Tunable Cation Release and Surface Oxides on Cu-Based Alloys: Investigating the Roles of Al and Sn in FCC Cu Solid Solutions with Implications Towards Anti-Tarnishing Combined with Antimicrobial Functionality

Copper alloys have received renewed interest in functional and structural materials and pathogenic microbiological communities due to recent studies which demonstrate its ability to kill/inactivate even antibiotic-resistant so-called ‘superbugs’, such as methicillin-resistant Staphylococcus aureus (MRSA), via soluble Cu ion release. A general rule is that copper alloys with >65% copper exhibit antimicrobial efficacy. However, not all alloy systems are equally effective, and many different alloys behave without clear dose-response relationships traceable to alloying content. The copper alloy surface itself is not antimicrobial, but the soluble Cu ions that it releases into the local environment through the corrosion process are responsible for the antimicrobial efficacy of the alloy. Corrosion is the essential link between alloying and antimicrobial performance, yet we still do not understand how alloying affects corrosion, oxide formation (tarnishing), and ion release for antimicrobial function. Moreover, the critical copper ion concentration necessary for timely reduction in bacteria colony population (i.e., dose-response relationship) is not clear. Fundamental understanding of how alloying affects corrosion and ion release as well as the dose-response relationship is crucial to the effective use and deployment of tunable Cu-based systems as antimicrobial alloys.
A critical balance is needed between corrosion, oxide formation, and ion release; corrosion rates must be sufficient to release enough Cu for desired antimicrobial function with simultaneous adequate suppression of undesired tarnish-forming or passivating oxides.
The objective of this dissertation is to explore two selected alloying elements. Al and Sn were specifically chosen to investigate the effects on corrosion and antimicrobial response as a function of alloy content in binary and ternary alloy systems designed with attributes favorable for Cu ion release (Sn) and corrosion/tarnish resistance (Al). This was accomplished by determining the fate of the elements by measuring corrosion rates, Cu ion release and quantitatively comparing the composition, structure, and molecular identities of all corrosion products including oxides and precipitated compounds. This data was utilized to understand pertinent phenomena, i.e. enhanced release and passivation, as regulated by Sn and Al.
Sn was shown to enhance Cu dissolution by affecting the anodic dissolution kinetics of Cu in Cu-Sn systems, but this could be mediated by formation of a protective inner layer of SnO2 under a Cu2O surface film. The minimum alloying amount of Sn needed for complete layer coverage of SnO2 was determined through a new theoretical framework for specifying the critical alloying content for passivation in duplex films based on solute enrichment theory and wetting phenomena governed by the interphase surface energies of insoluble oxides. This framework could not be applied to Al2O3 as this oxide was not found as a phase-separated oxide co-existing with Cu2O nor a Cu-Al-O phase as predicted from high temperature phase equilibria. Instead, Al3+ was observed to dope the Cu2O oxide as evidenced by structural, chemical, and electronic modifications to the Cu2O oxide indicating a metastable oxide. This dopant effect led to a formation of a thinner more tarnish-resistant oxide than seen in Al-free alloys. These doped Cu2O oxides, while tarnish-resistant, did not impede copper ion release. Investigations of ternary Cu-Al-Sn alloys over a range of compositions in the single-phase region demonstrated that the function of Sn-enhanced Cu dissolution operated concurrently and possibly synergistically with Al-doping of Cu2O and high tarnish-resistance. This combination of properties applied to the copper alloying system permitted both antimicrobial yet tarnish-resistant functions. A certain compositional range of solid solution Cu-Al-Sn alloys were found to be both antimicrobial and tarnish-resistant, able to kill all planktonic E. Coli under 24 hours in using a custom disinfection test.
The key engineering achievement of this thesis is the design of a specific alloy composition tuned to serve a very important societal function that avoids alloying elements which cause preferential solute release (dealloying), allergic responses, toxicity, or unaesthetic tarnishing.
The scientific achievements herein advance the science of alloy design moving away from trial and error alloy approaches towards integrated computation – experimental design where the dose-response behavior is established, and mechanistic understanding is developed regarding the role(s) of each element from phase equilibria, defect chemistry, electrochemical as well as surface science viewpoints.