Process control for stainless steel pickling and passivation

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1. Pretreatment for pickling and passivation: If there are surface contaminants on stainless steel workpieces prior to pickling and passivation, they should be cleaned mechanically, followed by degreasing. 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 corrosive 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 such as 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 halide content in the water used for the final cleaning. Deionized water is usually used. For water used in hydrostatic tests on petrochemical austenitic stainless steel pressure vessels, the Cl- content should be controlled to not exceed 25 mg/L. If this requirement cannot be met, sodium nitrate can be added to the water to bring the Cl- content within the specified limit. Excessive Cl- content can damage the passivation layer of stainless steel, thereby serving as a root cause for pitting corrosion, crevice corrosion, stress corrosion cracking, and similar issues. 3. Process control during pickling and passivation operations: Nitric acid solution is effective when used alone for removing free iron and other metallic contaminants; however, it is ineffective in eliminating mill scale, thick corrosion products, temper films, etc. Generally, an HNO3+HF solution should be employed. For the sake of convenience and operational safety, fluorides can be used as a substitute for HF. For a pure HNO3 solution, no corrosion inhibitor is needed; however, when performing pickling with HNO3 + HF, 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’s too high, HF will volatilize. For the passivation solution, the concentration of HNO3 should be kept between 20% and 50%. According to electrochemical tests, a passivation film formed at a HNO3 concentration lower than 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 obtained using the multi-step method. During the pickling process, the acid concentration, temperature, and contact time can be adjusted within certain limits. As the pickling solution is used over time, it is necessary to monitor changes in the acid concentration and metal ion concentration. Care should be taken to avoid over-pickling; the titanium ion concentration should be kept below 2%, otherwise severe pitting corrosion may 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 sensitization conditions of stainless steels. In some stainless steels, sensitization occurs due to improper heat treatment or welding. Pickling using HNO3+HF may lead to intergranular corrosion. The cracks resulting from this intergranular corrosion can concentrate halides during operation, cleaning, or subsequent processing, thereby causing stress corrosion. These sensitized stainless steels are generally not suitable for descaling or pickling with an HNO3+HF solution. If such acid cleaning 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, an appropriate corrosion inhibitor like Lan-826 should be added. When pickling with HNO3+HF cannot be used for stainless steel and carbon steel assemblies in a sensitized state, a solution of glycolic acid (2%) + formic acid (2%) + corrosion inhibitor at 93°C for 6 hours, or a neutral ammonium EDTA solution + corrosion inhibitor at 121°C for 6 hours may be employed. Subsequently, the assemblies should be rinsed with hot water and immersed in a solution containing 10 mg/L ammonium hydroxide and 100 mg/L hydrazine. 6. Post-treatment after pickling and passivation: After being pickled and rinsed with water, stainless steel workpieces can be immersed in an alkaline permanganate solution containing 10% (by mass) NaOH and 4% (by mass) KMnO4 at a temperature of 71–82°C for 5–60 minutes to remove any residues from the pickling process. Subsequently, the workpieces are thoroughly rinsed with water and dried. If mottling 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 passivated waste liquids shall 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; the quality of the passivation film is determined by 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, after passivation and while remaining in the air, 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, allowing for 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 piece is passivated, due to its smoothness, it is difficult for the passivation solution to adhere to that surface, causing it to evaporate quickly. Therefore, the time that the part spends in the solution as well as in contact with air during the passivation process needs to be increased appropriately; otherwise, the passivation layer on the part will be relatively weak. 8.5 Passivation of water-trapping parts: It is necessary to prevent any solution from accumulating, so as to avoid excessive consumption and waste of the passivation solution as well as environmental pollution; this also ensures the quality of the passivation film in that area. 8.6 Passivation of small parts: The entire set of parts 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.