How to Deal with Surface Spot Corrosion in Anodized Industrial Aluminum Profiles?

Exploring the pitting corrosion that occurs on surfaces after anodization, this study analyzes the nature and causes of pitting corrosion and discusses the key factors that contribute to its occurrence.

Due to inadequate processing techniques and alloy composition, 6063 aluminum profiles undergo anodization and form various types of corrosion defects on the surface. Among them, pitting corrosion is particularly common during production. After anodization, 6063 aluminum profiles exhibit excellent corrosion resistance and decorative properties.

In recent years, with the development of the national economy, the use of industrial aluminum profiles has become more widespread. However, many aluminum alloy profiles exhibit various forms of corrosion defects on their surface after a period of use, among which pitting corrosion is a common problem, significantly affecting the performance and decorative effects of industrial aluminum profiles. In order to improve the surface quality of industrial aluminum profiles effectively and achieve the goal of controlling pitting corrosion, we conducted an in-depth study of this type of defect.

The nature of pitting corrosion in industrial aluminum profiles

From a surface perspective, pit corrosion of anodized aluminum extrusions is caused by numerous pits that contain residual materials. When using SEM to analyze the composition of residual materials, excessive Si not only forms free Si phase, but also reacts with the matrix to form α phase (Al12 Fe2Si) and β phase (Al9Fe3Si2). The α and β phases have a significant impact on the corrosion resistance of the alloy, especially the β phase which significantly reduces the corrosion resistance of the alloy.

The residual materials at the pits primarily contain free Si phases and AlFeSi phases, while it is also found that chlorine elements have adsorbed onto the residual materials, indicating that Cl- is involved in the corrosion process of industrial aluminum profiles. Zinc, as an impurity element in the alloy, plays a role in the corrosion process since the zinc element content in the corrosion zone is much higher than in the base material.

What is advantageous can also be detrimental. To ensure that Mg elements can adequately form reinforced Mg2Si phases, the Si element is generally artificially excessive in the alloy composition. However, as the Si content increases, the alloy's grain size becomes finer, and its heat treatment effect becomes better. 

The surplus of industrial aluminum profile Si also has negative effects

The anode phase Mg2Si of the alloy is a pitting source for the anodizing process. During anodization alkaline washing, Mg2Si particles are preferentially dissolved, forming pits. Magnesium dissolves in the solution while silicon remains in the aluminum alloy. When pits aggregate on grains, the color of the grain darkens. Silicon is not readily removed in the sulfuric acid process or in other stages, and the silicon content at the bottom of the pitting corrosion pit is higher than that in other regions.

After examining 6063 aluminum profiles' surface, well-known scholars have discovered that the pitting corrosion is caused by anodizing phase Mg2Si segregation and coarsening in aluminum alloys, and impurity element Zn, Cl- concentration, and pH value of the solution in the alloy accelerate pitting corrosion's occurrence and development.

Therefore, implementing effective solutions include adjusting the proportion of magnesium and silicon elements in the alloy appropriately. Silicon should not be excessively high, and the aging system should be reasonably organized to prevent the agglomeration of Mg2Si particles, to avoid negatively affecting the corrosion resistance of industrial aluminum profiles. Controlling trace element Zn in the alloy and the Cl- concentration and pH value of the solution during the treatment process, reducing the negative impact of active elements.

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