Abstract:
This study investigates the influence of active elements on the occurrence and progression of spotted corrosion defects in the anodized surface of 6063 aluminum profiles. Specifically, the accelerated effects of zinc (Zn) and the activating role of chloride ions (Cl-) are analyzed, along with the promoting influence of pH value. The research aims to contribute to a comprehensive understanding of the complex factors contributing to spotted corrosion.
4.1 Accelerating Effect of Zn Element:
Zinc, when solid-solved in aluminum alloy, accelerates grain corrosion through a “dissolution-redeposition” mechanism. Zinc or iron deposited on the alloy surface, along with high-potential dissolution products such as FeSiAl and free silicon serving as cathodic particles, effectively catalyze the reduction of dissolved oxygen. This catalytic action propels the continuous expansion and deepening of corrosion.
During alkaline washing, Zn dissolves in the alkaline solution as Zn(OH)42- and Zn(OH)-3 ions due to the dissolution of aluminum. The positive potential of Zn (-0.76V) compared to aluminum (-1.67V) leads to selective deposition of Zn ions on residues within corrosion pits. The significant potential difference between Zn and Al contributes to a high corrosion current in the microcell, resulting in rapid dissolution of cathodic particles in Fe and Si-depleted regions (predominantly pure aluminum). This corrosion manifests as spotted corrosion.
4.2 Activating Role of Cl-:
Chloride ions (Cl-), as external factors, exhibit sensitivity in inducing and exacerbating pitting corrosion. Research findings indicate that Cl- in degreasing acid adsorbs at passive film defects, penetrating the passive film and adsorbing onto the substrate. Activated aluminum in this region rapidly dissolves, breaking down the passive film and forming an electrochemical cell structure. In an acidic environment, localized corrosion current increases significantly. With elevated Cl- concentrations, the complexation reaction Al3++Cl-+H2O→AlOHCl++H+ occurs, intensifying the solution’s acidity and worsening corrosion conditions. Higher Cl- concentrations result in increased active sites on the passive film, leading to severe spotted corrosion during subsequent alkaline washing.
4.3 Promoting Effect of pH Value:
The pH value of rinsing water plays a promoting role in the darkening process of grain color during the transition from gray to black. Spotted corrosion is less likely to occur when the pH of rinsing water is below 2 or above 4.
When the pH of rinsing water exceeds 4, the formed passivation film on the aluminum profile surface is more complete and dense, reducing the adsorption, activation, and destructive effects of H+ and Cl-. Consequently, corrosion rarely occurs, or may not occur at all. Conversely, when the pH is less than 2, the aluminum surface is in an actively dissolved state without the formation of a passivation film, preventing the occurrence of spotted corrosion.
5 Conclusion:
Spotted corrosion in 6063 aluminum profiles is primarily caused by the segregation and coarsening of the anodic phase Mg2Si in the aluminum alloy. The impurity element Zn, along with Cl- concentration and pH value in the solution, accelerates the initiation and progression of spotted corrosion. Adjusting the magnesium-silicon mass ratio in the alloy, avoiding excessive silicon content, and implementing a rational aging regimen to prevent the agglomeration of Mg2Si particles are recommended to maintain optimal corrosion performance in aluminum profiles. Additionally, controlling trace elements of Zn in the alloy and managing Cl- concentration and pH value during processing help mitigate the negative impact of active elements.
