[China Aluminum Industry Network] 1. Introduction With the rapid development of electronic technology, the electromagnetic shielding requirements for various electronic devices are increasingly strict, especially in the military industry, this requirement is particularly prominent. The traditional penta-acid passivation process, due to its good passivation film compactness, the film layer can be self-repair, etc., has excellent anti-corrosion performance, and is widely used in the surface protection of military components, but its high resistivity and conductivity Poor performance limits its use as an electromagnetic shielding coating. The process begins with this basic point and is dedicated to improving the conductive properties of military green passivation membranes (this article only uses galvanizing as an example) to meet the electromagnetic shielding requirements of military enclosures.
The improvement of this process originated from the development of new products in our factory. The surface treatment of an aluminum military connector requires that the shell be green, shielded, and protected from salt spray 96h. The passivation of penta-acid cannot meet the requirements of electromagnetic shielding. After improvement, the shielding performance has been greatly improved. The process has been used in small batch production, the effect is satisfactory, and the same process can meet the product housing grounding or most electronic products shielding requirements.
2. Process Introduction 2.1 Process Flow (Aluminum Housing as an Example)
Degreasing→hot water washing→flow washing→etching→flow cold water washing→secondary zinc bathing→water washing→potassium galvanization→water washing→hot alkaline washing→flow washing→lighting→water washing→passivation→water washing→0.2%→~ 0.5% CrO3 hot water closed → dry 2.2 passivation process and operating conditions chromic anhydride 40 ~ 50g/L
Sodium formate 10~20g/L
Acetic acid 40~55g/L
Metal salt M15～25g/L
θ room temperature t solution 30 ~ 60s, 15 ~ 20s in the air
2.3 Process description 1) Degreasing: At the same time as degreasing, aluminum is corroded by alkali. Some insoluble metals such as Cu, Mu, Si are exposed on the surface of the parts and removed during etching to ensure that the aluminum surface is uniform and fresh. .
2) Etching: Generally pure aluminum and hard aluminum use a 1:1 HNO31 solution, while cast aluminum with a higher Si content should use a solution of HNO3:HF=3:1 (volume ratio).
3) Zinc leaching: In order to improve the binding force of the coating, secondary zinc immersion can be used, that is, after immersing zinc for one time, immersing with 1:1 HNO3 and then performing secondary zinc immersion. A small amount of heavy metal salt (usually FeCl3) may be added to the zinc bath to alloy the substituted zinc layer to change the crystal structure of the zinc layer and improve the binding force between the zinc layer and the aluminum matrix.
4) Potassium salt galvanization: Since the galvanizing is a weakly acidic solution, in order to prevent the zinc immersion layer from being dissolved, the plating part must be charged with a lower groove. After plating, to increase the bonding strength of the passivation film, 50-60°C should be used, and the mass fraction should be 5% NaOH solution.
5) Passivation: Chromium anhydride is the main component of film formation, in which Cr3+ is reduced as the skeleton of the passivation film, and Cr6+ can automatically repair the damaged passivation film. A suitable amount of Cr3+ concentration is conducive to the formation of a stable military green.
Sodium formate and acetic acid participate in the formation of the film, which is beneficial to the stability of the solution PH. The higher the content of them, the faster the film formation rate and the higher the passivation film is darkened or grayish green.
The content of metal salt M will affect the conductivity of the film. Generally, the upper limit of the formula can be used, but if M is too high, the film will be black.
When the temperature is high, the film layer is not bonded tightly, and the passivation film is too thick for a long time, the resistivity of the film layer increases, and the shielding performance of the coating is unfavorable, and the time is too short to be a grayish rainbow color.
6) Closed: Use a 0.2% to 0.5% Cr solution to make the micropores of the passivation film closed to improve the corrosion resistance of the coating. The solution temperature is 50 to 60°C and the processing time is 1 to 3 minutes.
3·passivation film characteristics
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