Introduction to pickling and passivation of austenitic stainless steel pressure vessels

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Pickling and passivation: The phenomenon in which the corrosion rate of a metal decreases significantly after it is treated with an oxidizing medium is known as the passivation of the metal. The passivation mechanism can mainly be explained by the film theory: it is believed that passivation occurs because, when a metal reacts with an oxidizing medium, a very thin, dense passivation film is formed on the metal surface – a film with good covering properties that adheres firmly to the metal surface. This film exists independently; it is usually a compound of oxygen and metal. It serves to completely separate the metal from the corrosive agents, preventing direct contact between the metal and those agents and thus protecting the metal from corrosion. When austenitic stainless steel is exposed to oxidizing media, a passivation film on its surface is formed that meets the aforementioned requirements. It must be noted, however, that this passivation film is highly susceptible to damage under the action of halide ions such as Cl, Br, and F, which have an activating effect. This is one of the reasons why, for austenitic stainless steel pressure vessels that have undergone acid pickling and passivation treatment, it is still necessary to keep the chloride ion concentration in the water below 25 ppm if the water remaining after the hydrostatic test is not allowed to dry out. For austenitic stainless steels, passivation is generally carried out using a solution based on nitric acid, which has strong oxidizing properties. To ensure the effectiveness of the passivation process, the surface to be passivated is pickled prior to passivation. The entire processing procedure is known as acid washing and passivation, abbreviated as acid washing and passivation. Process: To ensure the quality of pickling and passivation, it is necessary to first consider using a pickling and passivation solution for immersion; only when immersion is not feasible should the use of a pickling and passivation paste be considered, though applying the pickling and passivation solution by brushing is not advisable. The standard process for acid washing and passivation is as follows: 1. Pretreatment – The oil and dirt on the surface of the parts to be acid washed and passivated must be removed through degreasing; if necessary, alkali washing can be used, after which the surface should be rinsed thoroughly with water. Spatter, slag, and undercut on the surface of the welded joint should be smoothed out. The workpiece surface must be free from scratches or damage; weld scars and defects should be ground down to meet relevant specifications. 2. Acid cleaning and passivation methods (1) Acid cleaning and passivation solution (tank washing) method: Acid cleaning (in a tank), followed by rinsing. The temperature and duration of acid cleaning are strictly controlled based on the composition of the acid cleaning solution, in order to prevent corrosion of the substrate due to insufficient or excessive acid cleaning; if necessary, a corrosion inhibitor can be added to the acid cleaning solution. For carbon steel components on the pickled products that can be removed, they should be taken off; those that cannot be removed need to be coated with protective paint or wrapped up. After pickling, it is essential to rinse thoroughly with clean water to ensure no residual liquid remains on the surface, which facilitates the subsequent passivation process. Passivation (pool washing) and rinsing: The passivation time is strictly controlled depending on the properties of the passivation solution and the temperature, in order to form a proper passivation film. After passivation, thorough rinsing is also necessary to ensure that no residual liquid remains. (2) Acid pickling and passivation (two-in-one) solution (tank cleaning) and rinsing method: Use the acid pickling and passivation (two-in-one) solution; the only difference from the aforementioned method is that pickling and passivation are carried out in one step, with the rest of the procedures remaining unchanged. (3) Pickling and passivation (2-in-1) paste (application) and rinsing method: The steps for using the pickling and passivation (2-in-1) paste are as follows: Apply the paste evenly to the surface that needs to be pickled and passivated (usually starting with the weld areas); apply a slightly thicker layer in areas where the scale is thicker. Keep the ointment in place for the duration required by the ointment instructions. Before the paste dries completely, use wire or cotton yarn to remove rust and oxides (a stainless steel brush can be used if necessary). Thoroughly rinse the surface until it achieves a uniform silver-white color. Post-treatment: After pickling and passivation, certain protective measures must be taken for the passivated surface to prevent damage to the passivation film. The passivated surface must not come into contact with hard objects; welding, grinding, and similar operations are prohibited. Quality inspection: The quality inspection of parts after acid washing and passivation can generally be considered from the following aspects: (1) After acid washing, there should be no obvious signs of corrosion on the surface of the parts, nor should any stainless steel particles be attached to them. (2) The surface must be free of oxide scale and uneven color spots. Oxidized colors are not allowed on the weld joints, heat-affected zones, and the surfaces of heat-treated parts. (3) After acid washing and passivation, any water accumulated inside and on the surface of the workpiece (equipment) must be removed immediately; if it cannot be completely removed, the chloride content in the rinsing water should be kept at no more than 25 ppm. (4) No residual liquid shall remain on the surface after pickling; a pH test strip dipped in water can be placed on the surface of the part under inspection, and a pH value between 6.5 and 7.5 is considered acceptable. (5) After passivation, the surface forms a matte, silver-gray film. (6) Blue dot test for checking the quality of pickling and passivation: The blue dot test can be used as needed to assess the quality; this method is generally applied to test pieces, and the passivation quality of these test pieces should be consistent with that of the actual workpieces. If spot testing on the workpiece is necessary, the area to be inspected must be treated again using the original pickling and passivation method until the specified requirements are met. Prepare a solution by using 1 gram of potassium ferrocyanide K3Fe(CN)6, 3 milliliters of (65%–85%) nitric acid HNO3, and 100 milliliters of water (it is advisable to prepare it just before use). Then, after soaking filter paper in the solution, it is attached to the surface to be tested, or the solution is directly dropped onto that surface; the condition of the surface is observed within 30 seconds, and no blue precipitate indicates a successful test result. It should be noted that this test can only be carried out after the surface that has been pickled and passivated is basically dry, and the testing solution should also be rinsed clean after the test.