Causes and Analysis of Surface Corrosion of Aluminum Extrusions

During the surface treatment process of 6063 aluminum alloy profiles, it is sometimes observed that there are irregularly arranged dark gray pitting corrosion spots of varying degrees on the surface of the aluminum profiles. These corrosion spots are completely different in shape from those caused by zinc elements and appear intermittently during the production of aluminum profiles. Some people believe the cause to be that the operator did not follow the correct surface treatment process; that the bath contains some harmful impurity ions; or that the material quality is poor with too many inclusions. Our analysis is as follows.

1. Analysis of the Causes of Corrosion Spots

Based on our years of production experience, examination of various process parameters in aluminum alloy profile production, and follow-up investigation on how operators execute the processes, we believe that the main reasons for the appearance of this type of dark gray corrosion spots are as follows:

(1) Sometimes, due to certain reasons during the casting process, the added ratios of magnesium and silicon are not appropriate, causing ω(Mg)/ω(Si) to be in the range of 1.0–1.3, which is much lower than the optimal ratio of 1.73 (generally controlled within 1.3–1.5). In this case, although the magnesium and silicon contents are within the specified ranges (ω(Mg)=0.45%–0.9%, ω(Si)=0.2%–0.6%), there is still some excess silicon. This excess silicon, apart from a small amount existing in free form, also forms ternary compounds in the aluminum alloy. When ω(Si) < ω(Fe), more α(Al12Fe3Si) phase is formed, which is a brittle compound. When ω(Si) > ω(Fe), more β(Al9Fe2Si12) phase is formed, which is a more brittle needle-like compound with greater harmful effects than the α phase, often causing the alloy to fracture along it. These insoluble impurity phases or free impurity phases in the alloy tend to gather at the grain boundaries, weakening the strength and toughness of the boundaries [1–3], becoming the weakest link in corrosion resistance, with corrosion typically initiating there.

(2) During the melting process, although the magnesium and silicon addition ratios are within the standard specified range, uneven and insufficient stirring can sometimes lead to uneven silicon distribution in the melt, resulting in local enrichment and depletion zones. Since silicon has very low solubility in aluminum, at the eutectic temperature of 577°C it is 1.65%, and at room temperature only 0.05%, after casting this results in compositional inhomogeneity. This directly affects industrial aluminum profiles: the presence of a small amount of free silicon in the aluminum matrix not only reduces the alloy's corrosion resistance but also coarsens the alloy grains [4].

(3) During extrusion, the control of various process parameters, such as excessively high billet preheating temperature, metal extrusion flow rate, air cooling intensity during extrusion, and improper aging temperature and holding time, can easily cause silicon segregation and free silicon, preventing magnesium and silicon from fully forming the Mg2Si phase, leaving some free silicon.

2. Corrosion Phenomena During Surface Treatment

Excess and free silicon in 6003 aluminum alloy profiles lead to the following phenomena during surface treatment: when the profiles are placed into an acidic bath (15%–20% sulfuric acid), many small bubbles can be clearly observed on the surface. As time passes and the bath temperature increases, the reaction rate accelerates, indicating the onset of galvanic electrochemical corrosion. At this point, if the profiles are removed from the bath for inspection, many spots with colors different from the normal surface can be found. During subsequent processes, such as alkaline etching, acidic neutralization brightening, and sulfuric anodizing, these dark gray corrosion spots become even more obvious and visually apparent.

The corrosion caused by zinc and the corrosion caused by silicon exhibit some differences in appearance. Zinc-induced corrosion spots resemble snowflakes, spreading outward along grain boundaries, and form pits of certain depth. In contrast, silicon-induced corrosion spots appear as dark gray inclusions, do not spread outward along grain boundaries, and feel depthless. Furthermore, the number of these spots increases over time until the reaction completes. These dark gray spots can be largely eliminated or reduced by extending the corrosion time or by film removal treatment.

3. Preventive Measures

The corrosion of 6063 aluminum alloy profiles caused by silicon can be completely prevented and controlled. Effective control over the raw material procurement and alloy composition, ensuring the magnesium-to-silicon ratio is within 1.3–1.7, along with strict control of process parameters (such as melting, stirring, casting cooling water temperature, billet preheating temperature, extrusion quenching and air cooling rate, and aging temperature and time) can prevent silicon segregation and free silicon, promoting the formation of beneficial Mg2Si strengthening phase.

If such silicon corrosion spots are detected, special attention should be paid during surface treatment. In the degreasing and oil-removing process, weakly alkaline baths should be used whenever possible. If this is not feasible, the soaking time in acidic degreasing solutions should be minimized (qualified aluminum alloy profiles can stay in acidic degreasing solution for 20–30 minutes without issues, whereas problematic profiles should only be placed for 1–3 minutes). Additionally, the pH of subsequent rinsing water should be relatively high (pH > 4, with controlled Cl- content), corrosion time during alkaline etching should be extended as much as possible, brightening should employ nitric acid solution, and anodizing in sulfuric acid should be performed as soon as possible. In this way, dark gray corrosion spots caused by silicon become less noticeable and the profiles can meet usage requirements.