Fisher Fisher-Stainless Steel Pickling Passivation Process Control

1. Pre-treatment of pickling passivation

If there is surface dirt before pickling and passivating the stainless steel workpiece, it should be cleaned mechanically, and then degreased and degreased. If the pickling solution and passivation solution can not remove the grease, the presence of grease on the surface will affect the quality of pickling passivation, so the degreasing and degreasing can not be omitted, and lye, emulsifier, organic solvent and steam can be used.

2. Control of Cl- in pickling solution and rinsing water

Some stainless steel pickling solution or pickling paste is used to add hydrochloric acid, perchloric acid, ferric chloride and sodium chloride and other chloride-containing erosion media as the main agent or auxiliary to remove the surface oxide layer, and chlorine-containing organic solvents such as trichloroethylene are used to remove grease, which is not suitable to prevent stress corrosion rupture. In addition, industrial water can be used for preliminary flushing, but the halide content is strictly controlled for final cleaning water. Deionized water is usually used. For example, the petrochemical austenitic stainless steel pressure vessel is used for hydrostatic test water, and the C1- content is controlled not to exceed 25mg/L, if this requirement cannot be met, sodium nitrate can be added to the water to make it meet the requirements, and the C1- content exceeds the standard, which will destroy the passivation film of stainless steel, which is the root cause of pitting, crevice corrosion, stress corrosion rupture, etc.

3. Process control in pickling passivation operations

Nitric acid solution alone is effective for removing free iron and other metal dirt, but it is ineffective for removing iron oxide scale, thick corrosion products, tempering film, etc., generally HNO3+HF solution should be used, for convenience and operation safety, fluoride can be used instead of HF. HNO3 solution alone can be done without corrosion inhibitors, but Lan-826 is required for HNO3+HF pickling. Pickling with HNO3+HF, the concentration should be maintained in a 5:1 ratio to prevent corrosion. The temperature should be below 49°C, if it is too high, HF will volatilize.

According to the electrochemical test, the quality of the passivation film treated with HNO3 concentration less than 20% is unstable and prone to pitting corrosion, but the concentration of HNO3 should not be greater than 50%, so as to prevent overpassivation. Although the one-step method is used to treat degreasing and pickling passivation, although the operation is simple and saves man-hours, there will be aggressive HF in the pickling passivation solution (paste), so the quality of its final protective film is not as good as the multi-step method.

The pickling process allows the acid concentration, temperature and contact time to be adjusted within a certain range. With the increase of the use time of the pickling solution, it is necessary to pay attention to the changes in acid concentration and metal ion concentration, care should be taken to avoid over-pickling, and the titanium ion concentration should be less than 2%, otherwise it will lead to serious pitting corrosion. In general, increasing the pickling temperature will speed up and improve the cleaning effect, but it may also increase the risk of surface contamination or damage

4. Control of pickling under stainless steel sensitization conditions

For some stainless steels due to sensitization caused by poor heat treatment or welding, the use of HNO3+HF pickling may produce intergranular corrosion, and the cracks caused by intergranular corrosion can concentrate halides during operation, or cleaning, or subsequent processing, causing stress corrosion. These sensitized stainless steels are generally not suitable for descaling or pickling with HNO3+HF solution. If this pickling is necessary after welding, ultra-low carbon or stabilized stainless steel should be used.

5. Pickling of stainless steel and carbon steel assembly

For stainless steel and carbon steel assemblies (such as stainless steel tubes, tube sheets and carbon steel shells in heat exchangers), if HNO3 or HNO3+HF is used for pickling passivation, the carbon steel will be seriously corroded, and a suitable corrosion inhibitor such as Lan-826 should be added. When the stainless steel and carbon steel assembly is in a sensitized state and cannot be pickled with HNO3+HF, hydroxyacetic acid (2%) + formic acid (2%) + corrosion inhibitor can be used at a temperature of 93 °C for 6h or EDTA ammonium neutral solution + corrosion inhibitor for 121 °C and 6 h, followed by rinsing with hot water and immersing in 10mg/L ammonium hydroxide + 100mg/L hydrazine.

