The breakdown in the barrier properties of organic coatings due to filiform corrosion

Filiform corrosion on two grades of aluminium, coated with two different polyester powder coatings has been studied. The growth rate of filiform corrosion was measured, and the change in coating porosity between the coating overlying filiform corrosion filaments and the bulk coating was determined using Electrochemical Impedance Spectroscopy (EIS). Scanning Electron Microscopy (SEM) was used to examine the surface morphology of the coating above filiform corrosion filaments and of the two different aluminium substrates studied. The corrosion rate of the bare metal substrates was measured using Tafel analysis of linear sweep voltammetry. The corrosion activity of the bare metal and surface roughness have an important effect on the growth rate of filiform corrosion. Results from EIS and SEM both suggest a breakdown of the coatings barrier properties above filiform corrosion filaments, with a large increase in porosity and the development of cracks in the coating. It was found that the coating that undergoes the greatest degree of breakdown, in terms of its barrier properties exhibits the greatest filiform corrosion rate.