Overview
In 2024 the use of hexavalent chromium (Cr(VI)) will be forbidden by REACH regulation. Many studies have been carried out to replace Cr(VI)-based coatings for lightweight metals such as aluminium and have resulted in the development of new protective layers (SAA, TSA, Cr(III)-conversion coating). However, the removal of protective oxide layers is still performed with Cr(VI)-based stripping processes. Cr(VI)-free stripping with NaOH leads to deterioration of the substrate if the process time exceeds the specification of the very narrow process window.
The Cr Free REAL project aims to develop an innovative process for Cr free removal of anodic layers and conversion layers, which does not deteriorate the aluminium substrate within an applicable process window.
Commercially available or literature mentioned Cr(VI) free stripping agents will be tested and evaluated.
In the proposed approaches, it is planned to develop innovative acidic or alkaline stripping agents containing additives. Corrosion inhibitors (e.g. oxoanions, cerates, thiourea to prevent corrosive attack during the stripping process), accelerators (e.g. nitrates, glycolic acid to enhance the removal of the oxide layer), surfactants and complexants (to help obtaining a complete and uniform removal of the oxide layer and for increase of bath stability) will be employed.
The best performing stripping process will be tested on six types of machined and cast aluminium alloys for removal of Cr(VI)-based and Cr(VI)-free protective layers. The experiments will be performed on lab scale and pre-industrial samples, including complex-shaped parts with a size up to 400*300*200 mm. Specimens which are re-treated with Cr(VI)-free anodic and conversion coatings shall exhibit a corrosion stability with no pits after 500 hours SST and a low fatigue drop, which shall be lower or equal to specimens treated with state-of-the-art processes.