1 . Problem Statement
In actual production, T5 state 6063 aluminum alloy extrusion profiles with high processing rates (ε>95%) and thin walls (δ≤1.5mm) exhibit regular (and sometimes irregular) distribution of white spots (or non-luminous marks) on their surfaces after anodizing with sulfuric acid.
In severe cases, dark spots appear – “white spots”.
The distribution and characteristics of “white spots” are as follows: they are a type of superficial defect that appears at approximately equal intervals, in a line or flattened quadrilateral or in an irregular star shape (flake), in planes parallel to the direction of extrusion, with a shallower depth forming a groove in relation to the base surface.
White spots are generally distributed on one or more surfaces of the profile, and sometimes distributed on all surfaces (for thin-walled hollow profiles, distributed on both sides of a given plane or curved surface).
two . Cause analysis
It was observed at the site that “white spots” form during the “alkali pickling” process and do not disappear after subsequent “neutralization” with dilute nitric acid (or sulfuric acid). After anodizing with sulfuric acid, they are even more clearly presented.
The author specially cut two samples of “white spots” with larger areas (F=30-40mm2) from the alkali-etched wash (the solution contains ω(Zn2＋)≥5×106). Next, a DV-5 atomic emission spark direct reading spectrometer was used to quantitatively analyze the components of the “white spot” areas of the two samples. The results are as follows (all data in the table are mass fractions):
From the results of the analysis in Table 1, it can be seen that the content of the elements Si, Mg and Zn in the “white spots” increased significantly. However, the results in Table 2 show that the content of Si and Zn elements in the “white spots” increased significantly, while the content of Mg elements decreased.
From the perspective of metal material corrosion, this surface defect of Mg2Si is essentially the result of “exfoliation corrosion” of the 6063 aluminum alloy material.
Exfoliation corrosion is a type of selective surface corrosion where corrosion occurs along the surface of the metal, and the volume of its products is often much larger than that of the corroded metal, thus expanding.
Generally speaking, when aluminum is adjacent to dissimilar metals with cathodic properties, “exfoliation corrosion” increases. Observations under an electron microscope have found that “exfoliation corrosion” generally occurs along insoluble constituents (such as Si, Mg2Si, etc.) or along grain boundaries.
2.1 Impact of Ingot Quality
The primary phase composition of 6063 aluminum alloy includes solid solution α (Al), free Si (anode phase) and FeAl3 (anode phase). When the iron content is high, β(FeSiAl)(anode phase) is present; when the iron content is low, α(FeSiAl)(cathodic phase) is present. Other possible impurity phases include MgZn2, CuAl2, etc.
During production, 6063 aluminum alloy ingot often exhibits macrosegregation or intracrystalline segregation due to the non-equilibrium crystallization process. Consequently, elements such as Si, Mg, Zn and Cu are unevenly distributed within the ingot.
Some aluminum profile processing companies, often for economic reasons, rarely carry out homogenizing annealing treatment on small-sized ingots (e.g. less than φ100mm) to eliminate the segregation phenomenon, thus paving the way for creating “ white spots.”
2.2 Impact of the Extrusion Heat Treatment Process
To improve production efficiency, low-temperature and high-speed extrusion is commonly adopted in production operations. The “thermal effect” caused by the extrusion speed significantly increases the quenching temperature of the product at the exit of the die.
When coming into contact with a graphite plate (or wheel) with a surface temperature of 80-110 (or slightly lower) on a fixed output table, the profile surface undergoes a “quick-cooling heat exchange”, causing concentration of the alloying elements Mg and Si in that part be higher than in normal areas.
In the subsequent artificial aging process, this area will precipitate the coarse β′ (Mg2Si) phase. Ingots of 6063 aluminum alloy, which have not undergone homogenizing annealing treatment and have a low heating temperature, due to the insufficient “thermal effect” caused by extrusion, cannot raise the profile quenching temperature above 500.
This not only results in a small portion of the β(Mg2Si) phase in the ingot remaining in the profile structure, but also triggers the previously mentioned changes in the Mg and Si elements which are high-temperature solid solutions in the α(Al) matrix. Internship. These factors prepare the structural conditions for the appearance of “white spots”.
2.3 Effect of surface alkaline corrosion treatment
For a Si content greater than the Fe content, excess Si tends to aggregate in the α (Al) crystal or near the crystal boundary, forming a free single-crystalline Si phase. The Si cathode phase and the segregated anode phase Mg2Si, or the matrix α(Al) anode phase and the coarse cathode phase Mg2Si, induce a “primary battery effect” in the alkaline etching solution.
The result is rapid dissolution of the α(Al) solid solution around the free Si or preferential dissolution of the thick Mg2Si phase compared to the α(Al) solid solution, leaving shallow, flat “etching pits” in the profile surface.
Furthermore, some researchers suggest that the white spots are related to the hydrolysis reaction of NaAlO2. When the ratio of Al3+ concentration to total NaOH concentration exceeds 0.35, the stability of NaAlO2 decreases and hydrolyzed Al(OH)3 precipitates on the surface of aluminum material.
Incomplete washing with water can also easily result in mottled or blocky “white spots”. However, this is believed to be mainly related to the effect of scale inhibitors (such as hydroxycarboxylates, sodium tartrate, etc.) contained in alkaline corrosion additives.
Specifically, under stable conditions of the alkaline corrosion process, hydroxycarboxylates can reversibly complex with Al(OH)3 to form soluble complex anions.
2.4 Factors that Influence the Anodizing Process with Sulfuric Acid
Generally, when the sulfuric acid concentration is too high, the electrolysis temperature is excessively high, or when the Al3+ content in the sulfuric acid solution of the oxidation tank exceeds 20g/L, the following ionization equilibrium condition at a temperature normal (about 20 degrees) is interrupted.
With the increase of Al3+, the Al(OH)3 in the sulfuric acid oxidation tank precipitates and adheres to the grooves on the profile surface or within the holes of the Al2O3 film in a flocculated form. It cannot be washed well with clean water, nor is it easy to seal the pores. After air drying, white spots appear on the surface.
3. Solutions
① Strictly control the chemical composition, requiring that the excess Si does not exceed 0.20% and the Zn content does not exceed 0.05%. In addition, strive to evenly anneal the ingots and apply rapid cooling to the ingots after treatment.
② Modify the shaft of the first graphite roller on the fixed unloading table to make its height adjustable. If possible, use a material with better insulation than graphite.
③ Employ low threshold temperature extrusion to avoid local overheating or minimize the duration of overheating, so that the 6063 aluminum alloy does not have enough time to precipitate the β′(Mg2Si) phase.
④ Add a precipitant, Na2S or sodium hydrosulfide, to the caustic etching solution, in an amount equivalent to twice the mass required to form the ZnS precipitate. When the Al3+ in the alkaline solution exceeds the control standard, supplement timely with caustic etching additives.
