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Volume 2 Extended Abstract 1

Submitted 26th August 1999

The Influence of Cleaning and Surface Treatment on Filiform Corrosion of Aluminium Alloys

A.J.Kayes, M.J.Robinson & S.Impey

School of Industrial & Manufacturing Science Cranfield University, Bedford MK43 OAL, UK

Keywords: filiform corrosion, cleaning, surface treatment, microstructure, contamination, electropolish

Introduction

Filiform corrosion is a localised form of attack that usually occurs beneath paint films and leads to the appearance of thread-like filaments. It is generally considered to result from contamination on the metal surface and will only propagate in humid conditions. The type of pretreatment used prior to painting has a very important influence on the subsequent occurrence of filiform corrosion. Chromate containing pretreatments have been widely used commercially, although alternatives are being actively sought on health and environmental grounds. A considerable range of susceptibility can be displayed between different alloys. It has been unclear whether this range of behaviour is caused by differences in the alloy compositions or by some feature of their microstructure or surface condition. In the work described in this paper the individual factors that influence an alloy’s susceptibility to filiform corrosion have been studied in detail. In particular, the effects of various cleaning methods and chromating treatments have been investigated, together with the influence of surface contamination, grain structure and mechanical and electrochemical polishing.

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 [1] Clean in acetone [2] Alkaline clean in Ridoline 1806 (30s) [3] Rinse in tap water [4] Acid rinse in 23% HNO3 (15s) [5] 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 treatments

Filiform 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.

Results

It was found that the most useful measure of the extent of filiform corrosion was the total length of the filaments that developed on the 70 mm long scribe during the 1000 hours exposure. In each case, these total lengths were expressed as Relative Filament Lengths (%) by comparing them with those that formed on 3003A when tested in the painted, mill finished condition, without any form of pretreatment. The results of the filiform testing are summarised in Fig 1. By performing tests in which different areas of the panels were polished in either the longitudinal or transverse direction it was possible to distinguish between effects caused by the surface layer and those due to the substrate. In other tests, the elongated grains were recrystallised by heat treatment at 535C to investigate the influence of the substrate microstructure. The results indicate that filiform corrosion behaviour is influenced most strongly by the properties of the original surface layer.

Fig 1. Summary of effects of surface treatments on filament lengths on 3003A

Conclusions

[1] The filiform corrosion susceptibility of aluminium sheet was shown to be more closely related to the condition of the original surface layer than to the microstructure of the substrate. This was due to the presence on the surface of high temperature oxides, rolling lubricants and other contaminants from processing.

[2] 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.

[3] 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.

[4] 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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