The method of using acid solution to remove oxide scale and rust on the surface of steel is called pickling. It is a way to clean metal surfaces. Generally, the parts are immersed in an aqueous solution such as sulfuric acid to remove oxides and other thin films on the metal surface. It is the pretreatment or intermediate treatment of electroplating, enamel, rolling and other processes. Pickling can remove the rust, oxide scale, etc. on the surface of the magnesium alloy, and at the same time roughen the surface to improve the bonding force between the substrate and the coating. Pickling includes chromium-containing acid washing and chromium-free acid washing. 1 Pickling with chromium Chromium-containing acid lotion usually uses CrO as the main component, which has strong oxidizing properties. After CrO3 pickling, electroless plating nickel is performed on AZ91D magnesium alloy at medium temperature. The polarization curve shows that the corrosion potential of the coating is higher than that of the substrate, and the corrosion rate is significantly reduced. The influence of chromium-containing pickling solution on coating performance was studied, and the best pickling process was determined as follows: 240g/LCrO3, 40mL/LHNO3, pickling time 30S. The chromium-containing acid washing solution has a good etching effect and will not cause large over-corrosion to the magnesium alloy matrix. It is widely used. However, chromic anhydride is a highly toxic substance and has serious effects on the environment and human health. Therefore, chromium-free pickling has become the research trend of pickling technology. 2 Chromium-free pickling Chromium-free pickling solution includes acid pickling solution and alkaline etching solution. Among inorganic acids, nitric acid has the best etching effect on magnesium alloys, phosphoric acid is slightly worse, and sulfuric acid is not suitable. Use chromium-free pickling solution (H3PO4200mL/L, Na2MoO4·2H2O5g/L) for 5～10S at 45℃, pickling and chemical plating after activationnickel , A NiP coating with good corrosion resistance was obtained. Using nitric acid decaphosphoric acid instead of chromium-containing acid lotion, a chromium-free and low-fluorine electroless nickel plating process for magnesium alloy was developed.  Figure 1 shows the pickling of process 1 (CrO125g/L, 68%HNO3110mL/L, room temperature treatment 30-60S) and process 2 (68%HNO330g/L, 85% H3PO4, room temperature treatment 30-40S) Compared with the effect, it can be seen that after the process 2 is processed, the magnesium alloy obtains a better corrosion morphology, which increases the bonding force between the plating layer and the substrate. Compared the pickling effect of 3 kinds of chromium-free pickling solutions on AZ91D magnesium alloy, and found that when HNO3+H3PO4 is used as pickling solution, it can fully remove the oxide scale and rough surface without causing excessive corrosion to the substrate. , See Figure 2. Adding potassium fluoride and molybdate to the phosphoric acid pickling solution can inhibit the corrosion of phosphoric acid on the magnesium matrix and reduce the amount of phosphorus in the pickling solution to 20-40mL/L, which also has a better pickling effect . The alkaline etching solution is mainly based on pyrophosphate, using P2O7 to coordinate with MgO and Mg(OH)2 to form MgP2Or, so that the magnesium alloy matrix is homogeneously etched to obtain a good coating. The chromium-free pickling process has attracted everyone's attention, but most of the current research is still in the experimental stage, which can remove rust stains and oxide layers. The process of roughening the surface of magnesium alloy without over-corrosion to the substrate needs further exploration. Activation The purpose of activation is to remove the oxide film and form a new film on the surface of the magnesium alloy. The film is generally a mixture of MgF2 and MgO. MgF2 protects the matrix. MgO can promote the deposition of nickel. The activation of magnesium alloy is divided into fluorine-containing activation and fluorine-free activation. ① Fluorine-containing activation Use HF (40%370mL/L) to activate the magnesium alloy at room temperature to improve the corrosion resistance of the Ni-P coating. Comparing the treatment effects of three activating solutions on magnesium alloys, it is found that after the magnesium alloys are activated with NH4H2PO480～100g/L and NH4F30-50g/L, the nickel plating layer obtained is uniform and fine. The plating layer has good bonding force and relatively high white corrosion potential. The base body is improved by 1.1v. Fluorine-containing activation is easy to control and has a good effect, but HF is very harmful to the human body and the environment. ② No fluorine activation Using Na2MoO4 and NaOH instead of acidic fluorine-containing activation solution, a Ni-P coating with corrosion resistance and good bonding strength is obtained at room temperature and pH≥12. Phytic acid (CHl8O24P6) is a new and environmentally friendly metal surface treatment agent. Research has found that after phytic acid treatment, the electrochemical performance of the film is improved and the corrosion rate is reduced. Research on the activation process of phytic acid, which means that after phytic acid treatment, the corrosion resistance of magnesium alloy is significantly improved. The optimal activation process is determined as: phytic acid 20g/L, 0=500C, pH=8, t=25min . Currently, there are many low-fluorine processes that use hydrofluoric acid or fluorides such as sodium fluoride and ammonium bifluoride instead of hydrofluoric acid. F-can etch the substrate and can form MgF2 protective substrate with magnesium alloy, so it is more used. Excess fluorine is also an element that must be strictly controlled for environmental protection. The increase in its content will definitely have a serious impact on the ecological environment, and a truly effective fluorine-free activation process is a future research trend. 3 One-step pickling activation method In order to simplify the steps and improve efficiency, the researchers completed the pickling and activation in one step. A one-step process based on phosphate and permanganate was developed to form a uniform and fine phosphate film on the surface of the magnesium alloy. After electroless nickel plating, the compactness and bonding force of the coating have been improved. Under the conditions of 0=25 and pH=2, 3uJ directly electrolessly plated the AZ91D magnesium alloy treated with the mixture of 3PO4, H3BO3, and NH4HF2 to obtain a corrosion-resistant coating. Use acetic acid and sodium nitrate as pickling solution to pickle AZ91D magnesium alloy, forming a silver honeycomb film layer on the surface of the substrate, which improves the bonding force of the coating. The one-step pickling activation method simplifies the process flow, and uses the experimental conditions of chromium-free and low fluorine ratio to reduce the harm to the environment and the human body. Reduce reagent consumption and subsequent processing steps, save costs, but one step The effect of pickling activation, process stability, proper range and coating performance, etc. need to be further improved. After talking so much, do you understand the method of electroless nickel plating and pickling activation? |
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