School of Industrial & Manufacturing Science Cranfield University,
Bedford MK43 OAL, UK
Keywords: filiform corrosion, cleaning, surface treatment, microstructure, contamination, electropolish
Surface Treatments. Filiform corrosion tests were performed on 3003 and 3105 aluminium alloys. Panels 145 mm x 80 mm of the mill finished alloys were prepared using a variety of cleaning and pretreatment methods. The standard pretreatment was  Clean in acetone  Alkaline clean in Ridoline 1806 (30s)  Rinse in tap water  Acid rinse in 23% HNO3 (15s)  Distilled water rinse. The influence of each experimental variable was investigated separately by omitting or adding steps in the standard treatment as shown in Table 1. For example, some panels were prepared by cleaning in acetone alone and others had the additional step of cleaning in 1M NaOH. The effects of different chromating treatments were investigated by varying the time of exposure to the Alochrom bath. In general, two replicate specimens were produced for each condition. The panels were then painted by bar coating with 3-5 microns of polyester primer, dried at 120C and given 13-15 microns of polyester top coat.
|Cleaning Treatment alone||Acetone||Ridoline 1806 30s||1M NaOH 20s|
|Pretreatment Alochrom 1225||45 seconds||90 seconds||180 seconds|
|Chromium VI Rinse||6 seconds||12 seconds||24 seconds|
|Surface Contamination||1 m g/mm2 NaCl|
|Initiation Time||30 minutes||60 minutes||120 minutes|
|Recrystallisation||Heat treatment at 535C|
|Mechanical Polish||45 microns||22 microns||5 microns|
|Electropolish||Phosphoric acid / methanol|
Table 1. Summary of surface treatmentsFiliform Corrosion Testing. The painted panels were tested using the standard filiform method BS X 32: 1991. A scribe 2mm wide and 0.15 mm deep was produced in the longitudinal and transverse directions. Filiform corrosion was initiated by exposure to HCl vapour for 1 hour and the panels were then tested at 42C and 82% RH for 1000 hours. The extent of filiform corrosion was quantitatively assessed by measuring the lengths of the filaments that developed on each scribe using a Leica 500QMC image analyser.
Fig 1. Summary of effects of surface treatments on filament lengths on 3003A
 Solvent cleaning of the aluminium prior to painting could reduce some of the subsequent development of corrosion filaments but those treatments that removed the original mill finished surface layer were found to be more effective.
 The use of appropriate alkaline cleaning, together with mechanical or electrochemical polishing, was able to control filiform corrosion to a similar extent to a chromate conversion coating.
 Electropolishing the surface gave the greatest reduction in filament development. This was attributed both to the efficient removal of the surface layer and to the production of a microscopically flat surface, free from the polishing irregularities that promote filiform corrosion.
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