ã€China Aluminum Network】 6063 aluminum profiles have good corrosion resistance and decorative properties after anodizing. In recent years, with the development of the national economy and the improvement of people’s living standards, the use of aluminum alloy doors and windows and aluminum alloy curtain walls More and more popular, however, many aluminum alloys have different forms of corrosion defects on the surface after use for some time, and spot corrosion is more common, which seriously affects the performance and decorative effects of aluminum profiles. In order to reasonably improve the surface quality of aluminum profiles and achieve the purpose of controlling surface spot corrosion, it is necessary to do in-depth analysis of spot defects. In this paper, the spot corrosion on the surface of 6063 aluminum profile after anodization was taken as the object of study. The nature, origin and formation mechanism of spot corrosion were analyzed, and the key factors for the occurrence of spot corrosion were discussed.
1 The analysis of the nature of spot corrosion is based on the 6063 aluminum profile used. In order to ensure that the Mg element sufficiently forms the strengthening phase Mg2Si, an appropriate amount of Si is artificially excessive when the alloy component is formulated. As the Si content increases, the grains of the alloy become finer and the heat treatment effect is better. On the other hand, the surplus of Si also has a negative effect, which reduces the plasticity of the alloy and deteriorates the corrosion resistance. Studies have shown that excess Si can not only form the free Si phase, but also form α phase (Al12Fe2Si) and β phase (Al9Fe3Si2) with the matrix, so that there are free Si phase, α phase (Al12Fe2Si), β phase in the aluminum alloy ( Cathode phase particles such as Al9Fe3Si2) and anode phase Mg2Si particles. The corrosion properties of α-phase and β-phase alloys are greatly affected, especially the β phase can significantly reduce the corrosion performance of the alloy. The composition of the residue at the spot is mainly the free Si phase and the AlFeSi phase. At the same time, it was found that the chlorine also adsorbed at the residue, which indicates that Cl- participates in the corrosion process. The content of zinc in the corrosion zone is much higher than that of the matrix, indicating that the zinc, an impurity element in the alloy, also participates in the corrosion process.
In the anodizing process, the anode phase Mg2Si is a pitting source of the alloy. In anodic oxidation, the Mg2Si particles preferentially dissolve to form pits, in which magnesium dissolves in the solution and silicon remains on the aluminum alloy. When the pits accumulate on the grains, the grains are dark in color. In the sulfuric acid neutralization process, silicon is not easily removed, so the silicon content at the bottom of the spotted etching pit is higher than in other regions.
2 Causes of Spot Corrosion Analysis The main factors affecting spot corrosion include alkali wash temperature during pretreatment, alkaline wash time, and Zn, Fe, and Si contents in the alloy composition and the alloy extrusion state. Among many factors, the extrusion state plays a key role. It affects the distribution of elements such as Zn, Fe, and Si, which have a great influence on the corrosion performance, and the location of the precipitation of particles such as intermetallic compounds. In the thicker squeezing strips, the spot corrosion distribution has obvious directionality, because the resistance in this area is larger and the stress is more concentrated in this area, where the metal lattice is seriously distorted and becomes a local high free energy. In the zone, during the subsequent recrystallization, nucleation is preferential. In order to reduce the interfacial energy and to maintain the stable state, the grains are not only abnormally grown, but also the Mg2Si anode phase, free Si, FeSiAl, FeAl3, and other cathode phases are preferentially precipitated. Spot corrosion created the conditions.
For the above reasons, a depleted region of ferrosilicon is present near the grain boundary of precipitated metal compounds such as free Si, FeSiAl, and FeAl3. This region is almost pure aluminum, and the negative potential is the anode, and it is an intermetallic compound (cathode). A microbattery is formed. Under the action of a corrosive medium, the Si and Fe-depleted regions (the anode phase) around the cathode phases (eg, free Si, FeSiAl, and FeAl3) in the microbattery preferentially dissolve, and Mg2Si also dissolves. As a result, the anode phase is dissolved. Dissolution of the surrounding Al forms an etch pit with a residue, and dissolution of the anode phase forms an etch pit without residue. When the corrosion conditions continue to deteriorate (such as temperature increase, long caustic wash time, etc.), the matrix Al continues to dissolve, the corrosion pit develops in the deep direction, so the surface morphology appears as part of the corrosion pits with residues and parts The residue-free etch pits constitute the aforementioned spot corrosion.
