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 causes are that the operator did not follow the correct surface treatment process, there are some harmful impurity ions in the bath solution, or the material quality is poor with too many inclusions. Our analysis is as follows.
60631, Analysis of the Causes of Corrosion Spots
Based on many years of production experience, the examination of various process parameters in aluminum alloy profile production, and follow-up investigations of operators' adherence to processes, we believe the main reasons for the occurrence of this type of dark gray corrosion spots are as follows:
(1) Sometimes, due to certain reasons, the proportions of magnesium and silicon added during the casting process are not suitable, causing the ω(Mg)/ω(Si) ratio to be in the range of 1.0–1.3, which is much lower than the optimal ratio of 1.73 (typically controlled within the 1.3–1.5 range). In this case, although the magnesium and silicon contents are within the specified range (ω(Mg)=0.45%–0.9%, ω(Si)=0.2%–0.6%), there is some excess silicon present. This excess silicon, aside from a small amount existing in a free state, also forms ternary compounds within 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(Al9Fe2Si) phase is formed, which is an even more brittle needle-like compound, and its harmful effects are greater than those of the α phase, often causing the alloy to fracture along its path. These insoluble impurity phases or free impurity phases in the alloy often accumulate at grain boundaries while weakening the strength and toughness of the boundaries [1–3], becoming the weakest points with the poorest corrosion resistance, and corrosion usually starts from these areas.
(2) During the smelting process, although the proportions of magnesium and silicon added are within the standard specified range, sometimes due to uneven and insufficient stirring, the distribution of silicon in the melt becomes uneven, resulting in local enrichment and depletion zones. Since the solubility of silicon in aluminum is very low-1.65% at the eutectic temperature of 577°C and only 0.05% at room temperature-this leads to compositional inhomogeneity after casting. This directly affects industrial aluminum profiles, where a small amount of free silicon in the aluminum matrix not only reduces the corrosion resistance of the alloy but also coarsens the alloy grains [4].
(3) During extrusion, control of process parameters-such as excessive billet preheating temperature, incorrect metal extrusion flow rate, inadequate air cooling strength 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 and resulting in the presence of some free silicon.
2. Corrosion Phenomena During Surface Treatment
Excessive and free silicon-rich 6003 aluminum alloy profiles exhibit the following phenomena during surface treatment: when the profiles are placed in an acidic bath (15%–20% sulfuric acid), numerous small bubbles can be clearly observed on the surface of the profiles. As time progresses and the temperature of the bath increases, the reaction rate accelerates, indicating that galvanic electrochemical corrosion has occurred. When the profiles are removed from the bath and inspected, many spots with colors different from the normal surface can be seen. During subsequent treatments, such as alkaline etching, acidic neutralization for brightening, and sulfuric acid anodizing, these dark gray corrosion spots become even more apparent and noticeable.
The corrosion caused by zinc and the corrosion caused by silicon have some differences in appearance. Corrosion spots caused by zinc resemble snowflakes, spreading outward along the grain boundaries and forming pits of certain depth. In contrast, corrosion spots caused by silicon look like embedded dark gray dots. They do not spread outward along the grain boundaries, and no depth can be felt. Moreover, as the treatment time increases, the number of spots increases until the reaction is complete and ceases. These dark gray spots can be largely eliminated or mitigated 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 entirely prevented and controlled. It is crucial to effectively control the incoming raw materials and alloy composition, ensuring that the magnesium-to-silicon ratio is within the range of 1.3–1.7. Additionally, the parameters of each process (such as melting, stirring, casting cooling water temperature, billet preheating temperature, extrusion quenching and air cooling strength, aging temperature and time, etc.) should be strictly managed to avoid silicon segregation and free silicon, and to maximize the formation of beneficial Mg2Si strengthening phases from silicon and magnesium.
If such silicon corrosion spots are observed, special attention should be paid during surface treatment. During degreasing and oil removal, weakly alkaline bath solutions should be used whenever possible. If not feasible, the soaking time in acidic degreasing solutions should be minimized (qualified aluminum alloy profiles can be left in acidic degreasing solutions for 20–30 minutes without problems, whereas affected profiles should only be in them for 1–3 minutes). Furthermore, the pH of subsequent rinsing water should be slightly higher (pH > 4, with controlled Cl- content). During alkaline etching, the etching time should be extended as much as possible; for the neutralization brightening process, nitric acid brightening solution should be used. During sulfuric acid anodizing, the electrical oxidation should be carried out promptly. In this way, the dark gray corrosion spots caused by silicon will be less noticeable and the profiles will meet usage requirements.
4. Conclusion
Although silicon is an indispensable main component of 6063 aluminum alloy profiles, improper addition or failure to fully form the Mg2Si strengthening phase with magnesium can lead to silicon segregation and free silicon, resulting in corrosion during surface treatment. Strict control of the main alloy elements, impurities, and process parameters during production is essential to prevent this phenomenon.
