Effects of fluoride ions in the growth of barrier-type films on aluminium

Highlights: •The effect of fluoride on growth of barrier-type anodic alumina films is studied. •Fluoride ions incorporated into the film migrate faster than oxygen ions. •A fluoride-rich layer is formed at the base of the film. •Fluoride promotes the field-assisted ejection of aluminium ions. •A sufficient fluoride concentration leads to formation of a porous film. -- Abstract: Fluoride ions are commonly present in solutions used in pre-treatments and conversion treatments of aluminium alloys. Because of the intrinsically reactive nature of aluminium metal, alumina layers are either already present on an aluminium surface or form within a matter of seconds on pre-etched aluminium. Hence fluoride ion transport through an alumina layer is an integral part of conversion coating formation. In order to understand the behaviour of fluoride ions in alumina films, the present study investigated the behaviour of fluoride ions during the growth of barrier-type anodic films formed on aluminium at 5 mA cm⁻² in 0.1 M ammonium pentaborate solution. The films were examined by analytical transmission electron microscopy, scanning electron microscopy and glow discharge optical emission spectroscopy in order to determine their composition and morphology. Fluoride ions were incorporated into the films by adding sodium fluoride to the electrolyte. Additions of up to 3.5 × 10⁻³ M sodium fluoride had a negligible influence on the film growth, which occurred at a high efficiency. In contrast, additions of 3.5 × 10⁻² M sodium fluoride reduced the efficiency to about 60%. Dissolution studies demonstrated that fluoride ions had promoted the field-assisted ejection of Al³⁺ ions from the film surface. Using sequential anodizing, the fluoride ions were shown to migrate inwards in the film at a rate about twice that of O²⁻ ions. Thus, a thin fluoride-rich layer of the film was formed next to the aluminium/film interface. Increasing the sodium fluoride content to 3.5 × 10⁻¹ M led to a porous film formed at a low voltage.