Filiform corrosion imaged beneath protection layers on Al alloys

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Abstract

Aluminium alloys used extensively in aircraft, ships and land transport vehicles are typically protected by a thin conversion coating based on chromium compounds followed by a surface protection layer of polymer paint. Breeching of the protection layer and exposure to a salt spray induces the growth of filiforms from the breech across the aluminium surface under the protective layers. The growth of the filiform is promoted by the formation of a galvanic cell based on chlorine chemistry. In this paper we study the elemental composition of the filiforms using a nuclear microprobe with 3 MeV proton beams. The deep penetration of this beam allows the composition of the intact filiform to be probed in situ through the surface layers. We present elemental maps of the intact filiforms that clearly highlight the presence of Cl in the growing end of the filiform, where the Cl concentration exceeds 20 wt.%, and the peculiar role of potassium in the trail of oxide left behind the growing filiform head.

Introduction

Standard methods for protecting aluminium alloys from corrosion include passivation of the raw metallic surface with a conversion coating, based on chromium compounds, followed by a sealer paint topcoat composed of a hydrocarbon polymer. In recent years, alternative conversion coatings, some based on cerium, have been sought that do not involve chromium because of the toxic properties of chromium compounds.

Breaching of the topcoat polymer, followed by exposure to a hostile corrosive ambient, containing salt spray, can initiate the growth of a filament of corrosion, termed a filiform [1], [2], that propagates along the interface between the conversion coating and the aluminium alloy substrate beneath the intact polymer topcoat. As the filiform grows along the interface it leaves behind a characteristic meandering trail of oxide. These processes are shown schematically in Fig. 1.

Study of the elemental composition of the growing filiform provides insights into the growth mechanism and may be used to evaluate the effectiveness of alternative conversion coatings. A variety of trace elements are responsible for the growth of the filiform and the precise mechanism for the catalysis of the growth are a matter of active research. Trace element studies are complicated by the fact that the filiform is buried beneath the polymer topcoat, typically 50 μm thick, which cannot readily be removed without disturbing the elemental composition and distribution within the filiform.

Nuclear microprobe analysis provides a simple method for probing the composition of the filiform without removal of the topcoat. The ion beam can penetrate through the topcoat to allow proton induced X-ray emission (PIXE) from the buried filiform as the induced X-rays from elements heavier than aluminium can typically escape though the topcoat and be detected.

Section snippets

Experimental method

Aluminium 2024-T3 alloy substrates samples (nominal composition by wt.%: 3.8–4.9 Cu, 1.2–1.8 Mg, 0.3–0.9 Mn, 0.5 Si, 0.5 Fe, 0.15 Zn, 0.15 Ti, 0.1 Cr), surface area 25×25 mm2, thickness 2 mm, were Cr or Ce conversion coated and sealed with a 50 μm polymer topcoat. The topcoat was scribed and the samples then exposed to a salt spray to promote the growth of filiforms. After the filiforms had grown to a nominal length of 15 mm, the samples were washed and dried then analysed in the Melbourne

Results and discussion

Signals in the X-ray spectrum with an energy below 3.5 keV were significantly attenuated due to the intact topcoat and X-ray detector filters. Despite this the Cl signal is significant. Images corresponding to the X-ray lines in the spectrum are shown in Fig. 2, Fig. 3. The filiforms are readily identified from the Cl and K characteristic X-ray distributions and, in the case of the stripped sample, also confirmed from images of the oxygen signal in the RBS spectrum (not shown).

Analysis of the

Conclusion

The filiform growth head is rich in chloride ions and the oxide residue has a chloride concentration greater than 20%. The fact that the highest chloride concentration was measured from an intact filiform in situ beneath the polymer topcoat suggests that there is some loss of material during the topcoat stripping process. Therefore probing the intact filiform through the topcoat provides a superior method for measuring the filiform composition. This is the first time that the chloride

Acknowledgments

This work was supported by a grant from the Australian Research Council: Strategic Partnerships with Industry – Research and Training Scheme.

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