1.Pre-treatment of pickling and passivation

If there are surface impurities on stainless steel workpieces before pickling and passivation, mechanical cleaning should be performed, followed by degreasing to remove oil. If the pickling solution and passivation solution cannot remove the grease, the presence of grease on the surface will affect the quality of pickling and passivation. Therefore, degreasing is essential and can be achieved using alkaline solutions, emulsifiers, organic solvents, and steam, among other methods.

2.Control of C1– in pickling solution and rinsing water

Certain stainless steel pickling solutions or pickling pastes use corrosive media containing chloride ions, such as hydrochloric acid, chloric acid, ferric chloride, and sodium chloride, as the main or auxiliary agents to remove surface oxide layers. Chlorine-containing organic solvents like trichloroethylene are used for degreasing, but they are not suitable for preventing stress corrosion cracking. In addition, industrial water can be used for initial rinsing, but the chloride ion content in the water used for final cleaning must be strictly controlled. Deionized water is commonly used for this purpose. For water used in hydrostatic testing of petrochemical austenitic stainless steel pressure vessels, the C1- content should not exceed 25mg/L. If this requirement cannot be met, sodium nitrate can be added to the water to meet the standards. Excessive C1- content can damage the passivation film of stainless steel, leading to pitting corrosion, crevice corrosion, stress corrosion cracking, and other issues.

3.Process control in pickling and passivation operations

Nitric acid solution alone is effective for removing free iron and other metal impurities, but it is ineffective against removing thick corrosion products, oxide scales, and tempering films. In general, an HNO3+HF solution should be used, and for convenience and safety, fluoride can be used as a substitute for HF. When using an HNO3+HF pickling solution, a corrosion inhibitor like Lan-826 should be added to prevent corrosion, and the concentration should be maintained at a ratio of 5:1 to avoid excessive corrosion. The temperature should be kept below 49°C to prevent HF from evaporating.

For the passivation solution, the concentration of HNO3 should be controlled between 20% and 50%. According to electrochemical testing, passivation film quality is unstable when HNO3 concentration is below 20%, leading to potential pitting corrosion. However, HNO3 concentration should not exceed 50% to prevent overpassivation. While a one-step method for degreasing, pickling, and passivation is convenient and time-saving, it may result in a less protective film quality due to the corrosive nature of HF in the pickling and passivation solution (paste).

During the pickling process, adjustments to acid concentration, temperature, and contact time are allowed within a certain range. As the pickling solution is used over time, attention must be paid to changes in acid concentration and metal ion concentration. Over-pickling should be avoided, and the concentration of titanium ions should be kept below 2% to prevent severe pitting corrosion. In general, increasing the pickling temperature can accelerate and improve the cleaning effect, but it may also increase the risk of surface contamination or damage.

4.Control of pickling under sensitization conditions in stainless steel

Certain stainless steels may become sensitized due to improper heat treatment or welding, and using an HNO3+HF pickling solution may lead to intergranular corrosion. Cracks caused by intergranular corrosion can concentrate halides during operation, cleaning, or subsequent processing, leading to stress corrosion. Sensitized stainless steels should generally not be pickled with an HNO3+HF solution to remove scale or for pickling purposes. If such pickling is necessary after welding, ultra-low carbon or stabilized stainless steel should be used to avoid sensitization issues.

5.Pickling of stainless steel and carbon steel combination components

For combination components of stainless steel and carbon steel, such as stainless steel tubes, tube sheets, and carbon steel shells in heat exchangers, pickling and passivation using HNO3 or HNO3+HF can severely corrode the carbon steel. In such cases, an appropriate corrosion inhibitor like Lan-826 should be added. When the stainless steel and carbon steel combination components are sensitized and cannot be pickled with HNO3+HF, a solution of hydroxyacetic acid (2%) + formic acid (2%) + corrosion inhibitor at a temperature of 93°C for 6 hours can be used. Alternatively, an EDTA ammonium-based neutral solution + corrosion inhibitor at a temperature of 121°C for 6 hours can be employed. Subsequently, rinse with hot water and immerse in a solution containing 10mg/L ammonium hydroxide + 100mg/L ammonia.

