to be combined.

3. Spray method:

It is suitable for pickling and passivation of a single product in a fixed site and a closed environment or equipment with a simple internal structure, such as the spray pickling process on the plate production line, and the pickling of the inner wall of the tank for the stainless steel chemical tanker. Chuandong Shipyard plans to adopt spray method for pickling and passivation of the cargo tank of Sinochem's 3450t stainless steel chemical tanker "Yingchun" under construction. The advantage is that the continuous operation speed is fast, the operation mode is simple, and the corrosion effect on workers is small. The pipeline can be pickled again during the pipetting process. The solution utilization rate is high. This method has many restrictions, such as:

1) The scaffolding in the tank must be completely removed, and the tank and piping system must be cleaned, and there must be no residues or impurities in the pipeline.

2) All liquid cargo systems such as deep well pumps, valves, and tank washing machines must be debugged and ready for use.

3) If the pickling liquid stays in the tank for too long, it will react and cause over-corrosion of stainless steel. Therefore, continuous operation must be carried out. The shipyard must cooperate with each other and prepare a large amount of cleaning water at any time. If there is a power outage, water cut, and shutdown, it will cause serious as a result of.

4) The waste acid and wastewater discharge must be filled with larger containers or a tank should be temporarily used instead, but they should be neutralized and discharged as soon as possible to avoid corrosion of the tank.

5) With the increase of the reaction time and the increase of impurities in the solution, the effective components of the pickling solution gradually decrease. The concentration of the solution must be checked at any time and new solution should be added in time.

6) The residence time of spray liquid on the deck, four walls, dead corners, back of escalators, etc. is shorter, and the pickling effect is slightly worse than that of the bottom plate.

Three, special attention

Pretreatment of pickling passivation: If there are grease and other dirt on the surface of stainless steel, it will affect the quality of pickling passivation. Generally, alkaline detergent should be used to clean the inner wall of the tank first.

Control of chloride ion concentration in pickling solution/paste and cleaning water: Excessive chloride ion content will damage the passive film of stainless steel. Some stainless steel pickling solutions/pastes use corrosion media containing chloride ions such as hydrochloric acid and perchloric acid as the main agent or Additives to remove the surface oxide layer are unfavorable for preventing corrosion and should be controlled during the inspection. For example, as stipulated in the ship standard "Stainless Steel Pickling Passivation Paste" CB/T3595-94, the chloride ion content of stainless steel pickling passivation paste Control in the range of 25ppm to 100ppm. In addition, the chlorine ion content of the cleaning water should be controlled to be less than 25PPM. During the construction, the chloride ion corrosion inhibitor sodium nitrate can be added to the tap water to meet the requirements.

Waste liquid neutralization and environmental protection discharge: the treatment of pickling passivation waste liquid should meet the national environmental protection discharge requirements. For example, fluorine-containing waste liquid can be treated with lime milk or calcium chloride; for chromium-containing waste liquid, sulfurous acid can be added Iron reduction treatment, etc.