The difference in the coefficients of linear expansion between the aluminum substrate and the oxide film is the primary cause of cracking. Aluminum has a much higher coefficient of linear expansion than anodic oxide films, resulting in greater expansion upon heating. This exerts strong tensile stress on the oxide film, causing it to "pull apart." The harder or thicker the oxide film, the higher the risk of cracking. For example, ordinary sulfuric acid anodic oxide films exceeding 8 μm in thickness are highly prone to cracking after high-temperature baking. To address this, the hardness of the film can be reduced by adjusting the oxidation process, or chromic acid anodic oxidation can be used to generate a soft, radially porous film structure, significantly increasing the coefficient of linear expansion and reducing the tendency to crack.
Controlling the internal stress of the oxide film is crucial. During anodic oxidation, film growth is accompanied by volume expansion. Initially, compressive stress dominates, but as the film thickness increases or the current density rises, the internal stress gradually shifts to tensile stress, increasing the risk of cracking. Reducing the current density, increasing the oxidation temperature, and shortening the oxidation time can reduce film brittleness, keeping the internal stress in a compressive stress state, thereby inhibiting cracking. For example, when using low current density oxidation, film growth is more uniform, the internal stress distribution is more reasonable, and the probability of cracking is significantly reduced.
The structural design during the fabrication of anodized plates must avoid stress concentration. If the curvature of protruding aluminum parts is too small, insufficient coverage during oxide film growth can easily lead to cracks. Therefore, rounding sharp corners and edges to create a high-curvature oxide film can disperse stress and reduce crack formation. Simultaneously, sawing, bending, and other external forces should be avoided during processing to prevent mechanical damage and cracking.
The choice of sealing process directly affects the weather resistance of the oxide film. Sealing treatment can fill the pores of the oxide film and improve corrosion resistance, but excessive sealing or inappropriate sealant selection can exacerbate cracking. For example, room-temperature nickel-based sealants are prone to micro-cracks under high-temperature exposure, while boiling water sealing or medium-to-high-temperature nickel-free sealing is more suitable for outdoor environments. Furthermore, the baking temperature and time must be controlled after sealing to prevent cracking due to excessive shrinkage or expansion of the film.
Optimization of post-processing can further improve the toughness of the oxide film. After sealing the pores, immersing the profile in 60-70℃ hot pure water promotes the hydration reaction, softens the oxide film, improves flexibility, and reduces the risk of cracking. Simultaneously, extending the aging time allows the reactive filler within the pores to fully absorb moisture from the air, achieving true pore sealing and enhancing the adhesion between the film and the substrate, further reducing the possibility of cracking.
Material selection is just as important as pretreatment. Aluminum alloys with high copper and magnesium content (such as 2-series and 5-series alloys) are prone to forming cavities during anodizing due to anodic dissolution, leading to discontinuous oxide films and increasing the risk of cracking. Therefore, it is necessary to select appropriate aluminum alloy grades according to processing requirements and perform metallographic analysis before oxidation to ensure that the material composition meets the requirements. Furthermore, before oxidation, it is essential to thoroughly remove oil, oxide layers, and other impurities from the aluminum surface to avoid uneven oxide film thickness caused by poor local conductivity, which can lead to stress concentration.
Preventing oxide film cracking during anodized plate processing requires a comprehensive approach encompassing materials, processes, structure, and post-treatment. By optimizing oxidation parameters, controlling internal stress, improving structural design, and rationally selecting sealing processes and post-treatment methods, the integrity and durability of the oxide film can be significantly improved, meeting the stringent quality requirements of high-end application scenarios.
