Sealing method of anodized film on aluminum alloy
The film generated by aluminum anodizing, including electrolytic coloring, is porous (with a porosity of about 5% to 30%). This porous film layer has low hardness, poor wear resistance, poor corrosion resistance, and is easily contaminated. Colored films can easily lose color if the pores are not sealed. In order to improve the performance of the membrane, it is necessary to seal the pores of the porous layer. The usual methods for sealing holes include hydration sealing, inorganic salt solution treatment sealing, and organic coating sealing.
（1） Hydration sealing treatment
Hydrated pore sealing is the process of placing an oxide film in contact with water at high temperatures, causing it to react and produce hydrated alumina, which undergoes volume expansion to shrink and seal the pores. The reaction equation is as follows:
Al203+nH20——，Al203·nH20
The n in the equation is equal to l or 3. If n=1, the density of the generated monohydrateAl203 · H20 is lower than the original Al20., Therefore, the volume of hydrated alumina increases by about 33%, while n= 2-3 , trihydrate alumina Al203 · H20 is generated, which has a lower density and an increase in volume of nearly 100%. Therefore, the membrane pores are closed due to the increase in volume . This process is an endothermic reaction and must be carried out at a temperature above 80 ℃.
The medium used for sealing holes includes hot water or steam. Hot water sealing is carried out in hot water or boiling water at ≥ 95 ℃, or steam sealing can be used. Hot water sealing is relatively simple and does not require burning a boiler. The pH value of the water is 6-7.5 and the sealing time is 20-40 minutes.
The water quality used for sealing holes must have strict requirements. Because ordinary tap water contains anions such as SOI, Cl~, and POi, it can reduce the corrosion resistance of the membrane layer. If there are metal ions such as Ca and M92+in the water, they will precipitate in the pores of the membrane, affecting the color or transparency of the oxide film. Therefore, deionized water or distilled water should be used for sealing holes.
Steam sealing must be carried out in a sealed container, connected to the boiler through pipelines. The pressure of the steam should be controlled at 0.1-0.3 MPa, the temperature should be about 100-115 ℃, and the time should be 20-30 minutes. Overall, the quality of steam sealing holes is better than that of hot water sealing holes, and the coloring film will not flow color. However, the equipment is complex, the investment is large, and the production cost is high. Generally, the selection should be based on product requirements. For products with high quality requirements and expensive prices, steam sealing should be used, while for ordinary products, hot water sealing should be used.
（2） Inorganic salt solution sealing is the process of soaking aluminum workpieces in a heated aqueous solution of inorganic salts for sealing. The operation method is similar to hot water sealing, but its sealing principle is not only hydration, but also the precipitation of inorganic salts in the sealing solution in the membrane pores, filling the membrane pores. Therefore, this type of sealing has a dual effect. The inorganic salts used include dichromate, molybdate, silicate, phosphate, acetate, etc. The most commonly used are dichromate, chromate, and hydrolyzed salts. From an environmental perspective, molybdate and acetate salts can be used. Inorganic salt sealing can be divided into heavy chromium salt sealing, hydrolyzed salt sealing, and inorganic salt room temperature sealing.
1. Dichromate sealing
Immerse the aluminum workpiece in a high-temperature aqueous solution containing dichromate and perform sealing treatment. It has a dual sealing effect of hydration sealing and inorganic salt filling. When the aluminum oxide that forms the oxide film reacts with water to form hydrated aluminum oxide, the following reaction also occurs:
2A1203+3K2Cr207+5H20-2AI (OH) Cr04+2AI (OH) Cr207+6KOH
It is the monohydrate alumina, trihydrate alumina, basic aluminum chromate, and basic aluminum dichromate generated in the reaction that together seal the pores of the oxide film. Before the dichromate sealing treatment, the surface of the aluminum workpiece should be thoroughly cleaned, and the anodic oxidation acid solution or oxidation coloring solution should not be brought into the sealing solution, which will affect the quality of the sealing. Due to the corrosion inhibition effect of dichromate and other substances on aluminum workpieces, the corrosion resistance of the oxide film produced by this method is improved, making it suitable for use in protective products. However, after this treatment, the appearance of the oxide film appears light yellow, making it unsuitable for sealing decorative films. Of course, if you prefer to use the color of this treatment film, the oxide film treated with this method is not only wear-resistant and corrosion-resistant, but also has certain gloss and decorative properties.
2. Hydrolyzed salt sealing is the process of sealing aluminum workpieces by heating them in a nearly neutral salt solution. This method can accelerate the hydration reaction of the oxide film and reduce the processing temperature appropriately. On the other hand, due to the hydrolysis of salt in the membrane, it fills the pores and also plays a dual sealing role. During the sealing process of a sealing solution containing nickel sulfate as the main salt, nickel sulfate hydrolyzes in the membrane pores to form nickel hydroxide precipitate, which deposits in the pores. The reaction equation is as follows:
NiS04+2H20 =Ni (oH) 2+H2S04
Other salts such as cobalt salts also have similar reactions. The formula and process conditions for the hydrolysis salt sealing solution The small amount of nickel hydroxide or cobalt hydroxide generated in the membrane pores almost does not show color, so the sealing of the hydrolysis salt does not affect the color of the anodic oxide film, nor does it affect the sealing of the decorative film. For workpieces colored by organic dyes, these hydroxide precipitates can also react with previous organic dyes to form metal complexes, thereby increasing the stability of the dyes and maintaining color stability. Therefore, the treatment of the color film is particularly advantageous.
