Volume 2 Paper 1

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

A.P.Kayes, M.J.Robinson & S.Impey
School of Industrial & Manufacturing Science Cranfield University, Bedford MK43 OAL, UK

Abstract

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

Introduction

Methods

Materials Filiform corrosion tests were carried out on the three alloys shown in Table 1. The two 3003 alloys were produced by different manufacturers and all were in the form of 0.8mm thick sheet.

Alloy	Si	Fe	Cu	Mn	Mg	Cr
3003A H24		0.60	0.14	1.10	0.10	-
3003B H26	0.20	0.50	0.06	1.00	-	0.20
3105 H25	0.44	0.60	0.20	0.50	0.40	0.03

Table 1. Composition of the alloys

Surface treatments 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 2. 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 painted by bar coating with 3-5 microns of polyester primer, dried for 2 hours at 60°C and then given 13-15 microns of polyester top coat and dried for 10 hours at 50°C.   

Table 2. 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 42°C and 82% RH for 1000 hours. The extent of filiform corrosion was quantitatively assessed by measuring the lengths and areas of the filaments that developed on each scribe using a Leica 500QMC image analyser.

Results and Discussion

Fig 1. Filiform corrosion on non-chromated panels 

  Fig 2. Comparison of filament growth rates on chromated panels 

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 finish condition, without any form of pretreatment. The filament lengths were consistent between the two replicate panels, being within 15% of each other in all cases and within 10% on panels that had been chromated. 

General effect of commercial cleaning and chromating treatments Fig 3 shows the effect of each step in the pretreatment process on the lengths of the filaments that formed on 3003A. The commercial alkaline cleaner Ridoline 1806 removes some aluminium from the surface and this is seen to be more effective than cleaning with acetone alone. A further reduction in filament length occurred when the surface was chromated. It is interesting to note the extent of this improvement when it is considered that the conversion coating is discontinuous [8] as shown in the TEM micrograph in Fig 4.

Fig 3. Effect of cleaning and chromate pretreatment on filament lengths on 3003A

Fig 4. Chromate conversion coating on alloy 3003B 

Cleaning treatments The effectiveness of the different cleaning treatments for each of the alloys can be seen in more detail in Fig 5. Cleaning in Ridoline for 30s removed 5-10 nm of aluminium from the surface. In contrast, the additional step of cleaning for 20s in 1M NaOH solution removed a thickness of approximately 200 nm and this resulted in reduced relative filament lengths.

Fig 5. Comparison of the effects of different cleaning methods 

Chromating time In each case, longer chromating times slightly reduced the extent of filiform corrosion as shown in Fig 6. This is assumed to result from the accompanying increase in the thicknesses of the chromate conversion coatings with treatment time. The chromating times of 45, 90 and 180s corresponded to coating thicknesses of approximately 60, 120 and 230 nm, respectively. These treatment times were considerably longer than those used commercially, where catalysts are added to the bath and where the time required to produce a 60 nm coating is typically in the order of 5 seconds. Even the thicker coatings were nodular in appearance and the discontinuities are sites at which corrosion initiation will occur. It is believed that the effectiveness of chromating is due, in part, to the improved paint adhesion.  

Fig 6. Comparison of the effects of different chromating and Cr VI rinse times

Chromating is usually followed by a chromium VI rinse and this gave a further small reduction in the filament lengths. It is probable that some residual Cr VI remains on the surface after the rinse and that this acts as a corrosion inhibitor, although it was not possible to detect its presence in this study. Further reduction of chromium during rinsing would improve the properties of the conversion coating but this reaction is believed to be slow and the use of the chromium VI rinse on non-chromated panels only reduced the filament lengths by 10%. 

Effect of surface contamination The influence of surface contamination was investigated by applying tracks of NaCl solution to pretreated panels prior to painting. The relative filament lengths that developed on contaminated and non-contaminated regions of chromated alloys are compared in Fig 7. Similar increases in filament lengths occurred on panels that had been mechanically abraded but not pretreated.

