Study of relative color stability and corrosion resistance of commercial copper alloys exposed to hand contact and synthetic hand sweat

• A hand baton was designed that contained many commercial copper alloys.
• During hand contact, some copper alloys discolored while others did not.
• SEM analysis showed that the discolored alloys were pitted.
• A synthetic hand sweat was designed based on salt and vinegar.
• The color-stable alloys had the highest corrosion rates in the synthetic sweat.

Commercial purity copper and four commercial copper alloys were studied to determine their color stability during prolonged hand contact and their corrosion resistance in synthetic sweat containing 0.8% sodium chloride and 3% acetic acid.The hand baton test showed that alloys C752 and C706 with high nickel content remained shiny after two years of exposure to hand contact whereas C110 and alloys C260 and C510 became dark and dull. Glancing Angle X-ray Diffraction showed the presence of cuprous oxide Cu2O on the copper alloys with a thickness between 50 and 230 nm, as determined by chronopotentiometry. The dark appearance of C110, C260 and C510 after hand contact was the consequence of the reflection of light from a rough and porous surface, as verified by Scanning Electron Microscopy. The nickel in the copper alloys kept the alloys shiny with good color stability by keeping a smoother surface.Electrochemical Impedance Spectroscopy showed that copper C110 and brass C260 had the best corrosion resistance, followed by C510 and alloys C706 and C752 had the lowest corrosion resistance, all tested in synthetic sweat. The nickel containing alloys corroded approximately 40 times faster than the C110 in the synthetic sweat. This suggests that the color stable surface of the Cu–Ni samples on the hand baton was the result of higher corrosion rates in hand sweat rather than better corrosion resistance. SEM studies of the samples after immersion, however, revealed very different surfaces and microstructures compared to the hand baton test. The laboratory test protocol could be improved by incorporating cyclic tests (wet and dry) and adding a rubbing component to better mimic actual hand contact.

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