As a common surface treatment technology, anodizing is widely used in aluminum alloys to improve its surface corrosion resistance and wear resistance. Due to high content of elements such as Si and Fe, conventional cast aluminum alloys have poor corrosion resistance and appear gray-black on the surface. They are generally considered unsuitable for surface treatment by anodizing. With development of casting technology, semi-solid forming and other technologies can be used to directly cast aluminum-silicon alloys with lower Si content. This solves to a certain extent problem of poor corrosion resistance of traditional cast aluminum alloys after anodization. Studies have shown that corrosion resistance of cast aluminum alloys after anodization can be significantly improved by changing morphology of Si particles, reducing Fe content, and appropriately controlling thickness of anodized film.
As a very important post-processing step in anodizing process, sealing treatment can change original porous structure of anodized film, better play the overall role of oxide film, and improve its corrosion resistance. It is widely used in anodizing process of aluminum alloys. Hydrothermal sealing (HTS), as one of the most common sealing treatments, can not only improve corrosion resistance of alloy, but also assist in dyeing of oxide film. However, most of existing research focuses on deformed aluminum alloys, and there are few reports on hydrothermal sealing treatment of cast aluminum alloys. Different from deformed alloys, surface of cast aluminum alloys, especially semi-solid formed aluminum alloys, has a surface liquid segregation layer (SLS) with an alloy composition much higher than normal value and containing a variety of structural phases, which will have an important impact on results of hydrothermal treatment.
In this study, rheological die-casting was used to prepare Al-2Si alloy samples, and 6082-T6 was used as a reference material. Effect of hydrothermal sealing treatment on corrosion resistance of surface oxide film was studied through electrochemical impedance spectroscopy experiments, corrosion mechanism was discussed based on microstructure of corrosion pits, aiming to provide reference for application of hydrothermal hole sealing treatment process for cast aluminum alloys.
Graphic and text results
Test Al-2Si alloy was prepared based on ZL101A alloy which was fully melted at 700℃, then added with a mass fraction of 99.9% pure Al and Al-10Sr master alloy. Its main chemical composition is shown in Table 1. Semi-solid formed Al-2Si aluminum alloy specimens were prepared using RheoMetal™ semi-solid flow forming process (see Figure 1). Specific operation steps include: after taking out 700℃ Al-2Si alloy liquid through a spoon, after temperature is cooled to 645℃, submerge heat enthalpy exchange material stirring block preheated at 200℃ and with a melt mass ratio of 5% into melt, and stir at a speed of 1000r/min for 18s; after slurry is prepared, slurry is poured into a die-casting machine with a clamping force of 500kN to obtain a semi-solid formed aluminum alloy sample. In order to facilitate observation of changes in anodized film before and after sealing treatment, removal of Fe-rich surface layer, before anodizing, surface of Al-2Si alloy sample was polished to remove about 20 μm, ultrasonically cleaned in an ethanol solution for 5 minutes to remove contaminants and impurities on sample surface. Sample after above treatment is immersed in electrolyte for anodization. Electrolyte uses 98g/L sulfuric acid solution, oxidation temperature is (23±2)℃, constant AC voltage is 20V, oxidation time is 20min, and cathode material is conductive oxide. After anodization, samples were ultrasonically cleaned in deionized water for 3 minutes. Hydrothermal sealing treatment method is as follows: place cleaned anodized sample in distilled water at 98℃ for about 40 minutes. After sealing, sample was placed in a drying oven at 40℃ for 30 minutes. As a comparison group, 6082-T6 sample was surface polished and then anodized, hydrothermally sealed using same parameters and processes.

Table 1 Chemical composition of Al-2Si alloy (%)

 

Figure 1 RheoMetal™ semi-solid rheoforming process

 

Figure 2 Microstructure of semi-solid formed Al-2Si aluminum alloy

 

