Copper brightness change
My take on the possible chemistry starting with the electrolysis of salt water, which can produce some chlorine. The latter can further react with water as follows:
$\ce{Cl2 + H2O &lt;=&gt; H+ + Cl- + HOCl}$
The created Hypochlorous acid may react with cuprous (here present as a $\ce{Cu2O}$ on the copper metal electrode). A fenton-type reaction based on $\ce{HOCl}$ could then proceed as follows for $\mathrm{pH &gt; 5}$:

$\ce{Cu(I) (s) + HOCl (aq) -&gt; Cu(II) (aq) + ^.OH (aq) + Cl- (aq) }$

The dissolving of the $\ce{Cu2O}$ results in a de facto cleaning of the copper metal surface, which per a reference to quote:

Copper is a beautiful burnished-gold color when it is clean and well maintained, but like all metals, copper can become discolored when exposed to air and water.

For background on the fenton-type reaction, see &quot;Fenton chemistry in biology and medicine&quot; by Josef Prousek, to quote reaction (15) on page 2330, to quote:

For $\ce{Fe(II)}$ and $\ce{Cu(I)}$, this situation can be generally depicted as follows [20,39]
$\ce{ Fe(II)/Cu(I) + HOX -&gt; Fe(III)/Cu(II) + HO^. + X- }$ (15)
where $\ce{X = Cl, ONO}$, and $\ce{SCN}$.