Chromate conversion coatings on aluminium: influences of alloying

Abstract

The growth kinetics of chromate/fluoride conversion coatings are examined for 99.99% aluminium, and Al–2.3at.%Cu, Al–1.9at.%Au and Al–20at.%Au alloys. The thickening of coatings and thinning of substrates, the latter deposited by magnetron sputtering, are determined by transmission electron microscopy. The results reveal consumption of substrates during the coating process, to the maximum immersion time of 24 min. Initial relatively rapid thinning in the period to 6 min is followed by slower, approximately constant thinning at about 9.3, 6.2, 6.0 and 5.4 nm min −1 for the 99.99%Al, Al–2.3at.%Cu, Al–1.9at.%Au and Al–20at.%Au alloys respectively. The ratio of the number of chromium atoms in the conversion coating, determined by Rutherford backscattering spectroscopy, to the number of oxidized substrate atoms is about 3, indicating a low efficiency of coating growth. Alloying decreases the coating thickness, as well as metal consumption. Alloying elements initially enrich beneath the coating, in alloy layers of a few nanometres thickness. Later, copper may be oxidized, and then enter the coating, whereas gold is occluded as nanoparticles. The gold nanoparticles act as markers to indicate formation of coating material at the metal/coating interface. Sudden loss of coating on the alloys, after about 6–9 min, occurs soon after entry of alloying element species into the coating. The loss may be related to the presence of nanoparticles of gold and copper near the alloy/coating interface, the latter possibly forming by reduction of copper species, that disrupt the bonding of the coating either directly though their presence or through their action as local cathodes.