Investigation of filiform corrosion of coated 7B04 aluminum alloy by scanning Kelvin probe and FTIR

Abstract:

Four types of samples were obtained by anodizing 7B04 aluminum alloy with sulfuric acid or chromic acid and then spraying primer or primer and topcoat. Then a standard filiform corrosion test was carried out and the corrosion behavior and mechanism of 7B04 aluminum alloy/coating systems were investigated by scanning Kelvin probe and Fourier transform infrared spectroscopy (FT-IR). Optical microscope observations reveal that the number of filaments on the four types of samples is small, and the length is no more than 3 mm, indicating that the four types of specimens all have good corrosion resistance. The results of SKP tests show that the variation of Volta potential of the scribe mark and adjacent area with the test time is similar among the four kinds of samples. The potential difference between the scribe mark and the adjacent coating-metal interface causes the occurrence of filiform corrosion, and the corrosion mechanism is anodic undermining. The Volta potential of a single filament decreases towards the head and increased towards the tail, and the potential difference between the head and the tail promotes the propagation of the filament. The results of FT-IR show that the main components of corrosion products at the scribe mark and under the filament are Al(OH)3, Al2O3, AlCl3，and its partially hydrolyzed products. The main chemical reactions in the filiform corrosion are similar to the pitting corrosion of 7B04 aluminum alloy under the erosion of chloride ion. Comparative analysis shows that the influence of two anodic oxidation modes on filiform corrosion is less different

Key words:7B04 aluminum alloy; coating system; filiform corrosion; scanning Kelvin probe; FT-IR