Corrosion of Cu in air environment. Magnetron sputtered 1 micrometer thick copper film was standing in dry air environment for 3 years and oxidized spots were forming. Optical microscopy image, width 200 micrometers, brightfield contrast mode.
<a href="//commons.wikimedia.org/wiki/User:Tavoromann" title="User:Tavoromann">Tavo Romann</a>
CC BY 4.0
<span class="int-own-work" lang="en">Own work</span>