(a) SEM (b) Spectrum A, EDS (c) Spectrum B, EDS (c) Spectrum C, EDS
Figure 3 SEM image and energy spectrum analysis of eutectic structure
Micromorphology of anodic oxide films on the surfaces of two samples before and after sealing is shown in Figure 4. It can be seen that eutectic structure dominated by Si phase in microstructure of semi-solid formed Al-2Si alloy sample cannot be dissolved during anodizing process and still remains in anodized film. Comparing micromorphology of anodized film before and after hydrothermal treatment, it can be seen that number of cracks on the surface of oxide film of Al-2Si alloy sample after hydrothermal sealing treatment increased significantly, and most of cracks were mainly located in eutectic region after oxidation; However, oxide film defects of 6082-T6 reference sample did not increase significantly after hydrothermal treatment, which shows that hydrothermal sealing treatment leads to an increase in Al-2Si alloy anodic oxide film defects. This is because hydrothermal sealing treatment causes volume expansion of oxide layer of semi-solid formed Al-2Si alloy sample, resulting in internal stress; since eutectic region contains a large amount of undissolved Si phase, this internal stress is more likely to be concentrated in eutectic region, causing crack defects in oxide layer. In order to further observe micromorphology of oxide film before and after hydrothermal sealing treatment, FIB was used to cut anodic oxide film, as shown in Figure 5. By comparison, it was found that oxide film after hydrothermal sealing treatment had a large number of defects in eutectic region, which further proved that this material caused many defects after hydrothermal sealing treatment.

Table 2 Oxide film thickness of two samples before and after hydrothermal sealing treatment

 

(a) Before treatment, low magnification (b) After treatment, low magnification

 

(c) Before treatment, high magnification (d) After treatment, high magnification
Figure 4 Morphology of anodized film on the surface of semi-solid formed Al-2Si sample under scanning electron microscope

 

Figure 5 SEM morphology of interface of anodic oxide film on the surface of semi-solid formed Al-2Si sample

 

Figure 6 Bode diagram of electrochemical impedance spectrum of sample immersed in 3.5% NaCl solution
It can be seen that as immersion time increases, the total impedance modulus and phase angle of anodized specimens gradually decrease, indicating that all specimens are gradually eroded by corrosive solution. Comparing impedance spectra of samples before and after hydrothermal sealing treatment, it can be seen that in control group 6082-T6, impedance modulus of sample after hydrothermal sealing treatment is higher in low frequency band; while in semi-solid formed Al-2Si alloy samples, impedance modulus in low frequency band of samples after hydrothermal sealing treatment is significantly lower than that of samples without hydrothermal sealing treatment. In order to further analyze corrosion resistance of samples, impedance data of all samples were fitted using Rel (QoxRox) (QinRpo) equivalent circuit, as shown in Figure 7, where Rel is electrolyte resistance; Qin and Rpo are capacitance behavior and polarization resistance (charge transfer resistance) of double electric layer at surface and solution interface respectively. Qox and Rox are capacitance behavior and resistance of oxide film respectively; polarization resistance Rpo can represent the overall corrosion resistance of sample.

 

Figure 7 Impedance data fitting equivalent circuit Rel(QoxRox)(QinRpo)

 

Figure 8 Equivalent circuit fitting sample polarization resistance Rpo value

 

(a) Without hydrothermal sealing treatment (b) After hydrothermal sealing treatment
Figure 9 Scanning electron microscope image of corroded oxide film on the surface of sample

 

(a) SEM morphology (b) Al surface scan (c) O surface scan (d) Cl surface scan (e) Si surface scan (f) Fe surface scan (g) Na surface scan
Figure 10 SEM morphology and elemental surface scanning analysis of corrosion pits on the surface of anodized sample without hydrothermal sealing treatment

 

(a) SEM morphology (b) Al surface scan (c) O surface scan (d) Cl surface scan (e) Si surface scan (f) Fe surface scan (g) Na surface scan
Figure 11 SEM morphology and elemental surface scanning analysis of corrosion pits on the surface of an anodized sample after hydrothermal sealing treatment
In conclusion
(1) After anodized sample undergoes hydrothermal sealing treatment, a large number of crack defects occur on the surface, and cracks are mainly concentrated in oxide film in eutectic region. This may be due to hydration reaction triggered by hydrothermal pore sealing treatment, which causes volume expansion of oxide film, thereby generating internal stress. Since eutectic region contains a large amount of undissolved Si phase, internal stress is more likely to be concentrated in eutectic region, leading to cracks.
(2) Analysis of electrochemical impedance spectrum of oxide film before and after hydrothermal sealing treatment shows that for Al-2Si aluminum alloy, hydrothermal sealing treatment causes a large number of cracks in oxide film, so corrosion resistance deteriorates significantly.
(3) Analysis of morphology of corrosion pits shows that surface of sample that has been treated by hydrothermal sealing is seriously corroded, and number of corrosion pits is significantly more than that of sample that has not been treated by hydrothermal sealing. These corrosion pits are mainly located in eutectic structure, and presence of Fe-rich intermetallic compounds in corrosion pits was observed. Therefore, main reason for formation of corrosion pits is presence of Fe-rich intermetallic compounds in matrix.