Causes and analysis of surface corrosion of aluminum extruded profiles

During the surface treatment process of 6063 aluminum alloy profiles, it is sometimes observed that there are irregularly arranged grayish-black 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 they appear intermittently during the production process of aluminum profiles. Some people believe that the reasons for this are that the operators did not implement the correct surface treatment process; there are harmful impurity ions in the electrolyte; or that the material quality is poor and contains too many inclusions. In this regard, we analyze as follows.

1. Analysis of the causes of corrosion spots

Based on years of production experience and investigation of various process parameters in the production of aluminum alloy profiles, as well as tracking the implementation of processes by operators, we believe that the main reasons for the occurrence of this type of dark gray corrosion spots are as follows:

(1) Sometimes, due to certain reasons, the addition ratio of magnesium and silicon in the melting process is not optimal, resulting in a ω(Mg)/ω(Si) ratio in the range of 1.0 to 1.3, which is much lower than the optimal ratio of 1.73 (generally controlled within the range of 1.3 to 1.5). Thus, although the content of magnesium and silicon is within the specified range (ω(Mg) = 0.45% to 0.9%, ω(Si) = 0.2% to 0.6%), there is some surplus silicon present. This surplus silicon not only exists in a small amount in the free state but 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 that is more harmful than the α phase and often causes the alloy to fracture along it. These insoluble impurity phases or free impurity phases formed in the alloy often accumulate at the grain boundaries, weakening the strength and toughness of the grain boundaries, becoming the weakest link with the worst corrosion resistance, leading to corrosion occurring first at these points.

(2) During the smelting process, although the addition ratio of magnesium and silicon is within the standard specified range, sometimes due to uneven and insufficient stirring, the distribution of silicon in the melt becomes uneven, leading to the presence of enriched and depleted zones. Because the solubility of silicon in aluminum is very low, at the eutectic temperature of 577°C it is only 1.65%, and at room temperature, it is merely 0.05%. After casting, this results in compositional inhomogeneity, which directly affects industrial aluminum profile products. When there are small amounts of free silicon present in the aluminum matrix, it not only reduces the corrosion resistance of the alloy but also coarsens the grain structure of the alloy.

(3) The control of various process parameters during extrusion, such as excessively high heating temperatures for the billet, improper control of the metal extrusion flow rate, cooling intensity during extrusion, aging temperature, and holding time, can easily result in silicon segregation and free silicon, preventing magnesium and silicon from fully forming the Mg2Si phase, and leaving some free silicon present.

2. Corrosion phenomena during the surface treatment process

The 6003 aluminum alloy profiles with surplus and free silicon exhibit the following phenomena during surface treatment: when the profiles are placed in an acidic bath (15% to 20% sulfuric acid), numerous small bubbles can be clearly observed on the surface of the profiles. As time extends and the temperature of the bath liquid increases, the reaction rate accelerates, indicating that original galvanic electrochemical corrosion has occurred. At this point, if the profiles are taken out of the bath for observation, many points differing in color from the normal surface can be found on the profile's surface. Continuing with subsequent treatments, such as alkaline corrosion, acidic neutralization for surface brightening, and sulfuric acid anodizing, these dark gray corrosion points will become more prominent and visible.

The corrosion caused by zinc and the corrosion caused by silicon have some differences in appearance. The corrosion spots caused by zinc resemble snowflakes, spreading outward along the grain boundaries and forming pits of considerable depth. In contrast, the corrosion spots caused by silicon appear as dark gray inclusions, which do not spread outward along the grain boundary and are not felt to have depth. Furthermore, as the processing time extends, their quantity increases until they stop only after complete reaction. These dark gray spots can be largely eliminated or reduced by extending the corrosion time or through film stripping treatment.

3. Preventive measures

The corrosion behavior of 6063 aluminum alloy caused by silicon can be completely prevented and controlled, as long as effective control is applied to the procurement of raw materials and the alloy composition, ensuring that the ratio of magnesium to silicon is within the range of 1.3 to 1.7. Additionally, strict control of the parameters of each process (such as melting, stirring, casting cooling water temperature, billet preheating temperature, extrusion quenching air cooling intensity, aging temperature and time, etc.) is necessary to avoid segregation and liberation of silicon, and to promote the beneficial formation of Mg2Si reinforcing phase.

If such silicon corrosion spots are found, special attention should be paid during surface treatment. During the degreasing process, weaker alkaline solutions should be used as much as possible. If conditions do not permit, the immersion time in acidic degreasing solutions should be minimized (qualified aluminum alloy profiles can be placed in acidic degreasing solutions for 20–30 minutes without issue, while problematic profiles can only be placed for 1–3 minutes). Additionally, the pH of the washing water afterwards should be kept higher (pH > 4, controlling Cl- content). In the alkaline corrosion process, the corrosion time should be extended as much as possible. When neutralizing for brightening, nitric acid brightening solutions should be used. For sulfuric acid anodizing, power should be supplied for oxidation treatment as quickly as possible. This way, dark gray corrosion spots caused by silicon will not be obvious and can meet usage requirements.

Silicon, although an essential component in 6063 aluminum alloy profiles, if added in improper amounts, may not fully combine with magnesium to form the Mg2Si strengthening phase. This can lead to segregation and free silicon, resulting in corrosion of the aluminum alloy profiles due to silicon during surface treatment. Strict control of the main alloying elements, impurities, and process parameters is crucial in production to prevent such occurrences.