Electrochemical Investigation of Filiform Corrosion on Aluminum Alloys

• Author / Creator

  Arango Vasquez,Daniela

• Filiform corrosion (FFC, also known as under-film corrosion) is commonly observed on surfaces
  of coated aluminum (Al) and aluminum alloys (AA), especially in environments combining marine
  and industrial atmosphere. The chemical composition of the substrate is believed to play an
  important role in FFC resistance of AAs. However, there is still a lack of electrochemical studies
  focusing on clarifying the FFC behaviour of commercially used Al alloys. This work aims to
  investigate the FFC response of different AAs. To do so, commercially pure Al (AA1100) and
  AAs such as 2024 (Cu), 3003(Mn), 5052 (Mg), 6061(Mg, Si) and AA7075(Zn) were selected as
  candidate materials. The FFC behaviour of AAs was investigated using electrochemical methods.
  To simulate the under-film electrolyte environment, polarisation measurements were carried in a
  deaerated anolyte (5% NaCl + 0.1M AlCl3, pH=2) and an aerated catholyte (phosphate buffer
  solution, pH=7), respectively. Three factors were defined, including the resistance to FFC
  initiation (ΔEPR, V), the driving force for propagation (ΔEcorr, V), and the FFC kinetics of
  propagation (iFFC, mA/cm2
  ). Results suggest that ΔEPR follows a tendency of AA 2024 (Cu) > 5052
  (Mg) > AA7075 (Zn) > 3003 (Mn) >AA6061 (Mg, Si) >AA1100; ΔEcorr followed the trend AA
  6061 (Mg, Si) > 7075 (Zn), 3003 (Mn) > 5052 (Mg) > 2024 (Cu), 1100 and iFFC followed the trend
  7075 (Zn) > 6061 (Mg, Si) > 3003 (Mn) > 5052 (Mg) > 1100 > 2024 (Cu).
  In addition, the influence of anodizing on the different substrates was evaluated using the same
  electrochemical parameters described for the bare alloys. AAs were pretreated with a three-step
  anodizing process: electropolishing in a 1:4 solution of HClO4:C2H5OH at 10 °C, sulfuric acid
  anodizing in 100 g/L H2SO4 at 18 °C and hydrothermal sealing in boiling DI water. EC
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  measurements showed that anodizing could effectively lower the susceptibility of AAs to FFC.
  Resulting in a decrease in both, the driving force for FFC propagation (ΔEcorr) and FFC kinetics
  (iFFC).
  Finally, the validation of the EC results was done by an accelerated exposure test. Bare and
  anodized AA disks were coated with a clear epoxy layer, scribed, and placed in an environmental
  testing chamber at 80% RH at 40 ºC for 1000h. Two parameters were defined (i.e., number of
  threads (N) and length of the filaments (L)), and calculated by visual examination with naked eyes
  and OM. The results from this section agreed with what was found through EC methods. The FFC
  susceptibility of bare samples predicted by EC measurements follows the same trend as that one
  found by accelerated exposure testing. Additionally, anodizing was confirmed to drastically
  increase FFC resistance on AAs, resulting in a strong decrease on N and L after 1000h of
  exposition.

• Subjects / Keywords

  • Filiform corrosion
  • Aluminum alloys
  • Electrochemical Measurements
  • Alloying elements
  • Corrosion

• Graduation date

  Spring 2022

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-2cr0-hs65

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.