3. Room temperature sealing process refers to the process of sealing holes in a specific sealing solution at room temperature or slightly above room temperature. Due to the traditional closed process being carried out under heating or high temperature conditions, the operating environment is harsh, especially when using chromate or acetate solutions, which have strong odors, toxicity, high energy consumption, and serious environmental pollution. Therefore, room temperature sealing has been welcomed and praised. Many patented formulations of room temperature sealing liquids contain metal salts such as nickel, cobalt, zinc, titanium, etc., especially nickel acetate, nickel fluoride, etc., which are commonly used, as well as fluorides. Its mechanism of action is due to the acceleration of nickel and cobalt ions, which enables water cooperation to continue normally at lower temperatures; At the same time as accelerating hydration, metal ions also undergo hydrolysis to form hydroxide precipitates for filling and sealing purposes; The sealing agent reacts with some slightly soluble aluminum ions inside the membrane pores to form a stable chemical conversion film, such as Al-Si-O, A1-Zn-F and other types of refractory compounds that also play a sealing role. This sealing method is equivalent to the superposition of several sealing effects, and the effect is very obvious.
At present, room temperature sealing agents have become specialized and confidential formulas for some units, with their main ingredients being publicly disclosed, while some additives are rarely disclosed and need to be formulated based on actual conditions. There are some pre prepared room temperature sealing agents available on the market, mostly patented formulas such as GKC-F, NF-5, etc. It can be directly diluted and used according to the instructions.
（3） Application examples of anodizing coloring of aluminum and aluminum alloys
The aluminum alloy products produced by an aluminum factory in the the Pearl River Delta adopt sulfuric acid anodizing, stannous salt AC electrolytic coloring and room temperature sealing methods. The entire process flow is as follows: aluminum workpiece - degreasing - warm water washing - alkali etching - warm water washing - cold water washing - neutralization (or light emission) - water washing - anodizing - cold water washing - pure water washing - AC electrolytic coloring - cold water washing - hot water washing - room temperature nickel salt sealing - water washing - drying - finished product.
1. Before anodizing, remove oil in 1609/L sulfuric acid solution, wash with water, and then etch the film in a 5% NaOH, 60 ° C solution for 3 minutes. After washing with water, expose to light in a 20% -30% nitric acid solution for 1-1.5 minutes.
2. After nitric acid treatment, the anodized aluminum parts are washed with water and enter the anodizing tank. They are then immersed in a sulfuric acid solution of 150-1809/L for anodizing at a temperature of 25-35 ° C. The current density is l.0～1.5A/dm2， Power on for 40-50 minutes.
3. After electrolytic coloring and oxidation, aluminum parts are colored by alternating current in a coloring solution of 10-129/L stannous sulfate, 12-159/L sulfuric acid, and 10-129/L stabilizer, with a voltage of 12V and a current frequency of 50Hz. The voltage and duration are determined by the desired color. Under these conditions, a bronze color is obtained.
4. Room temperature sealing process conditions: Nickel acetate 5-89/L solution pH 5.5-6.5 Sodium fluoride 1-1.59/L solution temperature 25-55 ℃ Surface activator 0.3-0.59/L Sealing time 10-15 hours Additive 5.5-6.59/L
5. Corrosion resistance of the product
(1) The soaking test involves immersing the product samples sealed at room temperature in 5% hydrochloric acid and 10% sodium hydroxide solution, respectively. After two months (60 days) of soaking, they are taken out, washed clean, and dried. The surface color of the product remains unchanged and there are no signs of corrosion, indicating good and stable corrosion resistance.
(2) Neutral salt spray test Put the product sample sealed at normal temperature into the YQ-250 salt spray test chamber, take 24h as a cycle, continuously spray for 8h, the gap is 16h, and the control temperature is 25-30 ℃. After 28 days, take out and observe, the product sample has no change, and the coloring and sealing quality are good.
（4） organic coating can be used to seal holes in aluminum and aluminum alloy workpieces that have been anodized or colored after anodizing. organic substances used for sealing holes include transparent paint, various resins, molten paraffin, dry oil, etc. Currently, electrostatic spray painting, powder coating, or electrophoretic painting are commonly used. The resulting film layer has good quality, a wide range of colors, dense coating, high hardness, good wear resistance, corrosion resistance, insulation, and is bright and pleasing to the eye. organic coating treatment generally has the following situations.
1. For anodized but uncolored parts, solid powder coatings of various colors can be sprayed by electrostatic spraying to obtain products of various colors; Alternatively, electrophoretic painting can be used to coat the surface with various colored coatings.
2. For parts that have undergone anodizing and electrolytic coloring or staining, transparent varnish can be impregnated or sprayed to seal the holes, or paraffin, dry oil, etc. can be used to seal the holes
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