First, the principle of pickling and passivation of stainless steel pipes.
The corrosion resistance of stainless steel pipes is mainly due to the addition of a high content of chromium (Cr). Cr is easily oxidized, rapidly forming a dense Cr2O3 oxide film on the steel surface. This causes a sudden increase in the electrode potential and corrosion resistance in oxidizing media. The corrosion resistance of stainless steel pipes mainly relies on this extremely thin (approximately 1 nm) dense passivation film covering the surface. This passivation film isolates the stainless steel pipe from the corrosive medium and is the basic protective barrier. If the passivation film is incomplete or damaged, the stainless steel pipe will still be corroded.
The handling, assembly, welding, weld inspection, and processing of stainless steel chemical tankers’ inner liner plates, equipment, and accessories can introduce surface contaminants such as oil, scratches, rust, impurities, low-melting-point metal contaminants, paint, weld slag, and spatter. These substances affect the surface quality of the stainless steel pipes, damage their passivation film, reduce surface corrosion resistance, and easily interact with corrosive media in the chemicals subsequently transported, leading to pitting corrosion, intergranular corrosion, and even stress corrosion cracking.
The passivation film of stainless steel pipes has dynamic characteristics and should not be considered as a complete cessation of corrosion. Instead, it forms a diffusing protective layer. Typically, the presence of reducing agents (such as chloride ions) tends to damage the passivation film, while the presence of oxidizing agents (such as air) can protect and repair it.
Stainless steel pipes form an oxide film when exposed to air, but this film is not sufficiently protective. Pickling removes an average 10μm thick layer from the surface of the stainless steel pipe. The chemical reactivity of the acid causes a higher dissolution rate at defective areas compared to other parts of the surface. Therefore, pickling can make the entire surface more uniform and balanced. More importantly, through pickling passivation, iron and iron oxides dissolve preferentially over chromium and chromium oxides, removing the chromium-depleted layer and enriching the surface of the stainless steel pipe with chromium. Under the passivation action of the oxidant, a complete and stable passivation film is formed on the surface. This chromium-rich passivation film has a potential of +1.0V (SCE), close to the potential of precious metals, improving corrosion resistance.
Second, pickling and passivation methods for stainless steel pipes. Depending on the operation method, there are six main pickling and passivation methods for stainless steel pipes: immersion, paste, brushing, spraying, circulation, and electrochemical methods. Among these, immersion, paste, and spraying are more suitable for pickling and passivation of stainless steel chemical tankers and equipment.
1. Immersion Method: This method is most suitable for stainless steel pipes, elbows, and small parts, and yields the best treatment results. The treated parts are fully immersed in the pickling and passivation solution, resulting in complete surface reaction and a dense, uniform passivation film. This method is suitable for continuous batch operations, but requires constant replenishment of the solution as the concentration decreases. Its disadvantages include limitations imposed by the shape and capacity of the acid tank, making it unsuitable for large-capacity equipment and excessively long or wide pipelines; long-term disuse can lead to decreased effectiveness due to solution evaporation; and the need for dedicated facilities, acid tanks, and heating equipment.
2. Paste Method: Pickling and passivation pastes for stainless steel pipes are widely used in China, with a series of products available. The main components consist of nitric acid, hydrofluoric acid, corrosion inhibitors, and thickeners in specific proportions. The marine standard “Stainless Steel Pickling and Passivation Paste” CB/T3595-94 provides specific inspection rules. This method is manual and suitable for on-site application. It applies to the treatment of weld seams, weld discoloration, deck tops, corners, backs of ladders, and large-area passivation within liquid tanks on stainless steel chemical tankers. The advantages of the paste method are that it does not require special equipment or sites, does not require heating equipment, is flexible in on-site operation, and the pickling and passivation are completed in one step, with strong independence; the passivation paste has a long shelf life, and each application requires a new passivation paste for one-time use. The reaction stops after the first passivation layer is completed, making it less prone to over-corrosion, not limited by subsequent rinsing time, and can further enhance passivation of weak points such as welds. The disadvantages are poor working conditions for workers, high labor intensity, higher costs, and slightly poor treatment effect on the inner walls of stainless steel pipelines, requiring a combination with other methods.
3. Spray method: Suitable for pickling and passivation of single products or equipment with simple internal structures in fixed, enclosed environments, such as the spray pickling process on plate production lines. For stainless steel chemical tankers, it can be used for pickling the inner walls of liquid cargo tanks. The solution utilization rate is high. This method has many limitations, such as:
(1) All scaffolding inside the liquid tank must be removed, and the tank and piping system must be thoroughly cleaned, with no residue or impurities in the pipes.
