Process control for stainless steel pickling and passivation

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1. Pretreatment for pickling and passivation: If there are surface contaminants on the stainless steel workpieces before pickling and passivation, they should be removed through mechanical cleaning, followed by oil and grease removal. If the pickling solution and passivation solution cannot remove grease, the presence of grease on the surface will affect the quality of pickling and passivation; therefore, degreasing is essential, and this can be achieved using alkalis, emulsifiers, organic solvents, and steam. 2. Control of Cl- in pickling solutions and rinsing water: Some stainless steel pickling solutions or pastes use chloride-containing agents such as hydrochloric acid, perchloric acid, ferric chloride, and sodium chloride as main components or additives to remove the surface oxide layer. Chlorine-containing organic solvents like trichloroethylene are used to remove grease, but these are not very suitable for preventing stress corrosion cracking. Furthermore, industrial water can be used for the initial rinsing water, but strict control is required over the chloride content in the water used for the final cleaning. Deionized water is usually used. For water used in hydrostatic testing of petrochemical austenitic stainless steel pressure vessels, the C1- content should be kept below 25 mg/L. If this requirement cannot be met, sodium nitrate can be added to the water to achieve the desired level. An excessive C1- content can damage the passivation layer of the stainless steel, leading to pitting, crevice corrosion, stress corrosion cracking, and other issues. 3. Process control in pickling and passivation operations: Nitric acid solution is effective for removing free iron and other metallic contaminants, but it is ineffective against removing iron oxide scale, thick corrosion products, and tempering films. Generally, a HNO3+HF solution should be used; for convenience and operational safety, fluorides can be used as a substitute for HF. A pure HNO3 solution can be used without a corrosion inhibitor, but when using HNO3+HF for pickling, Lan-826 must be added. Use HNO3+HF for pickling; to prevent corrosion, the concentration should be maintained at a 5:1 ratio. The temperature should be below 49°C; if it is too high, HF will evaporate. For the passivation solution, the concentration of HNO3 should be kept between 20% and 50%. According to electrochemical tests, a passivation film formed with an HNO3 concentration below 20% has unstable quality and is prone to pitting; however, the HNO3 concentration should not exceed 50% either, in order to avoid over-passivation. Although treating with the one-step method for degreasing, pickling, and passivation is simple and saves time, the pickling and passivation solution (paste) contains corrosive HF; as a result, the quality of the resulting protective film is inferior to that achieved by the multi-step method. During the pickling process, the concentration of the acid, temperature, and contact time can be adjusted within certain limits. As the acid cleaning solution is used for an extended period of time, it is necessary to pay attention to changes in acid concentration and metal ion concentration. Care should be taken to avoid over-cleaning, as the titanium ion concentration should remain below 2%; otherwise, severe pitting will occur. Generally, increasing the pickling temperature accelerates and improves the cleaning effect, but it may also increase the risk of surface contamination or damage. 4. Control of pickling under stainless steel sensitization conditions: Certain stainless steels become sensitized due to improper heat treatment or welding; pickling with HNO3+HF may cause intergranular corrosion. Cracks resulting from this intergranular corrosion can concentrate halides during operation, cleaning, or subsequent processing, thereby leading to stress corrosion. These sensitized stainless steels are generally not suitable for descaling or pickling using HNO3+HF solutions. If such pickling is necessary after welding, ultra-low carbon or stabilized stainless steel should be used. 5. Pickling of assemblies made of stainless steel and carbon steel For assemblies consisting of stainless steel and carbon steel (such as stainless steel tubes, tube sheets, and carbon steel shells in heat exchangers), the use of HNO3 or HNO3+HF for pickling and passivation can cause severe corrosion to the carbon steel. In such cases, a suitable corrosion inhibitor like Lan-826 should be added. When stainless steel and carbon steel assemblies are in a sensitized state and cannot be pickled using HNO3+HF, hydroxyacetic acid (2%) + formic acid (2%) + a corrosion inhibitor at 93°C for 6 hours can be used, or an ammonium EDTA neutral solution + a corrosion inhibitor at 121°C for 6 hours. After that, the components should be rinsed with hot water and immersed in a solution containing 10 mg/L of ammonium hydroxide + 100 mg/L of hydrazine. 