Investigation of novel Cr6+-free anodising pre-treatments for enhanced corrosion protection of aluminium alloy 2024-T3

Abstract

Hexavalent chromium has been the industry standard for corrosion protection for many years. Its unsurpassed active corrosion inhibiting capabilities, its incredible versatility and its economic benefits made it a popular all-rounder. Nowadays the widely known toxic and carcinogenic nature have restricted its use within the European Union. More and more research in the field of corrosion science has been focussing on finding safer alternatives, since hexavalent chromium was officially added the US annual report on carcinogens in the 1980s. Before it was used in almost every step of corrosion protective schemes consisting of a pre-treatment, a primer and a topcoat. In this work a novel approach to two industrial anodising pre-treatments (sulfuric acid anodising and tartaric sulfuric acid anodising) was investigated in order to improve the corrosion performance of the corrosion sensitive aluminium alloy 2024-T3. Both are currently used as alternatives to the historically often applied chromic acid anodising procedure, which contains hexavalent chromium compounds. In this work the effect of the anodising electrolyte viscosity, the anodising interelectrode distance and the addition of ceric sulphate to the anodising bath were investigated. Different fractions of ethylene glycol were used to vary the electrolyte viscosity. All anodising procedures used a fixed anodising voltage, temperature, acid concentration and agitation speed. These parameters were not changed. All samples were cleaned before anodising. In order to assess the corrosion performance linear sweep voltammetry, electrochemical impedance spectroscopy and immersion tests were deployed. Furthermore scanning electron microscopy with energy dispersive X-ray spectroscopy was used to evaluate the chemical composition of the anodised substrates.
It was found that an increase of the electrolyte viscosity results in a decrease of the anodising current density, which was related to a decrease in the overall thickness of the oxide layer created by the anodising process. The addition of 25 vol% ethylene glycol did not show any significant changes in corrosion performance although some indications of a slight improvement were found. Slightly smaller pores and a tighter barrier layer were proposed to be a possible explanation. A fraction of 75 vol% on the other hand dramatically deteriorated the corrosion performance, due to much slower oxide growth kinetics resulting in a much thinner oxide. The addition of ceric sulphate did not lead to any significant improvements in the corrosion performance for any of the tested procedures with one exception. The tartaric acid based procedure without ethylene glycol addition did show a significant improvement. Negatively charged complexes of cerium and tartaric acid compounds, which are supposed to be drawn towards the substrate during anodising, were proposed to be a possible explanation. The interelectrode distance did not show any significant differences except for the tartaric acid based procedure with ceric sulphate but without ethylene glycol addition. A higher electric field strength as a results of the smaller interelectrode distance was held responsible. The electric field strength should directly affect the amount of cerium complexes attracted towards the substrate, increasing the chance of cerium ending up as residues in the pores of the anodic oxide layer.