6. Post-treatment of pickling passivation

After pickling and water washing, the stainless steel workpiece can be soaked in alkali 1 permanganate solution containing 10% (mass fraction) NaOH + 4% (mass fraction) KMnO4 in 71-82 °C for 5-60min to remove the pickling residue, and then rinse thoroughly with water and dry. Spots or stains appear on the surface of stainless steel after pickling passivation, which can be eliminated by scrubbing with fresh passivation solution or higher concentration of nitric acid. The stainless steel equipment or parts that are passivated by final pickling should be protected, and can be covered or wrapped with polyethylene film to avoid contact between foreign metals and non-metals.

The treatment of acidic and passivated waste liquid shall comply with national environmental protection discharge regulations. For example, fluorine-containing wastewater can be treated with lime milk or calcium chloride. The passivation solution does not use dichromate as much as possible, and if there is chromium-containing wastewater, it can be reduced with ferrous sulfate.

Pickling may cause hydrogen embrittlement in martensitic stainless steels, which can be deoxygenated by heat treatment if necessary (heated to 200°C for a period of time).

Quality inspection

7. Stainless steel pickling passivation quality inspection

Since chemical inspection can damage the passivation film of the product, it is usually inspected on a sample. Examples of how to do this are as follows:

(1) Copper sulfate titration test

8g CuS04+500mL H20+2-3mLH2SO4 solution was dropped into the surface of the sample to keep it wet, and if there was no precipitation of copper within 6min, it was qualified.

(2) High-speed rail titration test

2mL HCl+1mL H2SO4+1g K3Fe(CN)6+97mL H20 solution was dropped on the surface of the sample, and the quality of the passivation film was identified by the number of blue spots generated and the length of time.

8. Application examples

8.1 Passivation of long strings

When the long string is passivated, the upper and lower ends of the plated parts enter and exit the passivation solution in a sequence, and when the workpiece swings in the solution, the swing amplitude of the lower end of the plated part is much larger than that of the upper end; On the other hand, when it stays in the air after passivation, the solution flows down from the upper end, and the surface of the lower plating part is more liquid than the upper plated part, and the lower end has more chemical reaction time with the solution. All of this makes the lower end of the passivation film darker than the upper end. In order to reduce the color difference, it is recommended that such parts enter and exit the passivation solution laterally (the lower end is hooked with a hook), and prevent the anode from being too long.

8.2 Passivation of long strip parts

If the passivation groove can not be accommodated when the long strip parts are passivated, temporary measures can be taken, and a frame that can accommodate the plated parts can be processed by using bricks or wooden strips, and the frame is lined with plastic sheeting, and can be used after injecting the passivation solution, and this method is convenient and can avoid quality problems such as uneven film layer or joint imprints.

8.3 Passivation of flat parts

Generally, when the plane part is passivated, the contact between the edge part and the passivation solution will be more intense than that of the middle part when swinging in the passivation tank, which causes the uneven color of the passivation film in this part, and this problem can be solved by compressed air stirring, and the effect is very good. In order to improve the uniformity of the passivation film, it is also necessary to pay attention to the uniformity of the current distribution in the galvanizing process, and if necessary, the edge of the plating part should be shielded to prevent the rough coating due to excessive current, which will affect the color of the passivation film.

8.4 Surface passivation

After the passivation of the surface smooth part, due to the smooth surface, the passivation solution is difficult to adsorb on its surface and is lost quickly, so the residence time in the solution and the air during passivation should be appropriately prolonged, otherwise the passivation film of the workpiece appears lighter.

8.5 Easy to passivate the water parts

It is necessary to avoid the solution being taken out, so as not to cause excessive damage, consumption and pollution of the passivation solution, and the quality of the passivation film can also be guaranteed.

8.6 Passivation of small pieces

The whole string of bound workpieces can be put in the plastic basket for passivation, so as to avoid detachment from the group due to shaking and falling into the groove during passivation

Fisher Fisher-Stainless Steel Pickling Passivation Process Control

Fisher Fisher-Stainless Steel Pickling Passivation Process Control

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