Recycled Aluminium Alloys and their Models: Role and Behaviour of Alloying Elements during Alkaline Etching

Abstract

Alkaline etching is a common pretreatment for aluminium surfaces. Etching behaviour was compared between an alloy based on post-consumer scrap (PCS) and several model alloys of rolled AA3005 and extruded AA6060 with systematically varied amounts of Mn, Cu, and Ni. Analysis of concentrations of alloy elements in the etching solution by inductively coupled plasma mass spectrometry (ICP-MS) shows that significant fractions of the investigated elements Cu, Fe, Mg, Mn, Ni, and Zn dissolve. Surface analysis of samples in different stages of the etching process show (i) an increase in oxide layer thickness with etching time, (ii) an enrichment of important alloy elements and impurities (Cr, Cu, Fe, Mg, Mn, Si) near the metal/oxide interface, and (iii) the deposition of Mg, Fe, Si-containing aluminium hydroxide on the surface. A comparison with open circuit potential measurements and time-resolved electrochemical polarisation resistance measurements enables a detailed analysis of the etching mechanism. The aluminium dissolution rate during etching is limited by the transport of species through the oxide precursor layer, making it potential-independent. Differences in etching rates between different alloy classes, evidenced by mass loss measurements, are related to differences in the cathodic or anodic reaction mechanisms (hydrogen evolution or metal dissolution) during etching.

Dynamic Electrochemical Impedance Spectroscopy for the Evaluation of Localised Corrosion on Aluminium Alloys

“…The inductive behaviour has been associated with metal dissolution without oxide formation related to the negative differential effect [57] but also with the adsorption of reaction intermediates in the case of Mg, [58] or Al corrosion [59] . A candidate intermediate for this system here could be a hydride species [2,28,60] . The evolution of the inductive loop with time or potential could be used for the study of the reaction mechanism that can in turn be related to different surface pretreatments or electrolyte conditions.…”

“…[59] A candidate intermediate for this system here could be a hydride species. [2,28,60] The evolution of the inductive loop with time or potential could be used for the study of the reaction mechanism that can in turn be related to different surface pretreatments or electrolyte conditions. Figure 5 demonstrates the possibility of following the evolution of parameters such as R ct , C dl , R pore and the CPE as a function of time and thus potential for the different samples.…”

“…One method to study such processes is electrochemical noise analysis which, despite of recent progress in the time/frequency resolution, [27] has the limit that it relies on rather uncontrolled signal generation. Another relatively simple method may be the interleaved, fast measurement of a quantity such as the polarisation resistance, [28] which can however be difficult to interpret.…”

Role of aluminium hydrides in localised corrosion of aluminium revealed by operando Raman spectroscopy

“…Both IMPs and NSDL near the surface are industrially removed by pretreatment such as alkaline etching. 25 A recent series of studies of FFC on coated Mg alloys showed a modification of the schematic mechanism as sketched above, and highlighted also the importance of the chloride concentration. 26,27 On magnesium, HER has been noted to drive a cathodic delamination type of process; the dominance of anodically driven FFC over cathodic delamination strongly depended on the salt used for initiation.…”

“…Both IMPs and NSDL near the surface are industrially removed by pretreatment such as alkaline etching. 25…”