Results in Materials (Dec 2022)
Effect of immersion time and pH variation on the inhibition of AA2198-T851 alloy by leaching of anticorrosion pigments from free-standing films of organic coatings
Abstract
In this work, organic coating films loaded with leachable anticorrosive pigments, cerium 4-hydroxycinnamate (Ce(4OHCin)3) and strontium zinc phosphosilicate (SZP 391) were immersed in 3.5% NaCl solution at pH 3, pH 5.6, and pH 10. Extracts from each bulk solutions were taken after 24 h, 72 h, 168 h, 240 h, and 336 h to quantitatively measure the leaching of the inhibitive species as a function of time using inductively coupled plasma optical emission spectroscopy (ICP-OES). Other part of the extracts was also employed as test solution to monitor the inhibitive properties of the leached species on AA2198-T851 substrates using electrochemical impedance spectroscopy (EIS). After the immersion process, the unexposed and exposed film surfaces were characterized using optical microscopy, transmission, and scanning electron microscopy (TEM & SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FT-IR). The leaching test results confirmed the release of inhibitive species from the coating films into the test solutions, whose rate increased with immersion time but decrease with increasing pH of the bulk solution. Greater leaching rate was observed at the lowest pH, whose release follows non-Fickian diffusion. Furthermore, the EIS results indicated significant protection of the alloy and the efficiency increased with increasing immersion time and decreasing pH. The extract with Ce(4OHCin)3 pigment shows comparatively higher release rate and better inhibitive properties, with the best at the lowest pH 3. This is attributed to its smaller cluster size and high solubility. The optical and electron Microscopy, XRD and FT-IR of coating films have further complemented the leaching process by revealing pores left behind due to dissolution of large clusters. The pores networks provided excellent transport path for both inward migration of aggressive solution and the release of dissolved inhibitive species.