(2) All deep well pumps, valves, tank cleaning machines, and other liquid cargo systems on the ship must be tested and ready for use.
(3) If the pickling solution remains in the tank for too long, it will react and cause excessive corrosion of the stainless steel pipes. Therefore, continuous operation is necessary, and the shipyard must cooperate closely, preparing large quantities of cleaning water at any time. Power outages, water outages, or work stoppages can have serious consequences.
(4) Waste acid and wastewater must be stored in large containers or temporarily disposed of in a separate tank, but must be neutralized and discharged as soon as possible to prevent corrosion of the tank hull.
(5) As the reaction time increases and the amount of impurities in the solution increases, the effective components of the pickling solution gradually decrease. The solution concentration must be monitored regularly, and fresh solution must be added promptly.
(6) The spray solution remains for a shorter time on areas such as the deck, walls, corners, and the back of ladders, resulting in a slightly less effective pickling than on the bottom plate.
Third, Special Precautions for Pickling and Passivation of Stainless Steel Pipes
Pre-treatment for pickling and passivation: If there is grease or other contaminants on the surface of the stainless steel pipes, it will affect the quality of pickling and passivation. Generally, the tank walls should be cleaned first using alkaline detergents or similar cleaning solutions.
Control of Chloride Ion Concentration in Pickling Solutions/Pastes and Cleaning Water: Excessive chloride ion content can damage the passivation film on stainless steel pipes. Some stainless steel pickling solutions/pastes use chloride-containing etching media such as hydrochloric acid and perchloric acid as the main agent or auxiliary agent to remove the surface oxide layer, which is detrimental to corrosion prevention. This should be controlled during inspection. For example, the marine standard CB/T3595-94, “Stainless Steel Pickling and Passivation Paste,” stipulates that the chloride ion content of stainless steel pipe pickling and passivation paste should be controlled within the range of 25ppm to 100ppm. Additionally, the chloride ion content of cleaning water should also be controlled below 25PPM. Adding sodium nitrate, a chloride ion corrosion inhibitor, to tap water during construction can meet this requirement.
Waste Liquid Neutralization and Environmental Discharge: The treatment of pickling and passivation waste liquid should meet national environmental discharge requirements. For example, fluoride-containing waste liquid can be treated by adding lime milk or calcium chloride, and chromium-containing waste liquid can be treated by adding ferrous sulfate, etc.
Fourth, Pickling and Passivation Quality Inspection of Stainless Steel Pipes.
The quality inspection of the pickling and passivation effect of stainless steel pipes can be carried out with reference to the ship standard “Stainless Steel Pickling and Passivation Paste” CB/T3595-94 and the international standard “Inspection of Surface Passivation Film Formed by Electrolytic Polishing and Grinding and Pickling Passivation of Stainless Steel” ISO15730, etc. It can generally be divided into: visual inspection and chemical testing. Chemical testing mainly includes artificial seawater corrosion test, copper sulfate titration test, and potassium ferricyanide titration test (blue spot test), etc.
(1) Visual Inspection: The pickled and passivated surface of the stainless steel pipe should be uniformly silvery-white, smooth and beautiful, without obvious corrosion marks. There should be no oxidation color in the weld and heat-affected zone, and no uneven color spots.
(2) Residual Liquid Inspection: Use phenolphthalein test paper to check the degree of rinsing of residual liquid on the surface of the stainless steel pipe. A neutral pH value is acceptable.
(3) Blue Spot Test: The basic principle of the blue spot test is that if the surface passivation film is incomplete or contaminated with iron ions, free iron ions will be present. When potassium ferricyanide solution encounters iron ions, it reacts to form a blue precipitate, as shown in the following reaction formula: K++Fe+2+[Fe(CN)6]-3→KFe[Fe(CN)6]↓. Dissolve 10g of potassium ferricyanide in 50ml of distilled water in a 100mL beaker. After dissolving, add 30ml of concentrated nitric acid, then transfer to a 1000ml volumetric flask and dilute to the mark with distilled water. This is the blue spot test solution, which has a shelf life of one week. Test method: Drop the blue spot test solution onto the test paper attached to the surface of the stainless steel tube. If a blue spot appears on the test paper after 30 seconds, it is considered unqualified. It should be noted that chemical tests will damage the passivation film of the stainless steel tube. Usually, the test plate and the product can be acid-washed and passivated simultaneously, and then the above test can be performed on the test plate.
Post time: Apr-29-2026