Fig 7. Influence of NaCl contamination on the filament lengths on chromated panels

Dissolved salts have an important effect on film breakdown in aluminium alloys and they are known to concentrate in localised corrosion cells, yet the increase in filament growth resulting from deliberate chloride contamination was surprisingly small. This could be related to the way that the filiform corrosion test was performed in that a high concentration of chloride was also deposited on the surface of the scribe during exposure to HCl vapour in the initiation stage. It was shown that increasing the initiation time caused longer relative filament lengths due to the higher concentration of salt deposited. 

Effect of surface polishing It is known that filaments tend to follow the surface irregularities introduced by the rolling process (typically 5 μm deep ) and that there is little development of filaments in the transverse direction [6]. For this reason, it was expected that the extent of filiform corrosion would be affected by polishing the surface with different grades of silicon carbide papers. Fig 8 shows the relative filament lengths for non-chromated panels that had been polished with 320 grit (45 μm), 800 grit (22 μm) and 4000 grit (5 μm) papers. The finer polishing had a beneficial effect in reducing the filament length and these results appear to support the view that filiform corrosion is enhanced by surface topography.

Fig 8. Effect of mechanical polishing using different sizes of abrasive particles

Although the corrosion filaments (typically 70 μm diameter) are larger than the surface irregularities [6,9], it is likely that the occluded cell at the head of the filament can propagate more easily along the irregularities and finer polishing would have the effect of reducing these pre-existing corrosion paths. This point was examined further by electropolishing the surface to remove all grinding and rolling marks and the results of these tests are discussed later. An alternative explanation for the effect of surface topography on the mill finished panels is that paint adhesion may be poorer at the bottom of the rolling marks. This could be the result of inadequate cleaning, particularly where rolling lubricant has been incorporated into the surface. 

Effect of polishing direction A further possible explanation for filiform corrosion following the rolling direction more easily than the transverse direction is because it is affected by the elongated microstructure of the substrate rather than by the surface topography. (The aspect ratios of the grains in the three materials are shown in Table 3.) This possibility was examined by testing panels in which different areas had been polished in either the longitudinal or the transverse directions. It was shown that the filaments followed the direction of the polishing marks rather than that of the elongated grains and the relative filament lengths were the same in each case. The results indicate that the directional filiform growth is caused by characteristics of the surface layer rather than by the properties of the substrate microstructure [10].

Table 3. Comparison of grain aspect ratios of the sheet materials

Effect of substrate recrystallisation The above conclusion was confirmed by conducting tests in which panels were recrystallised by heat treatment at 535°C prior to abrading down to 4000 grit paper (5 μm particle size), followed by painting. The results in Fig 9 show that the same relative filament length occurred in as-received and recrystallised material, again indicating that filiform corrosion was not controlled by the substrate grain shape. 

Effect of electropolishing The smallest relative filament lengths from all the tests occurred on panels that were electropolished. The results for non-chromated 3003B are shown in Fig 9. There was only a small difference between the results of tests on as-received and recrystallised material.

Fig 9. Effect of recrystallisation and electropolishing on non-chromated 3003B

It appears that the excellent performance of the electropolished surface could be attributed in part to the reduction in surface roughness. A further consideration is that each stage in polishing not only reduces the irregularities but it also involves the removal of more of the surface. The electropolished panels, which were the least susceptible to filiform corrosion, had more material removed during preparation. In a similar way, sequential mechanical polishing (45 μm - 22 μm - 5 μm), that was shown in Fig 8 to reduce the relative filament lengths, also required removing successively more material. However, the mechanical and electropolishing treatments removed approximately 100 μm and 200 μm, respectively. This was considerably greater than the depth of the original surface layer and it is clear that all of the polished surfaces would have been free of high temperature oxide films, impressed rolling lubricant and other sources of contamination arising from processing. It is considered that it was the removal of the surface layer and the resulting freedom from contamination that produced the greatest contribution to the improvement in filiform corrosion resistance but there were additional benefits in controlling filiform corrosion by polishing to reduce the surface irregularities. The effects of all the cleaning and pretreatments are summarised in Table 4.

Table 4. Summary of the effects of cleaning and pretreatment on relative filament lengths

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.

Acknowledgement

References

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