Corrosion and Critical Solute Content of Corrosion-Resistant Binary Solid Solution Cu-Al Alloys in Artificial Perspiration Solution: Implications Towards Tunable Cu-Based Alloys for Antimicrobial Surfaces
Authors: Hutchison Michael John , Zhou Peng , Ogle Kevin , Scully John R. 
Published: February 2020

Copper and its alloys as high-touch surfaces can be antimicrobial, capable of killing even antibiotic-resistant superbugs including Methicillin-resistant Staphylococcus aureus (MRSA)1–5 through controlled release of Cu(I)/Cu(II) cations as a result of corrosion. Therefore, there is interest in designing tunable Cu alloys with regulated dissolution and passivation properties. The inability to predict electrochemical behavior as a function of alloying content remains a critical challenge in both corrosion and electrochemical materials science. Yet, a lack of scientific understanding persists concerning how alloying controls soluble Cu cation release (antimicrobial function) of Cu-based alloys.6–10 A tunable alloy system is needed to balance antimicrobial efficacy, i.e. soluble Cu cation release, with tarnish-resistance which can both be regulated using alloy content optimization. Aluminum bronzes are of interest to study in this context for their relatively improved corrosion resistance due in part to formation of a tarnish-resistant oxide film.11–14The focus of this work is to investigate critical governing mechanisms associated with alloyed Al on protective oxide layer formation in FCC α-Cu, as a function of Al content. This investigation specifically addresses behavior in a realistic environment for high-touch surfaces where these pathogenic bacteria proliferate: human perspiration.

To quantitatively interrogate the effect of alloying FCC α-Cu with Al on corrosion and cation release, commercially pure Cu (Cu), and high-purity arc-melted solid solution alloys consisting of Cu-0.1Al, Cu-1Al and Cu-5Al (wt%) were evaluated in modified artificial human perspiration solution.15 Open circuit corrosion exposures were conducted for up to 130 hours to determine corrosion rate and the fate of Cu and Al. Released aqueous ion concentrations were monitored via inductively-coupled plasma – optical emission spectroscopy (ICP-OES). Al was not observed as a soluble ionic species within ICP-OES limits of detection (LOD). Corrosion products were characterized using grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and quantified with coulometric reduction (CR). Corrosion rates were assessed with mass loss and electrochemical impedance spectroscopy (EIS). Quantitative results of total anodic charge, from both mass loss and calculated from EIS-determined corrosion rates were converted to charge and directly compared to corrosion product charge (CR) and dissolution charge (ICP-OES) to determine the fate of the elements: Cu and Al, through charge balance as a function of Al alloying content. Diagnostic chemical environments were employed where only selected corrosion products are stable e.g., Al2O3 in citric/citrate buffer. For instance, HCl and deaerated citric/citrate buffer solution (CBS) were employed to determine the individual roles of Al in the metal as a metal solute and as an oxidized surface film, respectively.

Primary corrosion products were revealed to be Cu2O and an inner layer of Al2O3 for Al-containing alloys. Cu was dissolved as cuprous (Cu+) ions. Minor Al alloying in solid solution catalyzed Cu dissolution which was counteracted at higher Al contents by a partially passivating layer of Al2O3, achieving complete passivity at ~6 wt% Al according to graph theory.16,17 In contrast, Cu-Al alloys corroded more readily with increasing Al content in the absence of a protective oxide film. The effect of alloyed Al as a dissolution promoter for electrochemical Cu ion release, critical Al contents for stable passivity, and subsequent implications for functional antimicrobial alloys are discussed. These results point to the possibility of making tunable Cu-based alloys for antimicrobial function.