Feasibility study to probe the growth of chromate based conversion on Al by means of rotating ring disc electrode (RRDE) measurements

The use of a rotating ring disc electrode (RRDE) is introduced in this paper to study the formation of chromate–phosphate layers on rolled aluminium surfaces. A RRDE was developed containing an Al disc, which was converted with a chromate phosphate film, and a carbon ring, where the Cr6+ reduction rate was measured in situ. The principle of the method is that Cr6+ is transported from the bulk of the solution towards the disc and the ring electrode. At the disc electrode, a fraction of Cr6+ is consumed during the conversion of the Al disc electrode. This leads to a decrease in ring current, which is directly related to the amount of Cr6+ being reduced at the disc electrode. For proper operation of the RDDE system, an electrode with optimised geometrical characteristics is used. In order to be sure that the measured current on the ring is in relation with the Cr6+ reduction rate occurring at the aluminium disc, a number of requirements need to be fulfilled. After showing that the RDDE system is indeed able to monitor in situ the Cr6+ content during conversion, the amount of Cr6+ reduced at the disc electrode is calculated from the recorded ring current time curves. The method allows (i) to follow the growth of a conversion layer quantitatively in situ and (ii) to study the influence of important process parameters such as fluoride content, chromate content of the conversion bath and influence of mass transport.