6.Post-treatment of pickling and passivation

After pickling and rinsing stainless steel workpieces, immerse them in an alkaline permanganate solution containing 10% (mass fraction) NaOH + 4% (mass fraction) KMnO4 at 71-82°C for 5-60 minutes to remove pickling residues. Thoroughly rinse with water and then allow for drying. If spots or stains appear on the stainless steel surface after pickling and passivation, they can be eliminated by scrubbing with fresh passivation solution or a higher concentration of nitric acid. It’s important to protect the finally pickled and passivated stainless steel equipment or components by covering them with polyethylene film or bandaging to prevent contact between different metals and non-metals.

For the treatment of acidic and passivation waste liquids, compliance with national environmental emission regulations is necessary. Lime milk or calcium chloride can be added to fluoride-containing wastewater for treatment. It’s advisable to minimize the use of chromium-containing salts in passivation solutions. If there is wastewater containing chromium, it can be treated with ferrous sulfate reduction in the presence of sulfuric acid.

Pickling may induce hydrogen embrittlement in martensitic stainless steel. If necessary, this can be addressed through heat treatment for dehydrogenation (heating to 200°C and holding for a certain period).

Quality Inspection

7. Quality Inspection of Stainless Steel Pickling and Passivation

Due to the fact that chemical testing can damage the passivation film of the product, inspections are usually conducted on test panels. The methods are illustrated as follows:

Copper Sulfate Titration Test:

Drip a solution containing 8g CuSO4, 500mL H2O, and 2-3mL H2SO4 onto the surface of the test panel, keeping it moist.

If there is no precipitation of copper within 6 minutes, the passivation film is considered satisfactory.

Potassium Ferricyanide Titration Test:

Drip a solution containing 2mL HCl, 1mL H2SO4, 1g K3Fe(CN)6, and 97mL H2O onto the surface of the test panel.

Evaluate the quality of the passivation film based on the quantity and duration of the appearance of the generated blue spots.

Application Examples

8.1 Passivation of Long Parts

During the passivation of long parts, there is a sequential entry and exit of the plated part’s upper and lower ends into the passivation solution. Simultaneously, when the workpiece swings in the solution, the lower end of the plated part swings much more than the upper end. Additionally, after passivation, when staying in the air, the solution flows from the upper end to the lower end. The lower end of the plated part has more solution attached, leading to more chemical reaction time. All these factors result in a darker color of the passivation film at the lower end compared to the upper end. To reduce color differences, it is recommended for these parts to enter and exit the passivation solution laterally (with the lower end hooked up) and to avoid using excessively long anodes.

8.2 Passivation of Long Bar Parts

If the passivation tank cannot accommodate long bar parts, a temporary solution can be implemented. Use bricks or wooden strips to create a frame that can hold the plated parts. Line the frame with plastic cloth, inject the passivation solution, and then use it. This method is both convenient and helps avoid quality issues such as uneven film layers or joint marks.

8.3 Passivation of Flat Parts

During the passivation of flat parts, the edges of the parts come into more contact with the passivation solution than the central part when swinging in the passivation tank, resulting in uneven coloration of the passivation film in these areas. This issue can be addressed by using compressed air agitation, which proves to be very effective. To enhance the uniformity of the passivation film, it is also necessary to ensure even distribution of current during the galvanizing process. If needed, the edges of the plated parts should be shielded to prevent rough plating due to excessive current, which can affect the color of the passivation film.

8.4 Surface Passivation

For passivating smooth and polished parts, the passivation solution is less likely to adhere to the surface due to its smoothness, leading to rapid loss. Therefore, the immersion time in the solution and air should be appropriately extended during passivation; otherwise, the passivation film on these parts may appear lighter.

8.5 Passivation of Parts Prone to Collecting Solution

To avoid solution retention and prevent excessive consumption or environmental pollution, it is important to ensure the quality of the passivation film on the parts prone to collecting solution.

8.6 Passivation of Small Parts

For a batch of tied-up small parts, placing them in a plastic basket for passivation helps prevent individual parts from detaching and falling into the tank during agitation, reducing the need for rework.