In the process of surface treatment of 6063 aluminum alloy profiles, it is sometimes found that there are different degrees of irregular arrangement of spot-like dark gray corrosion points on the surface of aluminum profiles. The shape of such corrosion points is completely different from that caused by zinc elements, and it is intermittent in the production process of aluminum profiles. Some people think the reason is that the operator did not perform the correct surface treatment process; There are some harmful impurity ions in the tank. Bad material, too much inclusion. In this regard, we analyze as follows.
1. Cause analysis of corrosion points
Based on many years of production experience and investigation of various process parameters in the production of aluminum alloy profiles, as well as the follow-up investigation of the operator's execution of the process, we believe that the main reasons for this type of dark gray corrosion point are as follows:
(1) Sometimes due to some reasons in the casting process of magnesium, silicon addition ratio is not appropriate, so that ω(Mg)/ω(Si) in the range of 1.0 ~ 1.3, much smaller than the best ratio of 1.73 (generally controlled in the range of 1.3 ~ 1.5). So although magnesium, silicon content in rules (omega (Mg) = 0.45% ~ 0.9%, omega (Si) = 0.2% ~ 0.6%). However, there is some excess silicon, which can form ternary compounds in aluminum alloy at the same time except for a small amount of silicon in the free state. When ω(Si)<ω(Fe), more α(Al12Fe3Si) phase is formed, which is a brittle compound, and when ω(Si) > ω(Fe), more β(Al9Fe2Si12) phase is formed, which is a more brittle needle-like compound, which has more harmful effects than the α phase, often making the alloy easy to fracture along it. These insoluble impurity phases or free impurity phases formed in the alloy tend to gather at the grain boundary, and weaken the strength and toughness of the grain boundary [1-3], becoming the weak link with the worst corrosion resistance, and corrosion occurs from there first.
(2) In the smelting process, although the ratio of magnesium and silicon is within the range specified in the standard, the distribution of silicon in the melt is uneven sometimes due to uneven and inadequate mixing, and there are locally rich and poor areas. Because the solubility of silicon in aluminum is very small, and the eutectic temperature is 1.65% at 577℃ and only 0.05% at room temperature, the phenomenon of uneven composition occurs after casting rods, which is directly reflected in the industrial aluminum products. When there is a small amount of free silicon in the aluminum matrix, the corrosion resistance of the alloy will be reduced, and the grain of the alloy will be coarsened [4].
(3) The control of various process parameters during extrusion, such as excessive preheating temperature of the bar billet, improper control of metal extrusion flow rate, air cooling intensity during extrusion, aging temperature and holding time, are easy to produce silicon segregation and dissociation, so that magnesium and silicon do not completely become Mg2Si phase, but some free silicon exists.
2. Corrosion in the process of surface treatment
Surpluses and free silica in surface treatment of 6003 aluminium alloy profile when the following phenomena: when put profiles in the acid tank sulfuric acid (15% ~ 20%), the surface can be observed clearly in section there are a lot of small bubbles, as the extension and bath temperature, the reaction speed is faster and faster, which suggests that galvanic cell electrochemical corrosion have [5]. At this time, the profile is taken out of the tank and observed, and many points with different colors from the normal surface will be found on the profile surface. When future treatments, such as alkali corrosion, acid neutralization, and sulfuric acid anodizing, continue, this dark gray corrosion spot will be exposed more clearly and intuitively.
Corrosion caused by zinc and silicon have some differences in appearance and morphology. The corrosion spots caused by zinc are like snowflakes, which spread outward along grain boundaries and are pits with a certain depth [6,7]. However, the corrosion spots caused by silicon are like inclusion dark gray spots, which have no outward diffusion along the grain interface and no depth can be felt. And with the extension of processing time, the amount of more and more, until the complete reaction did not terminate. This dark gray spot can be basically eliminated or mitigated by prolonging corrosion time or film removal.
3. Preventive measures
The corrosion behavior of 6063 aluminum alloy profiles caused by silicon can be completely prevented and controlled as long as the purchase of raw materials and alloy composition are effectively controlled to ensure that the ratio of magnesium and silicon is within the range of 1.3 ~ 1.7. And the parameters of each process (such as melting, stirring, casting cooling water temperature, preheating temperature of bar billet, extrusion quenching air cooling intensity, aging temperature and time, etc.) are strictly controlled to avoid segregation and dissociation of silicon, and make silicon and magnesium form beneficial Mg2Si strengthened phase as far as possible.
If this silicon corrosion point phenomenon is found, special attention should be paid to the surface treatment, in the degreasing process, as far as possible to use weak alkaline tank, if conditions do not allow, should also be in acid degreasing soak time as short as possible (qualified aluminum alloy profiles in acid degreasing put 20 ~ 30min no problem, And there is a problem of profile can only be placed on 1 ~ 3 min), and after washing water pH value is higher than some (pH > 4, control the Cl - content), as far as possible to extend the time of corrosion, in the process of alkali corrosion using nitric acid and the light in the optical liquid, energized as soon as possible in the sulfuric acid anodizing oxidation treatment, in this way, dark gray corrosion caused by silicon points is not obvious, It can meet the requirements of use.
4. Closing Remarks
Although silicon is an indispensable main component of 6063 aluminum alloy profiles, if the amount of added silicon is not appropriate, the added silicon does not completely form Mg2Si strengthening phase with magnesium, resulting in segregation and dissociation of silicon, it will be prone to corrosion of aluminum alloy profiles caused by silicon in the surface treatment process. In the production of the main alloy components and impurities and process parameters should be strictly controlled to prevent the occurrence of such phenomena.
