Primary-equivalent corrosion protection of post-consumer scrap based aluminium

Abstract

Will increased amount of post-consumer scrap in aluminium increase the susceptibility of powder-coated aluminium alloys towards filiform corrosion? An answer to this so far unanswered question is crucial for large scale industrial implementation of post-consumer scrap based aluminium. Systematic variation of the pretreament consisting of different variants of alkaline etching, acidic oxidising desmutting, and conversion coating shows that variation between specimens depends on details of extrusion processes and oxide thickness instead of initial composition of 6060 aluminium alloys. Excessive Mg surface enrichment, as evidenced through glow discharge optical emission spectroscopy and transmission electron microscopy, prevents formation of ZrO₂ conversion coatings, requiring adaption of pretreatment processes. The observed Mg enrichment is unrelated to recycling. Industrial standardised tests for filiform corrosion according to a Qualicoat standard reveal that after alkaline etching of 2 g m⁻² using standard process chemicals, alloys with Cu and Zn content of up to ≈0.035 mass% fulfil industrial filiform corrosion resistance requirements. The content of Cu and Zn is above the currently considered industrial limit of 0.02 mass%; the exact upper limit of alloy element content has not been investigated in this study. According to results obtained here, increasing amount of post-consumer scrap will not increase susceptibility towards filiform corrosion. Hence, associated industrial standards should orient on performance and not arbitrarily limit alloy composition. Performance-oriented standards could become enablers for increased sustainability of materials production.