Investigation of the initiation and propagation of filiform corrosion on aluminum alloys by electrochemical techniques

Filiform corrosion (FFC) is a type of localized corrosion commonly observed on coated aluminum alloys (AAs). Preceding research has focused on qualitative approaches of FFC on pure and binary metals. However, there is a lack of studies clarifying the FFC behaviour of commercial AAs. The present study aimed to investigate the FFC response of different AAs via electrochemical (EC) measurements. To do so, AAs 1100, 2024(Cu), 3003(Mn), 5052(Mg), 6061(Mg, Si), and 7075(Zn), were selected. Polarization measurements were carried out and three EC factors were identified from the polarization diagrams: resistance to FFC initiation (Delta EPR), driving force for propagation (Delta Ecorr), and FFC kinetics (iFFC). To validate the EC results, accelerated exposure testing was performed. Coated AAs were placed in an environmental chamber and inspected; two parameters (number of threads (N) and filaments length (L)) were calculated and found to agree well with the EC factors. Results suggested that alloying elements Cu and Mg brought higher resistance to FFC initiation and slower FFC kinetics; that is, AA 2024(Cu) and 5052(Mg) showed the best resistance to FFC. In conclusion, the composition of AAs plays a significant role in their FFC susceptibility and EC measurements are effective for investigating the FFC behaviour of AAs.