As you will see, the stainless steel becomes cloudy and pitted wherever it’s touched by the cotton swab, but remains unchanged under the electrical tape. Congratulations: You’ve just etched metal via electrically induced corrosion.
Stainless steel gets its name from the fact that it doesn’t corrode—but that’s not strictly true. Like ordinary steel, stainless steel contains iron, which notoriously oxidizes when exposed to air, forming rust. Stainless steel, however, contains additional elements, notably chromium. Like iron, chromium does oxidize, but when it does, it forms chromium oxide, a sticky, protective, clear coating on the steel that prevents further oxidation.
Not many substances can break this protective seal of chromium oxide, but a chlorine ion (chloride, Cl-) can. The exact mechanism of action is complicated, but the short story is that the chlorine ion disrupts the oxide, allowing iron chlorides (FeCl2 and FeCl3) to be formed. Both are soluble in water and dissolve, leaving a pit behind. The iron then reacts with hydroxide ions in water, forming insoluble iron hydroxides and liberating the chloride ions to attack more iron.
This kind of etching will happen fairly slowly in saltwater without the addition of electricity, but electricity speeds it up mightily, for three reasons. First, the chloride and hydroxide ions are negative, and are attracted to the positively charged stainless steel. Second, electricity passing through water breaks the water molecules apart, creating extra hydroxide ions that can react with the iron and free the chlorides. Third, while iron is largely insoluble in water, the positive side of the battery removes electrons from the iron, converting it into iron ions, which are soluble.
Swapping the battery terminals inhibits all three of these effects. If you try this alternate arrangement, you’ll see little if any etching.
Electrical tape prevents contact with the chlorine and insulates against electricity, leaving the masked areas with less corrosion.