A recent article in npj Materials Degradation proposed a multilayer system for protecting aluminum cast alloy AlSi7Mg0.3 from corrosion. The system uses a hexafluoro-zirconated trivalent chromium coating (Zr-CrCC) and a polyacrylic/siloxane-silica (PEHA-SS) coating.
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Background
Alloys of the 3xx.x series constitute 90 % of all cast aluminum alloys produced, with AlSi7Mg0.3 being the most common. This lightweight material is widely used in engines, automobile wheels, aircraft, compressors, and pumps.
Although AlSi7Mg0.3 has good corrosion resistance, additional protection is beneficial under harsh conditions. Current chromate conversion coatings are environmentally hazardous. Zirconium-based coatings are more environmentally acceptable and enhance the corrosion properties of aluminum alloys.
This study investigated the corrosion and adhesion characteristics of Zr-CrCC on cast aluminum alloys, both as an individual coating and in a multilayer system with a thin organic sol-gel coating (PEHA-SS).
Methods
AlSi7Mg0.3 were cut into cuboids (6×4×1 cm) samples and ground to obtain even surfaces devoid of the oxide layer and other impurities. Coating with Zr-CrCC involved alkaline degreasing, acid de-smutting, and passivation.
The PEHA-SS coating was applied by dipping the alloy specimens in synthesized sols. The combined Zr-CrCC and PEHA-SS coatings formed the multilayer Zr-CrCC+PEHA-SS system.
The morphology of the specimens was analyzed using a field emission scanning electron microscope (FESEM). Adhesion was tested with a cross-hatch tester kit. Electrochemical measurements were performed in a three-electrode system at room temperature in 0.1 M NaCl solution.
Measurements were recorded after one hour for ground and Zr-CrCC samples, and after one week and four months for PEHA-SS and Zr-CrCC+PEHA-SS samples. Corrosion testing followed international standards in a salt-spray chamber with NaCl solution for up to seven days.
Results and Discussion
FESEM results showed that Zr-CrCC provided inhomogeneous coating coverage on AlSi7Mg0.3, with three regions of different compositions and thicknesses. Micro-cracks were observed, but the Zr-CrCC, despite being around 100 nm thick, enhanced corrosion protection for one week.
The PEHA-SS-coated surface was smooth, with no cracks or visible pores, and even coverage. The PEHA-SS coating contained small, randomly arranged silicon domains and was about 9 µm thick.
During adhesion tests, the PEHA-SS coating did not peel or flake, confirming strong adhesion due to covalent bonds. The higher impedance values further confirmed the efficient barrier protection of the alloy surface by PEHA-SS compared to the non-coated and Zr-CrCC samples.
However, the PEHA-SS coating’s impedance dropped significantly after one week of immersion in NaCl solution due to the swelling effect of the organic phase.
The multilayer Zr-CrCC+PEHA-SS system behaved differently from the PEHA-SS coating alone. It did not crack, but peeling occurred at certain spots along the scribe. The Zr-CrCC layer remained firmly adhered, enhancing the adhesion of the PEHA-SS coating.
The multilayer system showed less impedance drop over prolonged immersion compared to PEHA-SS alone, confirming its durability.
In salt spray corrosion tests, the uncoated alloy surface was heavily corroded after one week, while Zr-CrCC and PEHA-SS samples showed slowly progressing localized corrosion. The Zr-CrCC+PEHA-SS system demonstrated superior corrosion protection, with corrosion products visible only along the scribe.
Conclusion
The proposed multilayer system of Zr-CrCC and PEHA-SS for AlSi7Mg0.3 corrosion protection showed a complementary effect. Despite differing in thickness, morphology, and composition, these layers worked synergistically.
The active properties of Zr-CrCC provided long-term corrosion protection through self-healing, prolonging the coated alloy’s service life.
Notably, the self-healing capabilities of Zr-CrCC are generally effective only against minor damages, such as a scribe, and can vary with factors such as coating formulation and environmental conditions.
This study offers significant insights into the interfacial processes of the multilayer system and provides a basis for further research on combining different coatings for optimal protection of aluminum alloys.
Journal Reference
Rodič, P., Kapun, B., Milošev, I. (2024). Complementary corrosion protection of cast AlSi7Mg0.3 alloy using Zr-Cr conversion and polyacrylic/siloxane-silica multilayer coatings. npj Materials Degradation. doi.org/10.1038/s41529-024-00467-5,
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