Causes and Analysis of Surface Corrosion of Aluminum Extruded Profiles
[Chinese aluminum industry network] 6063 aluminum alloy profile surface treatment process, sometimes found in the aluminum profiles have different degrees, irregularly arranged point-like dark gray corrosion point, this corrosion point and zinc corrosion caused by The shape is completely different, and it is intermittent during the production of aluminum profiles. Some people think that the reason is that the operator has not performed the correct surface treatment process; there are some harmful impurity ions in the bath; the material is not good, too much inclusion. In this regard, we analyze the following.
1. Analysis of Causes of Corrosion Point Based on our many years of production experience and the inspection of various process parameters in the production of aluminum alloy profiles, as well as the follow-up investigation of the process performed by operators, we believe that the main reasons for producing this type of dark gray corrosion point are The following aspects:
(1) Sometimes the addition ratio of magnesium and silicon is not appropriate during the casting process due to certain reasons, so that ω(Mg)/ω(Si) is in the range of 1.0 to 1.3, which is much smaller than the 1.73 ratio (generally controlled at 1.3) Within the range of ~1.5). In this way, the content of magnesium and silicon components is in the range of (ω(Mg)=0.45% to 0.9% and ω(Si)=0.2% to 0.6%). However, part of the surplus silicon is present. In this part of the surplus silicon, ternary compounds are formed simultaneously in the aluminum alloy except for a small amount of silicon existing in the free state. When ω(Si)<ω(Fe), more α(Al12Fe3Si) phases are formed, which is a brittle compound, and when ω(Si)>ω(Fe), more β(Al9Fe2Si12) is formed. Phase, this is a more brittle needle-like compound, its harmful effect is greater than the alpha phase, tend to make the alloy easy to break along it. The insoluble impurity phase or the free impurity phase formed in the alloy tends to accumulate on the grain boundary, weakening the strength and toughness of the grain boundary, and becoming a weak link of poor corrosion resistance, and corrosion is first generated from this place.
(2) During the smelting process, although the addition ratio of magnesium and silicon is within the range specified in the standard, the uneven distribution of silicon in the melt sometimes leads to uneven distribution of silicon in the melt. Area. Because the solubility of silicon in aluminum is very small, the eutectic temperature is 577°C, which is 1.65%, and it is only 0.05% at room temperature. After the casting rod, the phenomenon of non-uniformity is also produced. It is directly reflected on the industrial aluminum profile products. When there is a small amount of free silicon in the aluminum matrix, not only the corrosion resistance of the alloy is reduced, but also the grain of the alloy is coarsened.
(3) The control of various process parameters during extrusion, such as excessively high preheating temperature of the billets, metal extrusion flow rate, air-cooling strength during extrusion, aging temperature, and holding time, are all likely to cause silicon segregation and dissociation. Magnesium and silicon do not completely become Mg2Si phases, but some free silicon exists.
2. Corrosion in the surface treatment process 6003 aluminum alloy profiles with excess free and free silicon appear the following phenomena in the surface treatment: When the profile is put into the acidic tank (sulfuric acid 15% to 20%), it can be clearly observed in There are many small bubbles on the surface of the profile. As time goes by and the temperature of the bath increases, the reaction rate becomes faster, which indicates that the electrochemical corrosion of the cell has already occurred. At this point, when the profile is lifted out of the bath, many points on the surface of the profile that are not the same as the color of the normal surface are found. The subsequent treatment, such as alkali corrosion, acid neutralization, and sulfate anodization, will expose the dark gray corrosion point more clearly and intuitively.
The corrosion caused by zinc and the corrosion caused by silicon have some differences in appearance. The corrosion caused by zinc is like a snowflake that spreads out along the grain boundary and is a pit with a certain depth [6, 7]. The corrosion caused by silicon is like a dark gray point, there is no outward diffusion along the crystal interface, and the depth is not felt. As the processing time is extended, the amount is increasing until the complete reaction is terminated. This dark gray point can be substantially eliminated or mitigated by prolonging the etching time or the film stripping process.
3. Preventive measures The behavior of silicon caused 6063 aluminum alloy profile corrosion can be completely prevented and controlled, as long as the raw material purchase, the alloy composition is effectively controlled to ensure that magnesium, silicon ratio in the range of 1.3 to 1.7, and for each process Parameters (such as smelting, stirring, casting cooling water temperature, rod blank preheating temperature, extrusion quenching air-cooling strength, aging temperature and time, etc.) are strictly controlled to avoid segregation and release of silicon, and silicon and magnesium are formed as beneficial as possible for Mg2Si. Strengthening phase.
If there is such a phenomenon of silicon corrosion, special attention should be paid to surface treatment. In the process of degreasing and degreasing, weak alkaline bath should be used as much as possible. If conditions do not allow it, it should be soaked in acidic oil. As short as possible (qualified aluminum alloy material in the acidic degreasing solution put 20 ~ 30min no problem, but the problem of the profile can only be placed 1 ~ 3min), and the pH value of the subsequent washing water is higher (pH> 4, control Cl-content), as long as possible in the alkaline corrosion process corrosion time, in the neutralization of light to use nitric acid light solution, in the anodizing of sulfuric acid should be energized as soon as possible oxidation treatment, so that the dark gray corrosion caused by silicon is not obvious , to meet the requirements of use.
4. Conclusion Although silicon is an indispensable component of the 6063 aluminum alloy profile, if the amount of silicon is not properly added, the added silicon does not completely form Mg2Si strengthening phase with magnesium, causing segregation and dissociation of silicon, and it will be easy in the surface treatment process. Silicon-induced corrosion of aluminum alloy profiles. In the production of the main alloy components and impurities and process parameters must be strictly controlled to prevent this phenomenon.
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