The corrosion resistance of stainless steel is mainly due to the high content of Cr (such as 316L w (CR) = 16.00-18.00), Cr The elements are easy to oxidize, and can form a compact Cr2O3 oxide film on the surface of the steel rapidly, which makes the electrode potential of the steel and the corrosion resistance of the steel in the oxidation medium increase abruptly. The corrosion resistance of the stainless steel mainly depends on the very thin (about 1nm) compact passivation film covered on the surface, which is isolated from the corrosion medium and is the basic barrier for stainless steel protection. If passivation If the film is incomplete or defective, the stainless steel will still be corroded.
When stainless steel is placed in the air, an oxide film will be formed, but the protection of this film is not perfect. A layer of stainless steel surface with an average thickness of 10 μ m will be corroded by acid pickling. The chemical activity of acid solution makes the dissolution rate of defect parts higher than that of other parts on the surface. Therefore, acid pickling can make the whole surface tend to be even and balanced. More importantly, iron can be passivated by acid pickling The oxide of iron dissolves preferentially than that of chromium and chromium, removes the chromium poor layer, enriches the surface of stainless steel, and produces a complete and stable passivation film under the action of oxidant passivation. The potential of this chromium rich passivation film can reach + 1.0V (SCE), which is close to the potential of precious metal, and improves the stability of anti-corrosion.
According to different operation modes, pickling and passivation treatment of stainless steel mainly includes six methods, namely, immersion method, paste method, brushing method, spray method, circulation method and electrochemical method.
①Immersion method:
Stainless steel pipeline, elbow, small parts, etc. are most suitable for this method, 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. This method is suitable for continuous batch operation, but new solution needs to be added continuously with the decrease of solution reaction concentration. Its 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 with too long and too wide shape; the effect will be reduced due to the volatilization of solution and other reasons when it is not used for a long time, so special site, acid pool and heating equipment are required.
② Plaster method:
Stainless steel pickling passivation paste has been widely used in China and a series of products are available. Its main components are nitric acid, hydrofluoric acid, corrosion inhibitor, viscous agent, etc. in a certain proportion. The ship standard "stainless steel pickling passivation paste" CB / t3595-94 There are specific inspection rules, manual operation, suitable for on-site construction. It is applicable to weld treatment, welding discoloration, deck top, corner dead corner, escalator back and large-area painting and passivation in the tank.
The advantages of paste method are no special equipment and site, no heating equipment, flexible field operation, pickling and passivation once completed, strong independence; the passivation paste has a long shelf life, and each application is treated with a new passivation paste once used, and the reaction stops immediately after the passivation of the surface layer, which is not easy to be corroded, and is not limited by the subsequent washing time. Weak links such as welds can also be Strengthen passivation. The disadvantages are poor working environment, high labor intensity and high cost. The treatment effect of inner wall of stainless steel pipeline is a little poor, so other methods should be combined.
③ Spray method:
It is suitable for pickling and passivation of single product or equipment with simple internal structure in fixed and closed environment, such as spray pickling process in plate production line. For stainless steel chemical ship, it can be used for pickling of inner wall of cargo tank. It has the advantages of fast continuous operation, simple operation mode and little impact on workers' corrosion. The pipeline can be pickled again during the liquid transfer process. The utilization rate of solution is high.
Pretreatment of pickling and passivation: if there is dirt such as grease on the stainless steel surface, it will affect the quality of pickling and passivation. Generally, the inner wall of cleaning tank such as alkaline detergent should be used first.
Pickling solution / Control of chloride ion concentration in paste and cleaning water: if the chloride ion content exceeds the standard, the passivation film of stainless steel will be damaged. Some stainless steel acid lotions / pastes will use the corrosive medium containing chloride ion such as hydrochloric acid and perchloric acid as the main agent or auxiliary agent to remove the surface oxide layer. It is not beneficial to prevent corrosion, and it should be controlled in the inspection, as specified in the ship standard stainless steel pickling passivation paste CB / t3595-94 The chloride ion content of stainless steel pickling passivation paste is controlled within the range of 25ppm-100ppm. In addition, for the cleaning water, the chloride ion content should be controlled below 25ppm, and the requirements can be met by adding the chloride ion inhibitor sodium nitrate into the tap water during the construction.
Waste liquid neutralization and environmental protection discharge: the treatment of acid cleaning and passivation waste liquid shall meet the national environmental protection discharge requirements, such as adding lime milk or calcium chloride to the fluorine-containing waste liquid, adding ferrous sulfate reduction to the chromium containing waste liquid, etc.
For the quality inspection of stainless steel pickling and passivation effect, please refer to the ship standard "stainless steel pickling and passivation paste" CB / t3595-94, the international standard "inspection of surface passivation film formed by stainless steel electropolishing, polishing and pickling and passivation" iso15730 Generally, it can be divided into appearance inspection and chemical test. The chemical test mainly includes artificial seawater hanging plate corrosion test, copper sulfate titration test, potassium ferricyanide titration test (blue point test), etc. the test on the passivation effect of "Ninghua 417" wheel pickling is carried out in the following three ways:
Appearance inspection: the pickling and passivation surface of stainless steel shall be uniform silver white, smooth and beautiful, without obvious corrosion traces, and there shall be no oxidation color or uneven color spots in the weld and heat affected area.
Inspection for residual liquid: use phenolphthalein test paper to check the flushing degree of residual liquid on stainless steel surface, and the pH value is neutral, which is qualified.
Blue point test: the basic principle of the blue point test method is that if the passivation film on the surface is incomplete or polluted by iron ions, there will be free iron ions. When the potassium ferricyanide solution encounters iron ions, it will react to form blue precipitates. The reaction formula is as follows:
K++Fe+2+[Fe(CN)6]-3→KFe[Fe(CN)6]↓
Use a 100ml beaker to dissolve 10g of potassium ferricyanide in 50ml of distilled water, add 30ml of concentrated nitric acid after dissolving, then transfer it into a 1000ml volumetric flask and dilute it to the scale with distilled water, that is, blue point test solution, with a storage period of one week. Test method: put the blue dot inspection liquid drop on the test paper pasted on the stainless steel surface, and it is unqualified if the blue dot appears on the test paper 30 seconds later. It should be noted that the chemical test will destroy the passivation film of stainless steel. Generally, the test plate and the product can be pickled and passivated at the same time, and then the above test can be carried out on the test plate.