6. Post-treatment of pickling and passivation: After pickling and rinsing with water, stainless steel parts can be immersed in an alkaline permanganate solution containing 10% (by mass) NaOH + 4% (by mass) KMnO4 at 71–82°C for 5–60 minutes to remove residues from the pickling process. Afterwards, they should be thoroughly rinsed with water and dried. If spots or stains appear on the surface of stainless steel after pickling and passivation, they can be removed by scrubbing with fresh passivation solution or nitric acid of higher concentration. Stainless steel equipment or components that have undergone final pickling and passivation should be protected by covering or wrapping them with polyethylene film to prevent contact between dissimilar metals and non-metals. The treatment of acidic and passivation wastewater must comply with **environmental discharge regulations**. For fluoride-containing wastewater, lime milk or calcium chloride can be used for treatment. Try to avoid using chromates in the passivation solution; if chromium-containing wastewater is present, ferrous sulfate can be added for reduction treatment. Pickling can cause hydrogen embrittlement in martensitic stainless steel; if necessary, deoxidization can be achieved through heat treatment (heating to 200°C and holding for a period of time). Quality inspection 7. Quality inspection of stainless steel pickling and passivation: Since chemical testing can damage the product’s passivation layer, inspections are usually carried out on samples. Examples of the methods are as follows: (1) Copper sulfate titration test – 8 g of CuSO4 + 500 mL of H2O + 2–3 mL of H2SO4 solution is applied to the surface of the sample and kept moist; if no copper precipitation occurs within 6 minutes, it is considered satisfactory. (2) Potassium cyanide titration test for high-speed steel: A solution of 2 mL HCl + 1 mL H2SO4 + 1 g K3Fe(CN)6 + 97 mL H2O is applied to the surface of the sample, and the quality of the passivation film is evaluated based on the number of blue spots that appear and the time it takes for them to appear. 8. Application Examples 8.1 Passivation of Long Parts: When passivating long parts, the upper and lower ends of the coated parts enter and exit the passivation solution at different times; moreover, as the part moves within the solution, the lower end moves more significantly than the upper end ； On the other hand, during the time spent in air after passivation, the solution flows from the top to the bottom; thus, the surface of the plated part at the bottom has more solution adhering to it than that at the top, giving it more time for chemical reactions with the solution. All these cause the color of the passivation film at the lower end to be darker than that at the upper end. To reduce color variation, it is recommended that such parts enter and exit the passivation solution horizontally (with a hook at the lower end to lift them), and that overly long anodes be avoided. 8.2 Passivation of long-shaped parts: When the passivation tank is not large enough to hold long-shaped parts, temporary measures can be taken by using bricks or wooden strips to construct a frame that can accommodate the parts; the inside of this frame is lined with plastic sheeting, and after filling it with the passivation solution, it can be used. This method is convenient and helps to avoid quality issues such as uneven coating or seams. 8.3 Passivation of flat parts: When passivating flat parts, as they move around in the passivation tank, the edges come into contact with the passivation solution more intensely than the central areas, which results in uneven coloring of the passivation film in those areas. This problem can be resolved by using compressed air for stirring, and this method proves to be very effective. To improve the uniformity of the passivation film, it is also necessary to pay attention to the uniformity of current distribution during the galvanizing process; if needed, the edges of the coated parts should be shielded to prevent rough coating in those areas due to excessive current, which could affect the color of the passivation film. 8.4 Surface Passivation: After the surface of a smooth part is passivated, the smooth surface makes it difficult for the passivation solution to adhere to it, causing the solution to evaporate quickly. Therefore, it is necessary to increase the time that the part spends in the solution as well as in contact with air during the passivation process; otherwise, the passivation layer on the part will be relatively weak. 8.5 Passivation of areas prone to water accumulation: It is necessary to prevent the solution from leaking out, so as to avoid excessive consumption of the passivation solution and environmental contamination, while also ensuring the quality of the passivation film in those areas. 8.6 Passivation of small parts: The entire set of components tied together can be placed in a plastic basket for passivation, so as to prevent them from separating from each other due to shaking during the passivation process and falling into the tank, which would lead to the need for rework.