The appearances of these alloys are very different: C72500 is very shiny and polished compared to the dull alloy C70600. This may be due to a passivating oxide layer, which may contribute to efficacy (this alloy is employed in marine engineering because of excellent antifouling properties). Hans et al. (
24) reported that a layer of cuprous oxide (Cu2O) was as effective at killing Enterococcus hirae as pure copper but found that copper II oxide, CuO, is inhibitory by forming a protective layer that reduces the rate of corrosion (passivation layer). We observed that if the surfaces of alloys C70600 and C72500 were abraded by rubbing with coarse sandpaper (
Fig. 3A), the efficacy of the former to inactivate MNV-1 was reduced following 30 min of contact at room temperature (
Fig. 3B). This suggests that the presence of an oxide layer on this alloy contributes to its antiviral efficacy.