Microstructural characterization and film-forming mechanism of a phosphate chemical conversion ceramic coating prepared on the surface of 2A12 aluminum alloy

Abstract

Phosphate chemical conversion (PCC) ceramic coatings on the surface of 2A12 aluminum alloy substrate have been fabricated by a simple and inexpensive chemical conversion process in CrO₃–NaF–H₃PO₄ solution. Microstructure characterization showed that the average diameter of micro-pores and the thickness of the PCC ceramic coating were about 50 nm and 4 μm, respectively, and the ceramic coating was compact and uniform when the conversion time was 60 min. Meanwhile, we found that the PCC ceramic coating mainly consisted of AlPO₄, AlOOH, AlF₃, and a few amorphous phases (CrPO₄ and CrOOH) via EDS, XRD, XPS analyses. TG-DSC results indicated that the PCC ceramic coatings had excellent thermal stability. Significantly, the adhesion strength (178.55 N) between the PCC ceramic coatings and 2A12 Al substrate was remarkably improved owing to the strong chemical bond between the PCC ceramic coating and 2A12 Al substrate and the increase of surface roughness. Furthermore, a lower corrosion current density (1.382 × 10⁻⁷ A cm⁻²) and a higher corrosion inhibition efficiency (99.91%) confirmed that PCC ceramic coatings had fantastic corrosion resistance because of the presence of crystalline AlPO₄/AlF₃/AlOOH and amorphous CrPO₄/CrOOH as a barrier layer. Additionally, a possible film-forming mechanism of the PCC ceramic coating was proposed during the chemical conversion process, which could be divided into four stages: dissolution of 2A12 aluminum substrate and hydrogen evolution; crystallization of insoluble phosphates and formation of an amorphous phase; growth of insoluble phosphates and dissolution of PCC ceramic coatings; growth and dissolution of PCC coatings to dynamic equilibrium.