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Anodized Aluminum: How Much Do You Know?

Anodized aluminum refers to a layer of dense aluminum oxide plated on the surface of aluminum and aluminum alloy in order to prevent further oxidation, with chemical properties that are the same as those of aluminum oxide. However, unlike a general oxide film, anodized aluminium can be dyed by electrolytic coloring.


Reaction Mechanism of Anodized Aluminum


Anode effect is a phenomenon unique to molten salt electrolysis, which is particularly pronounced in the production of electrolytic aluminum. When anode effect occurs during production, the cell voltage of the electrolysis bath increases sharply, reaching 20~-50V, sometimes even higher. Its occurrence has a great impact on the entire electrolysis series, reduces current efficiency, affects various technical indicators of electrolysis, and reduces the yield and quality of aluminum, which destroys the stable power supply of the entire electrolysis series. In terms of treatment, there are only two methods: extinguishing with an effect bar (wooden stick) or reducing the anode and increasing the amount of alumina feed to achieve the purpose of extinguishing the anode effect. No better treatment method has been found yet.


The occurrence of anode effect is due to the gradual decrease of oxygen ions in the electrolyte during the electrolysis process. When it reaches a certain level, fluorine precipitates and reacts with the anode carbon to form fluoride of carbon. The fluoride of carbon precipitates into fine carbon particles during decomposition. These carbon particles attach to the surface of the anode, preventing contact between the electrolyte and the anode, and making the electrolyte unable to wet the anode well, just like water cannot wet the surface of oiled paint, forming a layer of poorly conductive gas film between the electrolyte and the anode, and increasing the anode overvoltage, causing anode effect. After adding new alumina, oxygen is precipitated on the anode surface, reacts with carbon powder, gradually makes the anode surface calm, reduces the resistance, and the electrolysis process tends to normalize again.


Production Process of Anodized Aluminum


The following steps are how CNC anodized aluminum be produced:

  1. Mechanical polishing;

  2. Chemical treatment to remove copper components on the surface of certain alloys;

  3. Cleaning and degreasing (for parts that have been anodized but need to be anodized again, use alkali or special reagents to remove the original anodized top layer);

  4. Placing it in dilute sulfuric acid as an anode for electrification to generate a surface oxide layer;( it is porous and is a white semi-transparent film);

  5. Dyeing;

  6. Fixing (heating or using a chromate solution to seal the pores of the surface oxide layer).


Effect of Sulfuric Acid and Alloy Composition on the Oxide Film of Anodized Aluminum


In addition to affecting the corrosion resistance of the generated oxide film, the chemical composition of aluminium alloys also has a certain effect on the thickness of the final oxide film. Under the same oxidation treatment conditions, the oxide film obtained on pure aluminium is thicker than that on aluminium alloy. Aluminum-silicon alloy is difficult to oxidize, and the oxide film layer is dark and grey. Therefore, sheet metal parts with and without aluminium coating need to be anodised separately. Because under the same bath oxidation treatment, the oxide film on pure aluminium is generated faster and thicker, while that on naked aluminum is generated slower and thinner.


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