3 Spot corrosion formation mechanism analysis 6063 is an Al-Mg-Si alloy, and Mg2Si is an aging strengthening phase. In order to increase the strength of the alloy, the Si element content is often excessive in production, and free Si and FeSiAl phase particles are formed from excess Si. These particles are in the case of improper extrusion processes and non-standard heat treatment. It may lead to segregation (or segregation) at the grain boundary with the FeAl3 and Mg2Si particles, which constitutes a pitting source. According to the theory of corrosion, the anode aluminum around the cathode material dot will preferentially erode, and the generated Al3+ will diffuse toward the cathode. The OH- in the solution diffuses toward the anode, and white flocculent Al(OH)3 precipitates at the interface between the cathode and the anode. After drying, white spots are formed on the surface of the aluminum material. The so-called spot corrosion. The corresponding chemical equation is as follows:
Al→Al3++3e (anode)
Al3++3OH-→Al(OH)3↓ (cathode)
4 Influence of the active elements 4.1 Accelerating effect of Zn The solid solution of zinc in the aluminum alloy accelerates grain erosion by means of "dissolution-redeposition", zinc or iron deposited on the surface of the alloy and high-potential desolvable material FeSiAl and free. The cathode particles such as silicon can play an effective cathode role, accelerating the reduction process of dissolved oxygen, and promoting the continuous expansion and deepening of corrosion.
Alkali washing of the Zn element dissolves in the lye as Zn(OH)42- and Zn(OH)-3 as the Al dissolves. Since the potential of Zn (-0.76 V) is more positive than the potential of Al (-1.67 V), when the concentration of Zn ions in the alkaline solution increases to a certain value, Zn will selectively deposit in the pit. On the object, there will be an abnormal phenomenon of high Zn. On the other hand, due to the large potential difference between Zn and Al, the corrosion current in the microbattery is large, and the depletion region (basically pure aluminum) of the Fe and Si anodes of the cathode particles dissolves relatively quickly. For spot corrosion.
4.2 Cl-activation As an external factor, Cl- is very sensitive to spot corrosion and has the function of inducing and emphasizing erosion. The results of the study showed that the Cl- in the defatted acid adsorbed at the defects of the passivation film and penetrated through the passive film to adsorb on the substrate. Here, the aluminum element is rapidly dissolved due to activation, and the passivation film is destroyed to form a galvanic cell structure. Under the action of the acidic medium, the local corrosion current is large. At this time, Cl- and dissolved A13+ are complexed as follows: Reaction: Al3++Cl-+H2O→AlOHCl++H+, the acidity of the solution is further enhanced, and the corrosion condition is further deteriorated. When the Cl-concentration is increased, the complexation reaction proceeds to the right, and the active sites on the passivation film will greatly increase, and will be preferentially dissolved in the subsequent alkali wash process, thereby causing more severe spot corrosion.
4.3 Promotion of pH When the pH of the water is less than 2 or greater than 4, the spot corrosion rarely occurs. When the color is dark, the crystal grains change from gray to black, and the pH value in the washing tank plays a certain role in promoting.
When pH>4 in water washing, the passivation film formed on the surface of aluminum profiles is relatively complete and dense, and the adsorption, activation, and destruction of H+ and Cl- are greatly weakened. Therefore, the profile has little or no corrosion; when pH<2, The surface of the aluminum profile is in a state of active dissolution and no passivation film is formed, so no spot corrosion will occur.
5 Conclusion The spot corrosion of 6063 aluminium profile is caused by the segregation and coarsening of the anode phase Mg2Si in the aluminum alloy. The impurity element Zn in the alloy and Cl- and pH in the solution accelerate the occurrence and development of spot corrosion. The mass ratio of Mg and Si in the alloy should be properly adjusted, and the content of silicon should not be too high, and the aging system should be reasonably arranged to prevent the segregation of Mg2Si particles so as not to affect the corrosion performance of the aluminum profile. Control the trace element Zn in the alloy and the Cl-concentration and pH of the solution during the treatment to reduce the negative effect of the active element.
Fiberglass Tape Measure,Tape Line Measure,Magnet Tape Measure,Abs Fiberglass Tape Measure
Shangqiu Hengli Measuring Tools Co.,Ltd , https://www.jjmtools.com