Causes and Analysis of Surface Corrosion of Aluminum Extrusion Profiles

In the process of surface treatment of 6063 aluminum alloy profile, sometimes it is found that there are different degrees of irregularly arranged dark gray corrosion points on the surface of aluminum profile. This kind of corrosion point is completely different from the corrosion point caused by zinc. , It appears intermittently in the production process of aluminum profiles. Some people think that the reason is that the operator has not implemented the correct surface treatment process; there are some harmful impurity ions in the bath; the material is not good and there are too many inclusions. In this regard, our analysis is as follows.

1 Cause analysis of corrosion points

Based on years of production experience and investigation of various process parameters in the production of aluminum alloy profiles, as well as follow-up investigation of the operator’s implementation of the process, we believe that the main reasons for this type of dark gray corrosion points are as follows:

• (1) Sometimes, due to some reasons, the adding ratio of magnesium and silicon in the casting process is not suitable, so that ω(Mg)/ω(Si) is in the range of 1.0～1.3, which is much smaller than the optimal ratio of 1.73 (generally controlled in 1.3～1.5). In this way, although the content of magnesium and silicon components is within the prescribed range (ω(Mg)=0.45% to 0.9%, ω(Si)=0.2% to 0.6%). However, some surplus silicon exists. In addition to a small amount of silicon in the free state, this part of the surplus silicon will also form 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) is formed Phase, which is a more brittle needle-like compound, its harmful effect is greater than that of α phase, which tends to make the alloy easily fracture along it. These insoluble impurity phases or free impurity phases formed in the alloy tend to accumulate on the grain boundaries, and at the same time weaken the strength and toughness of the grain boundaries [1-3], and become the weakest link in corrosion resistance. Corrosion starts from there. produce.
• (2) During the smelting process, although the addition ratio of magnesium and silicon is within the range specified in the standard, sometimes due to uneven and insufficient mixing, the silicon in the melt is unevenly distributed, and there are local enrichment areas and poverty areas. Area. Because the solubility of silicon in aluminum is very small, the eutectic temperature is 1.65% at 577°C and only 0.05% at room temperature. After the rod is cast, the composition is uneven, which is directly reflected in the industrial aluminum profile products. When there is a small amount of free silicon in the aluminum matrix, it will not only reduce the corrosion resistance of the alloy, but also coarsen the crystal grains of the alloy [4].
• (3) Control of various process parameters during extrusion, such as excessively high bar preheating temperature, improper control of metal extrusion flow rate, air-cooling strength during extrusion, aging temperature and holding time, etc., will easily cause silicon segregation and liberation, causing Magnesium and silicon do not completely become the Mg2Si phase, but some free silicon exists.

2Corrosion phenomenon during surface treatment

The 6003 aluminum alloy profile with more surplus and free silicon has the following phenomena during surface treatment: When the profile is placed in an acid bath (sulfuric acid 15%-20%), it can be clearly observed that there are many small bubbles on the surface of the profile. The longer the time and the increase in bath temperature, the faster the reaction speed, which indicates that the electrochemical corrosion of the primary battery has occurred [5]. At this time, if the profile is lifted out of the bath and observed, there will be many points on the profile surface that are different from the normal surface color. When the subsequent treatments are continued, such as alkali corrosion, acid neutralization and sulfuric acid anodization, the dark gray corrosion points will be exposed more clearly and intuitively.

There are some differences in appearance between the corrosion caused by zinc and the corrosion caused by silicon. The corrosion points caused by zinc are like snowflakes, spreading outward along the grain boundaries, which are pits with a certain depth [6,7]. The corrosion points caused by silicon are like mixed dark gray points, which do not diffuse outward along the crystal interface, and do not feel the depth. And as the processing time increases, the number increases and does not stop until the complete reaction. This dark gray spot can be basically eliminated or reduced by prolonging the corrosion time or removing the film.

3 preventive measures

The corrosion behavior of 6063 aluminum alloy profiles caused by silicon can be prevented and controlled. As long as the purchase of raw materials and alloy composition are effectively controlled, the ratio of magnesium to silicon is within the range of 1.3 to 1.7, and the parameters of each process (such as melting , Mixing, casting cooling water temperature, bar preheating temperature, extrusion quenching air cooling strength, aging temperature and time, etc.) are strictly controlled to avoid segregation and liberation of silicon, and try to make silicon and magnesium form a beneficial Mg2Si strengthening phase.

If you find this kind of silicon corrosion spots, you should pay special attention to the surface treatment. In the process of degreasing and degreasing, try to use weak alkaline bath fluid. If the conditions are not allowed, you should also soak in the acid degreasing fluid for a period of time. Try to shorten it as much as possible (the qualified aluminum alloy profile can be placed in the acid degreasing solution for 20-30 minutes, while the problematic profile can only be placed for 1 to 3 minutes), and the pH value of the subsequent washing water should be higher (pH>4, control Cl- content), prolong the corrosion time as much as possible in the alkali corrosion process, and use nitric acid luminescence solution when neutralizing the light. When sulfuric acid anodizes, it should be energized and oxidized as soon as possible, so that the dark gray corrosion points caused by silicon are not obvious , Can meet the requirements of use.

4 Conclusion

Although silicon is an indispensable main component in 6063 aluminum alloy profiles, if the amount of addition is not appropriate, the added silicon will not completely form a Mg2Si strengthening phase with magnesium, causing segregation and liberation of silicon, which will easily cause silicon during surface treatment. Corrosion phenomenon of aluminum alloy profiles. In the production, the main alloy components, impurities and process parameters must be strictly controlled to prevent such phenomena from occurring.