(Corrosion Division H. H. Uhlig Award) The Remarkable Versatility of Copper: From its Corrosion Resistance to Antibacterial Properties
Authors: Milosev Ingrid 
Published: December 2024

Copper is one of the most important metals known from the prehistoric Copper Age and is still essential to our everyday lives and industrial applications today. Its remarkable properties include high ductility, malleability, thermal and electrical conductivity, mechanical strength, corrosion and biofouling resistance, fungicide and antibacterial properties. Copper is non-magnetic and easily alloyed with other metals, forming hundreds of copper alloys. It is one of the critical alloying elements of other alloys, increasing strength and facilitating precipitation hardening. In addition, copper is an essential nutrient in the human body. This lecture will explore through the many faces of copper, and selected examples of our studies will be presented, concentrating on three aspects: i) the corrosion resistance of copper and its alloys, ii) the role of copper-containing intermetallics in aluminium alloys, and iii) the role of copper-containing phases in titanium alloys (Figure 1).

1.) The corrosion resistance of copper in neutral and slightly alkaline solutions is based on forming a duplex layer containing an inner cuprous and outer cupric oxide layer. The oxide's thickness and structure depend on the electrode potential, electrolyte, and the presence of aggressive anions, such as chloride, which initiates pitting corrosion. Corrosion inhibitors are commonly used to protect copper alloys. Our experimental studies on organic inhibitors include about thirty formulations based on azoles, benzimidazoles and mercapto-based compounds. Various functional groups were explored; some were pinpointed as efficient, while others were not. One approach to address the challenges of poor or moderate inhibition is to search for synergistic combinations of inhibitors.

2.) In contrast to copper alloys, which are homogeneous mixtures, aluminium alloys are heterogeneous, consisting of numerous intermetallic particles (IMPs), constituent particles and phases. Such systems generally exhibit selective dissolution in aggressive solutions, with Cu-based particles as one of the main culprits for initiating this process. Dealloying Mg and Al from IMPs leads to Cu enrichment, thus contributing to a stronger cathodic character and intensive dissolution of the surrounding aluminium matrix. The mitigation of selective dissolution was tackled in our studies using rare earth, zirconium and chromium conversion coatings, sol-gel coatings and their combinations.

3.) Finally, the role of copper as an antibacterial agent is considered. The scientific challenge is to develop a new class of alloys for implants with long-term inherent antibacterial ability to reduce implant infection rates without jeopardising other biomaterial properties. Using novel additive manufacturing technology, modifying Ti-6Al-4V alloy by adding copper is possible. Scientific challenges include developing the Ti-6Al-4V-xCu alloy with an optimised content of copper, appropriate microstructure, minimum porosity and defined roughness, suitable mechanical properties, high corrosion resistance, antibacterial activity against clinically relevant bacterial strains and biocompatibility.

Studying copper's corrosion and corrosion protection mechanisms and other functional abilities remains challenging in future research.

Figure 1: Images of copper corrosion in slightly alkaline solutions containing (a) iodide, (b) fluoride and (c) corrosion products formed in the presence of chloride ions. (d) Corrosion of Cu-based intermetallic particles in aluminium alloy in sodium chloride solution. (e) Microstructure of additively manufactured Ti-6Al-4V-xCu alloy.

Figure 1