During the surface treatment of 6063 aluminum profiles, sometimes you might notice small, irregularly scattered dark gray corrosion spots on the surface of the aluminum. These spots look completely different from those caused by zinc, and they only appear intermittently during the aluminum profile production process. Some people think the cause might be that the operator didn't follow the correct surface treatment procedure, that there are some harmful impurity ions in the bath, or that the material quality is poor with too many inclusions.
1. Analysis of the Causes of Corrosion Spots
Based on years of production experience and examining various process parameters in aluminum alloy profile production, as well as tracking how operators follow procedures, we believe that the main reasons for the appearance of these dark gray corrosion spots are as follows:
(1) Sometimes, due to certain reasons, the added ratios of magnesium and silicon during the casting process are not properly adjusted, causing the ω(Mg)/ω(Si) ratio to fall within 1.0–1.3, which is much lower than the optimal ratio of 1.73 (generally controlled within 1.3–1.5). In this way, 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. Apart from a small amount existing in free form, this excess silicon will also form ternary compounds in the aluminum alloy. When ω(Si) < ω(Fe), more α(Al12Fe3Si) phase forms, which is a brittle compound. When ω(Si) > ω(Fe), more β(Al9Fe2Si2) phase forms, which is an even more brittle needle-like compound. Its harmful effect is greater than that of the α phase and often makes the alloy prone to cracking along it. These insoluble impurity phases or free impurity phases tend to accumulate at grain boundaries, weakening the strength and toughness of the boundaries [1-3], becoming the weakest point with the worst corrosion resistance, where corrosion starts first.
(2) During smelting, even if the magnesium and silicon addition ratio is within the standard range, sometimes due to uneven or insufficient stirring, the silicon distribution in the melt becomes uneven, creating local rich and poor zones. Since silicon has very low solubility in aluminum (1.65% at the eutectic temperature of 577°C and only 0.05% at room temperature), after casting, the composition becomes uneven. This directly affects industrial aluminum profiles. When a small amount of free silicon exists in the aluminum matrix, it 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 speed, air cooling strength during extrusion, and improper aging temperature or holding time, can all easily cause silicon segregation and free silicon, preventing magnesium and silicon from fully forming Mg2Si phase and leaving some free silicon.
2. Corrosion during the surface treatment process
When 6003 aluminum alloy profiles with a lot of free and excess silicon undergo surface treatment, the following phenomena can be observed: When the profiles are placed in an acidic bath (15%–20% sulfuric acid), many small bubbles can be clearly seen on the surface. As time passes and the bath temperature rises, the reaction speeds up, indicating that galvanic electrochemical corrosion has started. At this point, if you take the profiles out of the bath and inspect them, you'll notice many spots on the surface that are a different color from the normal surface. During subsequent treatments like alkaline etching, acidic neutralization polishing, and sulfuric anodizing, these dark gray corrosion spots become even more noticeable.
The corrosion caused by zinc looks different from that caused by silicon. Zinc-induced corrosion spots look like snowflakes, spreading outward along the grain boundaries and forming pits of a certain depth. In contrast, silicon-induced corrosion spots appear as dark gray inclusions, do not spread outward along the grain boundaries, and don't feel deep. As the treatment continues, more and more of these spots appear until the reaction is completely finished. These dark gray spots can basically be removed or reduced by extending the corrosion time or by doing a film removal treatment.
3. Preventive Measures
The corrosion of 6063 aluminum alloy profiles caused by silicon can be completely prevented and controlled. As long as the incoming raw materials and alloy composition are effectively monitored to ensure the magnesium-to-silicon ratio is between 1.3 and 1.7, and 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.) are strictly controlled, silicon segregation and free silicon can be avoided, and silicon and magnesium can form beneficial Mg2Si strengthening phases as much as possible.
If you notice this kind of silicon corrosion spot, you need to be extra careful during surface treatment. During degreasing, try to use a weakly alkaline solution. If that's not possible, keep the soaking time in acidic degreasing solution as short as possible (good-quality aluminum alloy profiles can handle 20–30 minutes in acidic degreasing solution, but problematic ones should only be left for 1–3 minutes). Also, the rinsing water afterward should have a higher pH (pH > 4, and control the Cl- content). During alkaline corrosion, try to extend the corrosion time. When neutralizing and brightening, use a nitric acid brightening solution. For sulfuric acid anodizing, start the electrolysis as soon as possible. This way, the dark gray corrosion spots caused by silicon won't be very noticeable and can meet usage requirements.
