The metallic luster of anodized plate surfaces is a core indicator of their appearance quality. Its formation and maintenance are influenced by multiple factors, including substrate characteristics, pretreatment processes, oxidation parameters, and post-treatment steps.
The substrate alloy composition is the fundamental factor determining gloss. Pure aluminum, due to its uniform crystal structure, forms a transparent and dense oxide film after oxidation, maximizing the retention of metallic luster. However, alloying elements can disrupt the uniformity of the oxide film. For example, when the magnesium content exceeds 3%, second-phase precipitates form, leading to uneven oxide film growth rates, thinner films, and poor gloss. When the silicon content exceeds 2%, the oxide film appears gray to black, losing its metallic luster. When the copper content exceeds 4%, the dissolution of the second phase creates pores, resulting in a rough and dull film. Furthermore, iron impurities significantly reduce gloss; when their content exceeds 0.1%, dark streaks or spots easily appear on the oxide film surface.
The quality of the pretreatment process directly affects the formation of the oxide film. Incomplete degreasing will leave residual grease, hindering the uniform oxidation reaction and leading to localized looseness in the film. Excessive pickling time or high acid concentration will over-corrode the substrate surface, creating a microscopic uneven structure that reduces the oxide film's light-reflecting ability. Improper sandblasting or polishing parameters will alter surface roughness, affecting the smoothness of the oxide film. For example, excessive sandblasting pressure can cause excessive surface roughness, resulting in severe light scattering after oxidation and reduced gloss.
The composition and concentration of the electrolyte during the oxidation process of anodized plates are crucial for controlling gloss. Sulfuric acid, a commonly used electrolyte, will accelerate oxide film dissolution if its concentration is too high, resulting in a loose and porous film with reduced gloss; conversely, a low concentration will lead to insufficient oxidation reaction rate, resulting in a thin and incomplete film. Impurities in the electrolyte significantly affect gloss. For example, copper ions can displace and deposit on the surface, forming a porous structure and reducing transparency and gloss. Excessive chloride ion content makes the oxide film rough and porous, leading to surface erosion and loss of gloss. Aluminum ion concentrations exceeding a certain range reduce the oxide film's corrosion resistance and abrasion resistance, making the surface prone to staining and affecting gloss uniformity.
Precise control of oxidation process parameters is crucial for gloss. Excessive oxidation voltage increases pore size and reduces the number of pores, making sealing difficult and resulting in white spots and patches, severely impacting surface gloss. Insufficient voltage slows oxide film growth, making it difficult to achieve the desired thickness and gloss. Excessively high oxidation temperature accelerates oxide film dissolution, making the film porous and reducing gloss; insufficient temperature leads to an inadequate oxidation reaction rate, resulting in rough and uneven crystallization. Insufficient oxidation time results in incomplete oxide film growth, a thin and structurally incomplete film with poor gloss; excessive time may lead to electrolyte erosion or overgrowth, resulting in an unstable structure and affecting gloss.
The sealing and cleaning quality in the post-treatment stage of anodized plates has a decisive impact on the final gloss. Improper selection of the sealing agent or inappropriate sealing time will lead to poor sealing results. For example, excessive sealing time may cause excessive deposition of the sealing agent, clogging the pores of the oxide film and affecting surface smoothness; insufficient time will result in incomplete sealing, making the oxide film susceptible to external erosion and reducing gloss stability. Inadequate cleaning, such as excessively high sulfuric acid concentration in the washing bath or poor water quality, will leave residual impurities adhering to the oxide film surface, reducing gloss.
The stirring and loading methods indirectly affect gloss by influencing current distribution and reaction uniformity. Insufficient stirring will lead to uneven concentration of the bath solution at the oxide film interface, large differences in local reaction rates, inconsistent film thickness and structure, and uneven gloss. Improper workpiece loading or overly dense arrangement will hinder uniform current distribution, causing differences in oxidation effects and affecting the overall consistency of gloss.
